The interfacial reactions between Sn-based solders and two common substrate materials, Cu and Ni, are the focuses of this paper. The reactions between Sn-based solders and Cu have been studied for several decades, and currently there are still many un-resolved issues. The reactions between Sn-based solders and Ni are equally challenging. Recent studies further pointed out that Cu and Ni interacted strongly when they were both present in the same solder joint. While this crossinteraction introduces complications, it offers opportunities for designing better solder joints. In this study, the Ni effect on the reactions between solders and Cu is discussed first. The presence of Ni can in fact reduce the growth rate of Cu 3 Sn. Excessive Cu 3 Sn growth can lead to the formation of Kirkendall voids, which is a leading factor responsible for poor drop test performance. The Cu effect on the reactions between solders and Ni is then covered in detail. The knowledge gained from the Cu and Ni effects is applied to explain the recently discovered intermetallic massive spalling, a process that can severely weaken a solder joint. It is pointed out that the massive spalling was caused by the shifting of the equilibrium phase as more and more Cu was extracted out of the solder by the growing intermetallic. Lastly, the problems and opportunities brought on by the cross-interaction of Cu and Ni across a solder joint is presented.

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The eutectic 99.3Sn-0.7Cu solder (wt%, Sn-0.7Cu) is the most promising lead-free replacement for the eutectic Sn-Pb solder in wave-soldering applications. In this study, the effect of a small perturbation in the Cu concentration on the reaction between the Sn-0.7Cu solder and Ni was investigated. Specifically, four Sn-xCu solders (x ‫ס‬ 0.2, 0.4, 0.7, and 1) were reacted with Ni at 250°C. A slight variation in Cu concentration produced completely different reaction products. When the Cu concentration was low (x ‫ס‬ 0.2), the reaction product was (Ni 1−x Cu x ) 3 Sn 4 . At high Cu concentrations (x ‫ס‬ 0.7 and 1), the reaction product was (Cu 1−y Ni y ) 6 Sn 5 . When the Cu concentration was in-between (x ‫ס‬ 0.4), both (Ni 1−x Cu x ) 3 Sn 4 and (Cu 1−y Ni y ) 6 Sn 5 , formed. The above findings were rationalized using the Cu-Ni-Sn isotherm. The results of this study imply that the Cu concentration must be strictly controlled in industrial production to produce the desired intermetallic at the interface.The eutectic Sn-Pb solder is the most widely used alloy for wave-soldering printed circuit boards and other electronic products. Environmental concern for the Pb toxicity has propelled the search for a Pb-free replacement. The eutectic Sn-Cu solder (Sn-0.7Cu, wt%) is considered the most promising candidate to replace the eutectic Sn-Pb for wave-soldering applications. 1 The Sn-0.7Cu alloy was also identified as a better alloy for flip-chip soldering compared to Sn-3.5Ag and Sn-3.8Ag-0.7Cu. 2 Moreover, Sn-0.7Cu is among the most inexpensive lead-free replacements. In addition to solder, printed circuit boards and component surface finishes also have to be lead-free. Nickel is used in several leadfree finishes, such as the Au/Ni and Pd/Ni, as a solderable diffusion barrier to prevent the rapid reaction between solder and the Cu conductor. Therefore, the interfacial reactions between Ni and solders have to be studied. Recently, Frear et al. studied the reaction between Sn-0.7Cu and Ni during reflow. 2 A thin layer of intermetallic was present at the interface after reflow, and this layer of intermetallic was reported to be Ni 3 Sn 4 . 2 No other study on the reaction between Sn-0.7Cu and Ni was found in the literature. The first objective of this study is to re-investigate the reaction product between Sn-0.7Cu and Ni in more detail. The second objective is to look into the effect of a small perturbation of Cu concentration on the interfacial reaction.In this study, the reactions between Ni and Sn-xCu solders were examined, with x being 0.2, 0.4, 0.7, and 1. The Sn-xCu solders were prepared from 99.99% pure elements. Nickel disks (6.35 mm diameter × 0.50 mm thick, 99.995% pure) were utilized to react with the solders. Before the reactions, each Ni disk was metallurgically polished and fluxed with a mildly activated rosin flux. Each disk was then reacted with a fresh molten solder bath (6 g) at 250°C. Reaction times were 10 min and 25 h. The compositions of the phases were determined by a JEOL JXA-8800M Electron Probe (...

The electronic industry has been using Pb-bearing solders for interconnection applications for over 50 years. These Pb-bearing solders include alloys such as 63Sn-37Pb (wt %, Sn37Pb), frequently used for soldering printed circuit boards (PCBs), and Sn95Pb, frequently used for chip-to-package connection. There are several pending national and international legislation proposals banning the use of Pb in these solders. The Sn3.9Ag0.6Cu solder is one of the most promising leadfree candidates to replace the Sn37Pb solder. 1 The allowance for concentration variation for each element in this solder is customarily taken to be (0.2 wt %.Nickel is one of the most common metals to be in direct contact with solder during soldering and during the normal life cycle of electronic products. Therefore, the interactions between Ni and Sn3.9Ag0.6Cu solder have to be considered. In the literature, no study on the reaction between Ni and Sn3.9Ag0.6Cu can be found. There are a few studies on the reaction between SnAg solders and Ni, [2][3][4][5] and in these studies it was found

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