Everett Charles Technologies Technical Paper
"Meeting the Challenge of ESD at the Test Fixture"

Written By Gary F. St. Onge, Vice President
Test Fixture Group, Everett Charles Technologies

Even if you're a casual reader of these pages ("Evaluation Engineering"), you know that the electronics industry is doing everything possible to cope with ESD. Ionizers, antistatic flooring, static dissipating mats, wrist and ankle straps and special shoes and clothing are in use wherever sensitive devices may be exposed to static. And with good reason.

That tingle when you touch a metal door frame in a low-humidity environment can pack 20 kV. As little as 250 V can destroy a CMOS device and it may be degraded at one-fourth of this value. We now realize that random ESD can cause significant device failures. The cost and reliability implications of these failures make it imperative to look at every potential solution to the problem.

Developing device technologies also are stimulating greater interest in ESD. Solid-state device junction thickness now is approaching the submicron range, resulting in a proportionate reduction in ESD tolerance. Manufacturers attempt to create a static-free environment in every process and assembly operation to prevent hard failures caused by ESD and soft failures related to electrical overstress (EOS).

ECT's Diss-Stat Fixture ECT's DISS-STAT" Fixture

Test Fixture Quandary
Although vacuum-actuated test fixtures generally are preferred for automatic board test equipment, they are an inherent source of ESD generated by the flow of air across insulating fixture surfaces during operation. The dielectric materials used to support the test probe field and the UUT are selected for their excellent insulating properties. But they also have the capacity for retaining an electrostatic charge level sufficient to damage devices on the UUT. The retained charge could be induced by a proximate charged body or by the triboelectric effect of the air flow or operator contact during test.

If the fixture is stored in a relatively static-free area protected by ionizers, the problem can be minimized. If ionizers are not well maintained and the ion balance is not checked regularly, the fixture can accumulate a potentially damaging charge.

In late 1985, ECT received reports from the field relative to ESD problems in ATE fixtures. The response from ECT engineering was to recommend using ground straps and ionizers. These measures did not provide sufficient protection for most customers. In fact, the military was shutting down some test equipment during site audits.

ECT's engineering staff began an extensive research project to identify the causes of ESD in test fixtures and eliminate the source. What ensued was a materials solution to an electrical problem.

In the new DISS-STAT" fixture, a proprietary chemical process makes appropriate interior and exterior surfaces of the fixture permanently static dissipative. The protection will not drift with time and works independently of humidity.

Resistivity in nominally 109 to 1010 ohms/sq. The fixture will not accumulate or retain a charge potentially damaging to sensitive devices on a UUT. The risk of a direct discharge from an accidentally charged UUT to the grounded test fixture also is eliminated. Any charge is bled off at a controlled rate, preventing an electrical overstress.

It is interesting to note that now, shown the solution, many customers have confessed to knowing about the problems for some time. With little more than slightly effective stop-gap measures with which to combat the problem, many chose to keep quite about the ESD troubles labeled by more than one as horror stories.

To keep these and other ATE horror stories from causing roadblocks to productivity, ECT continues research designed to reduce ESD/EOS factors at the test interface with the UUT. At present, the DISS-STAT fixture technology offers the only designed-in protection against damage from ESD in test fixtures.


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