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CCGA and overprinting?



CCGA and overprinting? | 12 October, 2001

Hello, I need some advice. I want to create apertures in my stencil for a IBM CCGA. I've heard the standard is 4800 cubic mils of paste per aperture. I usually use a 5mil stencil for this type of product. There is no way I'll get enough paste with the current ap size (27mils). It looks like I could get it if I overprint to a 35 mil aperture. That will give me about 15 mils gap between aps. Is this gap enough to prohibit bridging? Or, should I resort to a stepped stencil (which I've never done before either)? If so, what special precautions should I use for this type of stencil (UP-3000 printer with Rheo-pump or squeegee).

Thanks for any advice! Mike

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CCGA and overprinting? | 15 October, 2001

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.

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CCGA and overprinting? | 15 October, 2001

Dave, Thanks for the info! Yes, I have read the IBM document and realized that I was missinformed about the paste volume. I need 5000 to 5600 cubic mils of paste. Running the numbers I get a 32 mil aperture with a 7 mil step (the rest of the board needs to be 5 mils). So, I've ordered the stencil and will see how it goes... Best Regards, Mike

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CCGA and overprinting? | 15 October, 2001

I hate getting "roped" into math stuff, but here I go again ...

Area of aperture = volume of paste / stencil thickness = {[5000 mil^3] / 5 mil} = 1000 mil^2

Side of the square aperture = Sqrt of the area = [1000 mil^2]^1/2 = 32 mil

Diameter of aperture = 2 * Sqrt of the [area / pi] = 2*[1000 mil^2 / 3.1459]^1/2 = 36 mil

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