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Food grade silver solder is certified free of toxic metals. Typically cadmium is used to improve silver solder's properties, and it is toxic. Look for a NSF approval rather than just "Cadmium Free". One product is Harris Safety-Silv 56. However, I don't think that the 3A standards allow for it's use in food contact applications (gold solders only?). Flux should have been completely cleaned off before the machinery is put in a food processing environment, so I doubt it matters. I don't think there's such a thing as a NSF approved brazing flux.
If it's silver solder, very little. If it's not silver, nothing.
Silver jewelry requires that you use silver solder for repairs. It is silver based where common solder is lead or tin based. Standard solder is too acidic and will corrode the jewelry. It can probably be found on e-bay. Or ask a jeweler where he or she gets it.
If you're looking for treasure, it is not there. However, there are types of solder that have silver in it so there may be traces of silver in the solder joints on your motherboard.
silver alloy ,
Solder is manufactured in hundreds of different grades and compositions for thousands of different applications. Solder that might be described as "Silver solder" is most commonly commercially manufactured in about 50 different compositions with silver contents of up to 40% and is priced accordingly. Silver solder is often used in jewellery making and repair, engineering and many electrical manufacturing processes.
Typically one solders sterling silver with silver solders. There is not a solder called "sterling solder." You can choose from an array of silver solders ranging from easy (extra soft) through hard. Soft solders have lower silver content and melt at a lower temperature. Hard solders have higher silver content and flow at higher temperatures. If you are doing multiple solder joints on a single piece of solder you will need to use several grades of solder. However, if you are just creating a single solder joint than it is best to use a soft or medium solder.