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

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Stencil Design

Views: 1898


ray

#56524

Stencil Design | 16 September, 2008

Here is the scenario: We are using Lead Free Type III paste. We are using 5 Mil EFORM stencils The part we are having issues with is a SOT883. Stencil design is the SMALL PADs are 10x16mil Large PAD 14 x 24 mil. We are getting some see-sawing of the part and we are getting some shorts on the smaller pads. Does anyone have any suggestions?

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#56530

Stencil Design | 16 September, 2008

Try reductions in the "Y direction". For the SMALL PAD, go from 16-mils to 14-mils, and on the large pad, you can play around with reductions on the 14x24mil pad...or try mesh pattern. You'll be well with area-ratio guidelines with both approaches, and the reduced paste volume should reduce shorts.

Pictures would help too if you can.

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ray

#56538

Stencil Design | 16 September, 2008

Do you have any suggestions on what the area ratio should be for all PADS?

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#56539

Stencil Design | 16 September, 2008

Here are some guidelines that you can go by:

http://smt.pennnet.com/display_article/260261/35/ARTCL/none/none/1/Choosing-a-Stencil/

http://www.emasiamag.com/article-669-stencildesignforleadfreesmtassembly-Asia.html

My caveat is...I am merely acting as an encyclopedia and copying and pasting guidelines for you.

As for my personal experience, reductions usually work in solder short mitigation, coupled with optimizing screen printing parameters such as print speed and snap off (if you're equipment allows). Reduction in the 10-mil (x-direction) is a little risky, 'cause now, you're right on the border of violating area ratio. I've gone as low as 1.5 myself, but your process window tightens considerably and you need dead nuts print accuracy coupled with optimal paste transfer if you do this.

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#56541

Stencil Design | 16 September, 2008

All this nonsence talk of ratios drives me nuts.you guys should be do-ers and quit reading all these whitepapers. I fancy myself as be-ing from that school of hard knocks.No math or ratios needed for that. Speaking of hardknocks I'm in the Texas area just finished with a consultant assignment. Black's BBQ is to die for.Came up a little short on my tab and ending up washing some dishes. ::(

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ray

#56543

Stencil Design | 16 September, 2008

I am a doer myself but with the turn around and cost on EFORM stencils I prefer to be right and starting with math is always the best way. We have played with reducing apertures even stencil thickness as low as 4 mil. On the side that this component is on we have over 3500 apertures in a circuit that is about 1 x 1.5 inches.

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#56544

Stencil Design | 16 September, 2008

"....over 3500 apertures in a circuit that is about 1 x 1.5 inches..."

8O

Yeah, Larry would definitely be the go-to on this one. :P

I would think reducing stencil thickness would be your best bet to avoid that ratio problem, but how durable is a .004" stencil with more air than material in it?

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ray

#56548

Stencil Design | 16 September, 2008

10x16 by 5 mil stencil gives me a area ratio of .615 unless I am calculating incorrectly. I will try and attach some images tomorrow. By reducing the Y direction my area ratio s below .6 and I thought this was not advisable.

Your help is muc appreciated.

Ray

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#56565

Stencil Design | 17 September, 2008

Ray,

10x16 should give you .62 area ratio, Theoretical Solder Volume=800. You should increase the width and the length to get better solder release.

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