Understanding of Void Formation in Cu/Sn-Sn/Cu System During Transient Liquid Phase Bonding Process Through Diffusion Mo
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current trend of power electronic modules towards higher power densities and higher junction temperatures requires the development of new interconnect technologies that are able to withstand cyclic thermomechanical stresses at temperatures higher than 200 C. Indeed, classical packaging technologies, such as soft soldering and wire bonding, are the limiting factors for better performance of power modules.[1]
SYLVIE BORDE`RE is with the CNRS, Univ. Bordeaux, ICMCB, UPR 9048, 33600 Pessac, France. EMILIEN FEUILLET is with Innoptics, rue Franc¸ois Mitterrand, 33400 Talence, France. JEANLUC DIOT and RENAUD DE LANGLADE are with Composite Innovation, 2 alle´e du Doyen Brus, 33600 Pessac, France. JEANFRANC¸OIS SILVAIN is with the CNRS, Univ. Bordeaux, ICMCB, UPR 9048, and also with the Institut de Chimie de la Matie`re Condense´e de Bordeaux, ICMCB-CNRS, 87 Avenue du Dr A. Schweitzer, 33608 Pessac Cedex, France. Contact e-mail: [email protected] Manuscript submitted January 26, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
One promising interconnect technology for high-temperature applications is the Transient Liquid Phase Bonding (TLPB) process.[2] This technique relies on the formation of intermetallic compounds (IMCs) as a bonding medium involving a liquid phase which is progressively consumed. The process employs a binary system consisting of two metals (or metal alloy) with different melting points: a low-melting point interlayer, such as tin (Sn), sandwiched between two high-melting point substrates, such as copper (Cu). The choice of the Cu-Sn system was motivated first by the widespread use of Cu-metal in electronic packaging, in particular as metallization layers on ceramic substrates, and secondly by the low-melting point (232 C) of Sn-metal, together with its ability to form IMCs with Cu, and its low cost. Upon heating to typical bonding temperature, just above Sn melting point, the Cu substrates react with the liquid Sn trough inter-diffusion. According to the Cu-Sn phase diagram,[3,4] two IMCs, Cu6Sn5 and Cu3Sn, form and grow through isothermal condition. The goal of the process is to entirely transform the Sn interlayer into the Cu-rich IMC, Cu3Sn. This phase is
thermodynamically stable up to 350 C and has a re-melting temperature (676 C) much higher than the process temperature. The growth kinetics and the evolving morphologies of the IMCs during TLP soldering are highly dependent on the process parameters: heating rate and bonding temperature, compressive stress for layer contact[5] and initial thickness of the Sn foil.[6] Nevertheless, within the bonding temperature range [240 C to 280 C] the following growth sequence was observed. The Cu6Sn5phase growth dominates, that of the Cu3Sn-phase, in the early stages of the process. It is only when the Sn liquid phase is entirely consumed that the Cu3Sn layer grows at the expense of the previously formed Cu6Sn5-layer. Moreover, the Cu6Sn5-layer does not grow in a flat but rather in a rough way associated with a rounded grains morphology c
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