Many polymeric and inorganic materials have densely packed structure. Still water absorption takes place in mold compound, die attach, underfill, solder resist etc., which are polymeric in nature. The voids generated in the ‘free’ volume region comprise up to 2.5% of total volume. The absorbed moisture leads to pop corning, interfacial delamination and degradation of adhesion strength. This article gives the measure of diffusivity, solubility and hygroscopic swelling coefficient to critically assess the moisture behavior during solder reflow after preconditioning. The figure a) indicates weight gain histories of two mold compounds at three different temperatures and RH 75%. It is also observed that diffusivity and solubility of the same test mold compounds followed Arrhenius relationship over the entire temperature range of the experiment.
The diffusion of water molecules leads to increase in inter segmental hydrogen bond length and results in swelling. The hygroscopic strain developed has a linear relationship with moisture concentration. There are some methods to determine the coefficient of hygroscopic swelling; bending cantilever method, a micrometer, Michelson interferometry and thermo mechanical analyzer combined thermo gravimetric analyzer. In this review article , moire interferometry and digital image correlation have been utilized for experimental procedure. Fringe patterns were obtained for null field, zero and interval of 40 h.
The internal cavity of MEMS devices needs to be maintained at ambient conditions. In compliance to the performance and reliability standards, good hermiticity is the measure of maintaining stable and ambient conditions in the cavity. The commonly used sealing materials are Au-Sn, Au-Si and other tin-based alloys. More recently polymeric seals such as BCB, parylene and polyimide are used due to their advantages. The fictitious polymer with a very large diffusivity and an equivalent solubility are modeled to ensure proper vapor pressure within the cavity.
Han B, Kim D. Moisture Ingress, Behavior, and Prediction Inside Semiconductor Packaging: A Review. ASME. J. Electron. Packag. 2017;139(1):010802-010802-11. doi:10.1115/1.4035598.