Design of an efficient heat transfer path is important to keep microprocessor operating within its temperature limit, but is complicated by the various interfaces along the thermal transfer path. Thermal interface materials (TIMs) are often introduced to fill the gaps at the interface to minimize thermal contact resistance. Several types of TIMs have been developed, such as grease, gel, phase change material (PCM). However, these polymer-based materials have low thermal conductivities, e.g., below 5 W/m•K, which limits the improvement of the capacity of heat transfer in IC package. With increasing integration density in the microprocessor, heat dissipation has become a major issue.
Sn52In solder was chosen to be the TIMs in this work owe to its high thermal conductivity, and mechanical properties. Thermal resistance of Si/Sn52In/Cu sandwiched samples was measured by laser flash method after different stages of thermal cycling. It was found that the thermal resistance of Sn52In solder-TIMs was lower than the other traditional TIMs. After 200 thermal cycles, the thermal resistance increased linearly with number of thermal cycles. After 700 thermal cycles, thermal resistance increased from the initial 0.0258 cm2•K/W to 0.0297 cm2•K/W, about 15 %. The cross-section of the sample was examined by scanning electron microscopy (SEM). Cracks were observed in both solder bulk and interface between IMC and solder. The increase of the thermal resistance was related to widening of the crack segments which were inclined to the interface. Because of the difference of stress state, cracks initiation and propagation modes were different in the central and edge part of the sample.
FEA simulation of the stress and strain state of Si/Sn52In/Cu sample with different sizes during thermal cycling was performed. While the X-direction normal stress in solder was larger near the Si side than near the Cu side, the shear and peeling stress were concentrated near the free edge of the sample. The equivalent plastic strain of solder was greater near the Si side than near the Cu side after thermal cycling. The highest equivalent plastic strain was found near the Si/solder interface at the free edge of the sample. The equivalent plastic strain increased with the decrease of solder thickness.
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