Adhesion of underfills toward tin/lead and lead-free solders

Underfill materials are employed in flip chip assemblies to enhance solder joint reliability performance. The adhesion of underfills with solder materials is important to the integrity of the flip chip structure. We have studied the adhesion strength of two underfill samples with tin/lead (Sn/Pb) eutectic solder and tin/copper (Sn/Cu) lead-free solder, benchmarked with copper surface. It was found that the adhesion of underfills and both solder materials was about 1/3 the adhesion of underfills and copper. The effect of temperature and humidity aging as well as flux residue on adhesion strength was also investigated. A loss of adhesion was observed with pressure cooker test, but 85°C/85%RH aging and flux residue revealed only a slight influence on adhesion strength. Surface analysis was performed on copper, Sn/Pb eutectic solder, Sn/Cu lead-free solder and cured underfills by using the three-liquid-probe three-component surface tension method. The contact angles of liquid underfills on copper, Sn/Pb eutectic solder and Sn/Cu lead-free solder were also measured with a goniometer. The thermodynamic work of adhesion for underfills with copper and solder surfaces of different surface conditions was calculated. It was found that the thermodynamic work of adhesion calculated from either surface analysis approach was not correlated with the lap shear strength of underfills with copper and solder materials. Thus, the wetting property of an underfill on a substrate is not the determining factor for its practical adhesion strength. Various possible techniques for improving the adhesion of underfills and solder materials were considered, and the use of additives in underfill formulations was experimented. However, we have not observed any significant effect of adhesion strength enhancement from any of these additives. Further tests of additives with the base underfill formulation revealed a slight possibility to enhance adhesion of underfills and solders by proper formulation manipulation