Effects of Ni layer thickness of thin-ENEPIG surface finishes on the interfacial reactions and shear strength of Sn-3.0Ag–0.5Cu solder joints during aging

The effects of the thickness of the Ni layer in thin electroless-nickel electroless-palladium immersion gold (thin-ENEPIG) surface-finished printed circuit boards (PCB) with Sn-3.0Ag–0.5Cu (SAC305) solder joints during aging treatment were investigated. We evaluated the interfacial reactions and mechanical properties of 0.3–1.0 µm thick Ni layers in thin-ENEPIG PCBs with SAC305 solder joints at aging temperatures of 75, 100, 125, and 150 °C for 1000 h. A needle-type (Cu,aNi)₆Sn₅ intermetallic compound (IMC) layer formed at the interface of the SAC solder and Cu substrate. In addition, P-rich Ni layers were formed at the interface of the (Cu,Ni)₆Sn₅ IMC and Cu substrate for the 0.5–1.0 µm thick Ni joints. The thicknesses of the (Cu,Ni)₆Sn₅ IMCs with 0.3 and 0.5 µm Ni joints were strongly affected by the aging time and temperature. In contrast, the IMC growth rates of the 0.7 and 1.0 µm thick Ni joints were significantly lower than that of the 0.3 µm thick Ni joint. The reason for this behavior is that the P-rich Ni layer acts as a barrier for Sn and Cu diffusion at the joint and was maintained at the IMC/Cu interface after aging. In high-speed shear tests, the shear strengths of all Ni joints aged at 75 °C were similar. However, the rate of reduction in shear strength decreased with increasing Ni layer thickness as the aging time and temperature increased, but the shear strength of the 0.3 µm Ni joint rapidly decreased with increasing aging temperature and time. In contrast, the shear strengths of the 0.7 and 1.0 µm Ni joints slightly decreased with increasing aging temperature and time. The brittle fracture rates of the joints decreased with increasing Ni thickness, and we determined that this occurred because of the remaining P-rich Ni layer, which acts as a diffusion barrier during aging, at the interface of the 0.7 and 1.0 µm Ni joints. In low-speed shear tests, the shear strengths of the 0.7 and 1.0 µm Ni joints were slightly higher than those of the 0.3 and 0.5 µm Ni joints after aging at 125 and 150 °C for the entire aging period. Most fractures occurred via a ductile mode regardless of the Ni layer thickness. Therefore, a Ni layer thickness of more than 0.7 µm in thin-ENEPIG finished PCB with SAC305 solder joints is expected to yield high reliability on aging.