Study of Warpage and Mechanical Stress of 2.5D Package Interposers during Chip and Interposer Mount Process

Abstract

As data transmission rate increases, flip chip plastic ball grid array (FCPBGA) utilizing a interposer for multiple chips is gaining popularity because of high electrical performance, ease of chip design, ease of thermal management with thermal lid, etc. The authors assessed package design configuration and key design elements for two chips application assuming 1600 signal I/Os for logic and 800 signal I/Os for memory. Then, we studied warpage behavior of the interposer, and mechanical stress of solder interconnections and low-k dielectric layer under controlled collapse chip connection (C4) pad. We set three different mount process assumptions for chip to interposer and interposer to base organic substrate. The mount process assumptions are (1) two pass reflow of chip to interposer first, then interposer to base organic substrate, (2) reversed sequence of two pass reflow which is interposer to base organic substrate first, then chip to interposer, (3) one pass reflow of chip, interposer and base organic substrate all together. We also set three different interposer material assumptions of Si, glass and organic in this study. We analyzed warpage behavior and mechanical stress using finite element method (FEM) modeling technique with a set of combinations of coefficient of thermal expansion (CTE) and elastic modulus of the interposers. The study also includes an analysis for conventional multi-chip-module (MCM) FCPBGA as a reference. We show the analysis results of interposer warpage, first principal stress at low-k dielectric layer under C4 pad and von Mises stress at solder interconnections of chip joining and interposer joining.