Thermomechanical properties of overmold epoxies in MEMS packaging

摘要

Suitable selection of packaging materials is critical for microelectromechanical system (MEMS) devices for maintaining their performance, physical protection, and electrical connections. Several different materials can be used in MEMs packaging and different properties are required from these materials depending on their application. Overmold materials are commonly used to protect MEMS components. For these materials flexibility is one of the key properties to ensure their compatibility with different MEMS components. In addition to the flexibility, the thermal properties of the overmold materials are critical. The temperature of a MEMS package changes often during the assembly and usage of the MEMS components, which can easily lead to thermo-mechanical stresses and thereby failures. The source of these stresses is the mismatch between the coefficients of thermal expansion (CTE) of the different components in the package. Consequently, it is critical to know the thermal behavior of the materials and also how it is affected by their curing conditions. In this study thermomechanical behavior of two overmold epoxies, one low-T_g material and one high-T_g material, was investigated. Both materials were designed to be used as encapsulants in a MEMS device which is packaged according to Quad Flat No-lead (QFN) method. The CTEs of the materials were measured by thermomechanical analysis (TMA) instrument using a temperature range of -50°C to 270°C. Additionally, their thermal properties were measured using differential scanning calorimetry (DSC). The samples were cured using the temperature profiles suggested by the manufacturers. Two different curing profiles were used. Additionally, a part of the samples was exposed to three reflow cycles to study its effect on the material. TMA results showed differences in the coefficient of thermal expansions (CTE) of the different test samples. For the high T_g epoxy the reflow process decreased significantly its CTE below the glass transition. Similar behavior was seen in the low T_g epoxy with the shortened curing profile. Some changes were also seen in the T_g values, but they were relatively small. The values of the datasheets differed from the measured values.