Failure Verification

Failure verification is the process of validating (or invalidating) the failure of the sample.  The best way to verify a failure is to electrically test the samples using the automatic test equipment (ATE) used during production testing.  The advantages of using the ATE to verify the failure are:  1) the validation is correlated with production standards making it more 'official'; and 2) the ATE can give a datalog of results for each parameter, identifying the specific failure for the analyst.  

If ATE-generated datalog results are available, their careful analysis prior to the failure analysis proper will make a big difference in the efficiency of the FA process.  As early as datalog analysis, the analyst should be hypothesizing already the possible failure mechanisms and attributes affecting the sample, based on the test blocks failed by the sample.  

Failure verification is usually associated with failure characterization.  In fact, failure verification and characterization are sometimes indistinguishable, especially if ATE testing of the sample is not possible.  Bench testing and current-voltage (I/V) curve tracing are very good techniques for complementing ATE testing for failure verification and characterization. 

Bench testing is the process of characterizing the failure mode of the sample using various bench equipment for exciting the device and measuring its responses.  Since different test parameters require different test conditions, the analyst needs to revise the bench test set-up every time a new test parameter needs to be characterized.  As such, failure verification may entail the use of several set-ups before the nature of device failure is fully understood.

Equipment required for effective failure verification include various power supplies, multimeters, frequency counters, oscilloscopes, curve tracers, break-out boxes, and the like. Sometimes it is also necessary to build a circuit that simulates the application of the customer where the failure was observed.  The idea is to be able to observe the failure of the sample inside the FA lab without an ATE.

Fig. 1. Typical bench test equipment for failure verification: (left to right) power supply, multimeter, frequency counter, oscilloscope

Curve tracing is the process of analyzing the current-voltage characteristics of an electrical path using an equipment known as a curve tracer.  It can identify electrical failures that exhibit abnormal voltage-current relationships between pins.   Curve tracing can also be done on an electrical path inside the die circuitry itself, where the nodes defining the electrical path are not connected to any external pins. Microprobing is employed to achieve electrical contact with the selected nodes, with the probe needles also attached to the curve tracer.

Failure analysis proper should only be pursued if the analyst is convinced that the reported failure is valid.  Otherwise, the analyst should promptly inform the FA requestor that the failure is invalid, and that full FA will not be performed.  In such a case, the analyst should always have ample data to show that the sample is indeed electrically good.

See Also:  Failure Analysis;  All FA Techniques;  Curve Tracing;  Microprobing; 

FA Lab Equipment;  Basic FA Flows;  Package Failures;  Die Failures