Synergetic effect of Bi₂Te₃ alloys and electrodeposition of Ni for interfacial reactions at solder/Ni/Bi₂Te₃ joints

Ni is the most commonly used barrier in thermoelectric solder joints, and its role is to avoid excessive reaction between the bismuth telluride (Bi₂Te₃) element and the Sn-based solder. This study, for the first time, demonstrates a strong effect of the Bi₂Te₃ substrate on the electrodeposition of Ni layer, consequently affecting the interfacial reaction with the Sn-4 wt.% Ag-0.5 wt% Cu (SAC405) solder. Two types of Bi₂Te₃ substrates, prepared using zone-melting and ball-milling/hot-pressing methods, exhibit different crystal structures, and this structural difference significantly affects the microstructures of the Ni diffusion barrier electroplated on the Bi₂Te₃ substrates and the growth of their intermetallic compound (IMC) at the SAC405/Ni/Bi₂Te₃ solder joints. The Ni layer electroplated on a zone-melted Bi₂Te₃ substrate (anisotropic crystal structure) has a small grain size and a high grain-boundary density, and one IMC, (Cu,Ni)₆Sn₅, is formed at the interface between SAC405 and Ni after joining. The other (Ni,Cu)₃Sn₄ phase is formed at the (Cu,Ni)₆Sn₅/Ni interface after thermal aging and grows at a progressively faster rate with increasing aging time. In contrast, the Ni layer electroplated on a ball-milled/hot-pressed Bi₂Te₃ substrate (isotropic crystal structure) has a large grain size and a low grain-boundary density, and only the (Cu,Ni)₆Sn₅ phase is formed at the SAC405/Ni interface, with a very sluggish growth rate after joining and thermal aging. Since the IMCs are electrically resistive and mechanically brittle, the latter solder joint with a large-grained Ni layer on a ball-milled/hot-pressed Bi₂Te₃ substrate appears to have better reliability.