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

Abstract

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.