Effect of Free-standing Al/Ni Exothermic Film on Thermal Resistance of Reactively Bonded Solder Joint

In this study, a free-standing Al/Ni exothermic multilayer is fabricated and used as a heat source for reactive soldering, instead of an Al/Ni film directly deposited on the solder layer. The free-standing film enables us to reduce not only cracking in the reacted NiAl layer but also voiding at the NiAl-solder interface. These positive effects have successfully resulted in the reduction in the thermal resistance of the joint. Electron probe microanalysis (EPMA) and electron backscattering diffraction (EBSD) analyses suggest that reducing the thermal resistance has been caused by mechanical strain originating from volume shrinkage induced by the Al/Ni exothermic reaction as well as heat conduction from the reactive film to the solder layer. The mechanism of reducing the number of voids is discussed in light of stress distribution during reactive bonding.