I'm doing SMT
of lead-free solder BGA on FR4 Ni-Au PCB. The solder paste I'm using is Sn/Ag/Cu and reflow with peak temp at 240C. I found there are voids(1-2) in almost every solder joint(500 micron in dia) under xray. And the void size is violate the IPC-7095 standard. I tried to find out which parameter in my process cause the void. I tried different solder paste(different brand, composition, even Pb/Sn), differnt reflow peak temp(220 to 260), different heating rate, duration above melting point, multiple reflow.....There are still voids in my every solder joint. And finally I even have voids in my lead-tin solder joint. This is really a big headache to me. Could anyone please help. I've almost tried everything I can. Thanks and every input are welcomed.