Effect of Joint Size on Microstructure and Growth Kinetics of Intermetallic Compounds in Solid-Liquid Interdiffusion Sn3.5Ag/Cu-Substrate Solder Joints

Abstract

The effect of joint size on the interfacial reaction in the Sn3.5Ag/Cu-substrate soldering system was examined. An experiment was conducted in which parameters such as bonding time, temperature, and pressure were varied at multiple levels. The morphology and thickness of all intermetallic compounds (IMC) were analyzed using the scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) techniques. An examination of the microstructures of solder joints of different sizes revealed that the size of the solder joint has no effect on the type of IMCs formed during the process. It was found that the joint size significantly affected the thickness of the intermetallic layers. The Cu3Sn intermetallic layers formed in the smaller sized solder joints were found to be thicker than those in the larger sized solder joints. In all specimen sizes, the increase in the thickness of Cu3Sn intermetallic layers with soldering time was found to obey a parabolic relationship. Additionally, for the cases when eutectic solder is available in the joints, a similar soldering time and temperature dependency were found for the Cu6Sn5 IMC phase. The intermetallic growth of the Cu3Sn phase was under a volume-diffusion controlled mechanism. The growth rate constants and activation energies of intermetallic layers were also reported for different joint thicknesses. Furthermore, the growth rate constants of the Cu3Sn intermetallic layer were found to depend upon the size of the joints.

Critical Review of Size Effects on Microstructure and Mechanical Properties of Solder Joints for Electronic Packaging

“…Therefore, the underlying mechanism for the size effects of solder joints is highly likely to be related to the Cu flux. Abdelhadi et al [32] established an analytical model to investigate the growth rate of Cu 3 Sn, which was found to be joint-size dependent and obey the parabolic relation with time. Huang et al [33] measured the IMC evolution of Sn3.0Ag0.5Cu solder joints on Cu or Ni-P pads, and the Cu flux variation was inferred to be the cause of solder volume size effects.…”

Effects of Stress and Electromigration on Microstructural Evolution in Microbumps of Three-Dimensional Integrated Circuits

“…6(f), the hydrostatic stress over the entire domain is observed to be tensile, i.e., s H > 0, with the minimum tensile stress located in the region that separates the two Cu 6 Sn 5 grains. The interdiffusion flux tends to flow away from the top boundary and into the region between the two Cu 6 Sn 5 grains, which is driven by the gradient of s H according to (15). Consequently, more Sn atoms are available at the interface between the b-Sn and Cu 3 Sn grains to react and form a new Cu 6 Sn 5 phase.…”

“…Abdelhadi et al reported faster growth of the Cu 3 Sn IMC in smaller Sn3.5Ag solder joints and attributed this result to the effect of stress on the diffusion rates of Sn and Cu. [15]. EM also affects the microstructure within microbumps [17]- [19].…”

Local shear stress-strain response of Sn-3.5Ag/Cu solder joint with high fraction of intermetallic compounds: Experimental analysis

“…IMCs such as Cu6Sn5 and Cu3Sn form in between solder/Cu interface due to the diffusion of copper (Cu) into bulk solder (Sn). In Cu pillar micro bumps, major portion of bulk Sn converts into IMCs during the diffusion process, and therefore, the Cu pillar bump is typically left with small volume fraction of ductile solder alloy [1,2]. These IMCs are necessary for good metallurgical bonding.…”