Silver content is said to be necessary to get a proper bond to silver wires and connectors. It also raises the melting point making it more difficult to work with. The Cardas and others have 2-3% silver which doesn't require too much heat. Some have 4% or more, like jewelers use and that's a headache for the typical pencil iron. Both Kester "44" and Cardas are "eutectic", i.e. their tin/lead/silver alloys change coincidentally from 'solidus' to 'liquidus' at the lowest possible temp point . Non-eutectic solder has a 'plastic' phase during which one metal is still 'liquidus' while the other has hardened. If there is movement before all become 'solidus' cracks may develop and a 'cold' joint ensue. Lead-free solder is always non-eutectic and more difficult to work with. Silver and, I think, antimony are eutectic with lead and tin.
This whole lead-free thing by the EEC is kinda nuts; have you seen the 'safe' speaker terminals with plastic housings that make attachment a chore? How "green" are those plastic housings? Someday, the European countryside will be over-run with them.
There are entire threads on the Asylum devoted to the differing sonics of various solder as well as paeans to lead-free, some by manufacturers. This reviewer can't hear any difference mainly because I haven't bothered to listen for it. How do you listen to a component, take it to the bench, open it up, de-solder, re-solder, and then re-connect and have any memory of what the original sounded like?
