C. E. Ho

Effect of Cu concentration on the interfacial reactions between Ni and Sn–Cu solders

The eutectic 99.3Sn–0.7Cu solder (wt%, Sn–0.7Cu) is the most promising lead-free replacement for the eutectic Sn–Pb solder in wave-soldering applications. In this study, the effect of a small perturbation in the Cu concentration on the reaction between the Sn–0.7Cu solder and Ni was investigated. Specifically, four Sn– x Cu solders ( x = 0.2, 0.4, 0.7, and 1) were reacted with Ni at 250 °C. A slight variation in Cu concentration produced completely different reaction products. When the Cu concentration was low ( x = 0.2), the reaction product was (Ni 1– x Cu x ) 3 Sn 4 . At high Cu concentrations ( x = 0.7 and 1), the reaction product was (Cu 1– y Ni y ) 6 Sn 5 . When the Cu concentration was in-between ( x = 0.4), both (Ni 1– x Cu x ) 3 Sn 4 and (Cu 1– y Ni y ) 6 Sn 5 , formed. The above findings were rationalized using the Cu–Ni–Sn isotherm. The results of this study imply that the Cu concentration must be strictly controlled in industrial production to produce the desired intermetallic at the interface.

Reactions between Sn–Ag–Cu lead‐free solders and the Au/Ni surface finish in advanced electronic packages

The reactions between Sn–Ag–Cu lead‐free solders of various compositions and Au/Ni surface finish in advanced electronic packages were studied. Three solder compositions, Sn–3.5Ag, Sn4Ag–0.5Cu, and Sn–3.5Ag‐0.75Cu were used, and their performance was compared. It was found that the Sn–4Ag–0.5Cu solder gave the worst results in terms of shear strength. The poor performance of the Sn–4Ag–0.5Cu solder can be explained based on its microstructure. The types of intermetallic compounds formed at the interface were different for different solder compositions. When there was no Cu the reaction product was Ni3Sn4. For the Sn–3.5Ag–0.75Cu solder, the reaction product was (Cu1‐p‐qAupNiq)6Sn5 immediately after reflow, and two intermetallic compounds (Cu1‐p‐qAupNiq)6Sn5 and (Ni1‐yCuy)3Sn4 formed after aging at 180°C for 250 and 500 h. For the Sn–4Ag–0.5Cu solder, both Ni3Sn4 and (Cu1‐p‐qAupNiq)6Sn5 were present near the interface right after reflow, and there was a layer of solder between these two intermetallic compo...

Reaction of solder with Ni/Au metallization for electronic packages during reflow soldering

Gold over Ni is one of the most common surface finishes for Cu soldering pads in ball-grid-array (BGA) and other electronic packages. The Au layer is for oxidation protection, and the Ni layer serves as a solderable diffusion barrier. In this study, eutectic Pb-Sn solder-balls were reflowed on the Au/Ni/Cu pads, and the chemical interactions between the solder and the surface finish were studied. Quenched-in microstructures at different stages of the reflow were carefully examined using the scanning electron microscopy. It was found that the solder melted locally along the solder/pad interface at the very early stages of the reflow before the whole solder ball had reached the Pb-Sn eutectic temperature. This was because a ternary eutectic reaction L=(Pb)+(Sn)+AuSn/sub 4/ occurred at 177/spl deg/C, six degrees below the Pb-Sn eutectic temperature. Four distinct stages were identified for the reflow process. The four stages are: (1) partial melting of solder balls and the initial reaction of Au with Sn; (2) complete reaction of An with Sn; (3) separation of (Au/sub x/Ni/sub 1-x/)Sn/sub 4/ from the pad; (4) complete melting of solder balls and the reaction of Ni with Sn. After a typical reflow, with a 225/spl deg/C peak reflow temperature and 115 s reflow time, all the An and Au-bearing intermetallic compounds left the interface and the only intermetallic compound at the interface was Ni/sub 3/Sn/sub 4/ with a thickness of about 2 /spl mu/m.