The effect of underfill epoxy on mechanical behavior of flip chip assembly

The effect of underfill epoxy on mechanical behavior was investigated by measuring the thermally-induced warpage on both a real flip chip thermosonic bonded assembly and a simulated tri-layered assembly. The assembly´s mechanical behavior was dominated by the underfill epoxy rather than solder joints. Such a dominant role was not affected even when the assembly had 196 solder joints under a 5.8 mm×5.8 mm chip. Epoxy properties are well characterized by the curing and the glass transition temperatures, and these characteristic temperatures clearly divide the warpage levels into two distinctive regions. When the maximum temperature the assembly exposed to was less than the glass transition temperature (Tg), the mechanical behavior was characterized by the curing temperature. When the maximum temperature was higher than the Tg, the behavior was characterized by the Tg. Corresponding to different characteristic temperatures, e.g. 80°C for curing and 130°C for Tg, the warpage as well as the Von Misses stress each could increase by as much as a factor of two. Such an increase could affect device reliability for RF packages and alignment for optoelectronic packages. With the selected epoxy materials, mechanical behavior of a flip-chip with underfill epoxy is stable and predictable