The only risk I can see is if really old solder was used - it may contain lead. Still; there likely isn't enough exposure time in the boiling wort for leaching to be a concern considering the pH is in the 5+ range.
If you use solder on brewing equipment and it will be in contact with wort, be sure it is "lead-free" solder. New solder will be lead-free, which is what I used on my IC.