

   This is called silver bearing solder.   It is used to solder to
devices that have silvered contacts.   One application is soldering
to the ceramic terminal strips used in the old Tektronix scopes.   These
were notched ceramic strips that were silver plated in the notches.
If you used ordinary solder the molten metal would disolve the silver
off of the ceramic.   The silver was quite thin.   The 2% is a saturated
solution of silver in tin and lead.    Thus, no more silver can be
disolved in the solder solution.   Other devices that use silver contacts
are quartz crystals and ultrasonic transducers.