Can you mix leaded and lead-free solder on the same board? In general, no. Avoid this whenever you can, because if the two different types of solder ever do directly mix together on the same board, you run the risk of the solder joint becoming brittle over time and cracking, which will then need to be repaired.
Also, please note… that being said, if you are repairing old electronics, please please please prefer to use leaded solder. Future buyers will not like a board that has been ruined by mixing incorrect solder types.
Also, another good point in the discussion on the particular forum I’ve found.
20200406/DuckDuckGo mix leaded and lead-free solder on same board
20200406/https://www.eevblog.com/forum/beginners/mixing-lead-solder-with-lead-free-solder/
20200406/https://en.wikipedia.org/wiki/Tin-silver-copper
Notes on lead-free solder types for your own uses, avoid Tin-silver-copper solder for your own uses, it’s best only for automated assembly. Reworking and repairing with this type of solder is a real pain, prefer to use 99% tin or 100% tin solder instead. Not only is this much easier to work with, but it has also been proven to work well for automated assembly mass production.