On over printing, the general issues are: * Paste deposits being so large that when reflowing, collapse onto many, rather than a single solder mass. * Paste expanding during heating and being close to other solder deposits and bridging [as you say].
SOLDER BALLS: We can print our paste in 0.25� disks and not leave loose solder balls.
BRIDGING OF SOLDER DEPOSITS: We can�t guarantee your 35 thou over-print / 15 thou spacing won�t bridge. While not the same, but very similar, our scariest job was 47 thou apertures for a 50 pitch device. [I couldn't watch. Watta woose!!!]
We never have used a rheopump, but if you plan a stepped stencil consider putting the steps on the board side of the stencil when using it. A metal squeegee is fine, step up or step down, either side. We used to do a lot of stepping, but do none now, because we understand stencil design [ie, area & aspect ratios] better.
Two places to look for more information are: * IPC-7525, "Stencil Design Guidelines" gives good guidelines for keep-outs around steps, but only briefly touches on CCGA. * IPC-7095, "Design & Assembly Process Implementation for BGA's" give more information on printing paste for CCGA.
� but since you obviously have researched this already, these IPC documents may not add much.
IBM has an on-line CCGA application note that may help.
reply »