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5 Common Mistakes in ESD Control & How to Avoid Them

Image scsstaticcontrol July 26, 2018 1

Many companies implement an ElectroStatic Discharge (ESD) Control Program with the aim of improving their operations. Effective ESD control can be a key to improving:

Productivity
Quality
Customer Satisfaction

Problems arise when an organization invests in ESD protective products and/or equipment and then misuses them. Misuse of ESD protective products and/or equipment wastes invested money and can also be causing more harm than good. Today’s blog post will highlight some of the major issues we have come across and how you can avoid or fix them.

About ESD Control and ESD Protection

Remember that for a successful ESD control program, ESD protection is required throughout the manufacturing process: from goods-in to assembly all the way through to inspection. Anybody who handles electrical or electronic parts, assemblies or equipment that are susceptible to damage by electrostatic discharges should take necessary precautions.

Just like viruses or bacteria that can infect the human body, ESD can be a hidden threat unable to be detected by human eyes. Hidden viral/bacterial threats in hospitals are controlled by extensive contamination control procedures and protective measures such as sterilization. The same principles apply to ESD control: you should never handle, assemble or repair electronic assemblies without taking adequate protective measures against ESD.

Common Mistakes in ESD Control

1. Ionizers are poorly maintained or out-of-balance

If an ionizer is out of balance, instead of neutralizing charges, it will produce primarily positive or negative ions. This results in placing an electrostatic charge on items that are not grounded, potentially discharging and causing ESD damage to nearby sensitive items.

Remember to clean emitter pins and filters using appropriate tools. Create a regular maintenance schedule which will extend the lifespan of your ionizers tremendously.

Consider using ionizers with “Clean Me” and//or “Balance” alarms. These will alert you when maintenance is required.

“All ionization devices will require periodic maintenance for proper operation. Maintenance intervals for ionizers vary widely depending on the type of ionization equipment and use environment. Critical clean room use will generally require more frequent attention. It is important to set up a routine schedule for ionizer service.”

[ESD TR20.20 Handbook Ionization clause 15.8 Maintenance / Cleaning]

If you would like to learn more about how ionizers work and what type of ionizer will work best for your application, check out this post for detailed coverage.

2. ESD Garments are Ungrounded

We’ve seen it so many times: operators wearing an ESD coat (without appropriate wrist straps and/or footwear/flooring) thinking they are properly grounded. However, without proper electrical bonds to a grounding system they are not grounded!

Every ESD garment needs to be electrically bonded to the grounding system of the wearer. Otherwise it just acts as a floating conductor. There are a few options to choose from:

Wrist Straps
ESD footwear/flooring
Hip-to-Cuff grounding

“After verifying that the garment has electrical conductivity through all panels, the garment should be electrically bonded to the grounding system of the wearer so as not to act as a floating conductor.

This can be accomplished by several means:

Ground the garment to the body through a wrist strap-direct connection with an adapter.
Ground the garment through conductive wrist or heel cuffs in direct contact with the skin of a grounded operator.
Ground the garment through a typical separate ground cord, directly attached to an identified groundable point on the garment.
Garments should be worn with the front properly snapped or buttoned to avoid exposure of possible charge on personal clothing worn under the garment.”

[ESD TR20.20 Handbook Garments clause 19.4 Proper Use]

ESD clothing loses their ESD properties over time. It is therefore an important part of the ESD Control Program to incorporate periodic checks (see #3 below) of ESD garments.

If you need more information on ESD garments, we recommend having a look at this post.

3. No Compliance Verification Plan / Not Checking ESD Control Products

Companies can invest thousands of dollars in purchasing and installing ESD control products but then waste their investment by never checking their ESD items. This results in ESD equipment that is out of specification. Without the tools in place to check their ESD items, companies may have no idea if they are actually working correctly. Remember: ESD products (like any other product) are subject to wear and tear, and other errors when workstations get moved, ground cords get disconnected…etc. The list goes on.

When investing in ESD control products, make sure you also establish a Compliance Verification Plan. This ensures that:

ESD equipment is checked periodically
Necessary test equipment is available

“A compliance verification plan shall be established to ensure the organization’s fulfilment of the requirements of the plan. Process monitoring (measurements) shall be conducted in accordance with a compliance verification plan that identifies the technical requirements to be verified, the measurement limits and the frequency at which those verifications shall occur. The compliance verification plan shall document the test methods used for process monitoring and measurements. If the organization uses different test methods to replace those of this standard, the organization shall be able to show that the results achieved correlate with the referenced standards. Where test methods are devised for testing items not covered in this standard, these shall be adequately documented including corresponding test limits. Compliance verification records shall be established and maintained to provide evidence of conformity to the technical requirements.
The test equipment selected shall be capable of making the measurements defined in the compliance verification plan.”[ANSI/ESD clause 7.4 Compliance verification plan]

We provide detailed instructions on how to create a Compliance Verification Plan in this post.

4. Improperly Re-Using Shielding Bags / Using Shielding Bags with Holes or Scratches

ESD Shielding Bags are used to store and transport ESD sensitive items. When used properly, they create a Faraday Cage effect which causes charges to be conducted around the outside surface. Since similar charges repel, charges will rest on the exterior and ESD sensitive items on the inside will be ‘safe’. However, if the shielding layer of an ESD Shielding Bag is damaged, ESD sensitive items on the inside will not be protected anymore.

Re-using shielding bags is acceptable as long as there is no damage to the shielding layer. Shielding bags with holes, tears or excessive wrinkles should be discarded.

Use a system of labels to identify when the bag has gone through five (5) handling cycles. When there are five broken labels, the bag is discarded.

ESD shielding packaging is to be used particularly when transporting or storing ESD sensitive items outside an ESD Protected Area.

“Transportation of ESDS items outside an ESD Protected Area (hereafter referred to as “EPA”) requires enclosure in static protective materials, although the type of material depends on the situation and destination. Inside an EPA, low charging and static dissipative materials may provide adequate protection. Outside an EPA, low charging and static discharge shielding materials are recommended. While these materials are not discussed in

the document, it is important to recognize the differences in their application. For more clarification see ANSI/ESD S541. “

[ANSI/ESD Foreword]

This post provides further “dos and don’ts” when using ESD Shielding Bags.

5. Using Household Cleaners on ESD Matting

The use of standard household cleaners on ESD matting can put an ESD Control Program at risk and damage the ESD properties of items. Many household cleaners contain silicone or other insulative contaminants which create that lovely shine you get when wiping surfaces in your home. The problem is that silicone and other chemical contaminates can create an insulative layer which reduces the grounding performance of the mat.

Don’t spend all this extra money on ESD matting and then coat it with an insulative layer by using household cleaners. There are many specially formulated ESD surface and mat cleaners available on the market. Only clean your ESD working surfaces using those cleaners.

“Periodic cleaning, following the manufacturer’s recommendations, is required to maintain proper electrical function of all work surfaces. Ensure that the cleaning products used to not leave an electrically insulative residue which is common with some household cleaners that contain silicone.”

[ESD TR20.20 Handbook Worksurfaces clause 10.5 Maintenance]

Conclusion

There are many more issues we see when setting foot into EPAs and the above list is by no means complete. These are the most common issues we’ve found when assessing EPAs.

It is important to train all personnel using ESD products and/or equipment to follow proper ESD control programs, and maintenance procedures to avoid common ESD control mistakes. Basic ESD control principles should be followed for an ESD control program to be successful:

Ground conductors.
Remove, convert or neutralize insulators with ionizers.
Shield ESD sensitive items when stored or transported outside the EPA.

What mistakes do you commonly see when walking through an EPA? Let us know what you commonly see in the comments and your solutions for fixing them!

For more information on how to get your ESD control program off the ground and create an EPA, check this post.

6 Tips for handling “Class 0” Items

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When talking about ESD Classifications a little while ago, we identified a “class 0” item as withstanding discharges of less than 250 volts.

The introduction of ANSI/ESD S20.20 states: “This standard covers the requirements necessary to design, establish, implement and maintain an Electrostatic Discharge (ESD) Control Program for activities that manufacture, process, assemble, install, package, label, service, test, inspect or otherwise handle electrical or electronic parts, assemblies and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 volts Human Body Model (HBM) and 200 volts Charged Device Model (CDM).”

So how do you handle items that are susceptible to voltages of less than 100V? That’s what we’re going to answer in today’s blog post.

Introduction

Years ago, it was common for devices to be vulnerable to voltages greater than 100 V. As the need for smaller and faster devices increased, so did their sensitivity to ElectroStatic Discharges as circuit-protection schemes were removed to stay ahead of the market. These new extremely sensitive components are now susceptible to discharges nearing 0 V. This causes problems for companies handling these devices: while their ESD program may be in compliance with the ESD Standard, extremely sensitive devices require tighter ESD Control to protect them from ESD failures.

What is a “Class 0” device?

Before moving any further, we need to qualify the term “class 0”. As stated above, the HBM Model refers to any item with a failure voltage of less than 250 V as a “class 0” component. However, in recent times, the term has been used more and more to describe ultra-sensitive devices with failure voltages of less than 100 V. Whilst the following tips and tricks work on any “class 0” item, they are specifically designed to protect extremely sensitive items that withstand discharges of less than 100 V.

*Ultra-sensitive devices are extremely common*

Before Updating Your ESD Program

“Class 0” refers to a wide range of items and there are a few things you should remember before making any changes to your existing ESD program:

Verify what ESD Model your company/engineers/customers etc. are referring to. As we have learnt in the past, there are different ESD models (HBM, CDM, MM) as well as individual classifications for each model. A lot of people get confused when it comes to citing ESD classifications. There is only one “class 0” which refers to the human body model (HBM) but it’s always best to check.
Check the specific withstand voltage an individual part is susceptible to. “Class 0” refers to all items that withstand discharges of less than 250 V. However, there is a big difference between a failure voltage of 240 V or 50 V. You need to have detailed ESD sensitivity information available before being able to make decisions on how to improve your existing ESD control program. This step is part of creating a compliance verification plan.
A part’s ESD classification is only of importance until it is ‘merged’ into an assembly. So, the ESD classification of a device only refers to the stand-alone component. Once it goes into another construction, the classification of the whole assembly is likely to change.

Tips for handling “Class 0” Items

Below are 6 tips that will help your company to upgrade your ESD control program so you can effectively and efficiently handle ultra-sensitive items without risking ESD damage.

One thing to note: The best approach to stay ahead of the game is taking proactive actions. It is critical to figure out how to protect your components from ESD damage before you receive them. If actions are taken after components are received, the components are susceptible to receiving ESD damage.

1. Improve Grounding

Inside an EPA, all conductors (including people) are grounded. Now you’re probably thinking: “But I’ve already grounded my operators and worksurfaces. What else is there left to do?”. Firstly, well done for properly grounding the ‘objects’ in your EPA. The next step is to adjust and improve your current program to allow for even better protection. Here are some suggestions:

Personnel:

Decrease the wrist strap and ESD footwear upper limit. The ESD Association has test data showing charge on a person is less as the path-to-ground resistance is less.
Use continuous monitors and ESD smocks
Introduce/increase the use of ESD flooring
Use sole or full coverage foot grounders (rather than heel grounders)

*Full coverage foot grounders are recommended when handling ultra-sensitive devices*

Worksurfaces:

Reduce the required limit for Point-to-Point resistance of 1 x 10⁹ per the ESD Standard to 10⁶ to 10⁸ ohms (see #5). The reason for this reduction is simple: 1 x 10⁹is too high as it still produces thousands of volts of in electrostatic charges. However, the resistance cannot be too small either as this can lead to a sudden ‘hard discharge’ potentially damaging ESD sensitive components.

Other:

Improve grounding of carts, shelves and equipment to Ground
Minimize isolated conductors like devices on PCBs

2. Minimize Charge Generation

The best form of control is to minimize charge generation. First, you should always use shielding packing products like bags or containers (especially when outside an EPA) as these protect from generating charges in the first place. For more information on choosing the correct type of ESD Packaging, we recommend reading this post.

The next step is to eliminate charges once they are generated – this can be achieved through grounding and ionization. We’ll cover ionization in #3 and #4. We’ve already talked about improved grounding in #1. However, for ultra-sensitive components, we also recommend the following:

Personnel: Use low-charging floor finish
Surfaces: Use low-charging topical antistatic treatments

Both types of ESD products create a low tribocharging coating which allows charges to drain off when grounded. The antistatic properties will reduce triboelectric voltage to under 200 volts.

3. Remove Insulators

When talking about conductors and insulators, we explained that insulators cannot be grounded and can damage nearby sensitive devices with a sudden uncontrolled discharge. It is therefore critical to eliminate ALL insulators that are not required in your EPA: plastic cups, non-ESD brushes, tapes etc. How? Here are a couple of options:

Replace regular production supplies and fixtures with dissipative, low charging versions, e.g. ESD dissipative brushes, ESD dispensers, ESD tape, ESD Chairs etc.
Shield charges on clothing by using ESD smocks.

*Use ESD safe accessories whenever possible*

If an insulator is absolutely necessary for production and cannot be removed from the EPA, you could consider a topical treatment which will reduce triboelectric charges.

Is this not an option, then move on to tip #4.

4. Use Ionization

First, ionization is not a cure-all. We’ve learnt that ionizers neutralize charges on an insulator.

However, that does not mean that you can just have any insulator in your EPA because the ionizer will “just fix it”. No, in this instance, prevention is generally a better option than the cure. So, your priority should ALWAYS be to remove non-process essential insulators from your EPA – see tip #3. If this is not possible – then ionization becomes essential.

Ionization:

Ionizers can be critical to reduce induction charging caused by process necessary insulators
Ionizers can be critical in eliminating charges on isolated conductors like devices on PCBs
Offset voltage (balance) and discharge times are critical considerations depending on the actual application
Ionization can reduce ElectroStatic Attraction (ESA) and charged particles clinging and contaminating products.

It is recommended to use ionizers with feedback mechanisms, so you’re notified if the offset voltage is out of balance.

5. Increase ESD Training and Awareness

ESD Training is a requirement of every ESD Program. When handling ultra-sensitive devices, it is even more important to remind everyone what pre-cautions are necessary to avoid damage. Regular ‘refreshers’ are a must and it is recommended to verify the effectiveness of the training program, e.g. through tests. So, who, when and what should be taught?

*ESD Training is a vital part of every successful ESD Control Program*

ESD training needs to be provided to everyone who handles ESD sensitive devices – that includes managers, supervisors, subcontractors, visitors, cleaners and even temporary personnel.
Training must be given at the beginning of employment (BEFORE getting anywhere near a sensitive products) and in regular intervals thereafter.
Training should be conducted on proper compliance verification procedures and on the proper use of equipment used for verification.

6. Create an enhanced Compliance Verification Plan

We talked in a previous post about compliance verification, what it is and how to create a plan that complies with the ESD standard. So, if you already followed our steps and have a plan in place, here are a few tips to improve your compliance verification plan:

Use a computer data collection system for wrist straps and foot grounders testing
Increase the test frequency of personnel grounding devices from once per day to every time the operator enters the EPA
Use continuous monitors where operators are grounded via wrist straps. Consider computer based monitor data collection system, e.g. SMP. This should include continuous monitoring of the mat Ground.
Use Ground continuous monitors, e.g. Ground Master. At a large facility, the most frequent reoccurring violation is the ESD mat ground cord either becoming disconnected from the mat or grounding point. As Ground continuous monitors will only test the fact that the mat is grounded, it is still imperative that the Resistance to Ground of the mat is regularly tested. Remember that the use of improper mat cleaners can raise the mat surface resistance above the upper recommended level of <10⁹
Test ionizers more frequently or consider self-monitoring ionizers. Computer based data collection systems are a good alternative, too.
Increase the use of a static field meter and nano coulomb testing to verify that automated processes (like auto insertion, tape and reel, etc.) are not generating charges above acceptable limits.

Conclusion

“Class 0” items require additional measures of ESD protection due to their sensitivity to ESD damage. The best way to protect these ultra-sensitive components is to increase ESD protective redundancies and periodic verifications to all ESD Control technical elements.

To decrease the probability of ESD damage while handling ultra-sensitive items, additional precautions are required. This includes additional and/or more stringent technical requirements for ESD control products, increasing redundancies, and more frequent periodic verifications or audits.

Additionally, ESD control process systems should be evaluated as to their performance as a system. It is important to understand how the technical elements in use perform relative to the sensitivity of the devices being handled. Thus, tailoring the process to handle the more sensitive parts. For example: If the footwear/flooring allows a person’s body voltage to reach 80 volts and a 50 withstand voltage item gets introduced into the process, you must either allow only handling via wrist straps or would have to find a way to modify the footwear/flooring performance to get peak voltages below the 50 volt threshold.

Remember: The ESD Standard gives recommendations that will always be behind current/future developments. As soon as a Standard is published, technology will have progressed. In order to protect your devices and company reputation for reliable devices – it is recommended your company take responsibility to implement methods/procedures that exceed the recommendations of the ESD Standard to fit your sensitive component requirements.

References:

Dave Long: Seven Mandatory Considerations Before Handling Class 0 ESDS Devices
Stephen Halperin, David E Swenson, Craig Zander: The Truth About ESD Class 0
Ron Gibson: What is ESD Class Zero?
-Ing. Hartmut Berndt: Requirements on a Class “0” EPA – Basics, Standards, ESD Equipments and Measurements

ESD Symbols You Need To Know

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If you’ve been handling ElectroStatic Discharge (ESD) sensitive devices for a while, you’ve probably come across the various ESD symbols already. But do you know the difference(s) and when to use them?

If you are new to ESD protection or have just taken over responsibility for an existing ESD program, this is where to start!

Introduction

The ESD Standard ANSI/ESD S20.20 requires that “ESDS items, system or packaging marking shall be in accordance with customer contracts, purchase orders, drawing or other documentation.” [ANSI/ESD S20.20 clause 8.5 Marking]. If ESD sensitive items are not covered in any of these documents, each company has to decide whether marking is required. If it is deemed necessary, the ESD Control Program Plan needs to define the details.

ESD Susceptibility Symbol

The ESD Susceptibility Symbol is the most commonly known symbol which consists of a yellow hand in the act of reaching, deleted by a bar; all within a black triangle. It is intended to identify devices and assemblies that are susceptible to ESD.

When to use the ESD Susceptibility Symbol:

The ESD Susceptibility Symbol is correctly used as follows:

on individual components and related documents to state: ‘this device is static sensitive; do not touch without appropriate precautions’
on assemblies and related documents to state: ‘includes static sensitive components; take appropriate precautions’
as part of a sign identifying an area where sensitive devices are handled; to warn all who approach it that precautions are required

Color of the ESD Susceptibility Symbol

The color is optional except “the color red shall not be used because it suggests a hazard to personnel.” [ANSI/ESD S8.1 clause 4.2.1 color].

Normally, the hand and slash symbol is used on a black triangle on a yellow or orange background.

ESD Protective Symbol

Just like the ESD Susceptibility Symbol, the ESD Protective Symbol has a reaching hand in a triangle. However, note the arc and missing slash through the triangle! Because of these differences it has a very different meaning.

This symbol should be on ESD protective products identifying a specialty product that has at least one ESD control property.

The ESD Protective Symbol is also called the ESD Packaging Symbol.

If a letter is under the triangle, it should identify the most important ESD control property:

L = Low Charging
D = Static Dissipative
C = Conductive EPA (for use in the ESD Protected Area)

When to use the ESD Protective Symbol

The ESD Protective Symbol may be used to identify items that possess at least one ESD control property:

Low Charging (formerly referred to as astatic or antistatic)
Resistance (Conductive or Static Dissipative) able to remove electrostatic charges when grounded
Discharge Shielding

Color of the ESD Protective Symbol

The color is optional except “the color red shall not be used because it suggests a hazard to personnel.” [ANSI/ESD S8.1 clause 5.2.1 color].

Normally, a hand symbol is used on a black triangle on a yellow background.

ESD Common Ground Point and Earth Bonding Point Symbols

These two symbols identify where all ESD elements at an ESD workstation should be connected. There is a newer and older symbol; they are very different but basically have the same meaning:

The Earth Bonding Point Symbol has the earth ground symbol and concentric circles around the ground snap, plug or jack. This is the older symbol.
The ESD Common Ground Point Symbol has concentric circles with thick circle around the ground snap, plug or jack. This is the newer symbol.

Both symbols should include text identification.

ESD Common Ground Point (newer symbol) and Earth Bonding Point (older symbol)

Color of the ESD Common Ground Point and Earth Bonding Point Symbols

The color is optional except “the color red shall not be used because it suggests a hazard to personnel.” [ANSI/ESD S8.1 clause 5.2.1 color].

Conclusion

The ESD Standard S20.20 recommends that every organisation handling ESD sensitive items marks their products using the above 3 symbols for every. Correct use of markings will “indicate that an item or material is ESD susceptible and those that indicate that an item is designed to afford some degree of ESD protection.” [ANSI/ESD S8.1 clause 1.1 Purpose]

Symbol artwork for all of the above symbols can be downloaded at no charge from the ESD Association.

Best Storage Conditions for PCBs, and 5-Steps to Create a Dry Package

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ElectroStatic Discharge (ESD) can pose danger to a Printed Circuit Board (PCB). A standard bare PCB (meaning that it has no semiconductor components installed) should not be susceptible to ESD damage, however as soon as you add electronic (semiconductor) devices, it becomes susceptible according to each of the individual’s susceptibility.

While ESD damage can post a danger, there is another risk factor many operators forget: moisture.

Today’s blog post is going to address both risks and will explain how you can protect your PCBs from both when storing them.

The problem with moisture

If you have been following along with our blogs, you will be well aware of the problems ESD damage can cause.

Moisture, on the other hand, may be a new issue to you. Surface Mounted Devices (SMDs), for example, absorb moisture and then during solder re-flow operations, the rapid rise in temperature causes the moisture to expand and the delaminating of internal package interfaces, also known as “pop corning.” The result is either a circuit board assembly that will fail testing or can prematurely fail in the field.

*Moisture from air diffuses inside the plastic body & collects in spaces between body & circuit, lead frame and wires. Expanding vapor can crack (popcorn) the plastic body or cause delamination.*

Storing PCBs

All PCBs should be stored in a moisture barrier bag (MBB) that is vacuum sealed. In addition to the bags, Desiccant Packs and Humidity Indicator Cards must be used for proper moisture protection. This ‘package’ is also known as a dry package.

Most manufacturers of the Moisture Sensitive Devices (MSD) will dictate how their product should be stored, shipped, etc. However, the IPC/JEDEC J-STD-033B standard describes the standardized levels of floor life exposure for moisture/reflow-sensitive SMD packages along with the handling, packing and shipping requirements necessary to avoid moisture/reflow-related failures.

The ESD Handbook ESD TR20.20 mentions the importance of moisture barrier bags in section 5.4.3.2.2 Temperature: “While only specialized materials and structures can control the interior temperature of a package, it is important to take possible temperature exposure into account when shipping electronic parts. It is particularly important to consider what happens to the interior of a package if the environment has high humidity. If the temperature varies across the dew point of the established interior environment of the package, condensation may occur. The interior of a package should either contain desiccant or the air should be evacuated from the package during the sealing process. The package itself should have a low WVTR.”

Components of a dry package

A dry package has four parts:

Moisture Barrier Bag (MBB)
Desiccant
Humidity Indicator Card (HIC)
Moisture Sensitive Label (MSL)

	Moisture Barrier Bags (MBB) work by enclosing a device with a metal or plastic shield that keep moisture vapor from getting inside the bag. They have specialized layers of film that control the Moisture Vapor Transfer Rate (MVTR). The bag also provides static shielding protection.
Desiccant is a drying agent which is packaged inside a porous pouch so that the moisture can get through the pouch and be absorb by the desiccant. Desiccant absorbs moisture vapor (humidity) from the air left inside the barrier bag after it has been sealed. Moisture that penetrates the bag will also be absorbed. Desiccant remains dry to the touch even when it is fully saturated with moisture vapor. The recommended amount of desiccant depends on the interior surface area of the bag to be used. Use this desiccant calculator to determine the minimum amounts of desiccant to be used with Moisture Barrier Bags.
	Humidity Indicator Cards (HICs) are printed with moisture sensitive spots which respond to various levels of humidity with a visible color change from blue to pink. The humidity inside barrier bags can be monitored by the HIC inside. Examining the card when you open the bag will indicate the humidity level the components are experiencing so the user can determine if baking the devices is required.
The Moisture Sensitive Level (MSL) label tells you how long the devices can stay outside the bag before they should be soldered onto the board. This label is applied to the outside of the bag. If the “level” box is blank, look on the barcode label nearby.

5 Steps to Create a Dry Package

Now that we know the risks moisture poses to ESD components, follow these 5 steps to create a secure, dry package which will protect your PCBs against ElectroStatic Discharge and moisture:

Place the desiccant and HIC onto the tray stack. Trays carry the devices. Remember to store desiccant in an air tight container until it used.

Place the MSL label on the bag and note the proper level on the label.
Place the tray stack (with desiccant and HIC) into the moisture barrier bag.
Using a vacuum sealer, remove some of the air from the bag, and heat seal the bag closed. It is not good to take all the air out of the bag. Only slight evaluation is needed to allow the bag to fit inside a box.
Now your devices are safe from moisture and static.

With the steps taken above, your package should now be properly sealed from moisture and protected from ElectroStatic discharge.

Looking for a moisture barrier bag for your application? See the SCS Moisture Barrier Bag Selection Guide to find the packaging that fits your specifications!

Moisture Barrier Bags, Desiccant and Humidity Indicator Cards

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Are your static and moisture sensitive components protected by your packaging? Learn how to minimize potential product failures by protecting your products from Electrostatic Discharge (ESD) and moisture during the manufacture, transportation, and storage process.

Why are Moisture Barrier Bags important?

Moisture Barrier Bags (MBB) shield ESD sensitive devices from 2 potential risks:

The Faraday Cage created when using these bags protects contents from ESD Damage.
Specialized layers of film controlling the Moisture Vapor Transfer Rate (MVTR) also protect contents from moisture.

*Moisture Barrier Bags –* *more information*

“Desiccant” and “humidity indicator cards” must be used for proper moisture protection.

But what exactly are “desiccant” and “humidity indicator cards” and how are they used? These are the questions we will clarify in today’s blog post.

What is desiccant?

Desiccant is a drying agent that absorbs moisture from its surrounding area. Desiccant will stay dry to the touch even when it is fully saturated with moisture.

In a Moisture Barrier Bag it is used to ‘soak up’ moisture from the air inside the bag AFTER it has been sealed. Any moisture that gets through the bag from the outside will also be absorbed.

How is desiccant purchased?

Desiccant is available as a “unit” or fractional “unit”. A unit of desiccant absorbs a specific amount of moisture. One unit of desiccant weighs about 28g.

How is desiccant packaged?

Desiccant is packed in small sealed pouches made from a white plastic called “Tyvek” or brown “Kraft” paper. Tyvek pouches are very clean and Sulphur free. Kraft pouches are economical.

*A desiccant pouch –* *more information*

Pouches of desiccant are placed into metal pails – this ensures the desiccant is kept dry during transport and storage.

How much desiccant do you need?

There are 2 different methods you can use:

Method 1 per MIL-P-116
Formula: Unit = 0.011 x bag area in square inches
What you need: Bag area (2 times the surface area of your bag as there are 2 sides to a bag)
Example: 10” x 20” MBB bag
Apply formula: 0.011 x (10” x 20” x 2) = 4.4 rounded up to 4.5 units of desiccant
Method 2 per EIA 583 (allows you to tailor desiccant to your specific needs)
Formula: Unit = 0.231 x Bag Area x Bag MVTR x Months divided by Moisture Capacity
What you need: Bag area, Bag MVTR, Months of Storage, Maximum Interior Humidity (MIH), Moisture capacity table below:

10% MIH	3.0 g/unit
20% MIH	4.8 g/unit
30% MIH	5.8 g/unit
40% MIH	6.2 g/unit

Example: 10” x 20” bag with a 0.02 MVTR, a 12 month storage time and a MIH of 20%
Apply formula: 0.231 x (10″ x 20″ x 2) x (0.02) x (12/4.8) = 4.62 rounded down to 4.5 units of desiccant

What is a humidity indicator card?

A humidity indicator card allows for quick visual inspection of the relative humidity levels within its surrounding area. They are printed with moisture sensitive spots which respond to various levels of humidity with a visible color change from blue to pink.

In a Moisture Barrier Bag they provide a low-cost method of verifying the effectiveness of the moisture barrier packaging. If you are using Moisture Barrier Bags, moisture will be an issue in your application so you’re obviously aiming for as little moisture as possible. However, if you happen to open your MBB and the humidity indicator card shows a relative humidity of 60%, you’ll know that the contents of your bag have been exposed to moisture and may not be safe for use anymore.

How are humidity indicator cards purchased?

Humidity indicator cards come in many shapes and forms. Some will show relative humidity from 10% – 60%; others from 5% to 15%. Depending on the sensitivity of your application to moisture, the correct type of card should be chosen.

*A humidity indicator card –* *more information*

Bear in mind that not all humidity indicator cards are reversible. Some cards will measure the relative humidity only once and then halt at that reading. These types of humidity indicator cards are NOT re-usable. This is important to know so make sure you check before purchasing!

How are humidity indicator cards packaged?

Humidity indicator cards are sold in containers. It is recommended that cards are stored in their original un-opened canister in a dry, well ventilated room with a reasonably consistent temperature of 20°C. Humidity indicator cards should not be stored in ultraviolet sunlight, moisture or heat.

How many humidity indicator cards do you need?

One humidity indicator card per MBB is needed for proper verification of relative humidity.

Conclusion

Moisture Barrier Bags, desiccant and humidity indicator cards all play a very unique and important role when protecting ESD sensitive devices from moisture.

Protect your static and moisture sensitive components with proper packaging

They should always be used together to ensure maximum protection. However, remember that all three tools need to be used correctly as otherwise all your efforts have been in vain. And don’t forget: your Moisture Barrier Bag must be heat sealed with a vacuum sealer to eliminate the amount of “moisture laden air” within the package.

Find the right protective packaging for your sensitive components! Check out the SCS Moisture Barrier Bag Selection Guide and Humidity Indicator Card and Desiccant Chart to find the right packaging products for your application.

5 Tips For Using ESD Shielding Bags

Image scsstaticcontrol May 17, 2018 0

In a previous post we learnt how to select the correct ESD bag for your application, we want to focus on the next step: how to correctly use your ESD bag. We’ll use shielding bags as an example as they are the most commonly used ESD bags. However, the below can be applied to all types of ESD bags.

There are a few “dos and do-nots” you should keep in mind to ensure you get the most from your ESD bags. Nothing is worse than investing in all the right equipment and then using it incorrectly rendering all your efforts void. So, on that note, we have comprised a list of 5 tips for you on how to most efficiently use your shielding bags.

5 Tips On Efficient Use of Shielding Bags With ESD Sensitive Items:

1. Enclose Your ESD Sensitive Item with a Shielding Bag

Shielding bags should be large enough to enclose the entire product within. The shielding bag should be closed with a label or tape. Alternatively, you can use a zipper-style shielding bag. Following this advice ensures a continuous Faraday Cage is created which provides electrostatic shielding. This is the only way to ensure ESD sensitive devices placed inside the shielding bag are protected. If you are unfamiliar with the term “Faraday Cage”, scroll to the bottom of this page – we’ve included a more detailed explanation at the end of the post.

Enclose_Shielding_Bags — *Enclose your ESD sensitive item*

Please do not staple your shielding bag. The staple punctures the shielding layers and will provide a conductive path from the outside of the shielding bag to the inside. Charges outside the shielding bag could potentially charge or discharge to ESD sensitive components inside the shielding bag.

If you’re unsure as to what the correct size is for your application, catch-up on this post which will provide all the required information.

2. Remove Charges from Shielding Bags

When receiving an ESD sensitive device enclosed in a shielding bag, make sure you place the closed shielding bag on an ESD worksurface before removing the product. This will eliminate any charge that might have accumulated on the surface of the shielding bag.

3. Do Not Overuse Shielding Bags

Re-using shielding bags is acceptable as long as there is no damage to the shielding layer. Shielding bags with holes, tears or excessive wrinkles should be discarded.

Dont_overuse_shielding_bags — *Don’t overuse shielding bags*

4. Shielding Bags Are Not A Working Surface

Do not use a shielding bag as an ESD worksurface. Although a shielding bag is safe to use around ESD susceptible products, it is not intended to be a worksurface for product. When working on ESD sensitive devices, do so using ESD worksurfaces that are grounded correctly.

*Don’t use shielding bags as your ESD worksurface*

5. A Shielding Bag Is Not A “Potholder” Or “Glove”

Do not use a shielding bag as an “ESD potholder” or “ESD glove”. This type of use offers no ESD protection to the product.

If you need to handle ESD sensitive devices, make sure you are properly grounded using wrist straps or heel grounders.

Shielding_Bags_are_no-Gloves — *Shielding bags are no “ESD glove” or “ESD potholder”*

Some of you may have read through this post and have stumbled across the term “Faraday Cage” as you have not come across it before. We’ve also mentioned it before when talking about storing and transporting ESD sensitive items. However, we’ve never actually explained what a Faraday Cage is – so let’s rectify that!

What Is A “Faraday Cage” Or “Faraday Shield”?

A Faraday Cage or Faraday shield is an enclosure formed by conducting material or by a mesh of conductive material. Such an enclosure blocks external static and non-static electric fields. Faraday Cages are named after the English scientist Michael Faraday, who invented them in 1836.

What Is An Example of Faraday Cage Effect?

An impressive demonstration of the Faraday Cage effect is that of an aircraft being struck by lightning. This happens frequently but does not harm the plane or passengers. The metal body of the aircraft protects the interior. For the same reason, a car may be a safe place during a thunderstorm.

How Is A Faraday Cage Effect Used In ESD Protection?

In ESD Protection, the Faraday Cage effect causes charges to be conducted around the outside surface of the conductor. Since similar charges repel, charges will rest on the exterior and ESD sensitive items on the inside will be ‘safe’.

Examples of ESD control products that provide a Faraday Cage or shielding include Metal-In and Metal-Out Shielding bags.

When Is ESD Shielding Packaging Used?

ESD shielding packaging is to be used particularly when transporting or storing ESD sensitive items outside an ESD Protected Area.

ESD Packaging Standards For Outside An EPA

Per Packaging Standard ANSI/ESD S541 clause 6.2 Outside an EPA “Transportation of sensitive products outside of an EPA shall require packaging that provides:

Low charge generation.
Dissipative or conductive materials for intimate contact.
A structure that provides electrostatic discharge shielding.“

Additional ESD Definitions

Other helpful ESD related definitions from the ESD Association Glossary ESD ADV1.0 include:

Faraday Cage: “A conductive enclosure that attenuates a stationary electrostatic field.”
Electrostatic discharge (ESD) shield: “A barrier or enclosure that limits the passage of current and attenuates an electromagnetic field resulting from an electrostatic discharge.”
Electrostatic shield: “A barrier or enclosure that limits the penetration of an electrostatic field.”

So, hopefully we’ve clarified a few things today when it comes to the “shielding” property by explaining the phenomenon of the “Faraday Cage”. Don’t forget to implement our tips when it comes to using your ESD bags!

The Importance of Personnel Grounding Testers

Image scsstaticcontrol April 19, 2018 0

We already know that in an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same potential which is achieved by using ‘groundable’ materials that are then linked to ground. We have also learnt that the most common personnel grounding device to link people to ground are wrist straps. People who are moving around should instead wear ESD footwear.

So how do you know if your wrist straps and ESD footwear are working properly? Excellent question! And one we’ll answer in today’s post so let’s jump right in!

Purpose of Personnel Grounding Testers

Wrist straps and ESD footwear should be part of your Verification Plan. Each component in an EPA plays a vital part in the fight against electrostatic discharge (ESD). If just one component is not performing correctly, ESD sensitive devices can be damaged, potentially costing your company thousands of dollars.

Wrist straps and ESD footwear can fail and damage cannot always be detected by visual inspection. Just by looking at the items you would not know if they still provide sufficient protection. Personnel grounding testers should be used to provide feedback to verify the functionality of an operator’s wrist strap and/or footwear.

Your Personnel Grounding Checklist:

Wear your personnel grounding equipment such a wrist strap and/or footwear
Verify your personnel grounding system using a wrist strap and/or footwear tester. Wrist straps and footwear, need to be tested at least daily before handling ESD sensitive devices and should be worn while checking.
Log a record of each test. Records should be kept for quality control purposes.
Handle ESD sensitive components ONLY if your wrist strap and/or footwear pass(es) the test.

Types of Personnel Grounding Testers

Personnel grounding testers can be purchased in two configurations:

Wrist strap tester
Wrist strap and footwear tester

As wrist straps are the most commonly used personnel grounding device to ground operators, you will find a lot of testers on the market that check wrist straps only. Combined wrist strap and footwear testers will verify your wrist straps AND footwear.

In addition to WHAT the testers verify, you will also be faced with a wide range of devices differing in HOW they test. Below you will see a (by no means complete) list of options:

Continuous and split footplate: You will find testers with a continuous footplate which require each foot to be tested separately one after the other. Dual-footplate or independent footwear testers feature a split footplate which allows the unit to verify both feet independently at the same time. This can be an efficient time-saver if you have a number of operators in your company who are required to check their personnel grounding devices.
Portable, wall-mountable and fitted testers: Portable battery-powered (predominantly) wrist strap testers are suitable for small labs or for supervisors to spot-check workers and ensure compliance. Wall-mountable units are generally supplied with a wall plate which attaches to a wall; the tester is then mounted on to the wall plate. Some personal grounding devices are accompanied by a stand (and built-in footplate) which allow for a more freely positioning of the unit within a room.
Relay terminal: A few testers on the market are fitted with a relay terminal (electrically operated switch) that can be integrated with electronic door locks, turnstiles, lights, buzzers, etc. This can be of advantage if companies only want to allow personnel in an EPA that have passed their wrist strap and/or footwear test.
Data acquisition: A growing number of personnel grounding devices allow for test activity data to be logged in a database. The units link to a computer which records operator identification, test results, resistance measurements, time and more. Paperless data can enhance operator accountability, immediately identifying problems while reducing manual logging and auditing costs.

Operation of Personnel Grounding Testers

Wrist strap testing:

If you are not using a continuous or a constant monitor, a wrist strap should be tested at least daily. This quick check can determine that no break in the path-to-ground has occurred. Wrist straps should be worn while they are tested. This provides the best way to test all three components:

the wrist band
the ground cord (including the resistor)
the interface (contact) with the operator’s skin

The SCS Combo Wrist Strap/ Footwear Tester — *The SCS Combo Tester can be used to test wrist straps –* *more information*

“To ensure that the resistance to ground of personnel is within specification it is important to measure the entire system (i.e., wrist strap, person, and ground connection). The wrist strap system test method is described in ANIS/ESD S1.1. In general, the test method measures the resistance of the ground cord, wristband or cuff, and the interface of the band or cuff of the wearer.” [Handbook ESD TR20.20 Clause 8.2 Wrist Strap System]

“The wrist strap system should be tested daily to ensure proper electrical resistance. Nominally, the upper resistance reading should be < 35 megohms or a user-defined resistance. Daily test records can provide evidence of conformity. Daily testing may be omitted if continuous monitors are used.” [ANSI/ESD S1.1 Clause A3. Frequency of System Testing]

If the wrist strap tester outputs a FAIL test result, stop working and test the wrist band and cord individually to find out which item is damaged. Replace the bad component and repeat the test. Obtain a PASS test result before beginning work. For more information on troubleshooting failed wrist straps, check this post.

Footwear testing:

If you are using a flooring / footwear system as an alternative for standing or mobile workers, ESD footwear should be tested independently at least daily while being worn. Proper testing of foot grounders involves the verification of:

the individual foot grounder
the contact strip
the interface between the contact strip and the operator’s perspiration layer

“a) Place the foot grounders on the user’s shoes per the manufacturer’s instructions.
b) Place the left foot on the floor plate and touch the body contact area on the tester with one hand. Activate the tester per the manufacturer’s instructions.
c) Remove the left foot from the floor plate.
d) Repeat steps b and c with the right foot.”
[ANSI/ESD SP9.2 Clause 6.2.2 Procedure (Integrated Tester)]

*The SCS Dual Combination Tester is used to test wrist straps and footwear –* *more information*

If the footwear tester outputs a FAIL test result, stop working, and test the foot grounder and contact strip individually to find out which item is damaged. Replace the foot grounder. Obtain a PASS test result before beginning work.

Conclusion

Wrist straps and footwear need to be tested at least daily before handling any ESD sensitive devices. Personnel grounding devices need to be worn for verification using a wrist strap and/or footwear tester.

A record of each test has to be kept for quality control purposes.

Only handle ESD sensitive components if your wrist strap and/or footwear pass(es) the test.

Controlling ESD/EOS During the Soldering Process

Image scsstaticcontrol April 12, 2018 0

When the tip of a soldering iron comes into direct electrical contact with the pins of a sensitive component, there is a danger of voltage and/or current signal transfer between:

the grounded iron tip and the grounded PC board,
the ungrounded iron tip and the grounded PC board,
the grounded iron tip and the ungrounded PC board.

This can cause Electrical Overstress (EOS) and Electrostatic Discharge (ESD).

What is Electrical Overstress (EOS) and why is it important to detect?

EOS is the exposure of a component or PCB board to a current and/or voltage outside its operational range. This absolute maximum rating (AMR) differs from one device to the next and needs to be provided by the manufacturer of each component used during the soldering process. EOS can cause damage, malfunction or accelerated aging in sensitive devices.

ESD can be generated if a component and a board have different potentials and the voltage transfers from one to the other. When such an event happens, the component goes through EOS. ESD can influence EOS, but EOS can also be influenced by other signals.

Many people are familiar with Electrostatic Discharge (ESD) which is caused by the spontaneous discharge between two materials that are at different levels of ElectroStatic potential. Once electrostatic potential between the two materials is balanced, the ESD event will stop.

An EOS event on the other hand is created by voltage and/or current spikes when operating equipment; it can therefore last “as long as the originating signal exists”. [Source] The potentially never-ending stimulus of EOS is what makes it such a big concern in the electronics industry. Even though the voltage levels are generally much lower compared to an ESD event, applying this smaller voltage combined with a larger peak current over a long period of time will cause significant damage.

The high temperatures during an EOS event (created by the high current) can lead to visible EOS damage.

For more information on EOS and the differences to ESD, check-out this post.

Sources of EOS during the Soldering Process

When soldering components, it’s the tip of the soldering iron that comes into contact with the potentially sensitive device. Therefore, many people assume the soldering tip is the cause of ESD/EOS. However, the soldering iron and its tip are just some of the components used at a workbench. Other components on the bench like tweezers, wiring, test equipment, etc. can also be sources of ESD/EOS as they come into contact with the component or board.

There are many sources of EOS during the soldering process, which can include:

Loss of Ground
The tip of an ungrounded soldering iron can accumulate a voltage of up to ½ of the iron’s supply voltage. It can be caused within the soldering iron itself or in power outlets.
Noise on Ground
If a noise signal exists on ground, the tip of the solder iron will carry noise, too. These high-frequency signals, or electromagnetic interference (EMI), are disturbances that affect an electrical circuit, due to either electromagnetic induction or electromagnetic radiation emitted from an external source.
Noise on Power Lines
Noise not only generates via ground but in power lines, too. Transformers and power supplies that convert voltages to 24V are the main culprit. They regularly carry high-frequency spikes which end up on the tip of the soldering iron.
Power Tools
Although not technically related to the soldering process itself, it’s worth mentioning that the tips of power tools (e.g. electric screwdrivers) may not be properly grounded during rotation. This can result in high voltage on the tip itself.
Missing/Inadequate ESD Protection
ESD can be a cause of EOS damage. Therefore, it is essential to have proper ESD Protection in place. A voltage on the operator or the PCB board can otherwise lead to an ESD Event and expose the components on the PCB to EOS.

Detecting EOS during the Soldering Process

EOS/ESD events can be detected, measured, and monitored during the soldering process using a variety of diagnostic tools.

Diagnostic Tools

SCS CTM051 Ground Pro Meter
The SCS CTM051 Ground Pro Meter is a comprehensive instrument that measures ground impedance, AC and DC voltage on the ground as well as the presence of high-frequency noise or electromagnetic interference (EMI) voltage on the ground. It will alert if the soldering iron tip has lost its ground or has EMI voltage induced into the tip from an internal source on the soldering iron or from an EMI noisy ground or power lines.

The SCS CTM051 Ground Pro Meter
SCS CTM048 EM Eye – ESD Event Meter
The SCS CTM048 EM Eye – ESD Event Meter paired with the SCS CTC028 EM Field Sensor is a diagnostic tool for the detection and analysis of ESD events and electromagnetic fields and can identify sources of harmful ESD Events and electromagnetic interference (EMI).

The SCS CTM048 EM Eye – ESD Event Meter paired with the SCS CTC028 EM Field Sensor

EOS Continuous Monitors

SCS CTC331-WW Iron Man® Plus Workstation Monitor
The SCS CTC331-WW Iron Man® Plus Workstation Monitor is a single workstation continuous monitor which continuously monitors the path-to-ground integrity of an operator and conductive/dissipative worksurface and meets ANSI/ESD S20.20.The Iron Man® Plus Workstation Monitor is an essential tool when it comes to EOS detection. The unit is capable of detecting EOS on boards and alarms if an overvoltage (±5V or less) from a tool such as a soldering iron or electric screwdriver is applied to a circuit board under assembly.

The SCS CTC331-WW Iron Man® Plus Workstation Monitor

Data Acquisition

SCS Static Management Program
SCS Static Management Program (SMP) continuously monitors the ESD parameters throughout all stages of manufacturing. It captures data from SCS workstation monitors, ground integrity monitors for equipment, ESD event and static voltage continuous monitors and provides real-time data of manufacturing processes.The SCS 770063 EM Aware Monitor, which is part of SMP, can help during the soldering process by monitoring ESD events and change of static voltage that may result in EOS. The EM Aware alarms (visual and audibly) locally and sends data to the database of the SMP system if any of the ESD parameters are detected to be higher than user-defined limits.

The SCS 770063 EM Aware Monitor

Eliminating EOS during the Soldering Process

Once the source of ESD/EOS is known, there are many things that can be done to prevent it in the first place:

1. Managing Voltage on a PCB board

PCB boards contain isolated conductors and non-conductive (insulative) components. The only way to handle voltage on a PCB board is neutralizing potential static charges through ionization. An ionizer creates great numbers of positively and negatively charged ions. Fans help the generated ions flow over the work area to neutralize static charges (or voltage) on a PCB board in a matter of seconds.

For more information on ionization and how to choose the right type of ionizer for your application, please read these posts.

2. Managing Voltage on an Operator

Static voltage on an operator can be eliminated through proper grounding using a workstation monitor, e.g. WS Aware or Iron Man Plus Monitor, and proper grounding hardware. Sitting personnel are required to wear wrist straps. A wrist strap consists of a conductive wristband which provides an electrical connection to skin of an operator, and a coil cord, which is connected to a known ground point at a workbench, a tool or a continuous monitor. While a wrist strap does not prevent generation of voltages, its purpose is to dissipate these voltages to ground as quickly as possible.

Sitting personnel can also use continuous monitors – not only is the operator grounded through the continuous monitor, but they also provides a number of additional advantages:

Immediate feedback should a wrist strap fail
Monitoring of operators and work stations
Detection of split-second failures
Elimination of periodic testing

This post provides more details on continuous monitors.

Moving or standing personnel are grounded via a flooring/footwear system. ESD Footwear (e.g. foot grounders) are designed to reliably contact grounded ESD flooring and provide a continuous path-to-ground by removing electrostatic voltages from personnel.

3. Managing Current

One solution is the “re-routing of ground connection and separation of “noisy” ground from a clean one” as “connecting soldering iron and the workbench to the “quiet” ground often result in lower level of transient signals.“. [Source]

This will greatly reduce the high-frequency noise that could cause EOS damage.

If the noise on power lines and ground cannot be reduced manually, then the use of noise filters becomes necessary to reduce the risk of EOS exposure during the soldering process. Utilizing these filters suppresses the noise on power lines and will allow the solder iron to use “clean” power only.

In his papers, Vladimir Kraz, explains the set-up of a soldering station using a noise filter in more detail.

Conclusion

During the soldering process, current and voltage spikes between the solder tip and PCB can cause ESD/EOS. Sources are varied and can include:

Loss of Ground
Noise on Ground
Noise on Power Lines
Power Tools
Missing/Inadequate ESD Protection

ESD/EOS can be identified and controlled using diagnostic tools. SCS offers a number of tools that can detect current, voltage and EMI – all potentially leading to ESD and EOS.

Once the source of ESD/EOS is known, the next step is eliminating the source:

Managing voltage on a PCB board using ionizers.
Managing voltage on an operator using workstation monitors or foot grounders.
Managing current using noise filters.
Managing voltage on materials at the work bench.
Managing ESD generation during specific processes.
Managing grounding.

For more information regarding this topic, please see below for additional references.

References:

Industry Council on ESD Targets: White Paper 4: Understanding Electrical Overstress – EOS
In Compliance: Rethinking Electrical Overstress
EEWeb: Electric Overstress (EOS) and Its Effects on Today’s Manufacturing
EEWeb: EOS Exposure of Components in Soldering Process
Dangelmayer Associates/ESD Association: Electrical Overstress – Many Sources; Any Solutions?

How to Neutralize a Charge on an Object that Cannot be Grounded

Image scsstaticcontrol March 9, 2018 0

We have learned in a previous post that within an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential. We achieve this by using only ‘groundable’ materials.

But what do you do if an item in your EPA is essential to assembly and it cannot be grounded? Don’t sweat, not all hope is lost! Let us explain a couple of options which will allow you to use the non-groundable item in question.

Conductors and Insulators

In ESD Control, we differentiate items as conductors and insulators.

Materials that easily transfer electrons are called conductors. Examples of conductors are metals, carbon and the human body’s sweat layer.

Grounding cable snap with connection to a ground. — *A charged conductor can transfer electrons which allows it to be grounded*

Insulators are materials that do not easily transfer electrons are non-conductors by definition. Some well-known insulators are common plastics, polystyrene foam, and glass.

Plastic cup with charged electrons — *Insulators like this plastic cup will hold the charge and cannot be grounded and “conduct” the charge away.*

Both, conductors and insulators, may become charged with static electricity and discharge.

Electrostatic charges can effectively be removed from conductive or dissipative conductors by grounding them. A non-conductive insulator will hold the electron charge and cannot be grounded and “conduct” the charge away.

Conductors and Insulators in an EPA

The first two fundamental principles of ESD Control are:

Ground all conductors (including people).
Remove all insulators.

To ground all conductors per the first ESD Control principal, all surfaces, products and people are electrically bonded to ground. Bonding means linking or connecting, usually through a resistance of between 1 and 10 megohms.

Wrist straps and worksurface mats are some of the most common devices used to remove static charges:

Wrist straps drain charges from operators and a properly grounded mat will provide path-to-ground for exposed ESD susceptible devices.
Movable items (such as containers and tools) are bonded by standing on a bonded surface or being held by a bonded person.

If the static charge in question is on something that cannot be grounded, i.e. an insulator, then #2 of our ESD Control principles will kick in and insulators must be removed. Per the ESD Standard ANSI/ESD S20.20, “All nonessential insulators such as coffee cups, food wrappers and personal items shall be removed from the EPA.” [ANSI/ESD S20.20 clause 8.3.1 Insulators]

The ESD Standard differentiates between these two options:

If the field measured on the insulator is greater than 2000 volts/inch, keep it at a minimum distance of 12 inches from the ESDs or
If the field measured on the insulator is greater than 125 volts/inch, keep it at a minimum distance of 1 inch from the ESDs.

Moving an insulated keyboard away from ESD sensitive workspace — *Aim to keep insulators away from ESDs*

“Process-Essential” Insulators

Well, nothing in life is black and white. It would be easy if we were always able to follow the above ESD Control ‘rules’ but there are situations where said insulator is an item used at the workstation, e.g. hand tools. They are “process-essential” insulators – you cannot remove them from the EPA or the job won’t get done.

How do you ‘remove’ these vital insulators without actually ‘removing’ them from your EPA?

Here are four ways to reduce the ESD risk of these insulators:

Keep all insulators a minimum of 1 inch or 12 inches from ESDs at all times per recommendation of the ESD Standard.
This reduces the chance of insulators coming in contact with ESDs during workstation processes and assembly.
Replace regular insulative items with an ESD protective version.
There are numerous tools and accessories available that are ESD safe – from document handling to cups & dispensers, soldering tools, brushes and waste bins. They are either conductive or dissipative and replace the standard insulative varieties that are generally used at a workbench.
Periodically apply Topical Antistat on non-ESD surfaces.
After Topical Antistat has been applied and the surface dries, an antistatic and protective static dissipative coating is left behind. The static dissipative coating will allow charges to drain off when grounded. The antistatic properties will reduce triboelectric voltage to under 200 volts. It therefore gives non-ESD surfaces electrical properties until the hard coat is worn away.
Neutralization with Ionization
If these three options are not feasible for your application, the insulator is termed “process-essential” and therefore neutralization using an ionizer becomes a necessary part of your ESD control program. This allows for control of charged particles that can cause ESD events which we will cover next.

Neutralization

Most ESD workstations will have some insulators or isolated conductors that cannot be removed or replaced. These should be addressed with ionization.

Examples of some common process essential insulators are a PC board substrate, insulative test fixtures and product plastic housings.

Electronic enclosures are process-essential insulators (shown on ESD workstation) — *Electronic enclosures are process-essential insulators*

An example of isolated conductors are conductive traces or components loaded on a PC board that is not in contact with the ESD worksurface.

An ionizer creates great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. Ionization can neutralize static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.

The charged ions created by an ionizer will:

neutralize charges on process required insulators,
neutralize charges on non- essential insulators,
neutralize isolated conductors and
minimize triboelectric charging.

SCS Benchtop ionizer on a workstation removing charges from isolated conductors on PCB Board — *Insulators and isolated conductors are common in ESDs – Ionizers can help*

For more information on ionizers and how to choose the right type of ionizer for your application, read this post.

Summary

The best way to keep electrostatic sensitive devices (ESDs) from damage is to ground all conductive objects and remove insulators. This is not always possible because some insulators are “process-essential” and are necessary to build or assemble the ESDs.

Insulators, by definition, are non-conductors and therefore cannot be grounded, but they can be controlled to minimize potential ESD damage.

Insulators can be controlled by doing the following within an EPA:

Keep insulators a minimum distance from ESDS at all times (1 or 12 inch minimum distance depending on field voltage measurements of the insulator per ESD Standard recommendation)
Replace regular insulative items with ESD protective versions
Periodically apply a coat of Topical Antistat
Neutralize charges for “process-essential” insulators with ionization

With these steps added to your ESD control process, all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential in an ESD Protected Area (EPA) to reduce the risk of ESD events and ESD damage.

How to Reduce the Risk of Damaging ESD Sensitive Devices in Critical Applications

Image scsstaticcontrol February 22, 2018 0

Do your employees handle ESD-sensitive high-end components that are expensive to replace if they failed? If so, reducing the possibility of ESD damage is an important part of an ESD control program. Today’s blog post will look at one option of protecting your critical applications: Dual-Wire Wrist Straps.

Introduction

In an ESD Protected Area (EPA), all surfaces, objects, people and ESD sensitive devices (ESDs) are kept at the same electric potential. This is achieved by using only ‘groundable’ materials that are then linked to ground.

This is in line with the requirements of ANSI/ESD S20.20: “The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include:
– Training
– Product Qualification
– Compliance Verification
– Grounding / Equipotential Bonding Systems
– Personnel Grounding
– ESD Protected Area (EPA) Requirements
– Packaging Systems
– Marking”
[ANSI/ESD S20.20 clause 7.1 ESD Control Program Plan]

Wrist Straps

Wrist straps are the most common personnel grounding device and are used to link people to ground. They are required if the operator is sitting.

A wrist strap is made up of two components:

A wrist band that is worn comfortably around your wrist and
A coiled cord that connects the band to a Common Grounding Point.

Components of a Wrist Strap

Dual-Wire Wrist Straps

Dual-Wire Wrist Straps have two conductors (compared to single-wire monitors which have only one conductor inside the insulation of the coiled cord). They offer a reduced risk of damaging ESD sensitive devices because if one conductor is severed or damaged, the operator still has a reliable path-to-ground with the second conductor. For that reason, they dual-wire wrist straps are generally used in critical applications.

Advantages of using Dual-Wire Wrist Straps:

Elimination of intermittent failures
Extension of wrist strap lifespan
Compatible with high performance continuous monitors

The MagSnap 360™ Dual-Wire Wrist Strap and Coil Cord – more information

Dual-Wire Continuous Monitors

For maximum benefit, dual-wire wrist straps should be used together with dual-wire continuous monitors. Instead of connecting a coil cord directly to a common grounding point, the operator connects to a continuous monitor. The operator is grounded through the continuous monitor and the operator-to-ground connection is monitored.

The monitors provide operators with instant feedback on the status and functionality of their wrist strap and/or workstation. Continuous monitors detect split-second failures when the wrist strap is still in the “intermittent” stage. This is prior to a permanent “open” which could result in damage to ESD sensitive components. The “intermittent” stage is characterized by sporadic failures as the cord is not completely severed. Once the cord is fully split, the “open” stage is reached.

The WS Aware Dual-Wire Workstation Monitor – more information

“Since people are one of the greatest sources of static electricity and ESD, proper grounding is paramount. One of the most common ways to ground people is with a wrist strap. Ensuring that wrist straps are functional and are connected to people and ground is a continuous task.” “While effective at the time of testing, wrist strap checker use is periodic. The failure of a wrist strap between checks may expose products to damage from electrostatic charge. If the wrist strap system is checked at the beginning of a shift and subsequently fails, then an entire shift’s work could be suspect.” “Wrist strap checkers are usually placed in a central location for all to use. Wrist straps are stressed and flexed to their limits at a workstation. While a wrist strap is being checked, it is not stressed, as it would be under working conditions. Opens in the wire at the coiled cord’s strain relief are sometimes only detected under stress.“ [ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps]

Resistance (or dual-wire) constant monitors are “… used with a two wire (dual) wrist strap. When a person is wearing a wrist strap, the monitor observes the resistance of the loop, consisting of a wire, a person, a wristband, and a second wire. If any part of the loop should open (become disconnected or have out of limit resistance), the circuit will go into the alarm state.” “While the continuity of the loop is monitored, the connection of the wrist strap to ground is not monitored.” “There are two types of signals used by resistance based constant monitors; steady state DC and pulsed DC. Pulsed DC signals were developed because of concerns about skin irritation. However, pulse DC units introduce periods of off time (seconds) when the system is not being monitored.“ [ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps]

Conclusion

Dual Polarity Technology provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards. Dual-wire systems are used to create redundancy. In critical applications redundancy is built-in to have a backup if the primary source fails. With dual-wire wrist straps the redundancy is there as a protection rather than an alternative. If you are monitoring your dual-wire wrist strap and one wire fails, then the unit will alarm. You will still be grounded by the other wire, so there will be a significantly reduced risk of damaging ESD sensitive components if you happen to be handling them when the wrist strap fails. The wrist strap still needs to be replaced immediately.

And there you have it: dual-wire wrist straps together with dual-wire continuous monitors offer better protection than intermittent monitoring or testing if you have a critical application.

Check-out the SCS Wrist Strap Selection Guide and Workstation Monitor Selection Guide to find the correct products for your application.