Regradation Of Sn58Bi Solder Thermal Interface Material Under Thermal Cycling

With the increase of heat generated by microprocessors, the microelectronic industries need dissipate heat in a more effective way, which highly dependent on the ability of the thermal interface material (TIM) to transfer thermal energy efficiently from the back of the die to the heat spreader. Due to the high thermal conductivity, the Sn based solder alloys become attractive choice.In this paper, Sn58Bi solder was choose as the solder thermal interface materials. After different stage of thermal cycling, thermal resistances of Si/Sn58Bi/Cu sandwiched samples were measured, the microstructures of the samples were observed, and the mechanism of fatigue behavior for SnBi thermal interface materials during the thermal cycling were also discussed The experimental results show that after 1000 thermal cycles, the thermal resistance of the Sn58Bi alloy solder increased from the initial 0.0542 cm2·-k/w to 0.0628 cm2·k/w, about 15.87%. The increase of thermal resistance is mainly attributed to the initiation and expansion of cracks, in which, the initiation of the cracks is the main factor. During thermal cycling, cracks first initiated in the solder next to the Si side, along the phase boundaries, and then extend in width. Most of the cracks initiate parallel to the orientation at an angle of 45 degrees with the Si/solder interface, which is the orientation of the maximum tensile stress. While at the Cu side, cracks initiated late and expanded more slowly than that at the Si Side. Such phenomena may be attributed to the mismatch of the coefficients of thermal expansion between Si, solder and Cu. In addition, after thermal cycling, obvious coarsening observed in the SnBi solder.