Effect of Zn, Au, and In on the polymorphic phase transformation in Cu 6 Sn 5 intermetallics
- PDF / 255,180 Bytes
- 6 Pages / 584.957 x 782.986 pts Page_size
- 15 Downloads / 187 Views
Qinfen Gu Powder Diffraction Beamline, The Australian Synchrotron, Clayton, Victoria 3168, Australia
Kazuhiro Nogita Nihon Superior Centre for the Manufacture of Electronic Materials (CMEM), School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia (Received 4 April 2012; accepted 26 June 2012)
Cu6Sn5 is a critical intermetallic compound in soldering operations. Conventional equilibrium phase diagrams show that this compound is of either a hexagonal or monoclinic structure at temperatures above and below 186 °C, respectively. Under nonequilibrium conditions, the crystal structure is dependent on composition, temperature, and processing history. The effect of Zn, Au, and In on the hexagonal to monoclinic polymorphic transformation in Cu6Sn5 intermetallics is investigated using variable temperature synchrotron powder x-ray diffraction and differential scanning calorimetry. It is revealed that, as in the case of trace Ni additions, the alloying elements Zn and Au completely stabilize the hexagonal Cu6Sn5 and prevent the phase transformation. In contrast, In additions only partially stabilize the hexagonal Cu6Sn5. I. INTRODUCTION
Cu6Sn5 is an important intermetallic compound in leadfree soldering alloys and solder–substrate interfacial microstructures.1 Cu6Sn5 exists in at least two crystal structures in the solid state, with a polymorphic transformation from hexagonal Cu6Sn5 (space group P63/mmc) to monoclinic (space group C2/c) Cu6Sn5 at an equilibrium temperature of 186 °C.1–8 This transformation is considered to be an important factor influencing the reliability of the solder joint. Under nonequilibrium conditions, the crystal structure is dependent on composition, temperature, and processing history. During soldering and subsequent cooling to room temperature (RT) at typical rates, there is insufficient time for the hexagonal to monoclinic transformation and the high temperature hexagonal Cu6Sn5 remains as a metastable phase.9 However, at higher temperatures, it has been found that the transformation can proceed in relatively quick time frames1,10 (e.g., 100 s at 160 °C), and using synchrotron powder x-ray diffraction (PXRD), a time temperature transformation diagram has been established.1 The temperatures involved mean that the transformation is highly possible in electronic devices, which experience even short exposures to elevated temperatures. This can have implications for reliability, as the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.247 J. Mater. Res., Vol. 27, No. 20, Oct 28, 2012
monoclinic to hexagonal transformation of Cu6Sn5 results in a 2% volume expansion,11,12 which could result in crack initiation and/or propagation.1,11 It has been proposed that stabilizing the crystal structure of Cu6Sn5 could improve the joint strength and reliability.1,3,9 While the interfacial reactions and products of leadfree soldering have been the focus of recent research, only a few studies have dealt with the stabil
Data Loading...
