Testing and prevention of head-in-pillow

The electronic industry is moving toward smaller, faster, and cheaper. In the case of CSP or BGA, the components are reducing in pitch and ball size but increasing in package size. Due to the high complexity in design, many components inevitably suffer from warpage at soldering temperature, and consequently result in head-in-pillow (HIP) defect. The symptom of this defect is that the solder ball on BGA is sitting on solder dome formed from coalescence of solder paste. Although mechanical contact is established, solder joint is neither formed metallurgically nor exhibiting an adequate joint shape. Furthermore, electrical continuity may or may not be established. This defect type is a liability of devices, and is recognized to be related to type of components, processes, and solder pastes. However, as of today, an effective testing method has not been reported for predicting the propensity of HIP phenomenon under a given combination of process and materials. This becomes a desperate need when a robust solder paste is desired at solder paste selection phase. In this work, test methods have been developed for assessing the potential of having HIP issue for a specific solder paste. In the mean time, mechanisms of formation of HIP are also discussed. Accordingly, new solder pastes with excessive resistance against HIP were developed and assessed with these test methods, with results to be discussed in details.