Effect of Pd Thickness on the Interfacial Reaction and Shear Strength in Solder Joints Between Sn-3.0Ag-0.5Cu Solder and Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) Surface Finish

Intermetallic compound formation at the interface between Sn-3.0Ag-0.5Cu (SAC) solders and electroless nickel/electroless palladium/immersion gold (ENEPIG) surface finish and the mechanical strength of the solder joints were investigated at various Pd thicknesses (0 mu m to 0.5 mu m). The solder joints were fabricated on the ENEPIG surface finish with SAC solder via reflow soldering under various conditions. The (Cu,Ni)(6)Sn-5 phase formed at the SAC/ENEPIG interface after reflow in all samples. When samples were reflowed at 260A degrees C for 5 s, only (Cu,Ni)(6)Sn-5 was observed at the solder interfaces in samples with Pd thicknesses of 0.05 mu m or less. However, the (Pd,Ni)Sn-4 phase formed on (Cu,Ni)(6)Sn-5 when the Pd thickness increased to 0.1 mu m or greater. A thick and continuous (Pd,Ni)Sn-4 layer formed over the (Cu,Ni)(6)Sn-5 layer, especially when the Pd thickness was 0.3 mu m or greater. High-speed ball shear test results showed that the interfacial strengths of the SAC/ENEPIG solder joints decreased under high strain rate due to weak interfacial fracture between (Pd,Ni)Sn-4 and (Cu,Ni)(6)Sn-5 interfaces when the Pd thickness was greater than 0.3 mu m. In the samples reflowed at 260A degrees C for 20 s, only (Cu,Ni)(6)Sn-5 formed at the solder interfaces and the (Pd,Ni)Sn-4 phase was not observed in the solder interfaces, regardless of Pd thickness. The shear strength of the SAC/ENIG solder joints was the lowest of the joints, and the mechanical strength of the SAC/ENEPIG solder joints was enhanced as the Pd thickness increased to 0.1 mu m and maintained a nearly constant value when the Pd thickness was greater than 0.1 mu m. No adverse effect on the shear strength values was observed due to the interfacial fracture between (Pd,Ni)Sn-4 and (Cu,Ni)(6)Sn-5 since the (Pd,Ni)Sn-4 phase was already separated from the (Cu,Ni)(6)Sn-5 interface. These results indicate that the interfacial microstructures and mechanical strength of solder joints strongly depend on the Pd thickness and reflow conditions.