On the 3-D Shape of Interlaced Regions in Sn-3Ag-0.5Cu Solder Balls
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https://doi.org/10.1007/s11664-020-08508-w 2020 The Author(s)
TMS2020 MICROELECTRONIC PACKAGING, INTERCONNECT, AND PB-FREE SOLDER
On the 3-D Shape of Interlaced Regions in Sn-3Ag-0.5Cu Solder Balls A.A. DASZKI
1,2
and C.M. GOURLAY1
1.—Department of Materials, Imperial College London, London SW7 2AZ, UK. 2.—e-mail: [email protected]
The microstructure of Sn-Ag-Cu (SAC) solder joints plays an important role in the reliability of electronics, and interlaced twinning has been linked with improved performance. Here, we study the three-dimensional (3-D) shape of interlaced regions in Sn-3.0Ag-0.5Cu (SAC305) solder balls by combining serial sectioning with electron backscatter diffraction. In solder balls without large Ag3Sn plates, we show that the interlaced volume can be reasonably approximated as a hollow double cone with the common h100i twinning axis as the cone axis, and the h110i from all three twinned orientations making up the cone sides. This 3-D morphology can explain a range of partially interlaced morphologies in past work on 2-D cross-sections. Key words: Lead-free solder, interlacing, microstructure, electron backscatter diffraction (EBSD), twinning
INTRODUCTION bSn is the main solid phase in most electronic solder joints,1 and dictates much of the electrical and mechanical properties of joints. The bodycentered tetragonal (bct) structure is highly anisotropic. For example, the coefficient of thermal expansion2,3 and solute diffusivity4 vary between the a-direction and the c-direction of bSn. In leadfree solder joints, bSn usually presents only a few grain orientations, with single grain joints5–7 and three grain morphologies1,5,8–11 frequently reported in Sn-Ag-Cu (SAC) solders. The three orientations of bSn are related via cyclic twinning around a common h100i direction,1,6 and seem to come from a single nucleation event. These cyclic twin microstructures have been reported as either a beach-ball or interlacing morphology, or a mixture of the two.9,12 Electron backscatter diffraction (EBSD) techniques have been successfully used to identify crystal orientations and relationships
(Received July 16, 2020; accepted September 21, 2020)
between bSn grains, such as the common twinning axis.1,7,13,14 A SAC solder joint with an interlaced microstructure, composed of many small bSn grains, shows increased mechanical properties, such as hardness and creep,15 as well as better reliability compared to a beach-ball microstructure, by delaying crack initiation.16 Furthermore, a solder joint of a single grain will have anisotropic mechanical properties, corresponding to bSn anisotropy,17 and can result in early joint failure for certain grain orientations. The ability to control or predict interlacing in solder joints could therefore be beneficial in improving their reliability. Lehman et al.1 proposed that the beach-ball morphology comes from {101} cyclic twinning, and that the interlaced morphology comes from {301} cyclic twinning. However, many SAC microstructures have an interlaced morphology
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