Have you ever wondered about what happens to a Pb-free Ball-Grid Array (BGA), with Tin/Silver/Copper (SAC) spheres in a Tin/Lead (Sn/Pb) process? Well, after some recent testing, I can share my results with you.
The testing was performed with Sn63 solder paste, at 208°C (most testing is performed at 25°C above liquidus) and depicts the dissolution of a SAC (SnAgCu) solder sphere into the molten Sn63 solder, below the melting temperature of the SAC alloy sphere. The purpose of this research is to demonstrate that BGA assembly (using these materials) can be affected at temperatures BELOW standard Pb-free reflow temperatures (~245°C).
It also shows the incredible solubility of Tin (Sn), and its ability to dissolve higher temperature materials. This is also true for harder materials, such as Nickel and Aluminum. Given the proper flux, you can use tin-based alloys to for the intermetallics that you need.
We are not suggesting that all BGA attachment be done at 208°C. In fact, it is much easier to use a standard Pb-free reflow profile with the Sn/Pb solder alloy paste to ensure that it all melts and forms a strong intermetallic, but for those process' where either the components or solder pastes have issues at higher temperatures, we are demonstrating that going all the way up to 245°C is not always required. We have all had experiences with Sn/Pb solder pastes with flux residues that decompose at >220°C, or seen warpage of BGA's at the same temperature range that cause head-in-pillow (HIP) issues.
A standard Sn/Pb reflow profile, with a peak temperature of 205-210°C, and a time above liquidus (TAL) of >60s can be used to get acceptable solder joints using a mixed alloy process.