Huakai Shao

Elimination of pores in Ag–Sn TLP bonds by the introduction of dissimilar intermetallic phases

Low-temperature transient liquid phase (TLP) bonding is a very promising technology for achieving die attachment in high-temperature power devices. However, the Ag–Sn TLP-bonded joint has high sensitivity to shrinkage pores, which will lead to the deterioration of mechanical, thermal, and electrical properties. In this study, we have proposed two novel methods to solve the problem of pores in Ag–Sn TLP bonding through the introduction of second phases. The first method is replacing Ag substrate on one side with Cu substrate to create dual layers of dissimilar intermetallic compounds (IMCs). Consequently, the dual layers of Cu6Sn5 (or Cu3Sn) and Ag3Sn IMCs emerge on the cross-sections of bonded joint, resulting in the effective elimination of pores, which can be attributed to the change of the microstructure and the interface migration between two dissimilar IMC layers (e.g., Cu6Sn5/Ag3Sn). The second method is coating a thin Cu film on the Ag substrate to introduce Cu–Sn IMC particles. As a result, a great number of Cu6Sn5 particles disperse in the middle of the Ag3Sn layer, filling the micropores efficiently, such that a significant decrease in shrinkage pores is achieved. Both methods have been experimentally verified to improve the mechanical properties, and they have high potential to be implemented in other TLP systems.

Mechanism of Ag 3 Sn grain growth in Ag/Sn transient liquid phase soldering

Transient liquid phase (TLP) bonding is a potential high-temperature (HT) electron packaging technology that is used in the interconnection of wide band-gap semiconductors. This study focused on the mechanism of intermetallic compounds (IMCs) evolution in Ag/Sn TLP soldering at different temperatures. Experimental results indicated that morphologies of Ag3Sn grains mainly were scallop-type, and some other shapes such as prism, needle, hollow column, sheet and wire of Ag3Sn grains were also observed, which was resulted from their anisotropic growths. However, the scallop-type Ag3Sn layer turned into more planar with prolonging soldering time, due to grain coarsening and anisotropic mass flow of Ag atoms from substrate. Furthermore, a great amount of nano-Ag3Sn particles were found on the surfaces of Ag3Sn grains, which were formed in Ag-rich areas of the molten Sn and adsorbed by the Ag3Sn grains during solidification process. Growth kinetics of the Ag3Sn IMCs in TLP soldering followed a parabolic relationship with soldering time, and the growth rate constants of 250, 280 and 320 °C were calculated as 5.83×10−15 m2/s, 7.83×10−15 m2/s and 2.83×10−14 m2/s, respectively. Accordingly, the activation energy of the reaction was estimated about 58.89 kJ/mol.

Interactions at the planar Ag3Sn/liquid Sn interface under ultrasonic irradiation

The interactions at the interface between planar Ag3Sn and liquid Sn under ultrasonic irradiation were investigated. An intensive thermal grooving process occurred at Ag3Sn grain boundaries due to ultrasonic effects. Without ultrasonic application, planar shape of Ag3Sn layer gradually evolved into scalloped morphology after the solid-state Sn melting, due to a preferential dissolution of the intermetallic compounds from the regions at grain boundaries, which left behind the grooves embedding in the Ag3Sn layer. Under the effect of ultrasonic, stable grooves could be rapidly generated within an extremely short time (<10s) that was far less than the traditional soldering process (>10min). In addition, the deepened grooves leaded to the formation of necks at the roots of Ag3Sn grains, and further resulted in the strong detachment of intermetallic grains from the substrate. The intensive thermal grooving could promote the growth of Ag3Sn grains in the vertical direction but restrain their coarsening in the horizontal direction, consequently, an elongated morphology was presented. All these phenomena could be attributed to the acoustic cavitation and streaming effects of ultrasonic vibration.