I am just curious, is there a guidline that people follow for choosing their lead-free solder paste? In other words, how do people know whether they should use Indium alloy, or Castin (Sn/Ag/Cu), or whatever else?
Depends on your specific application. Can you handle lower reflow temps? Can your parts handle higher reflow temps? What environment will your assembled product have to endure? Lot of different factors come in to play when choosing the right alloy.