A Predictive Metallographic Means to Identify The Relative Risk of Failure for Plated Micro Vias

The use of laser formed microvias has increased significantly since their introduction, and they are now used extensively by todays PCB industry as a means of achieving the highest density of product designs. Over recent years however, there has been increasing concern regarding their long-term reliability performance, especially when used in a direct stacked configuration. In response to that concern, there have been many published investigations looking into not only the physical performance of microvias, but more recently into their metallurgical characteristics. Such metallurgical characterization has identified two significant microstructures, one being considered indicative of a “weakened” structure, the other being considered more of the ideal. While the differentiation of such extreme cases could be considered as being “straightforward” this is rarely the case, and it is now becoming more desirable to be able to give indications of the relative performance of two microstructures that look very similar. In this paper, we identify the two significant microstructures found in plated microvias, and then introduce a method for evaluating their relative degree of crystallographic epitaxy. This factor is then be compared to performance data recorded through testing of plated microvias, and so enables a method that can be used as a predictive tool to evaluate the potential performance of an interface within a microvia