Solid-State Interface Reactions between Silver and 95.5Sn-3.9Ag-0.6Cu and 63Sn-37Pb Solders
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THE development of a lead (Pb)-free soldering technology must address the long-term reliability of joints made to materials other than traditional copper (Cu) and Cu-based alloys. One such material is silver (Ag). Silver and Ag-based alloys are used widely in relay and switching components. Silver finishes are found in a variety of electrical and electronic devices.[1,2] A recent study documented the interface microstructure as well as rate kinetics of dissolution and intermetallic compound (IMC) layer growth between Ag base metal and liquid 95.5Sn-3.9Ag-0.6Cu (wt pct), 99.3Sn-0.3Cu, and 63Sn37Pb solders.[3] It is important to also systematically investigate solid-state interface reactions between these solders and Ag as the first step towards predicting the long-term reliability of fielded interconnections. Solid-state interface diffusion causes further growth of the IMC layer after solidification. Numerous studies have investigated such solid-state phenomena between Pb-free solders and the common base metals such as Cu and nickel (Ni).[4–7] However, similar studies addressing Ag base metals are few. Kay and Mackay investigated solid-state IMC growth between 100Sn electroplated films on Ag.[8] The present authors re-analyzed those data, using the Arrhenius equation format, tnexp(-DH/ RT), where t is time; DH is the apparent activation energy; R is the universal gas constant; and T is PAUL T. VIANCO, JOSEPH J. MARTIN, ROBERT D. WRIGHT, and PAUL F. HLAVA are with the Sandia National Laboratories, Albuquerque, NM, USA. Contact e-mail: [email protected] Manuscript submitted: April 11, 2007. Article published online September 13, 2007. 2488—VOLUME 38A, OCTOBER 2007
temperature. The time exponent, n, and apparent activation energy, DH, were 0.42 and 29 kJ/mol, respectively.[9] The unusually low value of DH suggested the activity of a very fast, fast-diffusion process (discussed later). The present study investigated the interface microstructure and IMC layer growth kinetics resulting from the solid-state aging of couples between Ag sheet and the Pb-free solder, 95.5Sn-3.9Ag-0.6Cu (wt pct). Parallel experiments evaluated 63Sn-37Pb/Ag couples for comparison purposes. The binary alloy phase diagrams were surveyed for potential IMC phases, although bi-metal couples are not equilibrium configurations.[10] The most likely IMC layer phase is Ag3Sn. In terms of the Pb-free solder, the other potential interaction is between Ag and Cu. In the case of the 63Sn-37Pb solder, Pb is not expected to be a component of any stoichiometric IMC phases; its demise can be the result of one of two scenarios. In the first scenario, as is observed in Sn-Pb/ Cu couples, Pb can be rejected during IMC layer growth to form a Pb-rich layer between the IMC layer and the solder field.[11] The second scenario, which is represented by 50In-50Pb/Au couples, is that the rejected Pb becomes incorporated in the IMC layer along with the stoichiometric phase.[12] In the present study, the interface microstructures were evaluated using optical microscopy and electron
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