We occasionally hear from customers who have ESD footwear (Foot Grounders or ESD Shoes) that is failing high on their ESD footwear tester (personnel grounding tester). There is often an assumption that the issue is a faulty tester or out of spec footwear. However, most of the time the problem turns out to be as simple as a “bad match”. This is between the upper resistance setting of the footwear tester and the resistance of the ESD footwear.
The first step in identifying the cause of the problem is to identify the upper limit setting of the tester being used.
Upper Test Limits of ESD Testers
The upper limit settings on SCS footwear testers are:
The upper limit is 1 gigaohm or 1 x 109 ohms however the factory setting 35 megaohm or 3.5 x 107 ohms
The upper limit 1 gigaohm or 1 x 109 ohms however the factory setting 35 megaohm or 3.5 x 107 ohms
Identifying the Resistance of the ESD Footwear
The next step is to identify the resistance of the ESD footwear being used, both by itself and with a person wearing the footwear.
Heel Grounders/Foot Grounders
Test Method 1 – Resistance of the Foot Grounder
Using a surface resistance tester and its alligator clip, clip the lead to the foot grounder tab
Clip the other lead to the cup material
This test will tell you the overall resistance of the tab, resistor (if there is one), and the cup material
If the foot grounder tab to cup resistance exceeds the upper limit of the footwear tester it’s very likely that you won’t pass on the tester.
Test Method 2 – Resistance of the Foot Grounder and Person Together
Use test method from TR53-01-15, 8.3.2 Meter, which has been summarised below:
Use a surface resistance tester, a handheld electrode and a foot electrode
Have the person hold the handheld electrode connect to one lead
Have the other lead connected to the foot electrode
Place one foot on the foot electrode making sure your other foot is not on an ESD floor or the electrode
This test will tell you the overall resistance of the whole foot grounder and person.
If the result is higher than the upper limit of your footwear tester you will not pass at the tester.
When we hear about ESD Footwear failing regularly at the personnel grounding tester, more often than not it is ESD Shoes that are failing rather than foot grounders.
Test Method 1 – Resistance of the ESD Shoes Using Foot Plate Electrode
Place the shoe on a metal plate.
Put the 5 lb weight from the surface resistance meter inside the shoe.
Place the other weight on the metal plate next to the shoe.
Lean on the weight inside the shoe and test.
This will give the resistance of the shoe itself from inside the shoe to the bottom of the shoe.
Test Method 2 – Resistance of the ESD Shoe Using Two 5 lb Electrodes
Put one of the 5 lb weights from the surface resistance meter inside the shoe.
Put the other weight on the sole of shoe, make like a sandwich
Press the Test
See what the resistance of the shoe is using either of these methods and compare it to the testers upper limit.
Perform the TR53 test below, see what the resistance is of the person and ESD shoe while the shoe is being worn.
The same test from TR53 used for foot grounders above can also be used for ESD Shoes. This will provide you with the total resistance is of the person and ESD shoe.
The tests above will identify a clear discrepancy between footwear resistance and tester settings, but there can also be other factors in play. These factors should be considered when the footwear resistance and the tester settings are close to being the same.
Weight of personnel
Sock thickness and material
Any questions regarding this post, please get in contact.
There are all kinds of variations and combinations of ESD Laminate and Continuous Monitors used by companies with ESD Programs.. There are some key factors that you need to be aware of if you intend to use ESD Laminate together with a continuous monitor that will monitor the worksurface connections.
It’s important to note that continuous monitors don’t monitor the status/condition of the outer surface of an ESD workstation. The continuous monitor monitors the connection between groundable points on the worksurface (often snaps), not the Resistance to Ground (RTG) of the surface itself. The RTG measurement must be taken separately, per ANSI/ESD TR53-01-18 – Compliance Verification of ESD Protective Equipment and Materials (Pages 6 and 7).
The basic technology is that the monitor applies a low test voltage to the scrim layer in the worksurface. Because the test voltage is so low, the resistance of the scrim layer of the mat must also be low so that the test voltage can complete a circuit of the scrim layer, worksurface connections (snaps) and ground cords. Completing that circuit indicates that the worksurface is electrically connected. The goal is to have the worksurface circuit fail because of a bad connection, not because the scrim layer resistance was too high. As mentioned earlier, testing the combination of the outer layer and the scrim layer together (RTG) is a separate test.
When choosing a continuous monitor and worksurface combination that will work together it is important to consider the following:
Does the worksurface have a separate scrim layer? (vs. a homogenous mat material)
Does the continuous monitor spec sheet note the resistance limit of the scrim layer required for the monitor/worksurface alarm system to pass?
See the excerpt below from the SCS 724 Continuous Monitor Technical Bulletin – https://www.descoindustries.com/PDF/724-Workstation-Monitor-User-Guide.pdf “Red Worksurface LED (M) This indicates that a high resistance condition (> 3.7 Megohms) exists across the conductive layer of theworksurface and/or the ground connections. Check the worksurface, ground cords and their connections for continuity. Note the audible alarm may also sound if enabled.”
What this means is if the resistance of the mat scrim layer is greater than 3.7 Megohms then the continuous monitor mat alarm would alarm for a high resistance condition even if the grounding hardware connections to the worksurface were intact. There are other SCS monitors available that will monitor a scrim layer with a resistance as high as 5 x 108.
In summary, it’s critical to know both the upper limit of the “pass” condition of the continuous monitor and the construction of the worksurface material (does it have a scrim layer and if so what is the resistance of the scrim layer?)
ESD Laminate requires extra attention when being considered for use with a continuous monitor for the following reasons:
Laminate material is rigid, which makes it more difficult for grounding hardware to make a good contact with the scrim layer.
Consider abrading the outer, decorative surface to expose the scrim layer for better contact. Consider a flat bottom drill for this process.
Most importantly, perform a test on the resistance between two points on the worksurface to determine if the resistance meets the requirements of the specified monitor (for the SCS 724 that requirement is less than 3.7 Megohms – 3.7 x 106 ). We recommend using an ohm meter with a test voltage similar, if not identical to the test voltage used by the continuous monitor. We recommend performing this test before the purchase/installation of any number of continuous monitor/worksurface combinations.
Welcome back to “A Minute with Miranda.” This week we will be covering how the Ground Master Monitor provides continuous monitoring of the path-to-ground impedance and electromagnetic integrity of eight metal ground connections of process tools in your SMT assembly work area.
The Ground Master Monitor
continuously monitors eight metal tools for electromagnetic interference (EMI).
EMI can cause equipment lockups and malfunction. The Ground Master Monitor will
alarm if EMI is detected. The Ground Master will also alarm if the grounded
metal tools have a high-frequency noise that can cause electrical overstress
(EOS) damage. The Ground Master Monitor provides both a visual and audible
alarm for the monitored ground connections. The Ground Master Monitor meets the
Continuous Monitor requirements of ANSI/ESD S20.20 in accordance with ESD TR53.
Sensitivity to electrostatic discharge (ESD) is an important concern in PCB assembly manufacturing. Pinpointing where ESD events are occurring on an SMT line can be difficult determine. ESD exposure can take place anywhere in the process, including PCB loading, component handling, soldering, and operator interventions. ESD events generate electromagnetic radiation. The stronger the ESD event, the stronger the electromagnetic radiation. Detecting and measuring the unique waveform generated by an ESD event can help determine where the ESD event is occurring and reduce the time it takes to identify and solve the problem. Watch this video to see how continuous ESD Event Detection monitoring can be setup on an SMT Line to monitor the processes and triangulate where static voltage and ESD events are occurring in the process.
Welcome back to “A Minute with Miranda.” This week we will be covering how the EM Aware Monitor provides continuous monitoring to detect and measure ESD Events in your STM machine.
The SCS EM Aware Monitor is a continuous monitor for three key parameters that allow you to verify your ESD process in an automated insertion machine; ESD events, change in static voltage field, and ionizer balance. The thresholds for all three of these parameters are fully adjustable by the user. The EM Aware Monitor is a miniature radio receiver tuned to detect and measure the unique waveform generated by an ESD event. The EM Aware Monitor meets the Continuous Monitor requirements of ANSI/ESD S20.20 in accordance with ESD TR1.0-01 and ANSI/ESD STM3.1. It meets the recommendations of ESD Handbook ESD TR20.20 which includes “if the products that are being produced are of such value that the knowledge of a continuous, reliable ground is needed, then continuous monitoring should be considered or even required.”
Welcome back to “A Minute with Miranda.” This week we will be covering how the WS Aware provides continuous monitoring for an operator at an SMT line.
The SCS WS Aware Monitor is a continuous monitor for operators, ESD Worksurfaces and metal tools. It will continuously monitor the path-to-ground integrity and body voltage of two operators. It also monitors the path-to-ground integrity of two metal tools. In addition, it continuously monitors for electromagnetic interference (EMI) on two metal tools, which may cause electrical overstress (EOS) damage. The WS Aware Monitor eliminates the need for periodic testing and record keeping of wrist straps.
Per ESD Handbook ESD TR 20.20 section 220.127.116.11.4 “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.”
View the full range of SCS WS Aware Monitors here.
Welcome back to “A Minute with Miranda.” This week we will be covering how to test ESD footwear entering an ESD Protected Area (EPA).
Per the ESD Handbook ESD TR20.20, A system test of the footwear in combination with the existing or proposed flooring materials in the plant should be made to ensure that the criteria for the facility are met. When using a footwear checker it is important to make sure the upper and lower resistance limits of the checker match the user’s requirements. When testing the footwear should test within the range of 1 x 106 to 1 x 108 ohms.
Heel, sole and toe grounders should be worn on both feet to ensure effective use. They should be worn by all personnel and visitors within an ESD controlled area. If worn improperly, the heel, sole and toe grounders become ineffective. ESD footwear should be tested daily before use within an ESD Protected Area (EPA).
Welcome back to “A Minute with Miranda.” This week we will be covering how the WS Aware Monitor provides for continuous monitoring of an ESD Workstation setup.
The SCS WS Aware Monitor is a dual workstation continuous monitor for operators, ESD Worksurfaces and metal tools. It will continuously monitor the path-to-ground integrity and body voltage of two operators. It also monitors the path-to-ground integrity for two conductive or dissipative worksurfaces and two metal tools. It also continuously monitors for electromagnetic interference (EMI) on two metal tools which may cause electrical overstress (EOS) damage. The WS Aware Monitor eliminates the need for periodic testing and record keeping of wrist straps.
Per the ESD Handbook ESD TR20.20 section 18.4.2 “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.”
View the full range of SCS WS Aware Monitors here.
Welcome back to “A Minute with Miranda.” This week we will be covering how to use the WS Aware Monitor for continuous monitoring of SCS wrist straps when at an ESD Workstation.
The SCS WS Aware Monitor is a dual workstation continuous monitor for operators, ESD Worksurfaces and metal tools. It will continuously monitor the path-to-ground integrity and body voltage of two operators. The WS Aware Monitor eliminates the need for periodic testing and record keeping of wrist straps. The WS Aware Monitor features operator body voltage detection. It will alarm if the operator generates or comes into contact with voltage that would be dangerous to an ESD susceptible item. Per the ESD Handbook ESD TR20.20 section 18.104.22.168.4 “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 continuous, reliable ground is needed, then continuous monitoring should be considered or even required.”
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.