AN INVESTIGATION ON THE INFLUENCE OF COPPER AND NICKEL SOLDERABLE SURFACES ON SOLDER JOINT DEGRADATION DUE TO GOLD EMBRITTLEMENT

摘要

The potential formation of the brittle AuSn_4 gold-tin intermetallic phase compound (IMC) that can result from soldering to gold surface finishes has been well understood in tin-lead soldering processes for many years. The use of gold printed circuit board finishes, gold-plated components and the implementation of lead-free soldering process have led to a renewed interest in the cause-and-effect of solder joint degradation due to gold embrittlement. This paper documents an investigation into the influence of soldering to copper or nickel surfaces on the formation and distribution of the AuSn_4 IMC using thermal cycling and shear testing. The solder joints of several resistor sizes were tested on printed circuit boards plated with gold from 10 to 60 microinches thick, with solder joint gold content from 2 to 6 weight%. SAC305 lead-free solder alloy was used in the testing. Gold content in the solder joints was determined by theoretical calculation and by scanning electron microscopy analysis. The investigation results revealed that the AuSn_4 diffused to the nickel layer of the nickel-plated samples creating a continuous (Au,Ni)Sn_4 deposit. In the copper-base samples, AuSn_4 remained in the solder joint microstructure. In both cases, at these gold concentrations, the shear testing and thermal cycle-induced solder joint failures were not influenced by the AuSn_4 IMC particles or layers.