Lead free Solder Paste troubles

2017/2/2 0:32:42

We are printing on an assembly using SAC305 Alloy Sn96.5 Ag 3 Cu .5 with a no clean flux and getting virtually NO spread with either a ramp to spike or straight ramp profile. We are printing to standard FR4 board with immersion Ag finish. I am trying both Indium and Senju pastes along with our normal No Clean paste which is an Aim no clean 293+ tin lead paste. The eutectic solder spreads fine, looks smooth and shiny, and wets fine to the lead free components, but we must use a no lead. Both lead free pastes are stiff, have poor release, stay put on the pad where printed and look very dull and very grainy. The indium stuff has tons of little solder balls on top of the joint on the flux residue. With the lead free both profiles are about 4 minutes with a peak of 245 C. Any suggestions ? We need to decide on a process and lead free paste for some proto boards which must ship next week.