Influences of packaging materials on the solder joint reliability of chip scale package assemblies

In the present work, a systematic study was performed in order to evaluate the influences of the materials of a chip scale package (CSP) on the reliability of the soldered joints. The type of adhesive interconnecting the top and the bottom layer of silicon was varied in this study. The materials alternatives evaluated comprise a hard epoxy materials with a high filler content, a rubber-like adhesive and an adhesive in medium state. These were combinated with the material of the substrates, FR4 and Al2O3 on which the CSPs are mounted. A unified viscoplastic constitutive model was applied to represent the temperature and time dependent deformation behavior of the solder material. The stress/strain responses of solder joints of CSP assemblies under thermal cycling conditions were evaluated by using the finite element method. Using these results, solder joint lifetimes were estimated with a crack propagation model. The results show that under thermal cycling conditions the lifetimes of CSP solder joints are lower when the CSP is fabricated with a highly filled adhesive compared to those with a rubber-like adhesive. Also, the solder joint reliability of CSP assemblies mounted onto FR4 substrates is inferior to those on Al2O3 substrates. This is a result of the fact that the internal adhesive layer in a CSP also serves as the source of thermal expansion mismatch. Furthermore a more flexible design of the CSP can reduce stresses in the soldered joints. Some discussions for material selection of epoxy adhesive in CSP are also presented. Consequently a structural adhesive with a low elastic modulus and a low CTE is more desirable than a harder material.