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Predicting Strength & Pad Cratering Failures Under BGA Pads
February 4, 2014 |Estimated reading time: 2 minutes
In the past few years, several papers, test methods, and methodologies have been developed to estimate pad cratering under BGA pads in PCB assemblies.
However, almost all the tests and methodologies proposed so far have the following shortcomings:
- They are destructive. The samples tested are broken and the failure mode(s) observed to determine the propensity for cratering.
- They require testing several samples using different test methods (bend, shock, pull, shear, or acoustic), but there is no easy correlation between the different test methods.
- They can be used for relative comparisons, but there is no easy way to translate the correlations into failures in actual functional board level assemblies.
While these tests have helped mitigate pad cratering significantly, the industry still needs the following:
- A non-destructive way to predict whether a certain PCBA design (BGA and PCB combination) is likely to result in mechanical strain induced pad cratering failures, long before the product has been built.
- An easy to use way to correlate PCB level pad pull tests done per IPC-9708 and monotonic bend tests per IPC-9702, so that the design can be optimized to mitigate pad cratering failures during qualification testing.
- An easy to implement design to detect pad cratering in a functional assembly, so that if a failure is observed in the field, it is easy to determine if the failure is due to pad cratering. Detecting a failure as soon as it occurs is critical in identifying the source of the mechanical strain that resulted in the failure, which in turn can help quickly resolve the issue.
In this study, using extensive experimental data, a detailed methodology is outlined to show the relationship between board strain, solder joint strain and force to failure in monotonic bend testing and pad pull testing. In addition, the correlation between monotonic bend testing and pad pull testing is shown, such that the results of pad pull testing can be used to estimate the likelihood of observing pad cratering failures in PCBAs during bend testing and manufacturing operations. The methodology can be used by designers to estimate the optimal combination of package and PCB design variables to minimize the likelihood of pad cratering and other solder joint failures during testing and field operation.
Finally, methodologies to predict and determine pad cratering failures quickly in field operations are also discussed and outlined.Read the full article here.Editor's Note: This article originally appeared in the January 2014 issue of SMT Magazine.