Abstract
Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) surface finish has been developed to overcome the “black pad” issue of Electroless Nickel/Immersion Gold (ENIG) recently. After reflow, the electroless Pd layer of ENEPIG would dissolve into solder matrix, which might reduce the growth of IMCs. However, the Pd distribution and detailed mechanism how Pd influences the interfacial reaction and reliability of solder joints are not yet clear. In the study, Sn3.0Ag0.5Cu (SAC305) solder doped with 0~0.5 wt.% Pd was used to reflowed with ENIG substrate. Before aging, most Pd atoms would dissolve in eutectic phase. After aging, Pd concentrated in Cu6Sn5 and the maximal solubility was around 0.16 at.%. In addition, nanoindentation testing revealed that the Pd doping would soften Cu6Sn5 phase, and then influence the hardness of SAC305-xPd solder. In the solid reaction of SAC305-xPd solder and ENIG substrate, Ni dissolved into solder and (Cu,Ni)6Sn5 formed at the interface. As the Pd concentration increased in the solder, the formation and growth of (Cu,Ni)6Sn5 were suppressed. After thermal aging, two types of (Cu,Ni)6Sn5 IMC, i.e. high Ni (H) and low Ni (L) were observed at the IX SAC305-xPd/ENIG interface. As compared to H-(Cu,Ni)6Sn5, more Pd dissolved in the L-(Cu,Ni)6Sn5. Besides, Pd doping enhanced the growth of H-(Cu,Ni)6Sn5 and slowed down the formation of L-(Cu,Ni)6Sn5, which would stabilize the IMCs. Based on the quantitative analysis by field emission electron probe microanalyzer (FE-EPMA), the correlation between Pd doping and interfacial reaction in solder joints was probed and discussed. In the liquid reaction of SAC305-xPd solder and ENIG substrate, Ni3P and Ni3SnP phases were observed between the (Cu,Ni)6Sn5 and Ni substrate. With the increased reflow time, the IMCs grew rapidly, especially the Ni3P layer. When Pd was added into the solder, not only the formation of interfacial (Cu,Ni)6Sn5 was restrained, but also the Ni diffusion from ENIG substrate was delayed. To evaluate the Pd effect on the reliability, the impact test was employed. The impact test showed that the Pd doping would increase the bonding strength, which was due to the reduced Ni3P thickness. The role of Pd in the solder joint reliability was addressed and proposed. This study aimed to evaluate the potential application of novel Pd-doped lead-free solders for future solder designs.