Electrical Test Confidence; ATE Test Confidence

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Electrical Test Confidence (Page 2 of 2)

Several retest cycles will eventually recover all the parts that have been invalidly rejected by the initial rounds of electrical testing, since all of them are good anyway. Such a process for recovering invalid rejects, however, is inefficient and not cost-effective. The value of recovering invalid failures through retesting is, therefore, determined by the system's test confidence as well.

The resulting first pass yield, Y1, is the product of the real or actual yield of the lot (denoted here as 'Y') multiplied by the test confidence of the ATE system (denoted here by 'C'), or Y1 = Y x C. Thus, if a test engineer knows the test confidence of a system, he can estimate the actual yield of the lot through the equation Y = Y1 / C.

If the engineer decides to recover the invalid failures, he'll have to retest all the fall-outs from the first test (denoted here by 'R2'). The retest quantity R2 is equal to the initial quantity (Q) multiplied by the first pass rejection rate, (1-Y1). Thus, R2 = Q (1-Y1).

The resulting yield of the retest (denoted here as 'Y2') is equal to the actual yield of the retest multiplied by C. The actual yield of the retest (denoted here as 'YY') equals the number of invalid failures R_invalid divided by the retest quantity R2. Thus, YY = R_invalid/R2 and Y2 = (R_invalid/R2) x C.

The number of invalid failures R_invalid is equal to the initial test quantity Q multiplied by the difference between the actual yield of the lot Y and first pass yield of the lot Y1, or R_invalid = Q x (Y-Y1). But Y = Y1/C, so R_invalid = Q x (Y1/C - Y1).

Thus, Y2 = {[Q(Y1/C-Y1)] / [Q (1-Y1)]} x C = [(Y1/C-Y1)/(1-Y1)] x C.

Simplifying, Y2 = (Y1(1-C))/(1-Y1), where Y2 is the expected yield of the retest based on the first pass yield Y1 and the test confidence C.

This equation can also be used by an engineer to compute for the test confidence exhibited by his test system, given first pass and retest yield data: 1-C = Y2(1-Y1)/Y1, or

C = 1 - [Y2(1-Y1) / Y1]

where C = confidence of your test system, Y1 is the first pass yield, and Y2 is the yield when the first pass rejects are retested.

According to Christopher Jones, author of the article "Analyze test Confidence to Enhance Throughput" upon which this article was based, their experience in testing millions of RF IC's every week has taught them the following:

1) never run a test if the test confidence is less than 85%;

2) retest of the rejects is not necessary if the test confidence exceeds 95%; and

3) retest of the rejects is recommended if the test confidence is between 85%-95%.

Of course, the above guidelines may not be applicable to every company, since different device groups and package types are subject to different economic factors, as well as exhibit different sensitivities to contactor degradation. It is the task, therefore, of a Test Manager to determine for his company how they can best apply the concept of test confidence management in improving their bottom lines.

Given the very competitive atmosphere of the IC testing industry today, every Test Manager must know when to do a retest, when it is not economical to do so, and when a test system should not be used at all. Being able to distinguish these situations from each other based on test confidence data and reacting to each of them appropriately is an important aspect of test engineering management.

Primary Reference: Christopher Jones, M/A-COM Division of AMP, Lowell, MA, "Analyze Test Confidence to Enhance Throughput"; Test & Measurement World, 9/1/1999; through http://www.reed-electronics.com

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