We report a case study for the optimization of a flip chip based stacked die array test package. We demonstrate the importance of package substrate design and substrate thickness on the processibility and package warpage control. We found that for thin substrates copper balancing of the top and bottom die is crucial. We show the impact of flip chip die thickness and substrate thickness on the die attach of the top die(s) in the stack. Investigations on different top die attach alternatives show that tape die attach can have advantages. We demonstrate the importance of the vertical stack structure (i.e. flip chip thickness) and material selection (i.e. mold compound) on the overall warpage control of the package. The results show that even small changes in the package structure can have large impact on the warpage characteristics of the stacked die package.

This paper is about warpage of laminate substrate based BGAs and fine pitch BGAs at lead-free board assembly reflow temperatures using shadow moiré technique. Various constructions (e.g. single die devices and multi chip modules) and different die and package sizes as well as different molding compound materials were considered in this study. A simple model for predicting high temperature warpage for single die devices based on the distance between package corner and die corner was developed. The comparison of the warpage results with and without moisture soaking according JEDEC MSL 3 did not exhibit a significant influence on high temperature warpage. The shadow moiré test results at elevated temperatures are compared with solder joint height distributions measured in cross sections after real lead-free board assembly. A clear correlation of the solder joint height distributions with the package warpage status at 170 °C during cooling phase was found. This is much lower than the mominal solidification temperature of lead-free solder. This study helps to understand the influencing factors of BGA warpage and its role for board assembly.

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