We describe the strength and fracture of Si wafers instantaneously bonded by exothermic reaction in an Al/Ni multilayer film. The Al/Ni multilayer film deposited by dual-source direct current (DC) sputtering is used as a heat source for the solder bonding of Si wafers. To reduce the numbers of voids and microcracks produced at the solder-reacted NiAl interface and the reacted NiAl layer, respectively, Al/Ni multilayer films are modified. By depositing the Al/Ni multilayer film with Ni layers on the top and bottom surfaces onto the base-side Si wafer, the number of voids at the base-side solder and reacted NiAl interface is reduced. Using a self-standing Al/Ni multilayer sheet with Ni layers on both sides reduces the number of microcracks introduced into the reacted NiAl layer. After dicing the bonded wafer to make a miniature rod having the bonded section, a four-point bending test is conducted. The improvements of the bonded section provide high fracture strength. The mechanical test determines the weakest portion of the bonded section. The effectiveness of modifying the Al/Ni reactive-bonding conditions is discussed, based on observations of fractured samples and results of energy-dispersive X-ray (EDX) analysis.