Flux Compatibility Study on Flip Chip Underfills

When flip chip is directly reflowed onto a laminate, an important observation becomes significant, the coefficient of thermal expansion (CTE) mismatch between the chip and the substrate. Due to the great difference in CTE, thermo-mechanical stresses are imposed on the solder joints during thermal cycling of the assembly. The stresses can subsequently result in early failure of the assembly. The function of the underfill is to firmly bond the chip and substrate together in order to reduce the stress on the solder joint due to the large thermal expansion.

As electronics manufacturing moves towards using a lead-free process, an elevated process reflow temperature and new interconnect metallurgies are imposing more challenges for underfills used. The properties of underfills, such as Tg, CTE, and modulus, have to be well balanced to overcome the internal stresses built into the flip chip system. In this study, we present data and analysis on the effect of these properties to the reliability tests, such as MRT and thermal cycling.