Electrical Test Confidence; ATE Test Confidence

Electrical Test Confidence

Every new test engineer eventually finds out that absolute certainty of test results can never be guaranteed, even if the most advanced automated test equipment (ATE) are involved. As such, there will always be instances wherein retest of units is necessitated, consuming valuable test capacity and increasing test cycle time without additional output.

Electrical test confidence, simply put, is a measure of how consistently an ATE system delivers the correct electrical test results. A test system with excellent test confidence makes retest unnecessary, saving precious test resources. Test confidence is therefore an indirect indicator of how efficient and productive a test process is.

If someone will collect all the test rejects after the first-pass screening of a lot and retest these, there is reasonable likelihood that some of these initially failing units will pass their second test. Since genuine failures can not be resurrected with retesting, the only explanation for their passing the second test is that they were never really electrically bad when they were first tested. Something, somehow, made them fail the first test.

An electrically good unit that fails electrical testing for whatever reason (and can therefore pass electrical retesting under more proper conditions) is often referred to as an 'invalid' reject or failure. One's test confidence in an ATE system may be measured in terms of the number of invalid failures encountered from the system.

Invalid failures occur due to a multitude of reasons, which include but are not limited to: 1) improper contact between DUT leads and the contactor; 2) poor design and fabrication of test hardware; 3) improper test equipment set-up; 4) oxidation and contamination of metal surfaces used for electrical connection in any part of the system; 5) condensation or excessive moisture build-up anywhere within the system; 6) tester/instrumentation repeatability and reproducibility. The performance variability of a marginally good device can also result in invalid failures.

Of the aforementioned causes of invalid rejections, improper electrical contact between the DUT and the contactor constitute a major slice of the pie for most companies. Improper contact between the DUT and the contactor can further be broken down into the following causes: 1) misalignment between the contactors and the DUT leads; 2) contactor wear-out or mechanical degradation; 3) oxidation/corrosion of any of the contactors or DUT leads; 4) moisture build-up or contamination on the contactors or DUT leads.

The fact that bulk of invalid failures are caused by contactor problems has led many companies to equate electrical test confidence to the confidence in making good electrical contact between the contactor and the DUT. After all, most of the other factors that contribute to invalid rejection can be corrected prior to the production release of the test process. Ways of achieving this include: 1) excellent test/software design and debugging; 2) proper test guardbanding; 3) use of good test equipment; 4) use of reliable boards and small hardware; 5) use of a well-designed test floor; and 6) a sound management system.

Based on the premise that electrical test confidence depends solely on the ability to achieve good electrical connection, 'test confidence' may then be defined as the probability that good connection will be achieved every time a DUT is tested. Thus, a test confidence of 90% means that for every 100 devices tested, 10 of these devices will encounter a contact problem that will prevent them from being tested properly.

If such a system tests 1000 devices, with 90% of them (or 900 units) being electrically good, only 810 units will pass the test. Ninety (90) of the 900 good units will experience contact issues during testing, becoming invalid failures. Subjecting these 90 invalid failures to retest will fail to recover all of them, since again, 10% of them (9 units) will become invalid failures.

<Proceed to Page 2 - Test Confidence and Yield Recovery Equations>

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