The Effect of Cu and Ni on the Structure and Properties of the IMC Formed by the Reaction of Liquid Sn-Cu Based Solders with Cu Substrate

The influence of Cu and Ni additions on the morphology of the intermetallic compounds (IMC) formed at the interface of the liquid Sn-Cu solder and the Cu substrate has been studied. At low Cu concentrations only a thin IMC layer is formed which remains almost unchanged in time and the substrate is dissolving quickly. Ni additions significantly accelerate the growth kinetics of the IMC layer, prevent the formation of a Cu3Sn layer and reduce the rate of substrate dissolution. Introduction The Sn-Pb alloy is a well-known, easily accessible solder material exhibiting very good mechanical properties. However, due to the toxicity of lead, it is now forbidden to use Pb in solders for electronic devices. Two recent European Union directives [1, 2] require new electric and electronic devices to be lead-free. Nowadays, a lot of low melting, usually Sn based, alloys are developed as lead-free alternatives for Sn-Pb solder. Several Sn-Ag-Cu alloys have been recently designed and their properties thoroughly studied by many authors [3-5]. On the other hand, relatively less effort has been given to the development of the much cheaper Sn-Cu alloys. It is now known [6] that the properties of these alloys could be improved by minor additions of other elements. For example, it has been shown, that the additions of Ni could improve Sn-Cu alloys soldering properties including a reduced tendency for bridging and an improved surface finish of the joints [6]. During the soldering process, nearly immediately upon contact of the melted solder with the solid substrate, the intermetallic compounds (IMC) starts to form at the interface. The formation of the interfacial intermetallic layer is desirable to attain good bonding between the substrate and the solder. On the other hand, because of the high brittleness of the intermetallics, excessive thickness of the layer may deteriorate the mechanical and electrical properties of the joint. Thermo-mechanical fatigue may cause additional growth of IMC layer, induce internal stresses and thus result in the degradation of the joint. Therefore, it is of extreme importance to control the growth of the IMC layer. Two IMC layers can be formed at the interface as a result of Sn-based solder reaction with a Cu substrate. A layer of a typical scallop morphology of the Cu6Sn5 (η-phase) is usually formed [3,4]. However, extensive soldering time and high temperature of the melted solder may result in the formation of an additional thin layer of the Cu3Sn (ε-phase) at the Cu/Cu6Sn5 interface. In this work, the results of a study aimed at assessing the effect of Cu content in Sn-Cu solder and the effect of Ni additions on the morphology of IMC layer formed during liquid solder/solid Cu substrate reaction in the temperature range of 240°C to 285°C and reaction time of 5s to 6000s is presented. Experimental Three Sn-Cu based lead-free solders were used in this investigation, in particular the near eutectic Sn-0.7Cu (SC), the Sn-1.4Cu (SC+) and the Sn-0.6Cu-Ni (SCN). The composition of these alloys is listed in Tab. 1. Rectangular plates of the dimensions of 50×5×0,5 mm were first cut from oxygen-free, high conductivity copper and used as the substrate for soldering. The specimens were chemically polished in a 40 vol. % HNO3 solution in distilled water to remove oxide film, then cleaned with ethanol and dried with hot-air blower. Next, mildly activated flux was applied on the polished surface. The 220 WDS'08 Proceedings of Contributed Papers, Part III, 220–224, 2008. ISBN 978-80-7378-067-8 © MATFYZPRESS