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Shear Strength and Thermo-mechanical Reliability of Sintered Ag Joints Containing low CTE Non-metal Additives for Die Attach
Keywords: Silver sintering for die attach, Low CTE non-metal additives, Shear strength, thermo-mechanical reliability
Ag sintering has been paid attention to considering as an alternative to soldering in die attach for decades, especially for high temperature power electronics packages due to its high melting temperature, highly thermal and electrical conductivity of the sintered silver joints, and low process temperature less than 275oC. The coefficient of thermal expansion (CTE) of silver (19.1ppm/oC), however, is much higher than that of silicon die (2.6ppm/oC) and the commonly used alumina substrate (7.2ppm/oC). CTE mismatch of the different materials in various components in power electronics package which would lead to the delamination at the interface between interconnection layer and chips or substrate, and/or cracking of the interconnection layer is one of the mostly common causes of failure of power electronics device during thermal cycling or high temperature operation. In recent years we have been developing a series of silver sinter paste containing low CTE non-metal particles as additives to reduce or adjust CTE of the sintered joints so as to extend the lifetime and reliability of power electronics device in high temperature application. In this present paper, we will report a new set of silver sinter pastes containing micro scale non-metal particles, sintering process, microstructural morphologies, thermo-mechanical reliability of the sintered joint and effect of the contents of non-metal particles on shear strength of the sintered silver joints bonding Ag metallized silicon die on Ni/Au DBC substrates. Shear test on the sintered joints with and/or without the low CTE non-metal additives has been conducted at room temperature, 200, 250, and 300oC. Thermo-mechanical reliability of the sintered joints was evaluated by thermal cycling, thermal shock, high temperature storage tests (HTS), respectively. X-ray inspection and scanning electronic microscopy (SEM) were used to characterize void, crack and microstructure morphologies of the sintered joints with and/or without the additives. Shear strength data obtained from high temperature shear test indicates that shear strength of the sintered joints with and/or without additives determined at the elevated temperature up to 250oC is quite similar to shear strength values measured at room temperature, which means the addition of the non-metal particles to silver sinter paste would not have significant influence on high temperature performance. Preliminary data obtained based on thermal shock test (TST) with a profile of -55 to 155oC and thermal cycling test (TCT) with a profile of -40 to 175oC reveal that both the sintered joints with and without the non-metal particles have excellent thermo-mechanical reliability. High temperature storage test (HTS) at 250oC indicates that the sintered joints with the non-metal additive would be helpful to reducing or eliminating the delamination of the metallization layers on die backside. X-ray inspection shows that both the sintered joints with and without the non-metal additives have very low voids and no crack before and after thermal shock test within 2000cycles. SEM cross-section microstructure morphologies of the sintered joints with and/or without the non-metal additives prove that there is no void and crack formation within 2000cycles of thermal shock using a profile of -55 to 155oC.
Guangyu Fan, Reseach Chemist
R&D, Indium Corporation of America
Clinton, NY

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