Influence of Die Backside Metallization on the Interfacial Intermetallic Compounds in Lead-Free Solder

Semiconductors are integral to wide array of daily activities like mobile phones and modern automobiles. With rapid advancements in the electronics industry, electronic devices are evolving towards miniaturization. This evolution drives to downsizing the existing semiconductor packages and imposes higher requirements on the quality and reliability of solder joints.

This study focuses on assessing the interfacial reactions between Sn-based solder and die-backside metallizations (BSM), Ti/Ni/Ag and Ti/Al/NiP. The dice with Ti/Al/NiP BSM underwent two processes: increased backside surface roughness and increased backside Aluminum content. They were assembled onto two distinct AMB substrates: Cu-exposed and Ni-plated.

Various intermetallic compounds (IMC) were identified near the die/solder interface. They were characterized using SAM, supplemented by mechanical cross-sectioning technique, SEM, and EDX.

The results show that the Ti/Ni/Ag BSM leads to the formation of fully fragmented IMC. The Ni-plated AMB substrate results in thinner IMCs compared to the Cu-exposed AMB. Increased die-backside roughness leads to a discontinuous IMC layer, while higher Al content promotes a thin IMC with scalloped and needle-like extensions. The SnAg₅ solder results in IMCs with distinct appearance compared to SAC305 due to its initial content of Cu. Finally, thermal cycling induces heterogeneous coarsening of IMCs, and formation of cracks and delamination at the interface and within the solder.

In conclusion, the study reveals that Ti/Al/NiP BSM provided a more reliable IMC. The increased BS roughness does not represent an optimal approach, this contrasts with the increase in Al content which results in a more favorable structure. The Ti/NiV/Ag BSM leads to highly fragmented IMC detached from the interface with the dispersion of plate-like Ag3Sn IMCs which represent a site for strain localization. Finally, the initial Cu content in SAC305 solder governs the IMC crystal growth.