The Stress State of BGA Solder Joints Influenced by the Grain Orientations of Neighboring Joints

Typical SnAgCu solder joints to ball grid array components (BGA) solidify in a single nucleation event, resulting in a cyclic twinned or single-grained structure. Due to the elastic and plastic anisotropy of these joints, coupled with the stochastic nature of the crystal orientation, significant joint-to-joint variations in damage progression during thermal cycling has been observed even for neighboring joints having the same distance to neutral point. Depending on package design, the stress state of a given solder joint can be strongly influenced by the orientations of the grains in neighboring joints. It is therefore evident that in a given area array component, there exists a "worst" and "best" combination of grain orientations. As a result, there is some doubt whether extrapolations to early failures from a typical qualification testing sample size can be trusted to be conservative. In this work, different combinations of solder joint grain orientations in a BGA component is investigated through thermo-mechanical simulations in an attempt to assess how the stress state can be affected. While it is improbable that such combinations of orientations are found in the small sample sizes of qualification testing, populations of typical manufacturing volumes may contain a significant number of early failures which is of great concern in high reliability and/or safety-critical applications.