Effect of underfill on BGA reliability

Abstract

The effect of underfill on various thermo-mechanical reliability issues in SBGA (Super Ball Grid Array) packages is studied in this paper. Non-linear finite element models with underfill and no underfill have been developed taking into consideration the process-induced residual stresses. In this study, the solder is modeled as time and temperaturedependent, while other materials are modeled temperature and direction-dependent, as appropriate.The stress/strain variations in solder joints due to thermal cycling are analyzed. The effect of underfill is studied with respect to magnitude and location of time-independent plastic strain, timedependent creep strain and total inelastic strain in solder balls. The possibility of delamination at the interposer-underfill interface as well as board-underfill interface is studied with the help of qualitative interfacial stress analysis. Results on SBGA packages indicate that the underfill does not always enhance BGA reliability, and that the properties of the underfill have a significant role in the overall reliability of the BGA solder balls. IntroductionWith the increasing need for higher performance, smaller size, and higher pin count, ball grid array (BGA) and chipscale packages (CSP) are being increasingly used in many applications. Burnette et al.[2] have studied the board-level reliability of underfilled ceramic BGA packages. In flip-chip on board (FCOB) assemblies, underfill is often used to enhance the reliability of solder joints. However, in the case of BGAs and CSPs, underfilling is done under select instances, typically to enhance reliability and/or to provide environmental protection to the solder balls.The Super Ball Grid Array (SBGA) is a BGA package developed for higher thermal and electrical performance. In an ongoing research program at Georgia Tech, test vehicles with SBGA packages are being studied for military aircraft applications. When SBGA packages are used in military aircraft applications, the second-level solder balls are often encapsulated with an underfill for preventing possible moisture condensation around the solder balls. Although the underfill is used for such environmental protection purposes, the effect of underfilling SBGA packages on thermomechanical reliability is not adequately studied. In the ongoing research program, computational models are developed to understand the reliability of SBGA packages with various underfills.

Taper pad design to improve electrical performance of BGAs on wafer level package (WLP)

“…The interconnection gap between them will expose stress, thermal, and signal integrity problems. Among them, stress and thermal problems concerning reliability issue can be solved by filling underfill below chips [5]. However, signal integrity for the interconnect gap resulting in current crowding and impedance mismatch is seldom discussed and treated.…”