One of the requirements of any quality ESD control program is to minimize the charge (also called voltage) on operators. When operators are charged, they can transfer this charge to the product, possibly causing damage or destruction. One effective way to reduce static charging on people is to ground them through the use of a wrist strap and coil cord. When working properly, the wrist strap system (band and coil cord connected to common point ground) can control voltages on people down to 10 volts and less and limit damage.
While wrist strap systems are the most common and effective way to minimize voltage on personnel, they are also the ESD control element that is most likely to eventually fail over time. A broken wire in the coil cord, a dirty cuff interior, incorrect wearing of the strap (too loose or placed over clothing), and high skin resistance are just a few of many causes of wrist strap failure. Should any of the previous examples occur, the resistance between the operator and ground increases, and subsequently, so does voltage. Therefore, it is imperative that measures are taken to assure that the operator is properly grounded when wearing a wrist strap.
ESD Test Stations
One way to assure that the wrist strap system is working correctly is for the operator to use a test station. The test station assesses the resistance of the wrist strap system, including the operator, and provides a pass/fail indication. Some more advanced test stations will also provide the actual resistance measurement and may even indicate that the system is nearing the failure level. Many companies institute this type of testing system. Some companies require testing just once at the beginning of the shift. Other companies may require re-testing when the operator leaves for break, and some companies have systems that require the operator to test and pass the wrist strap system before they are allowed to enter the ESD Protected Area (EPA). Regardless of how many times an operator tests, there is a risk that at some point while handling ESD sensitive devices or assemblies, the wrist strap system will fail.
Transforming Technologies offers several testing station options that range from wrist strap monitors (WST200), wrist strap and footwear test stations (GTS600K), wrist strap and individual foot test stations (GTS900K), test stations with “near-fail” technology (PDT700K), and test stations with “near-fail” technology and digital display (PDT800K). These testing stations function with a standard wrist strap and coil cord using a banana jack connector and standard heel grounders.
If a wrist strap fails the test at the beginning of a shift, the question arises, “When did the failure take place and how much product was handled after the failure?” Constant monitors provide assurance that the wrist strap system is working and alarms immediately if there is any disconnect from the operator to ground. There are several benefits to constant monitor systems:
- They provide immediate notification of a problem and alert the operator to not handle the product until the problem is resolved
- They monitor the wrist strap at the work location, eliminating the possibility of a functional wrist strap but a defective ground attachment
- They eliminate the need for recording and storing test results
- Some units will also monitor the work surface to verify that it is still connected to ground
Transforming Technologies offers several constant monitor options that range from monitoring a single operator (CM400), an operator and a work surface (CM410), and two operators and a single work surface (CM420). These monitors function with a standard wrist strap and coil cord using a banana jack connector.
Wrist straps are the primary method for removing charge from operators handling ESD sensitive products. Constant monitors provide assurance that the system is working as designed and the products are being handled safely.