Thermal resistance of low-melting-temperature alloy （LMTA） thermal interface materials （TIMs） was measured by laser flash method before and after different stages of heating. The results showed that the thermal performance of the LMTA TIMs was degraded during the heating process. It is suggested that the degradation may mainly be attributed to the interfacial reaction between the Cu and the molten LMTAs. Due to the fast growth rate of intermetallic compound （IMC） at the solid-liquid interface, a thick brittle IMC is layer formed at the interface, which makes cracks easy to initiate and expand. Otherwise, the losses of indium and tin contents in the LMTA during the interfacial reaction will make the melting point of the TIM layer increase, and so, the TIM layer will not melt at the operating temperature.
E.YangHongyan GuoJingdong GuoJianku ShangMingguang Wang
Plastic prestraining was applied to a solder interconnect to introduce internal defects such as dislocations in order to investigate the interaction of dislocations with electromigration damage. Above a critical prestrain, Bi interfacial segregation to the anode, a clear indication of electromigration damage in SnBi solder inter- connect, was effectively prevented. Such an inhibiting effect is apparently contrary to the common notion that dislocations often act as fast diffusion paths. It is suggested that the dislocations introduced by plastic prestraining acted as sinks for vacancies in the early stage of the electromigration process, but as the vacancies accumulated at the dislocations, climb of those dislocations prompted recovery of the deformed samples under current stressing, greatly decreasing the density of dislocation and vacancy in the solder, leading to slower diffusion of Bi atoms.
Fe-Ni films with compositions of 73 wt% of Ni and 45 wt% of Ni were used as under bump metallization （UBM） in wafer level chip scale package, and their reliability was evaluated through electromigration （EM） test compared with commercial Cu UBM. For Sn3.SAg0.7Cu（SAC）]Cu solder joints, voids had initiated at Cu cathode after 300 h and typical failures of depletion of Cu cathode and cracks were detected after 1000 h EM. While the SAC]Fe-Ni solder joints kept at a perfect condition without any failures after 1000 h EM. Moreover, the characteristic lifetime calculated by Weibull analysis for Fe-73Ni UBM （2121 h）, Fe-45Ni UBM （2340 h） were both over three folds to Cu UBM＇s （698 h）. The failure modes for Fe-Ni solder joints varied with the different growth behavior of intermetallic compounds （IMCs）, which can all be classified as the crack at the cathodic interface between solder and outer IMC layer. The atomic fluxes concerned cathode dissolution and crack initiation were analyzed. When Fe-Ni UBM was added, cathode dissolution was suppressed due to the low diffusivity of IMCs and opposite transferring direction to electron flow of Fe atoms. The smaller EM flux within solder material led a smaller vacancy flux in Fe-Ni solder joints, which can explain the delay of solder voids and cracks as well as the much longer lifetime under EM.
Li-Yin GaoHao ZhangCai-Fu LiJingdong GUOZhi-Quan Liu
