3D Package Solid-liquid Interdiffusion Of Cu-In System Chipbonding Mechanism Under Low Temperature

The chip package has moved to a time of3D package， withSolid-Liquid-Inter-diffusion （SLID） Bonding becoming a promising method for chipconnection due to its unique advantages. SLID bonding could realize bonding at verylow temperature because of the using of Sn or In as low-temperature solder, besides, thejoints’size is relatively small, which makes Intermetallic Compound （IMC） occupies allthe weld seam, and since the melting point of IMC is relatively high, the SLID bondingprocess could achieve low temperature bonding and high temperature service. Cu couldcontribute to good properties as connecting material inside chips, moreover, the meltingpoint of In is much lower than that of Sn, which helps bond at much lower temperature.So it is important to research in SLID bonding with Cu-In system.This article discussed the fabrication process of SLID bonding with Cu-In systemfirstly and optimized the parameters for film coating process. Then the comparisonbetween the microstructure evolution of bonding at high and low temperature wasanalyzed, together with the cracks and voids in joints during the bonding process. Atlast, the shear properties of the joint was studied, with the comparison of shearproperties for different process parameters, besides, the fracture mode and fracturelocation was confirmed by fractography study.It shows that after40min, only the joint bonding at200℃don’t obtain the fullIMC joint, while the joints bonding at260℃、310℃and360℃do. At260℃bondingcondition, the Cu₁₁In₉phase firstly generated at the welding process，then Cu₂In phasegenerated at the interface between Cu₁₁In₉and Cu, however, its nucleation and growthrate is very slow. At360℃condition, the Cu₂In phase firstly generated and thenchanged to Cu₇In₃phase, with faster nucleation and growth rate, especially along thedirection of grain boundary for Cu₂In phase. Kirkendall voids could be created at theinterface between Cu and weld seam, and both the number and size of them wouldincrease with the increasing of bonding temperature and bonding time. There arethrough cracks along the longitudinal direction in joints, which origins mostly from theKirkendall voids. The presence of Cu₂In phase could increase the shear properties ofjoints by study of shear strength at the joints, while Cu₇In₃phase could not improve theshear properties of joints. The cracks occurred at260℃bonding for360min wereproduced by the volume change from Cu₁₁In₉to Cu₂In at the joints. Cleavage fracture ismainly found for260℃bonding, with fracture usually happened at Cu₁₁In₉phase.There are many tongue-patterns for fracture in the joints at260℃bonding for360min, of which there is two kinds, one is in Cu₁₁In₉phase with smooth surface, and the otherone is apparent intergranular fracture with the fracture occurring at the interfacebetween Cu₂In and Cu₇In₃phase, which means the bonding force between these twophases is very weak. Cleavage fracture is preferred for360℃bonding for both40minand160min, with the cleavage occurring at Cu₂In phase. Comparatively, for360℃bonding for360min, there are both intergranular fracture and cleavage fracture, wherethe intergranular fracture happens at the interface between Cu₂In and Cu₇In₃phase, andthe cleavage fracture happens at Cu₇In₃phase.