Kinetics of Solid-State Reactive Diffusion in the (Pd-Ni)/Sn System

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 the alloying of Pd with Ni. To examine such influence of Ni on the compound growth, the kinetics of solid-state reactive diffusion in the (Pd-Ni)/Sn system was experimentally determined in the present study. Experiments were conducted using Sn/(Pd-Ni)/Sn diffusion couples with Ni mol fractions of y = 0.257, 0.505, and 0.746 which were prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed in the temperature range of 433 K to 473 K for various times up to 771 h. During annealing, different compounds are formed as rather uniform layers at the interface in the diffusion couple. In all the annealed diffusion couples, (Pd,Ni)Sn₄ was observed clearly. Furthermore, (Pd,Ni)Sn₃ and (Pd,Ni)Sn₂ were recognized for y = 0.257, and Ni₃Sn₄ was discerned for y = 0.746. However, no other compounds except (Pd,Ni)Sn₄ were detected for y = 0.505. The total thickness of the compound layers is proportional to a power function of the annealing time. The exponent of the power function is rather close to 0.5 for y = 0.257 and 0.505 but smaller than 0.5 for y = 0.746. Thus, volume diffusion is the rate-controlling process of the compound growth for y = 0.257 and 0.505, but boundary diffusion contributes to the rate-controlling process for y = 0.746. At the experimental annealing times, the overall growth rate of the compound layers is insensitive to y at y < 0.5 but decreases monotonically with increasing value of y at y > 0.5. Consequently, the compound growth is actually decelerated by the addition of Ni into Pd with y > 0.5 in the multilayer Pd/Ni/Cu conductor.