Development of Electroless Nickel-Iron Plating Process for Microelectronic Applications

Electroless nickel-phosphor (Ni-P) plating has been extensively used as a diffusion barrier layer in printed circuit board applications. However, the interfacial reactions between electroless Ni-P and lead (Pb)-free Sn solders during high temperature reflow are known to dissolve Ni-P excessively into Sn leaving behind a brittle Ni3P compound at the interface. This often causes various reliability issues in the solder joints. To mitigate this problem, an application of NiFe alloy is proposed with the concentration of iron (Fe) greater than 20% in the Ni barrier. The NiFe alloy produces a more robust diffusion barrier in the microelectronic applications. Development of a stable electroless NiFe plating process is therefore essential for its successful implementation. In this work, we developed an electroless NiFe alloy plating process with a high deposition rate and superior bath stability. The deposition rate of three micron per hour was achieved and five weeks of bath aging did not deteriorate the bath performance. Stable iron content around 30% in NiFe deposit was achieved in this timeframe. Factors that impact deposition rate and alloy composition were identified. The valence of the iron ions in the electroless NiFe bath was found to have a significant impact on the bath stability. The interfacial reaction between Pb-free SnAgCu solder and electroless NiFe(P) is discussed and compared with the electroless Ni-P process. Energy dispersed X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) were utilized to characterize electroless NiFe deposits and to analyze the interfacial reactions between Pb-free solder and electroless Ni-P or NiFe after multiple reflows.