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Cleaning Paste from 2nd Side Print

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John S.

#44795

Cleaning Paste from 2nd Side Print | 31 October, 2006

We've just started running a lot of double sided assemblies. What is the preferred method for removing paste from a board that is populated on side 1 if the side 2 print is poor? Thanks John S.

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#44798

Cleaning Paste from 2nd Side Print | 31 October, 2006

That depends, W/S or No-Clean, SnPb or Pb Free?

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John S.

#44800

Cleaning Paste from 2nd Side Print | 31 October, 2006

SnPb No-Clean.

We've done it on bare boards with alcohol and an ultrasonic bath, but that seems very agressive on a half populated board. Thanks John S.

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#44803

Cleaning Paste from 2nd Side Print | 31 October, 2006

In my other, better equipped life, we'd wipe them down manually w/dilute IPA then run them through the inline cleaner, nc or ws. I'd avoid US unless you've qualified your process to verify you're not damaging parts with it.

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#44813

Cleaning Paste from 2nd Side Print | 31 October, 2006

We never run raw solder paste through our board cleaner, because: * Flux floats on the water in the tank and contaminates everything, including the boards being 'cleaned'. [Prove this to yourself by observing the location of gray schmutz on the inside of your cleaner.] * Cleaning solder paste from the tanks is a horror show that's added to what starts as a fairly tough job. * Additional solder balls in the cleaning solution increases the potential for solder shorts on the cleaned boards. * Solder balls load the microfilters and increase maintence.

A stencil cleaner is a better alternative.

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#44814

Cleaning Paste from 2nd Side Print | 31 October, 2006

#44825

Cleaning Paste from 2nd Side Print | 1 November, 2006

Dave, your points certainly have merit and I wouldn't make a habit out of that method. We did it only because there were a lot of parts dependant upon chipbonder and we didn't think they'd all survive an ultrasonic cleaning, functionally or physically with respect to the bond. We did assess the cleanliness aspect and decided, oh, what the hell, it's due for a scrubbing soon anyway.

If you're going to do it this way, you wipe them down until they look clean enough to print on, then wash them.

Which reminds me of a story about the weekday shift coming in on Monday and finding a dark, filthy U/S cleaner, a week or so after the solution had been changed. It happened repeatedly and the weekend shift continually denied doing anything besides washing stencils and misprints (w/o parts). They finally had the facilities guys check out some security camera tapes and found that one of the guys was bringing dirty car parts in through the back door and taking clean ones back out.

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#44852

Cleaning Paste from 2nd Side Print | 2 November, 2006

John, As Dave says, never clean raw paste. Let it sit for at least 24 hrs if you can and use a plastic spatula to wipe off the bulk of it. The "drier" the paste the easier it is to remove without it travelling down vias etc. You will then have to choose a method to remove the residual. We apply used stencil wipes with Elecrtolube SSS and/or IPA and an air gun. You can even try some experiments with putting scrap boards through the oven at low temps so that each deposit will co-here to itself but not the pcb. Give the pcb a sharp rap on it's side and the paste falls off in complete patterns. You can then use this scrap to re-ball your BGAs! This is why God intended us to have misprints using a #200,000 printer, or even a $10,000 printer with a white hot operator. It's a beautiful world!

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