Whilst many people understand the basic concepts of electrostatic discharge (ESD), and as a result insist on a properly equipped ESD Protected Area (EPA), it is then forgotten about. The same products are used day-in, day-out, year after year, without knowing if they are still working properly. This article looks at the most common products in your EPA that you should be checking on a regular basis.
Why periodic verification?
Each component in an ESD-protected area plays a vital part in the fight against ESD. If just one component is not performing correctly, damage could be caused to ESD-sensitive devices potentially costing your company money. The problem with many ESD protection products (think wrist straps!) is that it is not always easy to see any damage. Just by looking at a coiled cord that has no visible damage to the insulation does not indicate whether the conductor on the inside is damaged. That’s where periodic verification comes into play.
ESD protected area (EPA) products should be tested:
• Prior to installation to qualify the product for listing in the ESD control plan.
• During initial installation.
• For periodic checks of installed products as part of IEC 61340-5-1 Edition 1 2007-08 clause 5.2.3 compliance verification plan.
Worksurface matting
The purpose of ESD bench matting is to ensure that when charged conductors (conductive or dissipative) are placed upon the surface, a controlled discharge occurs, and electrostatic charges are removed to ground. However, this only occurs if the ESD work surface is connected to ground. If the matting is out-of-spec, not grounded at all, the stud on the mat has become loose, or the ground cord has become disconnected, charges cannot be removed.
Many companies use a daily checklist, which includes the operator having to verify that ground cords are firmly connected.
Remember to regularly clean the bench matting to maintain proper electrical function. Cleaners with silicone should not be used as silicone build-up will create an insulative film on the surface. The company’s compliance verification plan should also include periodic checks of work surfaces measuring:
• Resistance point-to-point (Rp-p).
• Resistance-to-ground (Rg).
Surface resistance testers can be used to perform these tests in accordance with EN 61340-5-1 Electrostatics and its test method IEC 61340-2-3; if these measurements are within acceptable ranges, the worksurface matting and its connections are good.
Wrist straps
As discharges from people handling ESD-sensitive devices cause significant ESD damage, the wrist strap is considered the first line of ESD control.
Before handling ESD-sensitive items, the wrist strap should be visually inspected to see if there are any breakages, or other damage. The wrist strap should then be tested while being worn using a wrist strap tester. This ensures all three components are checked: the wrist band, the ground cord (including resistor) and the interface with the wearer’s skin. Records of each test should be kept.
Wiggling the resistor strain relief portion of the coil cord during the test will help identify failures sooner. Analysis and corrective action should take place when a wrist strap tester indicates a failure.
It is recommended that wrist straps are checked at least daily. An even better solution to daily wrist strap checks is the use of continuous monitors, which will sound an alarm if the person or work surface is not properly grounded.
Floor matting
A flooring/footwear system is an alternative for personnel grounding for standing or mobile workers. Foot grounders quickly and effectively drain the static charges which collect on personnel during normal, everyday activities. Foot grounders should be used in conjunction with floor surfaces which have a surface resistance of less than 1010 ohms.
As ESD floors get dirty, their resistance increases. For optimum electrical performance, floor matting must be cleaned regularly using an ESD mat cleaner. The same as bench matting, cleaners with silicone are not recommended as silicone build-up will create an insulative film on the surface.
A dissipative floor finish can be used to reduce floor resistance. Periodic verification will identify how often the floor finish needs to be applied. As the layer(s) of dissipative floor finish wear, the resistance measurements will increase. A cost-effective maintenance schedule can be established, therefore, after some amount of data collection.
Footwear
ESD shoes or foot grounders play an essential part in the flooring/footwear system. Before handling ESD-sensitive devices, ESD footwear should be visually inspected for any damage. Just like wrist straps, footwear should be checked while being worn using a wrist strap/footwear tester. Footwear needs to be checked daily.
ESD packaging
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. It is up to the user to determine if a shielding bag is suitable for re-use or not. The testing of every bag before re-use is not practical. Many companies will discard the shielding bag once used and replace it with a new one. Others will use a system of labels to identify when the bag has gone through five handling cycles:
• Non-reusable labels are used that require the label be broken to open the bag.
• The bag is then resealed with a new label.
• When there are five broken labels, the bag is discarded.
Ionisers
Ionisers are intended to neutralise static charges on insulators, thereby reducing their potential to cause ESD damage. However, poorly maintained ionisers, with dirty emitter pins and out-of-balance ionisers, can put a charge on ungrounded items. Ioniser emitter pins and filters need to be cleaned regularly.
An ionisation test kit can be used to measure the offset voltage and charge decay of ionisation equipment.
Testing the testers
What good do all the checks do, if the testers that are used are out-of-spec and show incorrect results?
Yearly calibration is recommended – many manufacturers offer a calibration service or, alternatively, calibration units can be purchased to allow the calibration to be performed in-house.
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