Three-Dimensional Effects of Solder Joints in Micro-Scale BGA Assembly

Abstract

Micro-scale BGA has attracted more and more interest due to their high performance/area ratio. This technology is especially important for mobile personal equipment, such as cellular phones and laptop computers. With the decrease of solder ball sizes, the reliability problem becomes the major concern for the package design, which requires more powerful tools to address this issue. Finite element method is a useful tool for packaging design and analysis. For some BGAs, the critical solder balls are sometimes not located at a symmetric plane and the deformations in three directions will affect the distribution of stress/strain in these solder balls. Therefore, a full three-dimensional analysis is necessary to address these effects. However, it is almost impossible to run a three-dimensional single finite element model to consider all design details even with simplified material properties, not to mention creep constitutive law. Therefore, the global/local modeling technique provides an alternate way, which can simplify the modeling work’ Significantly, reduce the computational time, and enhance the crucial information at critical locations. In this study, the global/local technique was used to study a micro-scale BGA assembly and to investigate the three-dimensional effects of stress/strain distribution inside the solder joints. It was found that the maximum creep strain is not located at the plane passing through the package center and solder joint axis. It was also found that the position of maximum creep strain will shift during first several cycles, but will stabilize after the fourth cycle. This result is also helpful in evaluating the reliability of the critical solder joints in micro-scale BGA assemblies.