Kinetics of Reactive Diffusion in the (Pd-Cu)/Sn System 
at Solid-State Temperatures
The kinetics of solid-state reactive diffusion in the (Pd-Cu)/Sn system was experimentally observed to examine how adding Cu into Pd influenced the growth behavior of compounds at the junction between the Sn-base solder and the multilayer Pd/Ni/Cu conductor during energization heating. In our experiment, Sn/(Pd-Cu)/Sn diffusion couples with Cu mole fractions (y) of 0.243, 0.497, and 0.735 were isothermally annealed at temperatures of 433 K to 473 K for various times up to 385 h. Annealing caused an intermetallic layer of rather uniform thickness to form between the Sn and Pd-Cu specimens in the diffusion couple. The intermetallic layer consisted of PdSn4, PdSn3, PdSn2, and Cu6Sn5 for y = 0. 243. However, PdSn3 and PdSn2 were not clearly detected for y = 0.497 or 0.735. The total thickness of the intermetallic layer was proportional to a power function of the annealing time, with the exponent of the power function being 0.33 to 0.53. Cases where the exponent was smaller than 0.5 indicated that boundary diffusion controlled the growth of the intermetallic layer. Cases where the exponent was equal to 0.5 implied that volume diffusion was the rate-controlling process. Even for exponents close to 0.5, however, the grain growth behavior of the intermetallic layer suggested that the layer growth was governed by boundary diffusion. For the annealing times tested in this work, the overall growth rate of the intermetallic layer was insensitive to the Cu concentration at y < 0.75, but decreased remarkably with increasing Cu concentration at y > 0.75. As a consequence, the growth of compounds at the Pd junction in the multilayer Pd/Ni/Cu conductor was markedly decelerated by adding Cu when y > 0.75.