Early-Stage Interfacial Intermetallic Growth and Local Micromechanical Response of Sn-3.0Ag-0.5Cu Joints on Immersion-Tin-Finished Cu during Thermal Cycling
DOI:
https://doi.org/10.66514/ssst34-2-150-164Keywords:
Electronic packaging, immersion tin, intermetallic compound, nanoindentation, SAC305Abstract
Sn-3.0Ag-0.5Cu (SAC305) solder joints were reflowed on immersion-tin (ImSn)-finished Cu pads. The joints were then cycled between 0 and 100 °C for 0, 100, 200 and 300 cycles. The aim was to characterize the earliest stage of interfacial and matrix evolution before macroscopic damage develops. Optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDS) and depth-sensing nanoindentation were used to characterize the samples. The mean thickness of the interfacial intermetallic compound (IMC) layer increased from 3.76 μm in the as-reflowed state to 3.98, 4.05 and 4.37 μm after 100, 200 and 300 cycles, respectively. This is a net increase of 16.2%. Over this early interval a linear dependence on cycle number described the data better (R2 = 0.95) than a parabolic, diffusion-controlled law (R2 = 0.85). The growth rate was about 1.9 nm per cycle. EDS of the interfacial band was 48.8 at.% Cu and 51.2 at.% Sn. This composition is consistent with Cu6Sn5 as the dominant reaction product. After 300 cycles, the hardness of the solder matrix decreased by 12.0%, from 0.221 to 0.195 GPa. The hardness of the solder adjacent to the IMC decreased by 7.2%, from 0.224 to 0.208 GPa. The near-interface solder remained harder than the matrix in both states. Both reductions agree within 1% with those predicted from the increase in maximum penetration depth. The reduced modulus scattered more than the difference between conditions. This scatter is consistent with β-Sn elastic anisotropy and indentation pile-up (hf/hmax ≥ 0.95). The results show that mild cycling thickens the Cu-Sn interfacial layer and softens the adjacent solder at the same time. Consequently, the local strength mismatch across the joint increases well before any crack is detectable.
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Copyright (c) 2026 Maria Abu Bakar, Azman Jalar, Mohamad Riduwan Ramli, Norliza Ismail (Author)
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