Solid–liquid Interdiffusion Bonding of Cu/Sn/Ni Micro-joints with the Assistance of Temperature Gradient

A novel solid–liquid interdiffusion (SLID) bonding method with the assistance of temperature gradient (TG) was carried out to bonding Cu and Ni substrates with Sn as interlayer. The element distribution and grain morphology of interfacial intermetallic compound (IMC) in Cu/Sn/Ni micro-joints during both SLID and TG-SLID bonding and in the final Cu/(Cu,Ni)6Sn5/Ni full IMC micro-joints were analyzed. Under the effect of Cu-Ni cross-interaction, interfacial (Cu,Ni)6Sn5 dominated the IMC growth at all the interfaces. The morphology of the (Cu,Ni)6Sn5 grains was closely related to Ni content with three levels of low, medium and high. The full IMC micro-joints consisted of L-(Cu,Ni)6Sn5, M-(Cu,Ni)6Sn5 and H-(Cu,Ni)6Sn5 grains after SLID bonding or TG-SLID bonding with Ni as hot end, while only L-(Cu,Ni)6Sn5 grains after TG-SLID bonding with Cu as hot end, showing that the direction of TG had a remarkably effect on the growth and morphology of the interfacial (Cu,Ni)6Sn5 during TG-SLID bonding. Thermodynamic analysis revealed the key molar latent heat and critical Ni content between fine-rounded-like (Cu,Ni)6Sn5 and block-like (Cu,Ni)6Sn5 were 17,725.4 J and 11.0 at.% at 260 °C, respectively. Moreover, the growth kinetic of the interfacial IMC was analyzed in detail during bonding with and without TG. Under the combination of TG and Cu-Ni cross-interaction, void-free full IMC micro-joints were fast formed by TG-SLID bonding with Cu as hot end. This bonding method may present a feasible solution to solve the problems of low formation efficiency and inevitable Cu3Sn growth of full IMC joints for 3D packaging applications.