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Screen Printing Print Validation

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James

#37495

Screen Printing Print Validation | 1 November, 2005

I've recently been subjected to a procedure that requires the first 2 solder paste prints of a production run to be wiped down and not used for the build. This is to condition the stencil aperture walls? and insure good print deposition. Is this a common pratice and if so are there alternatives to prevent the wipe down of production assemblies during this process. I realize that having solder samples available would be a viable option but they are not always available.

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RDR

#37497

Screen Printing Print Validation | 1 November, 2005

I've never heard of wiping the first two prints. We wipe when the paste is not printed well and only then. You may want to place a thin mylar over the board and print.

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#37498

Screen Printing Print Validation | 1 November, 2005

I've never heard of this before. If your solder paste is at the correct temperature, and your process is under control you should have an acceptable print the first time. This sounds like a terrible waste on time and paste. Set up a DOE to prove that there is no statistical improvement by doing this.

Jerry

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URL

#37511

Screen Printing Print Validation | 1 November, 2005

Wow, never heard of this. Hope your board cost isn't too much. Most printers have a kneeding procedure that does this while it prints the board. No waste of any kind when you do this. Conditioning the stencil - must've been in a magazine artile.

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#37515

Screen Printing Print Validation | 1 November, 2005

This smells of medical device manufacturing. Am I close?

At the very least you should be able to just double print one board, not single print two of them.

At best, go the DOE route as has been suggested and prove to them (whomever "they" are) it isn't necessary.

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#37521

Screen Printing Print Validation | 1 November, 2005

James, Never heard of this before. Purchase the cheapest transparency film you can find from your local office supply shop. Tape tightly to your board and print.

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KEN

#37523

Screen Printing Print Validation | 1 November, 2005

Um...for you mylar guys....esd safe?

Why worry about the first two prints. Worry if they power up or not when sent to funcitonal test. Zapp!

I think I know where this is comming from. It is not that uncommon to have a sacrificial print. However, this is generally refered to as "kneading" and is not considered a production print. It generally is sacrified to the wash process and reborn clean and ready for production. Its roots are from:

1. Screen printing where the screen is "primed" with material. 2. Providing a mixed thixotroic medium to the apertures. 3. conditioning the apertures meaning all the boards should print similar without detrimental effects of shaddowing or skipping (which were probably problematic at some suppliers site at some point in history with some process using some paste etc.).

If you don't want to do this process you're going to have to proove to your customer you have adequate process to quench their fears. Unfortunatly, you're probably going to need data like 2d / 3d inspection.

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Adnahn

#37532

Screen Printing Print Validation | 2 November, 2005

It is not common practice to perform this function.What is advisible is to cover the PCB surface with white copy paper.Screening onto the copy paper will give you a clear indication of your solder screening profile.This way your PCB's will stay clean.

Regards Adnahn

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Simon

#37538

Screen Printing Print Validation | 2 November, 2005

Hi James, I have worked in many SMT sites and this is not an uncommon practice. It is true that print deposit consistency will improve after the first few prints as the aperture walls become lubricated, but with modern stencils/paste and a well-designed process the first print 'should' be fine. As mentioned by other respondents, a thin transparent film over the pcb provides a simple solution if this practice is mandatory.

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