Microstructure characterization and mechanical behavior for Ag3Sn joint produced by foil-based TLP bonding in air atmosphere

Abstract

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.

Morphology transformation on Cu3Sn grains during the formation of full Cu3Sn solder joints in electronic packaging

“…Comparatively, under the interconnected height less than dozens of micrometers, the IMCs account for a larger proportion in the interfacial region after soldering. Even, when the soldering time is extended, the solder can be consumed completely to form IMCs, leading to the formation of full IMCs joints (Chiu et al, 2014;Flötgen et al, 2014;Kato et al, 1999;Li and Chan, 2017;Li and Agyakwa, 2010;Li et al, 2011;Luu et al, 2013;Mo et al, 2015;Shao et al, 2016Shao et al, , 2017aShao et al, , 2017bTian et al, 2014a;Yao et al, 2017aYao et al, , 2017b. In other words, when the interconnected height is less than dozens of micrometers, the interfacial structure of joints can be transformed from substrates/IMCs/solder/IMCs/substrates to substrates/IMCs/substrates with the increase of soldering time.…”

“…Undoubtedly, it is very essential to conduct studies regarding the formation of full IMCs solder joints. Currently, relevant studies mainly focus on growth kinetics of IMCs layers during the formation of full IMCs joints (Li and Chan, 2017;Li and Agyakwa, 2010;Mo et al, 2015), soldering process and interfacial phase evolution for the formation of full IMCs joints (Chiu et al, 2014;Flötgen et al, 2014;Kato et al, 1999;Li et al, 2011;Luu et al, 2013;Shao et al, 2016;Yao et al, 2017aYao et al, , 2017b and reliabilities of full IMCs joints Shao et al, 2017aShao et al, , 2017bTian et al, 2014a). For the studies regarding interfacial phase evolution, these can be divided into two parts.…”

Ag@Sn Core‐Shell Powder Preform with a High Re‐Melting Temperature for High‐Temperature Power Devices Packaging

“…The Ag particles are surrounded by ultrafine grained Ag 3 Sn with sizes ranging from 0.5 to 3 mm. While Ag-Sn TLP bonding usually requires at least 0.5 h to obtain a full Ag 3 Sn or Ag 3 Sn/Ag composite microstructure as reported previously, [20,24] the bonding time in the present study was only 5 min due to the high reaction area of the Ag@Sn in the preform. When the Ag@Sn particles were heated above the melting point of Sn, a liquid Sn coating was formed and flowed into the gaps between the particles through capillary action, resulting in the densification of the joint microstructure.…”

“…[11][12][13] In recent years, TLP has attracted increasing attention in the power electronic packaging field as a promising die attach technique due to its ability to increase the re-melting point, and many TLP systems, such as, Ag-Sn, Ni-Sn, Cu-Sn, Au-Sn, Au-In, and Ag-In, have been developed. [14][15][16][17][18][19][20] Generally, a TLP system consists of a low-melting-point interlayer and two substrates with high melting temperatures, where the interconnection can withstand a much higher temperature than the process temperature after the low-melting-point interlayer has been completely consumed and converted into intermetallic compounds (IMCs) with high re-melting temperatures. However, the small thickness of the bondline and the inherent brittle nature of IMCs limit their ability to absorb stress and strain, so that the reliability and service life of the bondline under thermomechanical stress is a serious concern for the TLP technique.…”

Effects of Sintering Pressure on the Densification and Mechanical Properties of Nanosilver Double-Side Sintered Power Module

“…As described by Khazaka et al [11], a metallic interlayer is usually used in the TLP process, such as Sn or In. This interlayer reacts under low pressure with a substrate that has a higher melting point, such as Cu, Ag, Ni, Au [12][13][14][15] T consists of full intermetallic compounds (IMCs) and its remelting temperature is much higher than the melting point of the corresponding solder material. But Shao et al [16] found that the TLP process is quite time consuming because of the diffusion dominated reaction between the interlayer and substrate, and IMCs are brittle causing performance issues.…”