Impact of Conformal Coating Material on the Long-Term Reliability of Ball Grid Array Solder Joints

Abstract

A conformal coating is a protective chemical material that can be applied as a thin layer on printed circuit board assemblies (PCBAs). The purpose of conformal coating is to prevent the interaction between external environmental factors and the electrical assembly. However, since conformal coating materials have different mechanical properties than electronic components, their use may induce additional mechanical stresses on the board. This study focuses on the effect of having a different coefficient of thermal expansion (CTE) for the conformal coating material from the solder joints of ball grid array (BGA) packages on printed circuit boards (PCBs). Typical lead-free solders have a CTE of less than 25 ppm/°C, while coating materials have a much higher CTE. For example, polyurethane has a CTE around 193 ppm/°C. This CTE mismatch generates stresses in different directions on the solder joints, especially when the assembly is subjected to temperature cycling during its service. In this effort, four samples of PCB assemblies that have BGAs were used in temperature cycling tests. The sample size is nine modules, with each module having at least 272 exposed BGA joints. Three of the samples were coated by a layer of polyurethane and one baseline sample was not coated. Coating thickness was selected to be the experimental design factor. Thus, these three samples, which had different thicknesses (30, 80, and $130~\mu \text{m}$ ), were evaluated and compared against each other and against the noncoated sample. By comparing the four samples, it was found that a thick coating material significantly reduced the lifetime for BGA solder balls by initiating early solder cracks, which propagated and caused complete separation in the solder joint.