ICSR (Soldering and Reliability) Conference Proceedings

Assessment of Microalloyed Sn-Zn as a Solder for Electronics Assembly

Authors: Keith Howell, Keith Sweatman, Takashi Nozu, Tetsuro Nishimura
Company: Nihon Superior Co., Ltd.
Date Published: 5/4/2011   Conference: ICSR (Soldering and Reliability)

Seika Machinery, Inc.

Abstract: Given the advantages of reduced damage to components and laminate and energy savings that could accrue as a result of a melting point 20ºC lower than the liquidus temperature of SAC305, the tin-9% zinc eutectic alloy has been attracting the attention of the electronics industry as a potential lead-free solder. A raw material cost that is approximately half that of SAC305 provides further motivation for finding a way of using this alloy in electronic assembly. Other advantages that zinc has over silver as an alloying addition to tin are that it is an abundant element of low toxicity so that a tin-zinc solder is more sustainable and has a lower environmental impact than SAC alloys. In the non-electronic applications in which tin-zinc solders have long been used, such as the joining of high voltage aluminum cables, their mechanical properties have been found more than adequate. However, the tin-zinc eutectic has found only limited usage as a solder in electronic assemblies because of early failures on copper substrates where the substantial difference in electronegative potential drives galvanic corrosion in the presence of even quite low levels of moisture and ionic contaminants.

If solders based upon the tin-zinc eutectic are to find wider application the problem of corrosion will have to be addressed and in this paper the authors report on a detailed assessment of the effect of a microalloying addition of manganese on the mechanical integrity of tin-zinc alloy joints on copper, nickel, and silver substrates after exposure to elevated temperature (150°C) and humid heat (85°C/85%RH) for up to 1000 hours. The nature and extent of corrosion on the surface and the solder/substrate interface was examined using SEM/EDX and found to vary considerably with the substrate. Other properties of the tinzinc solder relevant to its performance in thermal cycling, coefficient of thermal expansion, elastic modulus and flow stress were also measured. Evidence of some inhibition of corrosion and mechanical properties conducive to reliability under conditions of thermal cycling appear to justify further study of microalloyed eutectic and near-eutectic tin-zinc alloys as potential lead-free solders for use in electrical and electronic assembly.

Key words: Lead-free, Sn-Zn, Corrosion

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