Basic FA Flows 

Every experienced failure analyst knows that every FA is unique. Nobody can truly say that he or she has developed a standard FA flow for every FA that will come his or her way.  FA's have a tendency of directing themselves, with each subsequent step depending on the outcome of the previous step. 

The flow of an FA is influenced by a multitude of factors: the device itself, the application in which it failed, the stresses that the device has undergone prior to failure, the point of failure, the failure rate, the failure mode, the failure attributes, and of course, the failure mechanism.  Nonetheless, FA is FA, so it is indeed possible to define to a certain degree a 'standard' FA flow for every failure mechanism.  

This article aims to give the reader a basic idea of how the FA flow for a given failure mechanism may be standardized. 'Standardization' in this context does not mean defining a step-by-step FA procedure to follow, but rather what to look for when analyzing failures depending on what the observed or suspected failure mechanism is.

Basic Die-level FA Flow

1) Failure Information Review. Understand thoroughly the customer's description of the failure.  Determine: a) the specific electrical failure mode that the customer is experiencing; b) the point of failure or where the failure was encountered (field or manufacturing line and at which step?); c) what conditions the samples have already gone through or been subjected to; and d) the failure rate observed by the customer.

2) Failure Verification. Verify the customer's failure mode by electrical testing.  Check the datalog results for consistency with what the customer is reporting.

3) External Visual Inspection. Perform a thorough external visual inspection on the sample.  Note all markings on the package and look for external anomalies, i.e., missing/bent leads, package discolorations, package cracks/chip-outs/scratches, contamination, lead oxidation/corrosion, illegible marks, non-standard fonts, etc.

4) Bench Testing. Verify the electrical test results by bench testing to ensure that all ATE failures are not due to contact issues only. The ideal case is for the customer's reported failure mode, ATE results, and bench test results to be consistent with each other. 

5) Curve Tracing.  Perform curve tracing to identify which pins exhibit current/voltage (I/V) anomalies.  The objective of curve tracing is to look for open or shorted pins and pins with abnormal I/V characteristics (excessive leakage, abnormal breakdown voltages, etc.).  FA may then be focused on circuits involving these anomalous pins. Dynamic curve tracing, wherein the unit is powered up while undergoing curve tracing, may be performed if static curve tracing does not reveal any anomalies.

6) X-ray Inspection.  Perform x-ray inspection to look for internal package anomalies such as broken wires, missing wires, incorrect or missing die, excessive die attach voids, etc, without having to open the package.  Xray inspection results must be consistent with curve trace results, e.g., if x-ray inspection revealed a broken wire at a pin, then curve tracing should reveal that pin to be open. 

7) CSAM.  Perform CSAM on plastic packages to determine if the samples have any internal delaminations that may lead to other failure attributes such as corrosion, broken wires, and lifted bonds. 

8) Decapsulation.  Once all the non-destructive steps such as those above have been completed, the samples may be subjected to decapsulation to expose the die and other internal features of the device for further FA.

9) Internal Visual Inspection.  Perform internal visual inspection after decap.  This is usually done using a low-power microscope and a high-power microscope, proceeding from low magnification to higher ones. Look for wire/bond anomalies, die cracks, wire and die corrosion, die scratches, EOS/ESD sites, fab defects, and the like.  SEM inspection may be needed in some instances.

10) Hot Spot Detection.  If curve trace results indicate some major discrepancies between the I/V characteristics (especially with regard to power dissipation) of the samples and known good units, then the samples may have localized heating on the die.  For example, an abnormally large current flowing between an input pin and GND may mean a short circuit from this input pin to GND. Shorts such as this will emit heat that can be located by hot spot detection techniques. 

11) Light Emission Microscopy.  If the device does not exhibit abnormalities in power dissipation that may indicate hot spots, light emission microscopy may be performed to look for defects that emit light.  Note that an emission site does not mean that it is the failure site. 

12) Microprobing.  Microprobing becomes necessary if no hot spots nor abnormal photoemissions were seen from the samples.  Microprobing may entail extensive circuit analysis wherein the failure site is pinpointed by analyzing the die circuit stage by stage or section by section. The thought process used when troubleshooting a full-size circuit also applies to die circuit troubleshooting.

13) Die Deprocessing.  Perform die deprocessing to look for subsurface damage or defects if the above FA steps were not successful in locating the failure site.  

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