Modern Rhenium-Modified Ni-P Surface Finishes: Wetting and Interfacial Reactions with Sn-3.0Ag-0.5Cu Solder under Thermal Cycling Simulation

Abstract

The transition to lead-free soldering technology has driven significant research into advanced finishing layers with improved thermal stability and controlled interfacial reactivity. ENIG PCB finish is widely used in electronics, particularly in surface-mount technology for printed circuit boards. In the present work, previous studies on the reactivity of electroless Ni-P-Re finishing layers with varying phosphorus content were continued, this time using commercially available SAC305 solder (Sn-3.0Ag-0.5Cu). The addition of rhenium is expected to suppress or eliminate the massive detachment of intermetallic phases from the interface and their migration into the solder. To this end, three surface finish variants with different phosphorus content but constant rhenium content were deposited. The wettability of both Ni-P and Ni-P-Re finishing layers with SAC305 alloy was investigated using the sessile drop method. The obtained systems were subsequently subjected to cyclic heating and cooling to simulate typical operating conditions of electronic devices. The finishing layers, as well as the surface finish/solder systems before and after thermal cycling, were characterized using scanning and transmission electron microscopy together with energy-dispersive X-ray spectroscopy to evaluate microstructural and compositional changes. The results revealed a significant influence of both phosphorus content and rhenium addition on the sequence of interfacial phase formation and on the fraction of (Cu,Ni)₆Sn₅ phase spalling into the solder volume. The presence of both elements strongly affected the formation and thickness of the undesirable Ni₂SnP phase at the interface, both immediately after the wetting test and after thermal cycling.