Study of interfacial reactions in Sn–3.5Ag–3.0Bi and Sn–8.0Zn–3.0Bi sandwich structure solder joint with Ni(P)/Cu metallization on Cu substrate

In this paper, the coupling effect in Sn–3.5Ag–3.0Bi and Sn–8.0Zn–3.0Bi solder joint with sandwich structure by long time reflow soldering was studied. It was found that the interfacial compound at the Cu substrate was binary Cu–Sn compound in Sn–Ag–Bi solder joint and Cu 5Zn 8 phase in Sn–Zn–Bi solder joint. The thickness of the Cu–Zn compound layer formed at the Cu substrate was greater than or equal to that of Cu–Sn compound layer, although the reflow soldering temperature of Sn–Zn–Bi (240 °C) was lower than that of Sn–Ag–Bi (250 °C). The stable Cu–Zn compound was the absolute preferential phase in the interfacial layer between Sn–Zn–Bi and the Cu substrate. The ternary (Cu, Ni) 6Sn 5 compound was formed at the Sn–Ag–Bi/Ni(P)–Cu metallization interface, and a complex alloy Sn–Ni–Cu–Zn was formed at the Sn–Zn–Bi/Ni(P)–Cu metallization interface. It was noted that Cu atoms could diffuse from the Cu substrate through the solder matrix to the Ni(P)–Cu metallization within 1 min reflow soldering time for both solder systems, indicating that just 30 s was long enough for Cu to go through 250 μm diffusion length in the Sn–Ag–Bi solder joint at 250 °C. The coupling effect between Ni(P)/Cu metallization and Cu substrate was confirmed as the type of IMCs at Ni(P) layer had been changed from Ni–Sn system to Cu–Sn system apparently by the diffusion effect of Cu atoms. The (Cu, Ni) 6Sn 5 layer at the Ni(P)/Cu metallization grew significantly and its thickness was even greater than that of the Cu–Sn compound on the opposite side, however the growth of the complex alloy including Sn, Ni, Cu and Zn on the Ni(P)/Cu metallization was suppressed.