Quantifying the dependence of Ni(P) thickness in ultrathin-ENEPIG metallization on the growth of Cu–Sn intermetallic compounds in soldering reaction

• Microstructures in ultrathin-ENEPIG with various Ni(P) thickness are investigated.
• P-rich IMC layer formed when the Ni(P) thickness is greater than 0.18 μm.
• Secondary (Cu,Ni)6Sn5 formed when the Ni(P) thickness is between 0.18 and 0.31 μm.
• Cu diffusion flux without P-rich layer is larger than those with P-rich layer.
• P-rich layer in ultrathin-ENEPIG exhibits good diffusion barrier characteristic.

A new multilayer metallization, ENEPIG (Electroless Ni(P)/Electroless Pd/Immersion Au) with ultrathin Ni(P) deposit (ultrathin-ENEPIG), was designed to be used in high frequency electronic packaging in this study because of its ultra-low electrical impedance. Sequential interfacial microstructures of commercial Sn–3.0Ag–0.5Cu solders reflowed on ultarthin-ENEPIG with Ni(P) deposit thickness ranged from 4.79 μm to 0.05 μm were first investigated. Accelerated thermal aging test was then conducted to evaluate the long-term thermal stabilization of solder joints. The results showed that P-rich intermetallic compound (IMC) layer formed when the Ni(P) thickness was greater than a critical vale (about 0.18 μm). Besides, it is interesting to mention that the growth of (Cu,Ni)6Sn5 and (Cu,Ni)3Sn IMCs was suppressed with the formation of P-rich layer, i.e., Ni3P and Ni2Sn1+xP1−x phase, even though the electroless-plated Ni(P) layer was exhausted at initial stage of reflow process. The atomic Cu flux in solder joints without P-rich layer was calculated to be several times larger than that with P-rich layer formation after calculation, which implies that the P-rich layer and ultrathin Ni(P) deposit in ENEPIG served as diffusion barrier against rapid Cu diffusion.