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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Lead Free Stencil Design

Tom

#35457

Lead Free Stencil Design | 12 July, 2005

Most Lead Free Solder suppliers are saying that stencil aperatures should go to 1:1 or have no change from previous stencils design. Only one solder supplier seems to be complicating the process....is their solder very different?? Can anyone share stencil design experiences/tests? Do people generally agree that stencil aperatures should be 1:1 in lead free?

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Chris Ong

#35460

Lead Free Stencil Design | 12 July, 2005

Hi,

My stencil maker also say the same thing on 1:1 for lead free. I am using electro-form stencil. You can look for Mr Richard or Mr Chong at wkchong@dek.com.

Hope this info will be useful for you

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RDR

#35474

Lead Free Stencil Design | 13 July, 2005

The reason for going 1:1 with lead free is that it does not spread out like the pb solders. So 1:1 is really not necessary in my opinion. We go 1:1 anyway on discrete components along with other packages. You always have to make sure that the area and/or aspect ratio is acceptable for stencil design.

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Tom

#35477

Lead Free Stencil Design | 13 July, 2005

Thanks a lot folks! Seems like 1:1 in generally accepted. However Cookson/Alpha seem to be the only ones complicating homeplates to "roundy" instead of "triangular". I'll do some tests... Thanks again!

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Bob R.

#35480

Lead Free Stencil Design | 13 July, 2005

Are you seeing solder balling on your discretes when you print 1:1? Are you washing your boards?

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RDR

#35482

Lead Free Stencil Design | 13 July, 2005

We do have solder balling on occasion but it is usually very minimal. When and if this becomes an issue with us we will then utilize the homeplate aperture. We do not wash boards as a general rule. All of our pb free has been no-clean. We can't find a good water soluble lead free yet.

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Bman

#35483

Lead Free Stencil Design | 13 July, 2005

I know this is asking a lot, but could you please tell us what WS LF pastes you have looked at? What issues are you seeing with these WS pastes?

Thanks

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#35485

Lead Free Stencil Design | 13 July, 2005

I read the Cookson/Alpha paper "Optimizing stencil design for Lead-free SMT processing". I interpreted it to mean that if you had a mid chip solder ball (MCSB) problem the "radiused inverted home plate of 20/60/20 style" had the greatest impact on reducing MCSB's while not increaseing tombstoning.

Jerry

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RDR

#35489

Lead Free Stencil Design | 13 July, 2005

The problems we saw had to due with the Flux I believe. We saw what I would call poor coelescence and the residues were very hard for us to clean with our Water only cleaner. I will not lie and say I worked on the process very much. Since we already had/have a great no-clean process with the lead free I did not spend much time on it to see if the results could be improved. I would rather you E-Mail (click on my name) me off line for the manufacturers I tried.

Russ

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Darby

#35492

Lead Free Stencil Design | 13 July, 2005

This works for me with ENIG finish and a SAC305 NC paste Reduce all pads by -2mil on all sides ie 0.050" x 0.050" becomes 0.046" x 0.046" EXCEPT Anything smaller than 30mil; -1.5mil Anything smaller than 20mil; -1mil Anything smaller than 15mil; -0.5mil Anthing smaller than 11mil; no reduction. BGA - don't know. 0201 - don't know. Stencil thickness is 0.005" (5mil).

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Tom

#35518

Lead Free Stencil Design | 14 July, 2005

Thanks Again Everyone! One more question... Do you suggest to use 5 mil stencils? My existing stencils are all 6 mil and 8 mil. Should I change them?

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RDR

#35537

Lead Free Stencil Design | 14 July, 2005

Stencil thickness is related to PCB design and is not related to paste type. If you release with a 8 mil stencil then use an 8 mil stencil. Of course there are pastes that release bteer than other and what not but use area and aspect ratios to determine stencil thickness.

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