The Intermediate Barrier Performance of Electroless Co-W-B / Ni-P Stacked Deposits Between Cu and Solder Joint

ABSTRACT Nickel (Ni) is one of the most common surface finishing materials for solder joint and wire bonding because it can behave as the diffusion barrier for Copper (Cu) and bring excellent reliability. Electroless Ni plating has many advantages such as high productivity, good thickness uniformity, and the deposition ability for isolated patterns of printed circuit boards without supplying electrolytic power. Electroless Ni plating is widely used as the jointing material for Tin (Sn) or Sn alloy solder in printed circuit boards, lead frames, and so on. The amorphous deposits of electroless Nickel-Phosphorus (Ni-P) generate the intermetallic compounds (IMC) with solder after the reflow and the remaining Ni behaves as the barrier between Cu and solder. In this time, the P amount of Ni-P deposits generally have the impact to the growth of IMC. Electroless Ni-P with high P (high P Ni) such as 10-12 wt% has been selected to achieve both corrosion resistance and solder joint reliability. Because the growth of Sn-Ni IMC between high P Ni is faster than deposits with lower P, the target thickness of high P Ni has been set to approximately 10 μm to sustain the Ni-P layer as the barrier layer between Cu and solder during reflow. However, if the Ni-Sn IMC is repeatedly formed and dissolved by the diffusion after solder jointing, the Ni-P layer reduces and finally disappears. As a result, an additional barrier layer is desirable for higher reliability. In this study, we investigated adding electroless Cobalt-Tungsten-Boron (Co-W-B) as a new intermediate barrier between Cu and solder jointing. We evaluated the bonding strength and IMC analysis after mounting Sn-Cu-Ni-P type solder on Cu/CoW-B/Ni-P by using different reflow profiles. Results showed that Co-W-B 0.1-0.2 μm/Ni-P 1-2 μm stacked deposits had excellent bonding strength even though the Ni-P thickness is thinner such as 1 μm. After mounting solder, a P-rich layer was generated on the Co-W-B layer in cross-sectional Transmission Electron Microscope (TEM) observation. Scanning Transmission Electron Microscope (STEM) –Energy Dispersive X-ray Spectroscopy (EDS, STEM-EDS) analysis showed the P-rich layer and Co-W-B layer prevented diffusion between solder and Cu. Furthermore, even if after the thermal loading of high temperature, the bonding strength was excellent. Additionally, the cross-section analysis of STEM-EDS revealed that a thin layer composed of Ni, Co and P was formed between Co-W-B and the P-rich layer. This thin layer also might behave as the barrier for solder. In conclusion, Co-W-B / Ni-P stacked deposits exhibited excellent reliability as the additional intermediate barrier layer between Cu and solder.