Thermal Ageing Studies of Sintered Micron-Silver (Ag) Joint as a Lead-Free Bonding Material
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Thermal Ageing Studies of Sintered Micron‑Silver (Ag) Joint as a Lead‑Free Bonding Material K. S. Siow1 · S. T. Chua1 Received: 8 March 2019 / Accepted: 21 July 2019 © The Korean Institute of Metals and Materials 2019
Abstract The sintered silver (Ag) joint has proven to be a suitable die-attach material to be used under the operating conditions of wide bandgap semiconductors because of its high melting point and high thermal and electrical conductivities. However, to bond reliably, a sintered Ag joint needs a suitable metallized substrate (e.g. gold or silver) and the application of pressure during sintering. Hence, we investigated the evolving microstructure (i.e. the importance of pore shape factor) and shear strength of micron-Ag joints bonded without pressure on copper, Ag-plated substrate, and direct-bond copper (DBC) thermally aged at 300 °C for 1000 h. The DBC substrate maintained die-shear strength better because its coefficient of thermal expansion matched those of the sintered Ag and Si dies. Regardless of substrate, micron-Ag joints showed a decrease of large pores (> 0.16 µm2) and an increase of spherical pore shapes during the aging period. These favourable changes maintained the mechanical integrity of the micron-Ag joints. This evolving microstructure of the sintered Ag joint provides guidelines for packaging engineers to consider as part of their selection of metallizations and substrates for power electronic packaging. Keywords Sintered silver · Lead-free · Diffusion · Mechanical properties · Microstructure · Oxidation
1 Introduction A sintered silver (Ag) joint is one of the two high-temperature and lead (Pb)-free die-attach materials that can sustain the high-temperature applications of wide bandgap semiconductors used in the power electronics, the other being transient liquid phase sintering (TLPS). The recent International Electronics Manufacturing Initiative report proposed four different classes of die-attach materials, but only two sintered technologies, i.e. sintered Ag and TLPS, were able to reliably operate at temperatures above 200 °C for significant durations [1]. Compared to the intermetallic-based TLPS method, sintered Ag has the advantage of being adaptable to different package and die designs, along with excellent mechanical, electrical, and thermal properties due to its Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12540-019-00394-0) contains supplementary material, which is available to authorized users. * K. S. Siow [email protected] 1
Institute of Microengineering and Nanoelectronics, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
metallic and porous structure being easily tuned by selection of sintering profile. However, sintered Ag requires pressure during the die-bonding stage to stabilize its microstructure and enhance its reliability at high operating temperatures; this limitation is evident in comparisons of residual dieshear strength between pressure-assisted sintered and pressureless sinter
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