Protecting the Pb-free solder pot: it is best to begin with a new pot and impeller kit dedicated to Pb-free.

Engineers transitioning to Pb-free soldering have enough to worry about without the added burden of equipment reliability concerns. Unfortunately, the Pb-free transition for wave and selective soldering also means equipment modification. Pb-free equipment must not only handle higher temperatures but also neutralize the corrosive effects of molten tin on ferrous machine parts such as the solder pot and impellers.

The high tin content of Pb-free solders brings accelerated corrosion of steel parts in wave-soldering machines, particularly in areas with higher flow and contact rates, such as around impellers. Higher operating temperatures exacerbate the problem. The molten tin leaches iron from the steel, forming an intermetallic, Fe[Sn.sub.2]. The Fe[Sn.sub.2] crystals have a high melting point--508[degrees]C--so they remain solid and sink to the bottom of the pot where they accumulate. In time, if they get into the solder wave and find their way into solder joints, they can cause soldering defects and denigrate product reliability. Beyond that, the leaching of iron can cause impeller blade failure and possibly leaks in the solder pot, forming a machine safety issue.

The solution lies in material selection (since higher tin content SAC alloys will have a higher level of corrosive activity), and the identification and protecting of critical machine parts against this scavenging. A proven solution has been to coat the inside of the solder pot, impellers and parts in contact with the solder, particularly in high flow rate areas, with a proprietary coating that is not subject to degradation and leaching by the tin, and can also withstand the high temperatures associated with Pb-free soldering, particularly for the harsher environments of the selective soldering machines. This "high-endurance" coating does not de-bond, nor can the tin scavenge iron through the coating, even after several months of continuous use and testing at 400[degrees]C.

Wave soldering machines, operating at a lower temperature, have shown the same results and endurance with a less robust (and less expensive) coating for the solder pot; however the impellers and moving parts must still have the "high-endurance" coating. Wave machines can get by with the lower endurance coating because of the lower temperatures and thus less aggressive scavenging action within the pot itself. The lesser coating is still impervious to the scavenging on a par with the tougher coating in the selective soldering machine. Currently, there are nearly 70 lines in operation successfully running Pb-free wave soldering with this coating scheme.

Although many selective and wave soldering machines manufactured since 2001 have this protective coating scheme and are completely Pb-free compatible, pre-2001 models are usually not--check with the manufacturer.

Should you invest in a new solder pot and coated impeller kit, or flush and purge the existing pot and have it coated? There are potent arguments against trying to refurbish the old pot. Lead removal must be complete; sometimes up to three tin baths are required, and even then if all the Pb-bearing solder is not removed from all conduits, the new solder batch will be ruined--a complete and unrecoverable loss--at a cost of around $10,000. And it does not take much lead to do it. In the past year, I have seen a dozen instances of this. Cleaning and purging the pot is labor-intensive, hazardous, expensive in terms of labor, materials and downtime, and the result is uncertain.

It is preferable to replace the solder pot entirely with a new, coated one that is ready to fill with solder and begin manufacturing; this can be done quickly, easily and reliably. Indeed, some transitioning manufacturers maintain two solder pots on hand--one filled with Pb-free alloy and one with Pb-bearing alloy--that can be swapped depending on the product being run. Of course, it is important to ensure that the Pb-free pot is not contaminated inadvertently--a costly mistake! With new part-coating technologies, equipment reliability issues with Pb-free solders are no longer a concern. The recommendation is to start out with a new pot and impeller kit dedicated to Pb-free use, and to keep the pot free of lead contamination at all costs.

Dr. Denis Barbini is manager, advanced technologies at Vitronics Soltec (vitronics-soltec.com); [e-mail].