Retardation of Massive Spalling by Palladium Layer Addition to Surface Finish

Abstract

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. (Received June 29, 2010)