Glob top plastic ball grid array package encapsulation process development and its moisture sensitivity study

Liquid epoxy encapsulation is one of the important processes in the glob top PBGA assembly, which affect other subsequent process and package quality. The objective of the encapsulation is to protect the bonding wire and chip from harsh environment and make the device more reliable. In general, the basic concerns for encapsulation in glob top PBGA are encapsulation thickness, surface flatness, encapsulation area, voids in the encapsulation, and finished encapsulation surface. On the other hand, package reliability under accelerated condition should be addressed to assess the encapsulation quality as well as the assembly process reliability. In the encapsulation process development, design of experiment (DOE) is conducted in the first stage to help select the critical factors in the process, and then response surface methodology (RSM) is applied to optimize the process parameters. Additionally, statistic process control tools are used to monitor the process parameters and calculate the process capability. Basically, Plastic Ball Grid Arrays packages are known to have a higher moisture absorption rate when exposed to humid ambient conditions, so the moisture sensitivity becomes a concern when plastic packages are exposed to a humid environment for a prolonged time and then processed through reflow process. The objective of the moisture sensitivity study is to understand the effect of moisture ingress on the process and assembly reliability. The steam pressure pot test and Level-3 preconditioning test as well as temperature cycling test are used to evaluate the effects. The experimental result shows that the moisture contents inside the substrate will affect the die attach voids level and size. Moreover, the moisture level does play a part in the formation of delamination at the interface of die/substrate and encapsulation/substrate after acceleration test. However, there is no correlation between the die top delamination and the moisture contents absorbed by the package from the Level-3 preconditioning test. In addition, there was no delamination found at the die attach area or at the interface between the die and the substrate even when the package was moisture soaked fully saturated and went through temperature cycling. There is a trend to form more delamination in the interface of molding compound / solder mask, D/A after steam pressure test if more moisture is absorbed. Finally, it should be emphasized that plasma cleaning is found to have a significant effect on the package integrity for dry packed PBGA and moisture saturated units. Coupled with the stress test results, it implies that the most important thing in the PBGA assembly is to enhance the interface adhesion through plasma cleaning apart from the moisture preventing. The detailed experimental results and conclusions are explained in this paper.