Study of Electromigration of Solder Microbumps by 3D X-ray Microscopy

Abstract

In order to following Moore’s law, scaling down and piling up of chips through packaging processes are the most popular solution to raise performance of chips. So far, packaging in two-dimension (2D) tends to transform into 2.5D or 3D and even other novel and special structures. Due to the improvement of packaging technology, solder joints must shrink from about hundred to ten microns. The destructive analysis of reliability tests and failures will meet some troubles for small size of joints. Therefore, non-destructive inspection seems to be a good way for analysis. For the images of computed tomography (CT) done by X-ray, any view angles of joint array can be observed. The image can not only show the cross section of a row of joints but also check the distribution of failures for the advantage of CT technology.The reliability tests including temperature cycling test (TCT), high accelerated stress test (HAST), and electromigration (EM) are the common methods for improving the process or the design of structures. The size of solder microbumps will cause high current density for small cross section of joints, so the influence of EM becomes a big issue. With high current density, copper or nickel atoms will diffuse through interstitial site of tin, and cause the formation of voids and intermetallic compounds (IMCs). There are many factors affect the failure modes during EM test, such as the diffusivity of the materials in under bump metallurgy (UBM) and the tin grain orientation. Immortal microbumps might happen for the full IMCs joints. As a result, Voids formation is the most important failure mode during EM test. In the current study, the EM failures of microbumps were observed through 3D X-ray during various stages of EM, and evolution of voids can be examined.