Evolution of Viscoelastic Properties of Underfills Exposed to High Temperature

摘要

The electronics used in the automotive underhood applications may be subjected to very high operating temperatures of 125 to 150°C for prolonged period during normal operation. A number of advanced driver assistance systems are enabled by underhood electronics requiring the use of advanced semiconductor packaging including flip-chip ball-grid arrays. Underfill materials are used to diminish the solder-joint strains in under wide thermal excursions. The effect of prolonged high temperature exposure on the properties of underfills is not well understood. In this work, two different underfill encapsulants are subjected to very high isothermal aging conditions at three different temperatures which are near, below, and above the glass transition temperature of the underfill. To study the evolution in material properties, the underfills are aged at 100°C, 125°C, and 150°C for one year. The long-term aging of the underfill at high temperatures, oxidizes the underfill encapsulants and changes the material properties. The polarized optical microscopy was used to study the oxidation behavior of underfills by measuring the oxidation layer thickness. The changes in viscoelastic properties of the underfills and related glass transition temperatures were studied using DMA (Dynamic Mechanical Analyzer). The results show the presence of cracks on the boundary surfaces of underfills exposed to very high temperature of 150°C for more than 120 days. This causes the sudden increase in oxidation layer thickness there by glass transition temperature and altering the dynamic mechanical properties of the material.