Reliability of Die Attach on DBC Substrates With Different Ni Surface Finishes Using BiAgX Solder

Ni plating is a commonly used barrier layer for direct bond copper substrates used in high-temperature and high-power applications. The Ni plating method can be electrolytic or electroless and the composition options include pure Ni, Ni:P, Ni-Co, or Ni:B. BiAgX solder paste is a promising lead-free solder for applications below 200 °C. The reactivity of these different Ni layers with the BiAgX solder paste has been compared. Specially the reaction to form NiBi
₃
during 200 °C storage and -55 °C to + 195 °C thermal cycling test. The initial shear strength of all these Ni plating is ~4.8 kg/mm
²
. A degradation in shear strength was observed in further aging. Among them, the electrolytic Ni:P plated samples had the lowest degradation. The reason of degradation is determined to be the formation of NiBi
₃
intermetallic, since it had been found in all the other Ni plating samples beside the electrolytic Ni:P. Similar results were observed in thermal cycling test.