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SAC Alloy for RoHS Compliant Solder Paste: Still on Target


I read with dismay this week, an advertisement in an electronic assembly trade journal touting a "drop in" solder paste. The solder paste uses a patented alloy, that has four elements (e.g. tin, copper, silver, bismuth) versus the NEMI, IPC, JEITA etal recommended tin-silver-copper (SAC) alloy. The ad suggested that one need make no changes in their tin-lead based process to assemble PWBs with this "drop in" solder paste. Just as with the desire for RoHS exemptions for all your products, "hope springs eternal."

Why am I so down on the "drop in" solution? Here are the major points:

1. The alloys used don't really give a "drop in" solution. They typically melt at a little over 200C, admittedly a lower temperature than the mainstream SAC alloys (217C), but still about 20C higher than tin-lead. The 20C difference is just too much to enable a "drop in," most folks would need to develop a new process for this paste.
2. Four element alloys are hard to manufacture with consistency.
3. The alloys do not have a broad data base for reliability or processability.

The third point is the real kicker. You are on your own. The world is going lead-free using the SAC alloy. The vast majority of the reliability and process data in the world will be on SAC alloys. Even though preliminary data are good for both processability and reliability with SAC, as with all soldering systems there will be issues. We still have surprises with tin-lead solder after almost 100 years of use in electronics! If you are going to have issues with your solder, it is nice to have them with a solder that has a vast data base. That lead-free solder will be SAC.

Dr. Ron