HYGROTHERMALLY-INDUCED DELAMINATION AND CRACKING IN PLASTIC IC PACKAGES

Abstract

Plastic-encapsulated packages are susceptible to moisture-induced cracking due to high moisture permeability of the typical epoxy molding compound. During solder reflow, the high temperature together with the previously absorbed moisture will result in high hygrothermal stresses at material interfaces due to the mismatch in thermal expansion coefficient of the constituent materials. Due to manufacturing faults or contamination, very small defects or voids may exist in bonded interfaces. These small interfacial defects induce very high stress concentrations around the defects leading to debonding and delamination growth. If the total stress intensity factor generated by the loading due to the hygrothermal stresses and steam pressure is higher than the interfacial fracture toughness, the interfacial cracks will grow and finally cause popcorn cracking. The present investigation into moisture-induced delamination and cracking consists of six parts as follows: 1 Two-dimensional heat transfer analysis during moisture preconditioning (85°C/85<%RH ) and solder reflow. 2 Simultaneous heat transfer and moisture diffusion during moisture preconditioning and solder reflow. 3 Delamination and cracking of plastic IC packages using a stress-based criterion. 4 Three-dimensional bygrothermal stress analysis during moisture preconditioning and solder reflow. 5 Development of a modified J-integral which incorporates both thermal and hygro stresses and a methodology for predicting the onset of delamination in a plastic IC package based on elastic interfacial fracture mechanics fundamentals involving accurate numerical computation of the magnitude and phase angle of the complex stress intensity factor and the strain energy release rate. 6. Experimental measurement of interfacial fracture toughness by a three- point bending test and experimental verification for delamination propagation and cracking. The theoretical and experimental methodology reported in this study provides a powerful tool for determining whether packages will delaminate or crack. It can be easily used by engineers involved in the design and manufacturing of plastic IC packages. It is of great help in determining the effects of moisture exposure and prebaking processes in IC manufacturing