No abstract

The acute Zn concentration sensitivity of the reaction between Sn-based solders and Cu substrate is reported and explained in this article. Three Sn-xZn solders (x = 0.5, 0.7, and 2 wt%) were reacted with Cu substrates at 250 °C for 2–10 min. A slight variation in the Zn concentration changed the reaction product formed at the interface. When the Zn concentration was low (x = 0.5 wt%), the reaction product was Cu6Sn5. When the Zn concentration was slightly increased to 2 wt%, the reaction product became Cu5Zn8. When Zn concentration was in-between (x = 0.7 wt%), Cu6Sn5 and CuZn co-existed. The above findings are explained using the Cu–Sn–Zn phase diagram. The implication is that the type of compound forms at the interface can be controlled by adjusting the Zn concentration of the Sn-based solders.

Massive spalling of intermetallic compounds has been reported in the literature for several solder/ substrate systems, including SnAgCu soldered on Ni substrate, SnZn on Cu, high-Pb PbSn on Cu, and high-Pb PbSn on Ni. In this work, a unified thermodynamic argument is proposed to explain this rather unusual phenomenon. According to this argument, two necessary conditions must be met. The number one condition is that at least one of the reactive constituents of the solder must be present in a limited amount, and the second condition is that the soldering reaction has to be very sensitive to its concentration. With the growth of intermetallic, more and more atoms of this constituent are extracted out of the solder and incorporated into the intermetallic. As the concentration of this constituent decreases, the original intermetallic at the interface becomes a nonequilibrium phase, and the spalling of the original intermetallic occurs.

The microstructures of the eutectic Au20Sn (wt.%) solder that developed on the Cu and Ni substrates were studied. The Sn/Au/Ni sandwich structure (2.5/3.75/2 m) and the Sn/Au/Ni sandwich structure (1.83/2.74/5.8 m) were deposited on Si wafers first. The overall composition of the Au and the Sn layers in these sandwich structures corresponded to the Au20Sn binary eutectic. The microstructures of the Au20Sn solder on the Cu and Ni substrates could be controlled by using different bonding conditions. When the bonding condition was 290°C for 2min, the microstructure of Au20Sn/Cu and Au20Sn/ Ni was at wo-phase (Au 5 Sn and AuSn) eutectic microstructure. When the bonding condition was 240°C for 2min, the AuSn/Au 5 Sn/Cu and AuSn/Au 5 Sn/ Ni layered microstructure formed. After bonding, the Au20Sn/Cu and Au20Sn/ Ni diffusion couples were subjected to aging at 240°C. The thermal stability of Au20Sn/Ni was better than that of Au20Sn/Cu. Moreover, less Ni was consumed compared to that of Cu. This indicates that Ni is am ore effective diffusion barrier material for the Au20Sn solder.

The relatively fast diffusion of Au atoms in eutectic PbSn matrix is considered one of the contributing factors to the Au embrittlement problem. In this study, we further investigated the Au embrittlement problem in high-Sn solders. Experimentally, Sn3.5Ag (wt.%) spheres with 500-mm diameter were soldered over the Au/Ni soldering pads. It was found that some of the AuSn 4 needles that formed after reflow inside the solder migrated back to the solder/pad interface during thermal aging. However, the migration kinetics in high-Sn solders was slower compared to that in eutectic PbSn. The difference in migration kinetics of AuSn 4 in eutectic PbSn and SnAg was ascribed to the difference in the magnitudes of the Au flux and the Ni flux. In eutectic PbSn, the Au flux was much greater than that of the Ni flux, and the Au and Ni flux were in the same order of magnitude in eutectic SnAg. The relative magnitude of the Au and Ni flux changed in eutectic PbSn and SnAg because the homologous temperatures of PbSn and SnAg were different.