Stress and Thermal Characteristic Analyses for Advanced FCBGA Packages

Abstract

Flip chip ball grid array (FCBGA) was developed to meet the requirements of more functionality, excellent electrical performance, high input/output (I/O) density and high speed in semiconductor device market. In the past experience, the frequent coming challenge was bump fatigue crack caused by CTE mismatch between chip and substrate during the reliability tests. In order to elevate the resistance of bump fatigue crack in FCBGA, underfill was introduced where it can fill into the gap between chip and substrate to minimize the stress induced from mismatch deformation between die and substrate. However, with the functionality demand increases dramatically, the flip chip size grew up more than 20 mm by 20 mm in some particular application IC leading to insufficient bump protection supplied by underfill. The advanced FCBGA has been raised to ensure FCBGA success by the implementation of molded underfill (MUF) material to encapsulate the die and solder bumps that can very effectively reduce bump stress than traditional underfilled FCBGA. The molded FCBGA (MFCBGA) and MFCBGA with drop-in heat spreader (EDHS-MFCBGA) are introduced in this paper and aims to comprehensively study them in term of thermal and stress characteristics. This paper covers not only stress characterizations but also thermal dissipation capabilities among three FCBGA packages employing finite element method (FEM) modeling. In the stress characterizations, the warpage, die stress and bump stress were discussed and mold thickness was derived as optimal design guidelines corresponding to MFCBGA structure. Coefficient of thermal expansion (CTE) and glass transition temperature (Tg) were demonstrated to dominate both of bump and die stresses. The candidates picked up from modeling results were also applied into real parts with actual package warpage measurements and showed matched trends with modeling conclusions. The other key feature to ensure FCBGA success is the thermal capability so thermal modeling was further implemented to identify the levels of thermal dissipation for these three structures. The modeling results demonstrated mold thickness in range of 0.5 mm ~ 0.7 mm could guarantee less thermal performance losing but gain significant bump stress and warpage improvement