Currently manufacturing double-sided PCMCIA cards with a nickel/gold finish, which are 0.0235thou in thickness, using surface mount carriers. Problem we are seeing is when building the densely populated side on the second pass we are seeing lifting of the qfp's on the corners,but this problem is so intermittent I just can't put my finger on anything at the moment.The qfp's in question are 0.020 thou in pitch with standard stencil reductions. The printer is correctly set-up using a Indium paste,the reflow profile has been verified and we are using Nitrogen. On inspection the solder paste seems to have flowed and wetted really well ,but the corners still look lifted and a couple of leads just sit on top of the reflowed joints. I'm not ruling anything out at the moment, but am I looking at some kind of a thermal issue or a set-up issue? Also how critcal is it after the first pass that the boards are layed dowm flat and allowed to cool after reflow to ensure they don't bow or warp? could this have an impact ?
Any advice would be gratefully appreciated.