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Accepting Stencils


Accepting Stencils | 16 November, 2000

We want to improve our process for accepting SMT solder paste stencils received from suppliers.

Our process: * E-mail CAD aperatures to supplier with "readme" giving specific fabrication instructions to the supplier. * Receive stencil from the suppler. * Send the stencil to the stencil operators for acceptance. * Visually compare the stencil with a bare board. * Print paste on the bare board and judge the print quality.

What do you do? How would you improve this process?

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Early Moon


Re: Accepting Stencils | 16 November, 2000

I promised never to do this - BUT only for you needy folks when I have nothing else to do. And Dave's basically a good person though not as needy as most really needing this stuff.

1) Ensure internal Gerber data really matches board and stencil (Gerber is dumb data only indicating x/y stuff)

2) Email or whatever away to supplier with instructions and a drawing derived from your CAM/Gerber output - color coded or whatever to indicate special requirements as step downs, etc.

3) Do business with suppliers only having internal inspection capabilities based on CAM input (Gerber data to internal AOI)

4) Receive stencil for visual inspection especially looking for damage (pits, dings, dents, etc.) and have very responsible engineering personnel, whoever that is, do visual to ensure PCB matching characteristics and features (fids, pattern locations, etc.)

5) Install stencil and board in printer and observe no "green" showing - only silver, gold, or whatever solder termination areas match stencil apertures perfectly, unless otherwise required/specified.

6) At the printer, print paste on mylar (ESD preventive of course) and inspect/compare to board - first before printing board.

7) Print first article, determine print quality, if acceptable, then go for it.


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Re: Accepting Stencils | 17 November, 2000

Hi Dave,

I think your approach is more than some others do as incoming inspection. We just look for - damage of any kind - measure the thickness - measure a critical aperture for the correct size (reduction) - look for smooth walls - do Earls "look-for-green-test", often it is a B/W-test with a plot for the PCB�s sometimes take longer to arrive - check the receptacle holes and outline We do not do any testprinting ( good experience with our manufacturer )until the first run but that would be highly recommended if time ( to produce a new one) is a critical factor.

Can�t think of more


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Michael Parker


Re: Accepting Stencils | 17 November, 2000

Dave - Both Wolfgang and the MoonMan have valuable suggestions. I have taken it all one step further. It appears that assumptions are being made that could be making for further problems.

1. Besides relying on CAD gerber data to "spec." apetures, it is also wise to verify actual pad sizes on the PCB. Fab manufacturers have variation in their lots which could affect how well your stencil prints. I have found as much as 5-10% actual pad size loss from a fab vendor. This could negate the best efforts of aperture reduction and return you back to 1:1 ratio. Accurate measurements under > 20X magnification, using 0.5 mil accuracy, is helpful. I have a MicroVU system for geometric measurements in my Receiving Inspection, which is used to also measure the new stencils to ensure aperture reductions requested are actually provided by the stencil supplier.

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Re: Accepting Stencils | 21 November, 2000

Dave- this may not be much current help but for future needs: IPC-7525 Stencil Design Guidelines is due out real soon. Check with IPC on its status, last I heard it was in interim Final.What is nice about this Doccument is that it also covers Pin_Through_paste. Qualifying a vendor to this could be helpful.


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Re: Accepting Stencils | 27 November, 2000

Thanks guys. Following on Cal's point ... we ordered a recently released IPC guideline, seeking closure.

7525 - Stencil Design Guidelines

This new document provides guides for the design and fabrication of stencils for solder paste and surface-mount adhesive. This is the first time this important information has been collected and published in an industry consensus document. Stencil design for various surface-mount technology, as well as mixed technology with through-hole or flip chip components is discussed; this includes overprint, two-print and step stencil designs. A sample order form and a user inspection checklist are also included. 20 pages. Released May 2000.

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