Mechanical Robustness of Solder Connections to Thick Film Gold

摘要

Thick film gold metallization is required for many high reliability circuits, especially those subjected to operation inhigh temperature or high humidity environments. Traditionally, wire bonded bare die are used on these circuits, butthere is a trend to replace them with BGA packaged devices. State-of-the-art, chip scale packages increase circuitvolume by less than 20 percent, while their use greatly simplifies testing and repair, as compared to wire bonded die.The use of small, high density I/O pad arrays for attachment of BGA packages, necessitates very careful control ofthe solder reflow process to avoid excessive leaching of the gold into the solder. Also, unlike passive chipcomponents and leaded devices, the solder filet associated with a solder ball attachment does not distributemechanical loads over an extended area. Consequently, the stresses imposed on fine pitch, BGA pads are muchhigher than those imposed by other components. During aging, the gold metallization is converted to gold-tinintermetallic as inter-diffusion proceeds. This further reduces the mechanical integrity of the solder connection.This manifests itself in the observation that when BGA solder balls are subjected to accelerated aging followed byshear testing, the entire solder pad lifts off of the substrate, rather than failing in the solder joint.What we have done is construct a diffusion based model to estimate the conversion of a thick film gold metallizationpad to intermetallic and coupled this result with a finite element analysis to examine the effect of pad size and soldercomposition on the propensity of a pad to lift off the substrate, when subjected to mechanical or thermal inducedloading. We are designing experiments to compare the predictions of our model to experimental results obtainedfrom shearing solder balls, of different compositions and sizes, attached to substrates metalized with severaldifferent solderable, thick film gold materials.