SMTA International Conference Proceedings

Effect of Thermal Shock on Lead Free Alloy Combinations in Package on Package Assemblies

Authors: Andrew Daya and S. Manian Ramkumar, Ph.D.
Company: Rochester Institute of Technology
Date Published: 9/28/2014   Conference: SMTA International

Abstract: Package on Package (PoP) is a rapidly evolving 3D packaging technique involving stacking of one peripheral array BGA package on to another BGA package. This package assembly presents a myriad of technical challenges during the assembly process, especially in reflow soldering. The technology trend towards package miniaturization necessitates thinner substrates, reduced stand-off height, smaller die, thinner encapsulant materials etc. While all these design parameters help achieve the market need, they present additional challenges in the assembly, especially the reflow process. Higher melting temperatures used in lead free profiles increase the CTE mismatch among die, epoxy and substrates leading to warpage related defects.

A design of experiments was run on reflow process parameters to determine the best process parameters for PoP assembly reflow profiles. The profile chosen for PoP assemblies was based on first pass yield, defects found, and resultant package height. The profile chosen was used to assemble the boards for thermal shock testing.

This study of PoP components evaluates the effect of thermal shock on the alloy combinations of the package. Components will be thermally shocked (-55ºC to 125ºC) for 250, 500, 750, and 1000 hours. Using cross sectional analysis, the alloy structure between the various hours of thermal shock will be observed, and conclusions of the influence of thermal shock and alloy combinations will be made.

Daisy chain failures were not seen until the 700 hour mark of thermal shock, however thermal damage to components was seen at the 500 hour mark. Overall the SAC305 seemed to perform best, with the least amount of thermal damage observed in cross section analysis. SAC105 and SAC125Ni both saw significant thermal damage and components with this alloy combination were the only recorded daisy chain failures after 1000 hours of thermal shock.

Key Words: 

Package on Package, Thermal Shock, Lead Free, SAC Alloys

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