Solder Volume Effect on the Microstructure of Solder Joints Using Au/Pd(P)/Ni(P) Surface Finish

The liquid–solid reaction between Sn3Ag0.5Cu and Au/Pd(P)/Ni(P) tri–layer with three different joint diameters (djoint), 200 μm, 150 μm, and 60 μm, were investigated in this study. The reaction product(s) strongly depended on djoint. In the case of djoint = 200 μm, the predominant product after reaction at 245oC for 80 s was a continuous (Cu,Ni)6Sn5. When djoint slightly reduced to 150 μm, a mixture of (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 grew at the interface. When djoint further reduced to 60 μm, the (Cu,Ni)6Sn5 was completely replaced by (Ni,Cu)3Sn4. Moreover, a significant amount of isolated (Pd,Ni)Sn4 domains scattered inside the solder matrix near the interface. The transition of the microstructure was attributed to the decrease of Cu concentration (CCu) and increase of Pd concentration (CPd) as djoint reduced. This finding suggests that the solder volume is an important factor in determining the microstructure of solder joints.