Experimental characterization of underfill materials exposed to different moisture conditions

Abstract

Consistent and wide-ranging material property data are needed for microelectronics encapsulants for the determination of mechanical design, reliability assessment, and process optimization of electronic packages. Since, the vast majority of contemporary underfills used are epoxy based, they have the propensity to absorb moisture, which can lead to undesirable reductions in their mechanical and adhesion properties. In this study, the effects of moisture absorption on the stress-strain and creep behavior of an underfill encapsulant were evaluated experimentally. The material behavior changes have been characterized using 60 x 3 x 0.5 mm uniaxial test specimens that were cured with production equipment using the same conditions as those used in actual flip chip assembly. After curing, the samples were separated into 2 groups and subjected to various preconditioning: (1) no preconditioning, (2) prebaking at 85 oC for 24 hours. The fabricated and preconditioned uniaxial test specimens were then exposed to moisture for various durations (0, 1, 3, 10, 20, 30, 60 days) in an environmental chamber using three different sets of hygrothermal conditions including 85°C and 85 % RH (MSL 1, Moisture Sensitivity Level 1); 85°C and 60 % RH (MSL 2); and 30°C and 60 % RH (MSL 3). After the moisture exposures, a microscale tension-torsion testing machine was used to measure the room temperature stress-strain behavior of the material. In addition, the viscoelastic mechanical responses of the underfill encapsulant after the moisture exposures were also characterized via creep testing for a large range of applied stress levels. From the recorded results, it was found that the moisture exposures degrade the mechanical properties of the tested underfill including the effective elastic modulus, ultimate tensile stress, and tensile creep rate. Pre-baking was found to increase the initial material properties before moisture exposure, but the degradations due to subsequent moisture exposures occurred in an analogous manner. The moisture absorption into the underfill samples was also characterized by weight gain measurements for up to 60 days of exposure at the three different MSL conditions. From the results, it has been shown that maximum water weight gain in the material is about 1%. The results also showed that mechanical properties were only dependent on amount of water that has been absorbed by the material, regardless of how rapidly it was absorbed.