Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Bridging Problems

Views: 4631

#53992

Bridging Problems | 12 March, 2008

We have a lead free assembly with a 128-Pin PQFP with 20 mil pitch leads. We have tried stencil aperture reductions, step-down stencils, different thickness stencils, various printer setting changes, placement pressure changes, nozzle changes, moved the placement to a different machine, profile changes and still are having extreme difficulties with bridging. Most of the time, the bridging is not just between two leads but extends accross multiple leads. The paste job is consistent and looks very good as does the placement. Any less paste deposition and we'd have insufficient problems. If you look at a panel prior to reflow, you might see one placement that doesn't look as perfect as the rest and that might be the one that comes out the best - in other words, we don't see much consistency as to where the bridging might occur. We run the exact same product leaded and have very little trouble. Any ideas about what I should try next?

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#53994

Bridging Problems | 12 March, 2008

Check your board for solder resist between the pads. Without it, you get shorts very easily.

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#54028

Bridging Problems | 14 March, 2008

Pastes from different suppliers have different hot slump characteristics

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#54062

Bridging Problems | 17 March, 2008

Hi SWAG,

Bridging most contributed is from printing set up which is the parameter setting as well the board thickness setting. I would suggest you may check the parameter setting which mainly the squegee speed, stencil gap between PCB contact, printer machine humidity temperature cannot warm. I hope this would be help you.

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aj

#54076

Bridging Problems | 17 March, 2008

Hi,

Have you checked out the component ? you might have solderability issues or conflict between the LF paste & the lead coating.

If it looks like the component is shifting , my money would be on the component? do you have same issue with other components?

aj...

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#54078

Bridging Problems | 17 March, 2008

Thanks all for the replies. Chunks has hit the nail on the head as the distance between pads is so little that the boardhouse was unable to apply soldermask. We are in the process of having the pads resized to allow for soldermask. Hopefully this is the key - I'll let you know when we get new boards in.

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#54081

Bridging Problems | 17 March, 2008

You say, "We run the exact same product leaded and have very little trouble." Did someone remove the solder mask dams from the boards?

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#54084

Bridging Problems | 18 March, 2008

Yes - we got stuck in a train of thought that we have no troubles with the leaded product and should be able to do it lead-free. We overlooked what Chunks has pointed out and spent most of our time on process instead of looking at the boards closely. The lead free boards are a different part number than the leaded and might be a different supplier but I'm not sure about that...

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#54085

Bridging Problems | 18 March, 2008

Chunk be illin'!

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#54208

Bridging Problems | 28 March, 2008

When you say that the paste job "looks" good, are you actually taking a 3d measurement of the paste deposit, or are you doing it by sight? A good 3d paste measurement tool, in this case, is worth it's weight in gold. ASC International produces a line of 3d paste measurement tools that are very economical, and allow you to get a look at what is REALLY happening.

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#54210

Bridging Problems | 28 March, 2008

Chunks is truly an amazing woman. :)

She saves companies money just through all her wonderful advice.

Chunks, I'll see ya next week. Yes, I'm flyin' out of Riyadh tomorrow. :)

PS - remember Nepcon '97? :)

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PR

#54211

Bridging Problems | 28 March, 2008

Sounds like a love connection.....

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#54212

Bridging Problems | 28 March, 2008

KRIKIES, is this match.com?

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#54244

Bridging Problems | 1 April, 2008

Yes we have a 3D, in-line paste inspection system.

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#54245

Bridging Problems | 1 April, 2008

Just curious...What is the gap between the pads? If it's less than 3 mm, it's recommended to do gang relief. Otherwise the soldermask will peel off during rework.

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