Effect of Gd Content on Wettability on Si₃N₄ Ceramic Substrate of CuSnTi Copper-Based Silver-Free Solder

CuSnTi copper-based silver-free solders with mass fractions of 0, 0.25%, 0.50%, and 1.00% Gd were prepared by centrifugal stirring. The solder paste was dropped on the Si₃N₄ ceramic substrate and was subjected to high-temperature sintering. The effect of Gd content on the wettability of the solder on the Si₃N₄ ceramic substrate was investigated. The results show that with the increase of Gd content in the solder, the wetting angle of solder on the Si₃N₄ ceramic substrate first decreased, and then increased, and the width of the precursor film first increased and then decreased; the wettability first increased and then decreased. With the addition of 0.5% (mass fraction) Gd into the solder, the wetting angle of the solder was the smallest, and the width of the precursor film was the largest, which were 18.84° and 498.68 μm, respectively; the solder joint surface formed on the Si₃N₄ ceramic substrate was smooth and uniform, the width of the precursor film was uniform. The wettability of the solder containing 0.5% (mass fraction) Gd was the best. Gd with a mass fraction not higher than 0.50% promoted the diffusion of Ti in the solder and Ni and Si in Si₃N₄ ceramics, increasing the formation of compounds such as Cu₃Ti, TiN, Ti₅Si₃ and TiSi₂ at the interface, enhancing the thickness and continuity of the intermediate compound layer, and thereby improving the wettability of the solder on the Si₃N₄ ceramic substrate. Excessive Gd deposition on the surface of Si₃N₄ ceramics hindered the contact between Ti and ceramics, reducing the thickness of the intermediate compound layer and thus leading to a decrease in wettability.