SMTA International Conference Proceedings

Influence Of Alloy Combination and PCB Location On Package On Package (PoP) Component Assembly Drop Test

Authors: Andrew Daya and S. Manian Ramkumar, Ph.D.
Company: Center for Electronics Manufacturing and Assembly, Rochester Institute of Technology
Date Published: 10/14/2012   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.

Post assembly solder evaluation of PoP using AOI and X-Ray is extremely difficult due to the construction of the package. This places more emphasis on achieving higher First Pass Yield (FPY). Phase I of this study, presented at SMTAI-2010, involved the investigation of the influence of three important reflow parameters: (1) Preheat (2) Soak Time (3) Time Above Liquidus (TAL), on the PoP assembly process yield, using Design of Experiments (DOE). Head-in-Pillow was one of the prevalent defects that occurred exclusively in the bottom part. Other defects include bridging and open joints. Preliminary drop and thermal shock tests were performed to compare the reliability between profiles that yielded 100% FPY. Cross sections and dye penetration tests were done to identify solder joint integrity, before and after the drop tests.

Phase II of this research was carried out to identify the influence of various alloy combinations for both the top and bottom components, including the effects of PoP assembled at various locations on the PCB. The alloy combinations for top/bottom included SAC105/SAC125, SAC105/SAC305, and SAC305/SAC125. These assemblies were subjected to drop tests to determine the integrity of the solder joint. Results from this study are included in this paper.

Key Words: 

Package on Package, PoP, alloys, drop test

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