2015 Volume 56 Issue 1 Pages 30-39
The growth of compounds during energization heating at the interconnection between a Sn-based solder and a multilayer Pd/Ni/Cu conductor may be inhibited by alloying of Pd with Cr. To examine such influence of Cr on the compound growth, the kinetics of solid-state reactive diffusion in the (Pd–Cr)/Sn system was experimentally determined in the present study. Experiments were conducted using Sn/(Pd–Cr)/Sn diffusion couples with Cr mol fractions of y = 0.110, 0.215 and 0.323 which were prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed in the temperature range of T = 433–473 K for various times up to t = 360 h. During annealing, a compound layer consisting of different matrices and lamellar phases is formed at the original (Pd–Cr)/Sn interface in the diffusion couple. From the Pd–Cr side to the Sn side in the compound layer, the matrices are PdSn2, PdSn3 and PdSn4, and the lamellar phases are (Cr0.9Pd0.1)3Sn, (Cr0.95Pd0.05)2Sn, (Cr0.9Pd0.1)Sn and CrSn2. The total thickness of the compound layer is proportional to a power function of the annealing time. The exponent n of the power function is rather close to 0.5 independent of y at T = 453–473 K. At T = 433 K, however, n ≅ 0.5 for y = 0–0.110, but n = 0.7–0.8 for y = 0.215–0.323. Thus, the overall compound growth is controlled by volume diffusion for y = 0–0.323 at T = 453–473 K and for y = 0–0.110 at T = 433 K. On the other hand, for y = 0.215–0.323 at T = 433 K, the interface reaction at the moving interface contributes to the rate-controlling process. The overall growth rate of the compound layer is insensitive to y at y < yc but monotonically decreases with increasing value of y at y > yc, where yc = 0.1 and 0.2 at T = 433–453 K and 473 K, respectively. Consequently, the compound growth is actually decelerated by the addition of Cr with y > 0.2 into Pd in the multilayer Pd/Ni/Cu conductor.