Thermal deformations and stresses of flip chip BGA packages with low- and high-T_g underfills

The purpose of this study is to investigate the thermomechanical behaviors of flip chip EGA packages with low-and high-T_g underfills under thermal loads, after 85°C/85%RH conditions, and after solder reflow. The emphasis is placed on the deformations and stress state of the packages under these loading conditions. This study has presented that Suhir's solution and FEM can effectively predict the thermomechanical behaviors of flip-chip EGA packages.lt is also found that the stress-free temperature is at underfill-curing temperature (T_c) for both packages. Furthermore, the relation between curvature and temperature is nearly linear for packages with high-T_g underfills, while bi-linear (with turning point near T _g) for those with low-T_g underfills. It is also discovered that the packages with low-T_g underfills during cooling processes suffer stress relaxation and thus their deformations depend on cooling rate, but not for those with high-T_g underfills. The die stresses can be directly determined by Suhir's die stress solution associated with the curvature data from the experiments. The coplanarity of the packages has been also addressed by combining the experimental and FEM analyses, and then used to investigate the fillet effect. It is found that the fillet effect on the warpage is negligible for this flip-chip EGA and the 2-D axis-symmetrical model can be approximately used for addressing the global warpage. Regarding 85°C/85%RH environmental effect, it is shown that the packages with high-T_g underfills are insensitive to this environmental effect, while those with low-T_g underfills are minor sensitive (about 10% decrease). It is also found that solder reflow process does affect the curvature of the packages by increasing 18% and 15% for those with high-and low-T_g underfills, respectively.