Effect of PCB Surface Modifications on the EMC-to-PCB Adhesion in Electronic Packages

Abstract

The characteristics of interfacial adhesion between epoxy molding compound (EMC) and printed circuit board (PCB) were investigated. The surface conditions of solder resist (SR) layers, which were used as an outer skin of PCB, were varied within the range that would be encountered in the manufacturing process and reliability test conditions. First, the number of times of plasma treatment on the SR surfaces and the delay time prior to EMC molding on them were considered to examine the surface cleaning process and the aging effect, respectively, on adhesion. Second, moisture on the surfaces of PCB prior to EMC molding and moisture absorption and desorption at the interface were considered to investigate the environmental effect on adhesion. An unsymmetric double cantilever beam test method was devised by modifying the conventional symmetrical double cantilever beam. As a result, the phase angle of fracture could be controlled to achieve stable crack propagation along the desired interface, which enabled valid adhesion energy to be measured. The adhesion energy increased with plasma treatment by over 50%, from 55 to 86 J/m2. The improved adhesion was attributed to the increased the polar groups on the SR surface due to plasma treatment, which helped enhanced chemical bonding between the EMC resin and the SR resin. However, excessive plasma was counterproductive as it weakened the SR surface and caused cohesive crack propagation to occur within the SR layer. Adhesion remained nearly constant for delay time up to several hours between plasma treatment and EMC molding. However, small degradation of adhesion was observed when the delay time was extended to 12 h. Moisture on and in the SR material before EMC molding had a significant effect on adhesion. Absorbed moisture at the interface decreased the adhesion. However, when the moisture was baked out, adhesion was recovered almost to the original reference.