Basic FA Flows (Page 3) - Die Cracking FA Flow

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Basic Die Cracking FA Flow

1) Failure Information/Device and Lot History Review. Understand the customer's description of the failure, i.e., the failure mode, where it was encountered, what conditions the sample was subjected to, etc. Check the FA history of the device to determine if it has exhibited die cracking returns before.  Check the assembly and test history of the lot to determine if the lot has exhibited any yield or process issues potentially related to die cracking. 

2) Failure Verification. Verify the customer's failure mode by electrical testing. 

3) External Visual Inspection. Perform a thorough external visual inspection on the sample.  Note all package anomalies that may indicate the unit having been subjected to thermo-mechanical stresses, i.e., package cracks/chip-outs, tool marks, bent leads, discolored/burned package, etc.

4) Bench Testing. Verify the electrical test results by bench testing. 

5) Curve Tracing.  Perform curve tracing at ambient, elevated (125C-150C) and low temperature (-10C to -40C). Look for open or shorted pins which may indicate gross die cracking.  Note, however, that some die crack failures may only exhibit subtle I/V curve anomalies. 

7) X-ray Inspection.  Perform x-ray inspection on the sample.  Check for die attach problems such as excessive voids, die overhang, insufficient die attach coverage, and insufficient fillet. Check also for molding compound voids and cracks. Gross die cracks may also be found using sophisticated x-ray equipment.

8) CSAM.  Perform CSAM on plastic packages to determine if the samples have any internal delaminations that are indicative of the unit having been subjected to extremely high temperatures. Units with severe die attach abnormalities will exhibit die cracking upon exposure to temperature extremes.

9) Decapsulation/Internal Visual Inspection.  Perform internal visual inspection after decap to confirm the die crack.  The crack pattern on the die surface as well as the die edge must be fully understood through extensive optical and SEM inspection.  

10) Full Decapsulation.  Many die cracking issues involve die cracks that originate from the backside of the die.  If SEM inspection of the die surface and die edge indicates that the cracks most likely originated from the die backside, then full decapsulation must be done.  Full decapsulation consists of immersing the entire unit in acid to disintegrate the entire package, leaving behind the die only.  The die backside crack pattern may then be inspected freely once full decap is completed.

11) Fractography.  Fractography is the systematic and scientific process of determining the origination and propagation of the cracking mechanism by studying the attributes of the fracture surface of the die.  Fractography is a complicated process and can only be done reliably through years of study and experience. Once mastered, fractography would be an indispensable tool for analyzing die crack issues.  

Note that Steps 9, 10, and 11 all have one objective: to understand the crack origin and propagation pattern to determine what stresses were applied to the die.

12) Conclusion. As may be discerned from above, the basic flow of a die cracking FA consists of the following: a) taking note of all electrical and visual/mechanical attributes of the sample before decap; b) confirmation of the die crack after decap; c) determination of the point of origin and propagation pattern of the die crack; d) determination of the points of application and direction of the stresses most likely experienced by the die based on the crack origin and propagation; and e) subsequent investigations, simulations, or evaluations to identify the root cause of the stresses.