Moisture-induced interfacial delamination growth in plastic IC packages during solder reflow

Very small defects may exist at interfaces in IC packages due to random factors, manufacturing faults or contamination. During solder reflow, these defects give rise to stress concentrations due to thermal mismatch between the materials forming the interface and to the development of hygrostresses from moisture absorption in the plastic encapsulant. In this paper, the mechanics of interfacial delamination will be discussed and a methodology presented for analysing moisture-induced delamination growth during solder reflow. This paper also describes an experimental study which verified the methodology. In this experimental study, plastic packages which were fabricated with a known internal delamination, were divided into 3 groups and subjected to 3 different levels of moisture preconditioning, namely fully dry, 85 degrees C/60%RH and 85 degrees C/85%RH. Packages from each group were then subjected to oven temperatures varying from 180 degrees C to 230 degrees C in increments of 5 degrees C. Examining the specimens using a scanning acoustic microscope, the temperature at which delamination propagation occurred was determined and compared with that predicted using mixed-mode interfacial delamination mechanics. Generally good agreement was obtained. The growth of the delamination could be explained in terms of the variation of hygrothermal stress intensity factor and interface toughness with crack length.