Some "boutique" solder, Wonder, WBT, etc. is 4% silver and hard to work because of high melting point. Cardas is easier, has a lower melting point. I've been using Radio Shack 'High Tech Rosin Core' 62/36/2.Part #64-013, 2% silver. Very easy to work and cheap. Both the Welborne and Michael Percy sites play down any sonic advantage to silver content but both sell 3 or 4 "boutique" brands. Michael Percy, BTW, generally has best prices on most everything including connectors.
www.bainbridge.net/percyaudio
www.bainbridge.net/percyaudio