Hygroscopic swelling behavior of molding compound at high temperature

Many polymer based materials, such as molding compound, are commonly used in plastic encapsulated packages. In spite of many advantages, most of them, however, are hydrophilic and absorb moisture when exposed to a humid environment. This induces swelling and deformation mismatches in a package and leads to failure in some cases. Polymer materials swell upon absorbing moisture, while the adjacent non-polymeric materials, such as the lead frame and silicon chip, do not. This induces hygroscopic mismatch stresses in a package and this leads to failures in some cases. It has been reported that swelling induced stresses are comparable to thermally induced stresses. Thus, CHS (coefficient of hygroscopic swelling) is considered as one of the most important material properties in studying electronic package failure mechanisms. Recently, many researchers have realizing the importance of CHS in the package design and reliability analysis. Many efforts have been made in the past to obtain the CHS for the polymeric materials. The authors have performed the CHS measurements for a wide temperature range (-55°C to 180°C) using DIC (digital image correlation) measurement method and has shown that CHS is highly temperature dependent, especially at higher temperatures, while it is nearly constant at lower temperatures. There are few reports on CHS values at a high temperature. This study utilizes the TMA/TGA measurements to determine CHS at high temperature. FEA analyses for diffusion and hygro-stress models was used to correct the effect of non-uniform moisture distribution and initial moisture lost during temperature ramp up for high temperature CHS measurement. Corrections are necessary as the deformation depends on moisture distribution, and uniform moisture is not possible to maintain during the measurements, especially at high temperatures above 100°C, is demonstrated. The TMA/TGA results show a clear evidence of CHS dependency on moisture concentration. The results on CHS dependencies in both temperatures and moistures are presented in detail in this paper.