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Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


BGA footprint on PCB

#7049

BGA footprint on PCB | 24 June, 2001

Hello,

I need some information about footprint of BGA / fine BGA. As far as I know the pad on PCB should be the same as the pad on the BGA (much smaller than the ball dia.).

I need some information regarding the hole SMT process: - Paste printing: is it posible to get good printing in 16mil hole? (in 39.4 BGA). - Reflow results ? - Special problems / issues. - ICT yield .

Thanks Roni H.

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#7065

BGA footprint on PCB | 25 June, 2001

You are 100% correctamundo that the pad on PCB should be the same as the pad on the BGA interposer, but let�s not expend that thinking to the aperture of your solder paste stencil, right away. Generally, paste on large pitch BGA is often 1:1, but as the pitch gets smaller, expect to apply a lower proportionality of paste.

Before getting crazy about things, let�s take a deep breath and get basic. When designing a stencil to print fine pitch features, many people consider the aspect ratio and area ratio of the apertures they select, where: * Aspect ratio = aperture width/foil thick SB GT [1.5 : Chemically etched, 1.2 : Laser cut, 1.1 : Electroformed] stencils * Area ratio = (LxW)/(2x(L+W)xT) SB GT 0.6

Lemme give you several examples: * A BGA 50 thou pitch using 25 thou diameter circular aperture in a 6 thou thick stencil has: aspect ratio - 4.2 & area ratio - 1.0 [love it!!] * A uBGA 30 thou pitch using 13 thou square aperture in a 5 thou thick stencil has: aspect ratio - 2.6 & area ratio - 0.7 [getting difficult, but could be worse!!] * A uBGA 30 thou pitch using 11 thou square aperture in a 5 thou thick stencil has: aspect ratio - 2.2 & area ratio - 0.6 [see, this is worse!!]

When trade journals [eg, SMT magazine, Circuits Assembly, EP&P] talk about "paste printing", they generally mean stencil design. Stencil design usually means droning-on about stencil type [eg, chemically etched, laser cut, electropolished, electroformed] and about tailoring aperture size so that printing leaves adequate paste on the pad. So, there�s a tremendous amount of text on stencil design in the trade press. This increased fairly recently, with the publication of IPC-7525, "Stencil Design Guidelines" [http://www.ipc.org]. Several authors have written [and gotten published] their "Cliff Notes" version of these IPC guidelines in trade journals.

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Brian Sloth Bentzen

#7167

BGA footprint on PCB | 5 July, 2001

Yes, the solder pad on the PCB should match the pad on the part. This will minimize stress during the soldering process�.

Check out more on http://www.smtinfocus.com/papers_pbga.html Here you will fond some more informations on the process used in relation with BGAs.

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