Moisture absorption in no-flow underfill materials and its effect on interfacial adhesion to solder mask coated FR4 printed wiring board

Interfacial delamination of underfill from both chip and substrate is a prevalent failure mechanism in microelectronic packaging. Of particular interest is the role moisture plays in package interfacial adhesion. In this paper, the effect of moisture on the interfacial adhesion of two no-flow underfill materials with commercially available solder mask coated FR4 board is experimentally determined. Bilayer specimens with prefabricated interface cracks are used in a four point bending test to quantify the interfacial fracture toughness. Two groups of test specimens of varying underfill thickness were constructed. The first group was fully dried while the other was moisture preconditioned at 85°C/85%RH for 725 hours. In addition, a one dimensional diffusion model was developed to estimate the time at which the moisture first penetrated the interface. The results of this study show that the interfacial toughness is significantly affected by the presence of moisture and that amine functional groups considerably retard moisture penetration through underfill encapsulants