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

摘要

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 260 deg 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 260 deg 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.