November 2017

You are browsing the site archives for November 2017.

An Introduction to Wrist Straps

Image scsstaticcontrol November 30, 2017 3

We get a lot of inquiries regarding wrist straps: what they do, why there are different types, how they are used, etc. So, the purpose of today’s blog post is to answer all those questions for you. If there is something we did not cover in the blog post make sure you ask us in the comments!
Let’s get started!

Introduction
The ESD Standard S20.20 requires “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]

The most common personnel grounding device is a wrist strap which is used to connect people to ground.
A wrist strap in general is a conductive wristband which provides an electrical connection to skin of an operator and, in turn, is connected to a known ground point at a workbench or a tool. While a wrist strap does not prevent generation of charges, its purpose is to dissipate these charges to ground as quickly as possible. Wrist straps are required if the operator is sitting. They are not necessary if an operator is wearing two foot grounders on a conductive grounded floor and doesn’t lift both heels/toes at the same time. As some people lift both feet off the ground while seated, wrist straps are essential for sitting personnel.
A wrist strap is made up of two components:

a wristband that is worn comfortably around your wrist and
a coil cord that connects the band to ground.

Wristband and coil cord of a wrist strap

The key to the wrist strap is the intimate contact of the conductive band to the skin and of course the coil cord connecting to ground. It doesn’t matter if the contact point to your body is on your wrist, finger, forearm, ankle, etc., as long as it is in direct contact with your skin. The skin is electrically continuous over your whole body. The wrist is just a convenient place to couple the band to.

Styles of Wrist Straps
Operators can choose between elastic and metal wristbands:

Elastic wristbands are the most popular wristband as they are comfortable to wear and easy to adjust. Compared to metal wristbands they are also less expensive.
Some people prefer metal wristbands as they are generally longer lasting and easier to clean.

The key to personnel grounding is to have an adequate path to ground so that there is never a potential difference with respect to ground on the human body for longer than 150 milliseconds (ms) body movement time. Such rapid grounding is accomplished well by elastic or metal wrist straps. So, in terms of their effectiveness to protect against ESD, there is no difference between elastic and metal wristbands.

Both elastic and metal wristbands are (to a certain degree) adjustable. Metal wristbands offer less adjustment, so you will find those are generally available in different sizes depending on the circumference of your wrist. However, you are still able to adjust metal wristbands if you need a tighter/looser fit.
To adjust your wristband, follow the below steps:
1. Elastic wristbands:

Open the clasp by pulling upward on the “tail” of material that extends out from the clasp.
Tighten or loosen the elastic material through the clasp until the wristband fits snug but comfortably.
We recommend that you close the clasp and wear the band with the excess tail extended for a day to be sure the adjustment is snug, comfortable, and has the proper electrical contact with the skin before cutting.
Test the wrist strap system to be sure of proper electrical resistance and skin contact.
When you are ready to cut off excess material, mark with a pencil where excess material is to be trimmed.
Remove band from wrist. Open clasp. Cut off strip excess material about 1/4″ short of pencil mark so that the end of material is concealed by cap. This will eliminate the possibility of frayed ends.
Close clasp and use as a fixed elastic wristband.

Adjusting an elastic wristband

2. Metal wristbands:

Insert the link end of the wristband into the slotted opening on the cap. Insert it at a downward angle to allow the links to slide inside the channel in the backplate.
Change the size of the band by sliding the links in or out of the stainless steel backplate. For extra small size, you can cut off excess links with cutters.
Lock the links into place by pulling down on the band, seating the band securely over the lip on the edge of the backplate.
Test the wrist strap system to be sure of proper electrical resistance and skin contact.

Adjusting a metal wristband

1 megohm Safety Resistors
The purpose of the 1 megohm resistor found in series with wrist straps is solely to provide safety to the human body by limiting the amount of current that could be conducted through the body. The 1 megohm resistor is designed to limit the current to 250 microamps at 250 Volts rms AC. This is just below the perception level (and a bit before the nervous system goes awry) of most people. Physical perception of current traveling in/on the body varies depending on size, weight, water content, skin conditions, etc. Remember that the termination of the coil cord with the 1 megohm resistor must always be connected to the operator.
Such safety resistors are built into the wrist straps themselves and also in such wrist strap monitors as WS Aware, Iron Man® Plus and Ground Man Plus manufactured by SCS.

Typical Problems with Wrist Straps
Some of the typical problems with proper grounding of an operator using a wrist strap are:

worn out wrist strap which no longer has good electrical properties
stretched out wrist strap which doesn’t make good electrical contact with the skin
loosely-worn wrist strap which doesn’t make good electrical contact with the skin either
dry skin of an operator increasing electric resistance of a contact beyond specification
improper placement of a wrist strap, such as over the cuff of the garment

Also, another issue we often see is that wrist strap users connect their wrist cord to a stud on their ESD protective mat. This process is not recommended as it can increase the total system resistance to ground to over the 35 megohm limit required by ANSI/ESD S20.20 table 2.

Testing of Wrist Straps
Wrist straps need to be checked regularly to ensure they are faultless and ground the operator properly. Wrist straps should be worn while they are tested. This provides the best way to test all three components: the wristband, the ground cord (including the resistor) and the interface with the operator’s skin.
Wrist straps need to be checked before each use. Periodic testing is not required if continuous monitors are used. They provide instant feedback should the wrist strap fail while handling ESD sensitive devices.

Verifying a wrist strap using a wrist strap/footwear tester

If the wrist strap tester outputs a FAIL test result, stop working. Test the wristband and cord individually to find out which item is damaged. There are some methods to troubleshoot your wrist straps. First make sure your tester is properly adjusted and calibrated.

If the operator and wrist strap system fails low:

Make sure that the person is not directly connected to ground via another path, i.e., touching a grounded metal structure.
The most common cause of a fail low is a shorted resistor in the wrist strap coil cord. Replace the coiled cord with a new one and repeat the test.

If the operator and wrist strap system fails high:

Make sure the coiled cord has a secure connection both the banana jack/socket to tester and the stud snap to wrist strap buckle.
Ensure there is continuity in the coiled cord (you can test with an ohmmeter).
Remove the wrist strap and hold the bottom part of the band tightly between the operator’s thumb and index finger and test. If the test fails high, the band may be soiled and needs cleaning or the buckle to band connection may be suspect. Either replace the band or clean and then retest.
If the above test is okay, then the skin of the operator’s wrist may be too dry. Apply ESD lotion to the wrist to re-moisturize the skin thereby increasing its conductivity. Retest. Operators with dryer skin should wear metal banded wrist straps to minimize the contact resistance. If their skin is very dry, application of an ESD lotion may be required as part of their donning process.

You need to obtain a PASS test result before beginning work.

Now that we’ve covered the basics of wrist straps, we will dive into the different types of wrist straps – but that will have to wait until next time as this post is already very long. Stay tuned!

The importance of employees in the fight against ESD

Image scsstaticcontrol November 16, 2017 0

People pose the biggest threat to ESD sensitive components. However, when properly trained, operators can become the key weapon in the fight against ESD. Every person coming into contact with ESD sensitive items should be able to prevent ESD related problems before they occur or provide immediate action when they do occur. Today’s blog post will explain in detail the role operators play in ESD Protection and how your company can support them in the fight against ESD.

Introduction
As an employee, the invisible threat of ESD should be of great concern to you. ESD damage can significantly reduce your company’s profitability. This may affect your company’s ability to compete in the marketplace, your profit sharing and even your employment. Everyone likes to take pride in their work, but without proper ESD controls, your best efforts may be destroyed by ElectroStatic discharges that you can neither feel nor see.

motherboard doctors — People are often a major factor in the generation of static charges

Perhaps the most important factor in a successful static control program is developing an awareness of the “unseen” problem. People are often a major factor in the generation of static charges. Studies have shown that personnel in a manufacturing environment frequently develop 5000 volts or more by just walking across the floor. This is “tribocharging” produced by the separation of their shoes and the flooring as they walk.
A technician seated at a non-ESD workbench could easily have a 400-500 volt charge on his or her body caused not only by friction or tribocharging but additionally by the constant change in body capacitance that occurs from natural movements. The simple act of lifting both feet off the floor can raise the measured voltage on a person by as much as 500-1000 volts.
Educating your personnel is therefore an essential basic ingredient in any effective static control program. A high level of static awareness must be created and maintained in and around the ESD protected area. Once personnel understand the potential problem, it might help to reinforce this understanding by hanging up a few static control posters in strategic locations. No technician needs an unprotected person wandering over and touching things on the service bench.

The invisible enemy
The biggest issue with ElectroStatic discharges is that you can neither see nor feel the threat. Daily life has other examples of hidden enemies where careful procedures must be followed to regularly obtain positive results. One example is sterilization which combats germs and contamination in hospitals.
Damage caused by invisible and undetectable events can be understood by comparing ESD damage to medical contamination of the human body by viruses or bacteria. Although invisible, they can cause severe damage. In hospitals, the defense against this invisible threat is extensive contamination control procedures including sterilization.

A medical team performing an operation — Would you consider having surgery in a contaminated operating room?

We are aware of the benefits of sterilization in medicine. We must develop the same attitude towards ESD control and “sterilize” against its contamination. Just as you would never consider having surgery in a contaminated operating room, you should never handle, assemble or repair electronic assemblies without taking adequate measures against ESD. For the hospital to sterilize most of the instruments is not acceptable; actually, it may waste money. Every single instrument needs to be sterilized. Likewise, it is not acceptable to protect the ESD sensitive items most of the time. Effective ESD control must occur at each and every step where ESDS items are manufactured, processed, assembled, installed, packaged, labelled, serviced, tested, inspected, transported or otherwise handled.
Everyone handling sensitive components should:

“recognize ESD threat
know what equipment to use, and how to use it
know the correct ESD procedures, and work to them
know how to check equipment
know which packaging to use
take corrective actions when required.” [Source]

It is obvious that ESD training of personnel is a prerequisite for a functioning ESD control program.

Training
ESD training needs to be provided to everyone who handles ESD sensitive devices – that includes managers, supervisors, subcontractors, cleaners and even temporary personnel. Training must be given at the beginning of employment (BEFORE getting anywhere near an ESDS) and in regular intervals thereafter.
“Initial and recurrent ESD awareness and prevention training shall be provided to all personnel who handle or otherwise come into contact with any ESDS [ESD sensitive] items. Initial training shall be provided before personnel handle ESDS items. The type and frequency of ESD training for personnel shall be defined in the Training Plan. The Training Plan shall include a requirement for maintaining employee training records and shall document where the records are stored. Training methods and the use of specific techniques are at the Organization’s discretion. The training plan shall include the methods used by the Organization to verify trainee comprehension and training adequacy.” [ANSI/ESD S20.20-2007 section 7.2]

ESD training should include:

an introduction to ESD – what it is, what it’s caused by and how to control it
how to handle sensitive devices and what precautions to take when coming into contact with them
how to identify and mark ESD sensitive items
an overview of the ESD Standard.

For operators working in assembly, repair or field service, job specific training will be required, too.
If visitors are entering an EPA, they must possess basic ESD awareness and understand how to use their wrist straps and footwear.

Operator’s safety comes first
One final word of warning: while ESD control is important, it is of secondary importance to employee safety. ElectroStatic charges or static electricity can be everywhere; however, conductors can be effectively grounded and charges removed to ground. A fundamental rule in ESD control is to ground all conductors, including people. BUT: Personnel should not be grounded in situations where they could come into contact with voltage over 250 volts AC.

How to choose the right ESD Bag

Image scsstaticcontrol November 2, 2017 1

Last time, we discussed the storage requirements of ESD sensitive items. Today we want to focus on the overall selection process for ESD bags: from choosing the correct type of material all the way through to determining the right size for your application. Sounds complicated? Honestly – it’s not and once you have the right tools (you’re welcome!), you’ll be an expert in no time. So, let’s go!

Choose the correct material for your ESD bag
Before you get started, you need to be clear about the purpose of your ESD bag and the environment it’s being used in. Make sure you have the answers to the following questions:

What do you intend to put inside the ESD bag? ESD sensitive items? Non-ESD sensitive items?
Is moisture an issue you need to consider?
Do you need your ESD bags to be self-sealable? Or will you be using ESD tape/labels to close your bags?
Are the items inside your ESD bag sensitive to physical damage?
Are the items you’re storing in your ESD bag particularly sharp which could potentially damage the material?
Will the ESD bags (and obviously the items inside) be stored on a grounded shelf inside an EPA or are they being transported outside of an EPA, as well?

Once you have answers to ALL of the above questions, you can move on to the below selection chart and choose the right material for your application.

	ESD sensitive items	Moisture protection	Self-sealable	Physical protection	Inside/ Outside EPA
Metal-In	x		x		Both
Metal-Out	x		x		Both
MBB – High Barrier	x	x	x		Both
MBB – Low Barrier	x	x	x		Both
Bubble Shielding	x			x	Both
Conductive Black	Only if used on a grounded surface				Inside only if used with ESD sensitive items

A few more details on the different types of materials listed in the above chart:

Metal-In Shielding Bags
ESD bags which protect ESD sensitive items. The ESD shielding limits energy penetration from electrostatic charges and discharge. The dissipative outer layer dampens any discharge and therefore reduces damaging electrostatic events. They offer good see-through clarity. Available with and without dissipative zipper.

Metal-In Shielding Bag – more information
Metal-Out Shielding Bags
Integral antistatic and low tribocharging bags which will not electrostatically charge contents during movement. Bags have a protective coated aluminum metal outer layer of laminated film; this promotes a more rapid discharge of static fields creating the event which the metal layer then attenuates. If RF field sensitivity is an issue, metal-out bags may be unsuitable. The rapid discharge to the highly conductive outer layer can create a higher radiated field which in return can cause issues for objects inside or near the bag. Available with and without dissipative zipper.

Metal-Out Shielding Bag – more information
Moisture Barrier Bags (MBB) – Low Barrier
Offers ESD and moisture protection and can be used to pack SMD reels or trays. Available with and without dissipative zipper (except Dri-Shield® 2700).

Low Barrier Moisture Barrier (MBB) Bag – more information
Moisture Barrier Bags (MBB) – High Barrier
Offer ESD and moisture protection and are ideal for applications where high moisture protection or conformance to IPC/JEDEC J-STD-033 is required. Available with and without dissipative zipper (except Dri-Shield® 3000).

High Barrier Moisture Barrier (MBB) Bag – more information
Cushioned Bags
These bags combine the “Faraday Cage” and mechanical protection. They shield about twice as well as normal shielding bags of equivalent size. Bubble cushion layer provides heavy-duty protection that absorbs impact and prevents product damage.

Cushioned Bag – more information
Conductive Black Bags
Black conductive film is made of virgin low density materials with black conductive compound to achieve high toughness and strength. This is commonly used for material handling, shipping and storage.

Conductive Black Bag – more information

Calculating the correct size for your ESD bag
Once you have selected the correct type of material, it’s time to choose the right size for your ESD bag. There are different ways to determine this based on the type of material you use:

Shielding and Black Conductive Bags:
A. Bag Width = Item’s Thickness + Item’s Width + 25mm
B. Bag Length = Item’s Thickness + Item’s Length + 50mm

Moisture Barrier Bags (MBB):
A. Bag Width = Item’s Thickness + Item’s Width + 25mm
B. Bag Length = Item’s Thickness + Item’s Length + 76mm

Cushioned Bags:
A. Bag Width = Item’s Thickness + Item’s Width + 76mm
B. Bag Length = Item’s Thickness + Item’s Length + 76mm

Bonus Tip: Measuring a bag
It might seem obvious to some of you but given that we do get these types of queries on a regular basis, we thought this would be a good opportunity to include. Imagine you already have ESD bags that you use in your company. Someone has just taken the last one off the shelf and you need to order some more. How do you know what size ESD bag you have in front of you so you can place a new purchase order? No worries – we have the answer:

A. The width is measured from inside seam to inside seam. This is also your opening.
B. The length is measured from the top of the opening to the bottom of the bag.

Bonus Tip 2: Remember your ESDS items
Outside an ESD protected area, the objective of ESD protective packaging is to prevent a direct electrostatic discharge to the ESD sensitive item contained within and allow for dissipation of charge from the exterior surface. In addition, the packaging should minimize charging of the ESD sensitive item in response to an external electrostatic field and triboelectrification. If the user does not know the sensitivity of the items being used, we would always recommend static shielding packaging to be on the safe side.