Retardation of massive spalling by palladium layer addition to surface finish

The reactions between a Sn-3.0Ag-0.5Cu solder alloy and electroless Ni/electroless Pd/immersion Au (ENEPIG) surface finishes with various Pd layer thicknesses (0, 0.05, 0.1, 0.2, 0.4 μm) were examined for the effect of the Pd layer on the massive spalling of the (Cu, Ni)₆Sn₅ layer during reflow at 235°C. The thin layer deposition of an electroless Pd (EP) between the electroless Ni (7 μm) and immersion Au (0.06 μm) plating on the Cu substrate significantly retarded the massive spalling of the (Cu, Ni)₆Sn₅ layer during reflow. Its retarding effect increased with an increasing EP layer thickness. When the EP layer was thin (≤0.1 μm), the retardation of the massive spalling was attributed to a reduced growth rate of the (Cu,Ni)₆Sn₅ layer and thus to a lowered consumption rate of Cu in the bulk solder during reflow. However, when the EP layer was thick (≥0.2 μm), the initially dissolved Pd atoms in the molten solder resettled as (Pd, Ni)Sn₄ precipitates near the solder/(Cu, Ni)₆Sn₅ interface with an increasing reflow time. Since the Pd resettlement requires a continuous Ni supply across the (Cu, Ni)₆Sn₅ layer from the Ni(P) substrate, it suppressed the formation of (Ni, Cu)₃Sn₄ at the (Cu,Ni) ₆Sn₅/Ni(P) interface and retarded the massive spalling of the (Cu, Ni)₆Sn₅ layer.