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

In this work, a systematic study was performed in order to evaluate the influences of chip scale package (CSP) materials on the reliability of the soldered joints. The type of adhesive interconnecting the top and bottom layers of silicon was varied in this study. The materials alternatives evaluated comprise a hard epoxy material with high filler content, a rubber-like adhesive and an adhesive in medium state. These were combined with the substrate materials, FR4 and Al₂O₃, on which the CSPs are mounted. A unified viscoplastic constitutive model was applied to represent the temperature and time dependent deformation behaviour of the solder material. The stress/strain responses of CSP solder joints under thermal cycling conditions were evaluated using the finite element method. From the results, solder joint lifetimes were estimated with a crack propagation model. The results show that under thermal cycling conditions, the CSP solder joint lifetimes 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 CSPs mounted on FR4 substrates is inferior to those on Al₂O₃ substrates. This is due to the fact that the internal adhesive layer in a CSP also serves as the source of thermal expansion mismatch. Furthermore, a more flexible CSP design 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 low CTE is more desirable than a harder material