Welcome back to “A Minute with Miranda.” This week we will be covering how to properly wear a wrist strap.
ANSI/ESD S20.20 requires seated personnel to be
connected to the grounding / equipotential bonding system via a wrist strap.
The total resistance of the Wrist Strap System needs to be less than 3.5 x 10^7 ohms. The key to a wrist strap is
the intimate contact of the band to the skin and that the coil cord is
connected to ground. Wrist straps need to be tested at least daily before
handling any ESD sensitive devices.
Operators can choose between elastic and metal wristbands. Elastic wristbands are comfortable to wear and easy to adjust. Metal wristbands generally last longer and are easier to clean. View the full range of SCS Wrist Straps here.
In today’s connected world, we are surrounded by home monitoring networks, fitness trackers and other smart systems. They all use an IoT platform to keep us up to-date with the current temperature in our house or the number of steps we have taken in a day. There are many different applications of IoT: Consumer, Commercial, Industrial, and Infrastructure, but is there a way to use this incredibly smart technology to improve ESD Control? Let’s take a look!
What Is The Internet of Things (IoT)?
The Internet of Things (IoT) is used everywhere today – from medical devices, to vehicles, to homes and more! Simply put, IoT:
Connects “things” in the physical world to the internet using sensors.
Collects data for these “things” via sensors.
Analyses the collected data and provides a deeper insight into the “things”.
Another broad definition provided for IoT is:
“The Internet of Things (IoT) is the network of physical devices, vehicles, home appliances, and other items embedded with electronics, software, sensors, actuators, and connectivity which enables these things to connect and exchange data, creating opportunities for more direct integration of the physical world into computer-based systems, resulting in efficiency improvements, economic benefits, and reduced human exertions.” [Source]
As mentioned previously, there are many different applications for IoT, but The Industrial Internet of Things (IIoT) applies specifically to manufacturing and industrial processes.
It has slightly different requirements compared to consumer IoT products but the principle is the same: smart machines (incorporating various sensors) accurately and consistently capture and analyze real-time data allowing companies to pick-up problems as soon as (or even before) they appear.
Internet of Things (IoT) and Industry 4.0
IoT helped push the 3rd industrial revolution (machine automation) one step further. “Cyber Physical Systems (CPS) dominate the manufacturing floor, linking real objects with information processing, and virtual objects via the internet. The goal is to converge Operational Technology (OT) and Information Technology (IT).” [Source]
The 4th industrial revolution is also referred to as “Industry 4.0”. “At the very core Industry 4.0 includes the (partial) transfer of autonomy and autonomous decisions to cyber-physical systems and machines, leveraging information systems”. [Source]
Industry 4.0 as fourth industrial revolution [Source]
So, how can companies use the power of IoT and create accessible, real-time feedback on the status of their ESD Control Protected Area (EPA) and ESD control items?
Industry 4.0 IoT Platforms in ESD Control
ESD damages can be extremely costly – especially when it comes to latent defects that are not detected until the damaged component is installed in a customer’s system. Conventional ESD control programs incorporate periodic verification checks of ESD control products to detect any issues that could result in ESD events and ESD damage. The problem is that ESD control products (and the EPA as a whole) are not constantly monitored.
Take an ionizer for example: if a company uses ionization to handle process-essential insulators, the ionizers need to be fully reliable at all times. If an ionizer passes one check but is found to be out of balance at the next, the company faces a huge problem: nobody knows WHEN exactly the ionizer failed or if contributed to a charged insulator potentially causing ESD damage.
The Industry 4.0 IoT platform will be a game changer when it comes to creating a reliable and dependable ESD control program. Sensors collecting vital ESD information like field voltage, Electromagnetic Interference (EMI), temperature, humidity etc. in an EPA will help detect potential threats in real-time allowing supervisors to act even before an ESD threat occurs.
Advantages of Internet of Things (IoT) in ESD Control
Here is a (by no means exhaustive) list of advantages, IoT can bring to ESD Control:
The day in an EPA can be busy. Taking the time to capture and record measurements of ionizers, wrist straps, work surfaces, automated processes etc. can be disruptive and is prone to errors. IoT allows data to be collected automatically without any input from users. This helps to increase the accuracy of data and allows operators and supervisors more time focusing on their actual jobs.
Supervisors have all the essential data in one place right in front of them and can make informed decisions; they can provide feedback and give suggestions in case of an ESD emergency. IoT allows to pinpoint areas of concern and prevent ESD events.
IoT continuously monitors processes and provides a real-time picture of them – no manual checks required. If a potential threat is detected, warnings will show-up immediately. There is no need to worry about potentially damaging sensitive devices because the next scheduled check of ionizers, wrist straps etc. has not been completed yet.
The number one reason for adapting an ESD control program is to reduce costs by:
Enhancing quality and productivity,
Improving customer satisfaction,
Lowering repair, rework and field service costs and
Reducing material, labor and overhead costs.
Reduced Workload and Increased Productivity
IoT pushes all the above even further with the additional benefits of:
Reduced workload for operators: Data is collected remotely without any input from users. Operators are not disrupted in their day-to-day activities.
Reduced workload for supervisors: Supervisors don’t have to collect and analyze data from personnel testers, field meters, monitors etc. The system does it for them and will highlight any issues.
Further increases in productivity and cost reductions: An ESD program can be managed better and with fewer resources.
Static Management Program (SMP): the next generation of ESD Process Control – more information
IoT will no doubt change ESD control and the way EPAs are monitored. Quantifiable data allows companies to see trends, become more proactive and improve the efficiency of their ESD process control system. IoT will support organizations’ efforts to make more dependable products, improve yields, increase automation and provide a measurable return on investment. Not only will this benefit users and supervisors, but the company as a whole.
SCS Static Management Program (SMP) is the only smart ESD system on the market that continuously monitors your entire ESD process control system throughout all stages of manufacturing. SMP captures data from SCS workstation, equipment and ESD event continuous monitors and provides a real-time picture of critical manufacturing processes.
For more information on how to continuously monitor your ESD control program and/or improve an existing program, request a free ESD/EOS Assessment or SMP demo at your facility by one of our knowledgeable local representatives to evaluate your ESD program and answer any ESD questions!
Electronic devices and systems can be damaged by exposure to high electric fields as well as by direct electrostatic discharges. A good circuit layout and on-board protection may reduce the risk of damage by such events, but the only safe action at present is to ensure that devices are not exposed to levels of static electricity above the critical threshold.
This can only be achieved by introducing a static control program which usually involves setting up an ESD Protected Area (EPA) in which personnel are correctly grounded and all meet the ESD Standard. However, setting up an EPA does not of itself guarantee a low static environment. Production procedures may change, new materials may be introduced, the performance of older materials may degrade and so on.
Measuring Effectiveness of an ESD Control Program
To ensure the effectiveness of any static control program it is important that regular measurements are carried out:
to determine the sensitivity to ESD of devices being produced or handled.
to confirm that static levels are lower than the critical level, and that new or modified work practices have not introduced high static levels.
to ensure that both new and existing materials in the EPA meet the necessary requirements.
Only after an ‘operational baseline’ has been established by regular auditing will it become possible to identify the origin of unexpected problems arising from the presence of static.
1. Determining the sensitivity of ESD sensitive Devices
It is important to understand the sensitivity of ESD sensitive devices before an action plan can be created. Once you know the sensitivity of the items you are handling, can you work towards ensuring you’re not exceeding those levels.
Part of every ESD control plan is to identify items in your company that are sensitive to ESD. At the same time, you need to recognize the level of their sensitivity. As explained by the ESD Association, how susceptible to ESD a product is depends on the item’s ability to either:
dissipate the discharge energy or
withstand the levels of current.
2. Measurements to prove the effectiveness of an ESD Control Program
Measuring electrostatic quantities poses special problems because electrostatic systems are generally characterized by high resistances and small amounts of electrical charge. Consequently, conventional electronic instrumentation cannot normally be used.
Measuring Electrical Field
Wherever electrostatic charges accumulate, they can be detected by the presence of an associated electric field. The magnitude of this field is determined by many factors, e. g. the magnitude and distribution of the charge, the geometry and location of grounded surfaces and the medium in which the charge is located.
“The current general view of experts is that the main source of ESD risk may occur where ESDS can reach high induced voltage due to external fields from the clothing, and subsequently experience a field induced CDM type discharge.” [CLC TR 61340-5-2 User guide Garments clause 18.104.22.168 Introductory remarks]
A static field meter is often used for ESD testing of static fields. It indicates surface voltage and polarity on objects and is therefore an effective problem-solving tool used to identify items that are able to be charged.
A field meter can be used to:
verify that automated processes (like auto insertion, tape and reel, etc.) are not generating charges above acceptable limits.
measure charges generated by causing contact and separation with other materials.
demonstrate shielding by measuring a charged object and then covering the charged item with an ESD lab coat or shielding bag. Being shielded the measured charge should be greatly reduced.
Measuring ESD Events
ESD events can damage ESD sensitive items and can cause tool lock-ups, erratic behavior and parametric errors. An ESD Event Detector like the EM Eye ESD Event Meter will help detect most ESD events. It detects the magnitude of events and using filters built into the unit, it can provide approximate values for some ESD events for models (CDM, MM, HBM) using proprietary algorithms.
Solving ESD problems requires data. A tool counting ESD events will help carry out a before-and-after analysis and will prove the effectiveness of implementing ESD control measures.
3. Checking Materials in your EPA
When talking about material properties, the measurement you will most frequently come across is “Surface Resistance”. It expresses the ability of a material to conduct electricity and is related to current and voltage. The surface resistance of a material is the ratio of the voltage and current that’s flowing between two pre-defined electrodes.
It is important to remember that the surface resistance of a material is dependent on the electrodes used (shape as well as distance). If your company implements an ESD control program compliant to the ESD Standard ANSI/ESD S20.20, it is therefore vital to carry out surface resistance measurements as described in the Standard itself. For more information on the definition of resistance measurements used in ESD control, check out this post.
Surface resistance testers can be used to perform these tests in accordance with ANSI/ESD S20.20 and its test method ANSI/ESD S4.1; if these measurements are within acceptable ranges, the surface and its connections are good. For more information on checking your ESD control products, catch-up with this. It goes into depth as to what products you should be checking in your EPA and how they should be checked.
Measurements form an integral part of any ESD control program. Measuring devices help identify the sensitivity of ESD devices that ESD programs are based on, and also are used to verify the effectiveness of ESD control programs set in place. High quality instruments are available commercially for measuring all the parameters necessary for quantifying the extent of a static problem.
We hope the list above has introduced the techniques most commonly used. For more information on how to get your ESD control program off the ground, Request a free ESD/EOS Assessment at your facility by one of our knowledgeable local representatives to evaluate your ESD program and answer any ESD questions!
SCS Static Management Program (SMP) continuously monitors your ESD process control system throughout all stages of manufacturing. SMP captures data from SCS workstation, equipment and ESD event continuous monitors and allows you to pinpoint areas of concern and prevent ESD events. Quantifiable data allows you to see trends, become more proactive and prove the efficiency of your ESD process control system.
We will also be featuring a selection of our ESD monitoring equipment:
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.
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 22.214.171.124.2 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)
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.
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
“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.
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)]
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.
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.
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.
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.
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).
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.
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.
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.
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
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.
For more information regarding this topic, please see below for additional references.
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.
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:
– Product Qualification
– Compliance Verification
– Grounding / Equipotential Bonding Systems
– Personnel Grounding
– ESD Protected Area (EPA) Requirements
– Packaging Systems
[ANSI/ESD S20.20 clause 7.1 ESD Control Program Plan]
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.
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.
“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]
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.
Imagine this scenario: you come to work in the morning and test your wrist strap per your ESD program’s recommended test frequency procedure. The wrist strap passes and you start work on your ESD sensitive devices. 3 hours later, when you come back from your tea break, you test your wrist strap again before continuing work and the wrist strap fails.
What to do? It is unknown when exactly the wrist strap failed in those 3 hours after your first periodic test in the morning and it is possible the devices you worked on during that time frame have been damaged. You don’t know which products have been damaged – latent defects are not visible and failures may only occur at a later time, reducing the potential reliability of the products.
Periodic testing is commonly used in an ESD program, however using continuous monitoring while working on those sensitive devices will alert the operator as soon as their wrist strap and/or workstation path-to-ground connection fails. Today’s blog post will highlight various benefits of continuous monitoring.
Wrist straps are considered the first line of ESD Control. They are used to link people to ground ensuring operators are kept at the same potential as surfaces, objects and ESD sensitive devices (ESDs). Before handling sensitive items, wrist straps need to be visually inspected and checked (while worn) which will alert the operator to potential faults.
Per ESD Handbook TR 20.20 paragraph 126.96.36.199.4 Test Frequency, “Because wrist straps have a finite life, it is important to develop a test frequency that will guarantee integrity of the system. Typical test programs recommend that wrist straps that are used daily should be tested daily. However, if the products that are being produced are of such value that knowledge of a continuous, reliable ground is needed, then continuous monitoring should be considered or even required.”
Continuous monitors come in different styles and sizes but are intended to be kept on the workstation. Some units just ‘sit’ on the bench; others are attached to the working surface matting; some can even be attached underneath the workbench so they don’t take away valuable workspace. Operators connect their wrist strap to the unit to allow for real-time continuous monitoring. If the wrist strap fails, the unit will alarm. Many continuous monitors also feature a parking snap providing a means for the operator to disconnect when leaving their workstation.
Types of Continuous Monitors
There are two different types of continuous monitors available:
Single-wire continuous monitors allow the use of any standard, single-wire wrist strap and coiled cord. The monitor / wrist strap system life-cycle costs are significantly lower compared to dual-wire systems. While they would not be suitable for the most critical applications, single-wire continuous monitors are an economical way to monitor both the operator’s wrist strap and/or workstation surface.
Dual-wire continuous monitors provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards. Dual-wire continuous monitors provide redundancy because even if one dual-wire wrist strap conductor is severed, the operator still has a reliable path-to-ground with the other conductor. Dual-wire technology requires the use of dual-wire wrist straps and coiled cords.
Benefits of Continuous Monitors
1. Instant Feedback
Continuous monitors provide operators with instant feedback on the status and functionality of their wrist strap. The instant an operator’s wrist strap or cord fails, the monitor will issue audible and visual (LEDs) alarms alerting the user and supervisor of the problem. The faulty wrist strap can be replaced with a new one from stock.
2. Monitoring of Operator AND Workstation
When the monitor is connected to an ESD working surface, the amount of current that flows is a function of the total resistance between the monitor and through the working surface to ground. When the resistance of the working surface is below a pre-set threshold*, the monitor will indicate good. Conversely, if the resistance level is high when compared to the monitor’s reference*, the unit will alarm. This is an integrating resistance measuring circuit, therefore it is relatively insensitive to externally induced electromagnetic fields.
*The resistance threshold limits can vary between brands and models (and can sometimes also be adjusted by the user) so make sure you do your homework before committing to a particular unit and check the limit meets your individual requirements.
“Some continuous monitors can monitor worksurface ground connections. A test signal is passed through the worksurface and ground connections. Discontinuity or over limit resistance changes cause the monitor to alarm. Worksurface monitors test the electrical connection between the monitor, the worksurface, and the ground point. However, the monitor will not detect insulative contamination on the worksurface and test methods such as those outlined in ESD TR53 can be used to isolate this problem. ” [ESD TR20.20 Continuous Monitors Clause 18.4.2 Worksurface Ground Monitoring].
3. Detection of Initial Flex Fatigue
Unlike wrist strap testers, 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.
“During operation, wrist straps might be stressed and flexed to their limits at a workstation. While a wrist strap is being checked it is typically not stressed, as it would be under working conditions. Openings in the wire at the coiled cord’s strain relief are sometimes only detected under stress. Even if the wrist strap is working properly, a bad or intermittent ground connection will render the wrist strap system less than 100% effective.” [ESD TR20.20 Continuous Monitors Clause 18.2 Wrist Strap Checkers]
4. Elimination of Periodic Testing
Many customers are eliminating periodic touch testing of wrist straps and are utilizing continuous monitoring to better ensure that their products were manufactured in an ESD protected environment. Continuous monitors also eliminate the need for users to test wrist straps and log the results.
When using continuous monitoring, operators:
Don’t have to waste time queuing at a wrist strap test station before each shift.
Don’t have to remember to complete their daily test logs.
If your company manufactures products containing ESD sensitive items, you need to ask yourself “how important is the reliability of our products”? Sooner or later a wrist strap is going to fail. If your products are of such high value that you need to be 100% sure your operators are grounded at all times, then you should consider a continuous monitoring system.
Advantages of Continuous Monitors are plentiful:
Immediate feedback should a wrist strap fail
Monitoring of operators and work stations
Detection of split-second failures
Elimination of periodic testing
All of the above advantages of Continuous Monitors will lead to a reduction in overall costs.
Savings comes from:
Eliminating time/labor required in verifying a wrist strap before handling ESDs
Reducing damage to ESDs from broken wrist straps that may go unnoticed with standard wrist strap testers.
For more information and an overview of SCS Workstation Monitors, have a look at our Selection Guide.