Low-temperature transient liquid phase (TLP) bonding for Ag-plated substrates was systematically investigated by using foil-based interlayer of pure Sn foil or preformed Sn/Cu/Sn sandwich structure in air atmosphere. The influences of bonding process, such as bonding temperature, bonding time and foil thickness, on the microstructure characterization and mechanical behavior of TLP joint were discussed. Experimental results show that Ag-plated substrates can be successfully TLP bonded in air atmosphere by the protection of flux. The formation of pores in intermetallic compounds (IMCs) is a serious problem for Ag/Sn/Ag TLP bonding, which is attributed to the volume shrinkage of isolated Sn areas during isothermal solidification. Prolonging homogenization time, properly increasing bonding pressure, decreasing temperature, or reducing interlayer thickness can effectively reduce the shrinkage porosity, but is still incapable of eliminating pores thoroughly. Both shear bands and intergranular facets are simultaneously observed on the fracture surface of Ag 3 Sn joint. Since the micro voids distributing along Ag 3 Sn grain boundaries weaken the cohesion strength between two neighboring Ag 3 Sn grains in some areas. Using preformed Sn/Cu/Sn interlayer is available to enhance the mechanical integrity, which is strongly depended on the Cu thickness and Sn thickness. The joint shear strength can be increased by even more than 100% by the introduction of Cu foil. Moreover, the remained Cu layer in the IMCs can act as a buffer layer during fracture process, leading to the improvement of the ductility of TLP joint.
Rapid transient liquid phase (TLP) bonding process on Ag/Sn/Ag system is achieved in air by the assistance of ultrasonic, which has great potential to be applied to high-temperature power devices packaging. In this study, the influence of ultrasonic effect on the morphology and growth kinetics of AgSn grains, and the joint microstructure, mechanical property and thermal reliability were systematically investigated. Experimental results indicated that the rapid consumption of the "dynamic" transient liquid phase was attributed to the accelerated dissolution of Ag substrate and the extrusion of liquid Sn, which were entirely induced by the complex sonochemical effects on the liquid/solid intermetallic compounds (IMCs) interface. An elongated scallop-like morphology of AgSn grains was developed during Ag/Sn interfacial reaction with ultrasonic, accompanied by widening of grooves between neighbored grains. This phenomenon is called as a strengthening thermal grooving, in which the grooves at grain boundaries provide stable molten channels for Ag atoms diffusion from the substrate. Consequently, the improved elemental diffusion was evaluated through the growth kinetics of AgSn IMCs, with conservative estimation of 6-16.5 times faster than the traditional TLP process. In addition, both excellent mechanical property and thermal reliability of the Ag-Sn intermetallic joint were experimentally verified by shear test and high-temperature storage test, respectively.
Low temperature transient liquid phase (LTTLP) bonding is a promising technology to enable in high temperature electronic packaging. In this study, interfacial reaction and mechanical characterizations for Cu/Sn/ Ag system LTTLP bonding at temperatures ranging from 260 to 340°C for various time were investigated. Experimental results showed that Cu and Ag substrate independently reacted with molten Sn, and the growth of IMCs on one side was hindered by the opposite IMCs layer after scalloped Cu 6 Sn 5 contacted with the Ag 3 Sn, and there was no ternary alloy phase formed all the time. Pores were found and distributed at the Cu 6 Sn 5 /Ag 3 Sn interface or between grain boundaries after the residual Sn was fully consumed, however, they gradually disappeared with continuing reaction of that Cu 6 Sn 5 phase converted into Cu 3 Sn phase. Shear strength of the LTTLP joints increased with increasing bonding time, and the adhesive strength of Cu 6 Sn 5 /Ag 3 Sn interface was weaker than that of the Cu 3 Sn/Ag 3 Sn interface. The rupture behaviors were also discussed with a fracture model. As follow, cracks initiated in the pore and mainly propagated along the Cu
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