Question posed by the Moderator to Dongkai Shangguan:
The materials community has paid much attention to the reliability of solder joint interfaces. Can you comment on how progress in these materials-centric reliability studies affects the design and performance of industrially significant 3-D packaging?

Dr. Shangguan responded: That’s a very good question. If you look at 3-D packaging, especially for hand-held applications, drop tests are very significant loading conditions for the reliability of such a product. In 3-D packaging, such as package-on-package, because now the package mass is very large, when the product is dropped, it’s of course bending the ball and that creates stress and strain on the solder joint. The mass of the package is directly related to a magnitude of the mechanical stress and strain in the solder joint. Also, because in such miniaturized products the solder joint is very small. Therefore, the ratio of solder joint to the solder-copper interface is very small. That means the interface is playing an increasingly important role as we continue to miniaturize the product, including the solder joint. So, the loading condition is getting more severe, the solder joint is getting smaller, meaning the interface is becoming more important, and under such loaded conditions, most of the failures are interfacial failures as we all see. All this means that we must focus more of our research on understanding the interfacial failure in such small-miniaturized solder joints, especially under mechanical loading conditions, such as drop and the bending.