Moisture Absorption Analysis of Interfacial Fracture Test Specimens Composed of No-Flow Underfill Materials

Abstract

In a previous study, we found that moisture preconditioning strongly influenced the interfacial fracture toughness of the underfill/solder mask interface, decreasing the interfacial adhesion by approximately one-half for both classifications of underfill/solder mask interfaces after 725 h of exposure at 85°C/85%RH. To better understand the rate and mechanisms for moisture transport through the interfacial fracture test specimens, a diffusion analysis was implemented based on traditional, analytical solutions of Fick’s second law of diffusion. Test specimens were constructed to experimentally determine the diffusion coefficient for each underfill. Since both underfill encapsulants proved to exhibit non-Fickian behavior at 85°C/85%RH, the application of the analytical Fickian solution for the test specimens was limited to the associated JEDEC criteria of 168 hours for 85°C/85%RH. A finite element analysis was performed to illustrate the moisture concentration in the interfacial fracture test specimens for initial times of exposure to the humid environment. The results of this study demonstrate that the presence of amine functional groups considerably retard moisture penetration through underfill encapsulants.