Influence of latent heat released from solder joints II: PCB deformation during reflow and pad cratering defects

Abstract

This work is focused on the deformation of FR4 laminate, a material used to form many basic components of electronic assemblies, during the reflow process. The thermal expansion of FR4 was assessed from room temperature up to 250 °C. The dilatation characteristics of the laminate were found to be highly anisotropic in three directions. Special attention was paid to the irreversible changes in the laminate after the first thermal cycle, and the results were used to analyze the effects of latent heat on the deformation of FR4 during reflow. The latent heat released during solidification of a solder joint heats not only the joint but also its surrounding, leading to localized expansion of all materials around the joint and potentially causing pad cratering. Thermal expansion measurements demonstrated that the coefficient of thermal expansion (CTE) of the FR4 laminate in the z direction was at least 10 times that of the adjacent materials in the electronics assembly. This large difference in the CTE leads to residual strains in the assembly. An analysis of this problem is presented, and a model for the effect of latent heat on the FR4 deformation is proposed.