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Long-term, solid-state intermetallic compound (IMC) layer growth was examined in 95.5Sn-3.9Ag-0.6Cu (wt.%)/copper (Cu) couples. Aging temperatures and times ranged from 70°C to 205°C and from 1 day to 400 days, respectively. The IMC layer thicknesses and compositions were compared to those investigated in 96.5Sn-3.5Ag/Cu, 95.5Sn-0.5Ag-4.0Cu/Cu, and 100Sn/Cu couples. The nominal Cu 3 Sn and Cu 6 Sn 5 stoichiometries were observed. The Cu 3 Sn layer accounted for 0.4-0.6 of the total IMC layer thickness. The 95.5Sn-3.9Ag-0.6Cu/Cu couples exhibited porosity development at the Cu 3 Sn/Cu interface and in the Cu 3 Sn layer as well as localized "plumes" of accelerated Cu 3 Sn growth into the Cu substrate when aged at 205°C and t Ͼ 150 days. An excess of 3-5at.%Cu in the near-interface solder field likely contributed to IMC layer growth. The growth kinetics of the IMC layer in 95.5Sn-3.9Ag-0.6Cu/Cu couples were described by the equation x ϭ x o ϩ At n exp [Ϫ∆H/RT]. The time exponents, n, were 0.56 Ϯ 0.06, 0.54 Ϯ 0.07, and 0.58 Ϯ 0.07 for the Cu 3 Sn layer, the Cu 6 Sn 5 , and the total layer, respectively, indicating a diffusion-based mechanism. The apparent-activation energies (∆H) were Cu 3 Sn layer: 50 Ϯ 6 kJ/mol; Cu 6 Sn 5 layer: 44 Ϯ 4 kJ/mol; and total layer: 50 Ϯ 4 kJ/mol, which suggested a fast-diffusion path along grain boundaries. The kinetics of Cu 3 Sn growth were sensitive to the Pb-free solder composition while those of Cu 6 Sn 5 layer growth were not so.

The compression creep behavior was studied for the ternary solder alloy 95.5Sn-3.9Ag-0.6Cu in the as-cast condition. Samples were tested under stresses of 2-45 MPa and temperatures of Ϫ25-160°C. There was a significant variability in the creep curve shape, strain magnitude, and steady-state strainrate properties. A multivariable linear-regression analysis of the steady-state strain-rate data, using the sinh-law stress representation, indicated two mechanisms distinguished by low-and high-temperature regimes of Ϫ25-75°C and 75-160°C, respectively. The sinh-law stress exponent (n) and apparentactivation energy (∆H) in the Ϫ25-75°C regime were 4.4 Ϯ 0.7 kJ/mol and 25 Ϯ 7 kJ/mol (63% confidence intervals), respectively. Those same parameters in the 75-160°C regime were 5.2 Ϯ 0.8 kJ/mol and 95 Ϯ 14 kJ/mol, respectively, for the high-temperature regime. The values of ∆H suggested a short-circuit diffusion mechanism at low temperatures and a lattice or bulk-diffusion mechanism at high temperatures. The stress dependency of the steady-state strain rate did not indicate a strong power-law breakdown behavior or a threshold stress phenomenon. Cracks and grain-boundary sliding were not observed in any of the samples. As the creep temperature increased, a coarsened particle boundary and particle depletion zone formed in the region of fine Ag 3 Sn particles that existed between the Sn-rich phase areas. The coarsened particle boundary, as well as accelerated coarsening of Ag 3 Sn particles, were direct consequences of the creep deformation process.

Compression creep tests were performed on the 95.5Sn-3.9Ag-0.6Cu (wt.%) solder. The specimens were aged prior to testing at 125°C, 24 h or 150°C, 24 h. Applied stresses were 2-40 MPa. Test temperatures were Ϫ25°C to 160°C. The 125°C, 24-h aging treatment caused the formation of coarsened Ag 3 Sn particle boundaries within the larger ternary-eutectic regions. The 150°C, 24-h aging treatment resulted in contiguous Ag 3 Sn boundaries in the ternary-eutectic regions as well as a general coarsening of Ag 3 Sn particles. The 125°C, 24-h aging treatment had only a small effect on the strain-time curves vis-à-vis the as-cast condition. Negative creep was observed at 75°C for time periods Ͼ10 5 sec and stresses of 3-10 MPa. The creep kinetics exhibited a sinh term (stress) exponent, p, of 5.3 Ϯ 0.6 and an apparent-activation energy, ∆H, of 49 Ϯ 5 kJ/mol when data from all test temperatures were included. A good data correlation was observed over the [Ϫ25-125°C] temperature regime. Steady-state creep kinetics exhibited a greater variability in the [125-160°C] regime because of the simultaneous coarsening of Ag 3 Sn particles. The aging treatment of 150°C for 24 h resulted in a more consistent stress dependence and better reproducibility of the creep curves. Negative creep was observed in samples aged at 150°C for 24 h when tested at Ϫ25°C, 25°C, and 75°C. The values of p and ∆H were 4.9 Ϯ 0.3 kJ/mol and 6 Ϯ 5 kJ/mol, respectively. Only a slight improvement in the data correlation was observed when the analysis examined separated [Ϫ25-75°C] and [75-166°C] temperature regimes. Creep testing did not cause observable deformation in any of the sample microstructures.