The mechanics and impact of hygroscopic swelling of polymeric materials in electronic packaging

A reliable technique for characterising the hygroscopic swelling of materials has been developed and used to characterise a number of packaging materials. The hygro-swelling of these materials, though varing in magnitude, exhibits a common behavior in that it increases with temperature and decreases sharply across T_g. Using this data, hygroscopic stress modeling was performed on leaded and substrate based packages. The hygroscopic stress induced through moisture conditioning was found to be significant compared to the thermal stress during solder reflow. Hygroscopic stress in over-molded wire bond PBGA & molded flip chip PBGA was found to be 1.3 to 1.5 times that of thermal stress. Hygroscopic swelling of the underfill in flip chip PBGA was found to be the main failure driver during autoclave test. Autoclave performance of flip chip PBGA package assembled with different underfills & chips was analysed. Excellent correlation was found between autoclave performance and the hygroscopic swelling characteristics of the underfills