Cu-Cu low temperature bonding based on lead-free solder with graphene interlayer

In order to suppress the immoderate growth of intermetallic compounds (IMCs) at the Sn-Cu bonding interface, we transferred multilayer graphene between the Sn solder and the Cu film sample as an interlayer. Then, an external pressure of ∼350 N was applied to the two Cu film samples at 150 °C for 5 min to realize bonding using a thermo-compression bonding machine. After bonding, the sample was placed in an oven at 150 °C for an aging test. The IMC layer grew at the bonding interface gradually with the aging duration increasing for both cases. The growth rate of the IMC layer with the graphene interlayer is much lower than that without the graphene interlayer. For 72 h aging, the thickness of the IMC layer at the interface of Sn-graphene-Cu bonding reduces by about 55% relative to that of Sn-Cu bonding. The reduction rate of bonding strength with the graphene interlayer is also lower than that without the graphene interlayer. For 72 h aging, the shear strength of Sn-Cu bonding is lower than that of Sn-graphene-Cu bonding. Through comparison of data measured by the 4-point probe test, it is found that the contact resistances of Sn-graphene-Cu bonding have no obvious difference from those of Sn-Cu bonding after aging. Thus, the graphene transfer and low temperature process can suppress the interfacial reaction and the immoderate growth of IMC to retard aging, and the multilayer graphene interlayer has no significant negative influence on the electrical properties of the bonding.