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Stencil design for flex-rigid PWB

#30548

Stencil design for flex-rigid PWB | 15 September, 2004

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

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C Lampron

#30556

Stencil design for flex-rigid PWB | 16 September, 2004

Hello Chris,

"It sticks up about 3/8" or so above the surface of the board." Is this over the ridgid portion of the PCB or in between? Is it possible to create a support fixture for printing that has relief that will allow the flex to bow downward instead of upward? If the bow is on the top of the ridgid PCB, I would suggest doing what you had mentioned, recutting the stencil with relief around the bulge.

Also, a solder dispener may prove to be usefull in this situation.

Good Luck

Chris

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#30562

Stencil design for flex-rigid PWB | 16 September, 2004

Actually, it's not even close to 3/8". It's more like 0.075"...not sure what I was smoking last night (I tried to guess on dimensions from my house, and now I have one in front of me).

Anyway, to answer your question, the bulge is in between side A and B. So, from a squeege perspective, it travels fine until it nears the bulge, rises up some, then back down, and is fine across the rest of the other side. We can't push the bulge down the other way (through the bottom), and I really don't like to push on the bulge at all...worried about conductor damage inside the flex.

Chris

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C Lampron

#30578

Stencil design for flex-rigid PWB | 17 September, 2004

Hi Chris,

It sounds like you may have to go with your idea of a relief cut into the stencil. Are there any components that need to be printed in close proximity? Is this a high volume?I used to work at a flex house and we had a similar situation. We ended up cutting a relief in the stencil and actually modifying a squegee blade to print around the relief. I'm not sure if that approach is a posibility without seeing the application.

Good Luck

Chris

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#30598

Stencil design for flex-rigid PWB | 19 September, 2004

This (along w/ everything else we do) is not high volume. Actually, we're dropping a batch of 7 the first of the week, and that's fairly sizeable for us. I talked with my stencil design folks and sent them a scrap board and all my gerber info. I think I'm going to try to turn the board 90 degrees and cut a hole (like you said) in both the stencil - to allow the flex to poke through, and in the blade - to pass by the flex sticking up.

I don't know enough about the hole in the blade though. Is that going to be a big problem? My stencil design guys also make blades, so I'm gonna let the pros look into it. Did you have any problems with the blade and stencil you modified (Chris)?

Thanks

Chris

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C Lampron

#30602

Stencil design for flex-rigid PWB | 20 September, 2004

Hi Chris,

Acually, it worked pretty good. We made a new blade from SS. We were not sure if it would work so we diddn't want to commit a good sqeegee blade. You will want to try to keep the cutaway toward the center of the blade. And don't hit the flex. It usually doesnt hold up well to being run over by the sqeegee.

Best of Luck

Chris

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