A New Method to Measure the Moisture Expansion in Plastic Packaging Materials

Moisture induced failure in plastic encapsulated packages is one most important failure mechanisms in microelectronics. This failure is driven by the mismatch between different material properties, such as CTE, CME (Coefficient of Moisture induced Expansion) caused by moisture absorption in plastic packaging materials. Therefore, it is important to know moisture effects on mechanical properties of plastic packaging materials, especially CME. Moisture induced expansion can be calculated using epsilon = beta . c, here epsilon is the strain, beta is the CME and C is the moisture concentration. Traditionally using the combined TGA (Thermal Gravimetric Analyzer)/TMA (Thermal Mechanical Analyzer) technique, the CME of plastic packaging materials is characterized. TGA is used to measure the weight change and TMA is used to measure the length change. By combining both the TMA and TGA measurements, the CME can be determined. This method is often used in industry and it is observed that the CME value is often over estimated. In order to get precise CME values a high precision DMA (Dynamic Mechanical Analyzer) is used to measure the length change of a sample while a humidity generator is used to regulate the relative humidity. Therefore, temperature and relative humidity are controlled in the DMA chamber and can be used to measure the length change under different relative humidity conditions. CME values measured by the DMA plus humidity method are much lower than that of the TGA/TMA method. In order to find out which method is more reliable, a third experiment was done. A bi-material sample is created to verify our measured CME value. TDM equipment (oven + camera system to detect vertical displacement of the sample) is used to measure the warpage of the bi-material sample. Using our measured CME value, finite element model simulation result shows that the hygromechanical warpage of the model fits well with TDM test result.