Intermetallic growth and mechanical behavior of low and high melting temperature solder alloys
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I.
INTRODUCTION
TRADITIONALLY, the predominant solder alloys used in electronic applications have been the eutectic, or near-eutectic, Sn-Pb solders. In the past, electronic systems have been designed to use these alloys, based upon their fairly low melting temperature and good wetting behavior. However, other solder alloys have been used in specialized assembly processes where, for example, concerns such as heat sensitivity of particular electronic components are important. Lower and higher melting temperature solders are also used in step soldering applications where subsequent soldering operations must occur without melting previously soldered connections. Recent environmental concerns over lead contamination are causing a closer examination of lead-free solder alloys that can be substituted for lead-bearing materials. The formation of intermetallic compounds at the solder-substrate interface signifies the formation of a metallurgical bond. The presence of intermetallics is not a strict necessity for wetting to occur, although almost all solder systems of interest do form intermetallics. The interfacial intermetallic increases rapidly in thickness, while the solder is molten and continues to grow through solid state diffusion after the joint has solidified. The integrity of solder joints can depend upon the properties of the intermetallic layers. Excessive growth of the layer can cause the base metal, or finish, to be consumed by the solder. Consumption of the solderable D.R. FREAR and P.T. VIANCO, Senior Members of the Technical Staff, are with Sandia National Laboratories, Albuquerque, NM 87185-0340. Manuscript submitted April 7, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
metallization in the liquid state can result in dewetting of the solder. The transformation of solderable coatings into intermetallics by solid state reaction can also result in fracture of the joint. The mechanical properties of a solder joint can be degraded by the presence of interfacial intermetallics because the layer is brittle and can, therefore, initiate failures during deformation. This is important for solder joints in microelectronic applications, because the joint is required to accommodate mechanical strain that results from the constraint of materials with differing coefficients of thermal expansion. Studies (Section B) have confirmed that interfacial intermetallics play a role in the deformation and failure behavior of solder joints. However, this information is largely confined to the Sn-Pb solder/copper substrate system. Relevant studies from the literature on intermetallic growth and mechanical behavior are summarized in the following sections. A. Growth Kinetics
Numerous studies have examined the intermetallic growth kinetics of the eutectic and near-eutectic tin-lead solders on copper.ll.2,3] Zakraysek[4] evaluated the growth kinetics of intermetallic layers between 100Sn, 97.5Sn2.5Sb, and 95Sn-5Ag solder and copper for temperatures in the range of 100 ~ to 200 ~ and time periods of 0.25 hours to 32 days. The sam
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