Improving thermal shock response of interfacial IMCs in Sn–Ag–Cu joints by using ultrathin-Ni/Pd/Au metallization in 3D-IC packages

The feasibility of novel ultrathin-Electroless Ni/Electroless Pd/Immersion Au (ultrathin-ENEPIG) metallization containing ultrathin electroless Ni layer for 3D-IC packaging application was evaluated by thermal shock test in this study. Via employing ultrathin-ENEPIG pad, the joints showed higher resistance against thermomechanical stress due to minor degradation in solder and the strength enhancement of interfacial IMC. In the as-fabricated joints, larger Sn grain formed in the ultrathin-ENEPIG joints in contrast to those in the conventional-ENEPIG one, which caused the distinct failure modes between two different systems under the test. Additionally, a dual-layer structure of high/low Ni–(Cu,Ni)₆Sn₅ and a single layer of low Ni–(Cu,Ni)₆Sn₅ could be observed in the conventional-ENEPIG and the ultrathin-ENEPIG joints, respectively, after testing. The dual layer (Cu,Ni)₆Sn₅ degraded the thermal shock performance of conventional-ENEPIG joints as a result of the weak bonding of interface between high/low Ni–(Cu,Ni)₆Sn₅ layers. However, through employment of ultrathin-ENEPIG substrates, the electroless Ni layer was completely exhausted and thus only one layer of low Ni–(Cu,Ni)₆Sn₅ intermetallic formed in the ultrathin-ENEPIG joints. The formation of dual-layer compound was suppressed and the fracture between layers was inhibited, leading to a stronger bonding at interface. Furthermore, the overall thickness of interfacial intermetallic compound was also reduced due to the suppression of high Ni–(Cu,Ni)₆Sn₅ layer. Influences of Ni thickness on the related mechanisms behind dual-layer intermetallic suppression and the Sn grain structure in ultrathin-ENEPIG joints were addressed and discussed in details.