Vol.:(0123456789) 1 3 J Mater Sci: Mater Electron DOI 10.1007/s10854-017-6674-3 Low temperature and low pressure bonding of plateless Cu–Cu substrates by Ag-based transient liquid phase sintering Khairi Faiz Muhammad 1  · Takehiro Yamamoto 1  · Makoto Yoshida 1,2   Received: 17 October 2016 / Accepted: 27 February 2017 © Springer Science+Business Media New York 2017 temperature shifted from eutectic temperature of Sn–Bi to approximately 262 °C, allowing the application of mod- ules at higher operating temperature than the processing temperature. 1 Introduction Recent advancements of the wide band gap devices such as SiC (silicon carbide) and GaN (gallium nitride), have ena- bling power modules for not only achieving higher power density compared to the conventional Si-based devices, but also allowing miniaturization of the power modules, which finally lead to the weight reduction of the vehicles. However, one major issue to be tackled for the next gen- eration power modules (epoxy-molded SiC/GaN-based power modules) is an efficient heat dissipation system. From indirect liquid cooling, which used thermal grease as thermal interface material, recent power electronics modules adapted direct liquid cooling which used Sn- based (tin) solders to bond the power module to the heat spreader and/or heat sink [1]. Formerly, Sn–Ag (tin–silver) solder has been used as the thermal interface material [2] but recently Sn–Sb (tin–antimony) solder received great attention due to higher heat resistance and higher mechani- cal strength compared to the former [3, 4]. Sn–Sb solder exhibited two types of mechanical strengthening, i.e. solid solution strengthening and precipitation strengthening [3], and also showed a longer fatigue lifetime compared to the Sn–Ag solder [4]. On the other hand, one considerable drawback when applying Sn–Sb solder is its high process- ing temperature. The processing temperature when using Sn–Sb solder may vary depending on the weight percentage (wt%) of the Sb contained in the solder which corresponds to the liquidus temperature, e.g. the processing temperature Abstract Bonding silicon carbide/gallium nitride (SiC/ GaN) based power modules, particularly epoxy-molded modules to heat-substrate and/or heat sink, requires low processing temperature preferably lower than 250 °C, and low pressure as low as 0.1 MPa to prevent damage to the modules. In addition, due to the impracticality of deposit- ing metal-plating to the epoxy-molded module, bonding of plateless Cu-substrates is in great demand. Furthermore, post-processing residual flux cleaning, which is costly and unfavorable to industry need to be avoided as possible by opting out the usage of flux. Up to authors’ knowledge, our study is the first to fulfill all requirements stated above. Transient liquid phase sintering (TLPS) of mixed fluxless Sn–Bi (tin–bismuth) eutectic alloy and Ag (silver) particles was applied to bond plateless Cu–Cu substrates. Sinter- ing temperature of 250 °C, sintering pressure of 0.02 MPa, and reducing environment were applied during process- ing. The effects of addition amount of Sn–Bi and sinter- ing holding time to the shear strength and microstructure were investigated. The remelting temperature after sinter- ing was also examined. Shear strength of 30 wt% added Sn–Bi was over than 20 MPa, which qualify the require- ment of MIL-STD-883K, and larger than conventional Pb- based solder. Formation of intermetallic compounds are thought to strengthen the interface and matrix. Remelting * Khairi Faiz Muhammad khairifaiz296@akane.waseda.jp; mkhairifaiz@gmail.com 1 Department of Modern Mechanical Engineering, Graduate School of Creative Science and Engineering, Waseda University, 3-4-1 Shinjuku Okubo, Shinjuku, Tokyo 169-8555, Japan 2 Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-6, Nishi-Waseda, Shinjuku, Tokyo 169-0051, Japan