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Fine Pitch Printing Issues

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rem

#61864

Fine Pitch Printing Issues | 19 May, 2010

I am currently experiencing major issues when printing fine pitch parts. Pad width is .008 Even starting with a new screen, fresh paste, etc. I see where the paste is not breaking away from the screen apts consistently and when inspecting the pads I see where the paste is missing on some pads and peaking on others as if its not breaking away.

In the past I have done a reduction on fine pitch but have never experienced this problem before. It is on most every board we run and even on a blank pcb with no pads. My stencil guy says they electro-polish and laser etch. Any thoughs would greatly be appreciated. I have also worked with seperation speed, 1 to 1.5 of sqeegie lenght on pressure ( 14" blades I have been from 13 to 16 on pressure). Print gap has been from .000 to .010.

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#61865

Fine Pitch Printing Issues | 19 May, 2010

How thick is the stencil? Also what type of paste?

We have a nice little microscope that has gradients on the lens so we can measure whether an aperture is 8 thou or not. Almost always it is but then we buy from a couple of good stencil suppliers.

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#61887

Fine Pitch Printing Issues | 20 May, 2010

What is the area ratio for the aperture? What size paste are you using? Stencil thickness & aperture size?

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rem

#61889

Fine Pitch Printing Issues | 20 May, 2010

I just got word from my board designer that the actual pad size is 11.8 mil and he is reducing the paste layer to 7.8 mil. To me this is way to much of a reduction. I am was accustomed to doing 20% reduction on any thing 20mil pitch or less. The screen is 5mil. Doesn't this sound like a significant reduction of paste? I was thinking of enlarging to 9.5 mil (20%) on my end and give it a try.

Thanks Rich

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#61890

Fine Pitch Printing Issues | 20 May, 2010

I believe industry standard is 1/2 pitch for fine pitch devices. If your part is 20 mil pitch, apts would be 10 mil wide. This should gasket fine to the pcb pad and with a 5 mil stencil should print well.

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rem

#61899

Fine Pitch Printing Issues | 21 May, 2010

Thanks I will give this a shot.

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#61910

Fine Pitch Printing Issues | 24 May, 2010

A guideline that I use agrees with MikeS: For 20mil pitch, apertures should be 9-10mil with a stencil thickness of 5-6mils to achieve an acceptable aspect ratio of 1.7. Just be sure you are using a mesh Type 3 or 4 solder paste. Otherwise, larger particles in Type 2 pastes will tend not to release properly.

Dean

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rem

#61927

Fine Pitch Printing Issues | 25 May, 2010

Yes, I use Indium Mesh 3. I have a re-cut stencil on the way.

Thanks for the input!

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