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Journal of ELECTRONIC MATERIALS, Vol. 42, No. 2, 2013
DOI: 10.1007/s11664-012-2222-3
2012 TMS
Inuence of Composition on the Morphology of Primary Cu6Sn5 in Sn-4Cu Alloys
STUART MCDONALD,1,3 KAZUHIRO NOGITA,1 JONATHAN READ,1
TINA VENTURA,1 and TETSURO NISHIMURA2
1.Nihon Superior Centre for the Manufacture of Electronic Materials School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. 2.Nihon Superior Co., Ltd., Osaka, Japan. 3.e-mail: [email protected]
Alloys from the composition range Sn-(0.7 wt.% to 7.6 wt.%)Cu consist of primary Cu6Sn5 surrounded by a eutectic Sn-Cu6Sn5 mixture. The primary
Cu6Sn5 intermetallics commonly adopt a coarse elongated morphology, which is not optimal for the mechanical properties of the soldered joint. This paper investigates the effect of trace elemental additions on the size and morphology of the primary Cu6Sn5 in Sn-4 wt.%Cu alloy with and without Ni additions.
Elements investigated include ppm additions of Al, Ag, Ge, and Pb. It is shown that Al has a marked effect on the solder microstructure and renes the size of the primary Cu6Sn5, even at very low addition levels, in both binary Sn-Cu alloys and those containing additional Ni. The effect of Al is conrmed using real-time x-radiographic synchrotron observations of solidication.
Key words: Lead-free solder, Cu6Sn5, grain renement, solidication, synchrotron
INTRODUCTION
Many Pb-free soldering alloys containing both Sn and Cu have a microstructure characterized by large volume fractions of Sn-Cu6Sn5 eutectic or eutectic variants containing these phases. In the composition range of 0.7 wt.% to 7.6 wt.% Cu the liquidus temperature increases from 227C to 415C, respectively, and solidication of the bulk alloy commences with the nucleation and growth of primary Cu6Sn5 intermetallic compounds (IMCs). However, it is generally accepted that large/coarse Cu6Sn5 IMCs have the potential to embrittle the microstructure and can be detrimental to the mechanical properties and reliability of the soldered joint.13
Cu6Sn5 is a relatively brittle phase in bulk solders, and interfacial soldersubstrate IMCs often display visible cracking after thermal cycling.4 This cracking can occur due to volumetric changes experienced during the polymorphic transformation between the hexagonal (high-temperature) and monoclinic (low temperature) Cu6Sn5 crystallographic variants,5
which under equilibrium conditions, occurs at 186C.6 At practical cooling rates, hexagonal Cu6Sn5 is still present at room temperature but can undergo a transformation to monoclinic during typical service...