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RoHS and stencils

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RoHS and stencils | 3 March, 2006

We have a customer that is transitioning to RoHS compliant assemblies. Their layouts are not changing from the old boards to the compliant ones so the stencils will remain unchanged. They are going through a transition phase where both will be built. Have there been any studies done regarding any potential cross contamination of lead to lead free assemblies from the stencil? We ultrasonically clean the stencils after every job. Instead of using the same stencil, would it be beneficial to order a new stencil just for the compliant assembly?

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RoHS and stencils | 3 March, 2006

Haven't done any studies other than how clean can you get a stencil. I would clean 'em and inspect the apertures and see if you have any residues. If so, I would clean again the amount left would be so miniscule that I don't beleive it would cause a non-compliance.

If you have the money to buy a new stencil for this I would go ahead if you are worried.

I personally would not however.

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RoHS and stencils | 3 March, 2006

We've looked at our stencil under magnification after hand cleaning. There is some paste left behind. Machine washing removes all of the paste. Your best bet is to clean like normal, then look at the aps under magnification. Adjust accordingly (apply, lather, rinse, repeat). Like others have posted, the small amount you would mix in is probably so small you'd never know it, but why cause yourself all them sleepless nights.

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RoHS and stencils | 3 March, 2006

i think a more important issue here would be the reduction ratio. as LF solders have a higher surface tension, they do not flow as well as their leaded counterparts. therefore the stencil design may have to be changed (hence a new stencil) if the reduction ratio on the old stencil produces insufficient wetting of the pad/insuifficient fillet heights etc.

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RoHS and stencils | 6 March, 2006

Our CMs require a new stencil to be purchased, even if it is no different, or else sign a waiver. We purchased a new stencil.

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