Influence of moisture on humidity sensitive material parameters of polymers used in microelectronic applications

Most used polymers show a visco-elastic behaviour. The significant change of the material properties is a function of time and temperature, and needs to be taken into consideration. When exposed to humid environments, epoxy resins can absorb water up to a 10 weight -%, depending on the chemical nature and structure, stress state, exposure time, water concentration and temperature. The use of temperature and time dependent material parameters is state-of-the-art in microelectronic packaging. This paper presents various modified measurement methods for analysis of the effect of moisture of thermo-mechanical properties of MEMS-relevant polymer systems. In the Micro Materials Center´s lab, an advanced measurement method was developed - which modified DMA multifrequency and hygroscopic swell analysis for the determination of moisture-dependent parameters for micro and nano scale samples. The time-moisture superposition principle has been introduced for some polymers where a humidity shift factor is used along the frequency axis at constant temperature. The relationship between the humidity shift factor and the equilibrium water content is analogically described by the WLF (Williams-Landel-Ferry)-type equation on the time-temperature-superposition. Due to moisture absorption, the glass transition temperature was reduced for fully saturated samples, and visco-elastic properties which are influenced by moisture prefer the state of entropy elasticity already to lower temperature significantly. Furthermore, diffusion in unfilled epoxies follows the conventional Fickian diffusion of polymers.