Strong Zn concentration effect on the soldering reactions between Sn-based solders and Cu
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Strong Zn concentration effect on the soldering reactions between Sn-based solders and Cu S.C. Yang, C.E. Ho, and C.W. Chang Department of Chemical & Materials Engineering, National Central University, Jhongli City, Taiwan
C.R. Kaoa) Department of Materials Science & Engineering, National Taiwan University, Taipei, Taiwan (Received 18 May 2006; accepted 12 July 2006)
The acute Zn concentration sensitivity of the reaction between Sn-based solders and Cu substrate is reported and explained in this article. Three Sn-xZn solders (x ⳱ 0.5, 0.7, and 2 wt%) were reacted with Cu substrates at 250 °C for 2–10 min. A slight variation in the Zn concentration changed the reaction product formed at the interface. When the Zn concentration was low (x ⳱ 0.5 wt%), the reaction product was Cu6Sn5. When the Zn concentration was slightly increased to 2 wt%, the reaction product became Cu5Zn8. When Zn concentration was in-between (x ⳱ 0.7 wt%), Cu6Sn5 and CuZn co-existed. The above findings are explained using the Cu–Sn–Zn phase diagram. The implication is that the type of compound forms at the interface can be controlled by adjusting the Zn concentration of the Sn-based solders. The SnAgCu family of solders has firmly established itself as the leading replacement for the eutectic PbSn solder for electronic applications. At this stage, main research efforts in lead-free solder development are focused on adding minor alloy elements to enhance or to fine-tune the various properties of the SnAgCu solders. For example, Mn, Ni, Ge, Ti, Si, Cr, and Zn had been evaluated for their potential for suppressing the void coalescence during the reactions between solders and Cu substrate.1 The most noteworthy alloying element is Zn. Adding Zn had been shown to improve the creep resistance of SnAgCu.2 The addition of Zn could also restrain the formation of large Ag3Sn plates.3 Recently, Kang et al.4 pointed out minor Zn additions (0.1 and 0.7 wt%) into SnAgCu solders could suppress the growth of Cu3Sn, which was believed to be responsible for the formation of Kirkendall voids over the Cu substrates. It was proposed that the suppression of the Cu3Sn growth was probably associated with the accumulation of Zn at a)
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs.org/ jmr_policy DOI: 10.1557/JMR.2006.0320 2436
J. Mater. Res., Vol. 21, No. 10, Oct 2006
the Cu3Sn/Cu interface.4 In the literature, it had also been reported that Zn could in fact completely inhibit the formation of both Cu3Sn and Cu6Sn5, and produced Cu–Zn compounds instead.5 In short, more studies on the Zn effect are needed. The objective of this study was to investigate this Zn effect in detail. In this study, the reactions between Cu substrates and the Sn-xZn solders with three different Zn concentrations (x ⳱ 0.5, 0.7, and 2.0 wt%) were investigated. No Cu or Ag was added t
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