We've had a few problems with printing a flex-rigid PWB, and I'm hoping some of you could throw some opinions/suggestions my way.
Our PWB is about 8" x 11" divided into two parts (side A & B) by 2 wide flex cables. Each of these flex cables is about 3 inches wide with an inch or so gap between them, so they consume most of that 8 inches in width.
This PWB is later folded over a heatsink, sort of like a sandwich. So, to allow the PWB to fold properly, there is a bulge in the flex which won't allow my stencil to rest on the PWB (or even slightly above). It sticks up about 3/8" or so above the surface of the board.
Now, the previous engineer had the operators push the center of the flex bulge down as much as possible w/o damaging it, and just let the stencil blade wipe over it. Obviously, this causes problems on those components w/in about 1/2" of either side of where the blade comes off the stencil. The remainder of the board actually gets a pretty decent print. The operators then would remove the paste from those areas, and place more by syringe (mostly chip components in those areas)...shady sounding, I know.
I've just been moved in to support this area, and don't much care for this method, but am not sure what a good alternative would be. I've thought about doing 1 side at a time, but have not experimented with that yet. I've also thought about cutting a gap in the stencil so the bulge can stick through (actually, I'd order a new stencil - not actually cut the current one). What would you suggest? ...besides a new design!
Thanks in advance,
Chris
reply »