Tektronix coincidentally used lead free solder for their special ceramic solder strips in their oscilloscopes, because the lead could interfere with the bond between the actual metal lug and the ceramic. My Tek 454a contains a tiny roll of that solder to ensure repair people don't use lead based solder on those solder strips, because back in the day no one had lead free solder on hand.
Sorry, but you are not right here. The problem with ceramic strips is just the silver layer quickly dissolving in molten solder. A common solution to this is to use a solder already saturated with a bit of silver. Leaded or lead-free, does not matter. A quote from vintagetek.org :
"Special silver-bearing solder is used to solder wires and components directly to the
notches in ceramic strips and has a composition of 60% tin, 37% lead, 3% silver. This silver-bearing solder should be used when soldering or repairing ceramic strips which prevents the silver bonded to the ceramic from dissolving in the molten solder. Failure to use the correct silver-bearing solder on Tektronix ceramic strips can result in failure of the silver to ceramic bond. Tektronix would include a small spool of silver-bearing solder for repair in some of its instruments."
https://vintagetek.org/ceramic-strips/