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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Stencil Design Question

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#53630

Stencil Design Question | 13 February, 2008

Greetings everyone. We are currently doing some experimentation with solder paste printing over ground planes. In this application, the entire perimeter of the PCB is a ground plane. It's about 4mm wide and wraps around the whole PCB; There is no solder mask anywhere on the ground plane itself, but soldermask seperates the ground plane from the rest of the PCB. I hope thats clear... We've been asked to try and print solder paste over this ground plane area so that it is completely covered after reflow. We are also using lead free paste. My question is regarding the stencil design. Does anyone have any ideas about how to design the apertures so that we can have sufficient paste depoist while providing complete reflow coverage and without severely weakening the stencil? Also, I understand that lead free paste does not really "move" during reflow, so i'm wondering if anyone thinks this is even possible. I wish i could answer why we are doing this, but I can't...it was handed down to us by RnD. Any input would be appreciated.

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#53642

Stencil Design Question | 14 February, 2008

Maybe what you could do is have the stencil contain a tightly spaced mesh or grid of openings over the ground plane areas that you need to be solder covered. Also, to help the solder flow out better, you might want to decrease the percentage of metals in your solder paste which will increase the flux volume and help the solder flow out. I've attached a image of what I'm thinking. The stencil should be pretty stable even with the mesh or grid...

Steve Gregory

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#53644

Stencil Design Question | 14 February, 2008

What Steve posted is what I was picturing. I think that the stencil would certainly not be as strong due to the surrounding apertures, but stencils can be replaced. Hopefully the assembly volume will allow for replacements as necessary due to the peculiar demands being put on production.

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aj

#53645

Stencil Design Question | 14 February, 2008

take a look at what solder preforms are available. You can get ones that can be machine placed.

This might allow you to add paste to areas that you cant open up around the perimeter.

aj...

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#53646

Stencil Design Question | 14 February, 2008

Don't worry about the paste print. Just omit it from the stencil. Mask over all the SMT components after reflow with water washable WSOL masking. Then run it thru the wave to deposit the solder. Then wash to remove masking. Or you could manually run it over a solder pot.

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#53648

Stencil Design Question | 14 February, 2008

Thanks for the ideas! We're gonna try em all, I'll let you know how it turns out.

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