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Fine Pitch Stencil Design

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Please give me some guidelines on designing Stencils for Fin... - Dec 08, 2005 by armin  

#38285

Fine Pitch Stencil Design | 8 December, 2005

Please give me some guidelines on designing Stencils for Fine Pitch.

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#38291

Fine Pitch Stencil Design | 8 December, 2005

Hi,

How fine pitch are you going? What kind of stencil printer are you using?

We had a manual stencil printer and needed to print 1-mm pitch BGA and needed to go to laster cut and electro polish at 4 thou to get good paste release. We also went 1:1 reduction.

However with the DEK being a much better printer, we went back to 5 thou, and don't have problems.

Stencil design is really the most important part of your process, apart from perhaps the oven temps. But a bad print will cause most problems. If you have more info on what kind of parts your placing, it might help some suggestions.

However if your having paste release problems, try 1:1 reduction on the problem part if possible, and go to a thinner stencil such as 5 thou, and don't use chem etch stencils. Go for laster cut with electro polish. That should help.

Regards,

Grant

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#38293

Fine Pitch Stencil Design | 8 December, 2005

I'm with Grant on the .005" foil thickness.

The key is to keep your aspect ratio (aperture width/foil thickness) over 1.5, laser cut, and I like trapezoidal aps. too.

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#38299

Fine Pitch Stencil Design | 8 December, 2005

Any good stencil house should be able to recommend the proper configuration on a fine pitch aperture design. They can also advise you as to which foil thickness works best.

The norm foil thickness is .005" - .006". There are always special instances where the norm may not apply. IE: The area and aspect ratios do not fall within acceptable limits.

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URL

#38303

Fine Pitch Stencil Design | 8 December, 2005

Start with your board first. Do you have resist/mask between the fine pitch? Is the finish HASL or a flat surface? If there is no mask/resist between the fine pitch leads you may want to reduce the apertures 5%. If the board is HASL, that means your pads are probably not flat, so trapezoids may cause gasketing problems, which can lead to defects. How wide is are your pads?

Next check your parts. J lead or Gull wing fine pitch. Are the leads as wide as the pads or smaller?Are there any large parts, like a TO-220? Larger parts may require a bit more paste than a 5 mil stencil can offer.

Next, what type of screen printer process do you have? Can it print accurately? Does it have snap-off capability? Can it perform 2D inspection to catch insufficients? Does it have understencil wipe capability? Are you using fids? A name brand screen printer such as DEK doesn�t guarantee good printing. Get to know your process.

NOW, once you have all the above answered can you finally start thinking stencils.

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Samir Nagaheenanajar

#38305

Fine Pitch Stencil Design | 8 December, 2005

Yes, yes, I agree with American man, URL, on all his points about having to know the overall process 1st. I'm surprised he doesn't have his own free consulting website.

Other things you can try are:

* "Dog-bone" apertures... taper down the aperture where paste isn't needed, but make sure the taper is within aspect ratio guidelines.

* Reduce aperture on all 4 sides to reduce overall paste volume - especially the "Y" direction. Some design guys love to use super-long pads for QFP's.

* Step your stencil down to 5-mils in those problem areas. You can get away with a couple mils step-down even using metal blades.

In Riyadh, that's the way we roll.

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URL

#38308

Fine Pitch Stencil Design | 8 December, 2005

Also the zipper pattern will work too.

Samir, why are you calling me a man? Last I checked, the only thing hotter than these double D�s were my behind.

That�s how we roll in Cali.

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Rob

#38330

Fine Pitch Stencil Design | 9 December, 2005

Madam, that's rather forward!

By zipper pattern do you mean offsetting the apertures on every other ultra fine pitch qfp pad?

Thanks,

Rob.

(we don't roll in England, we amble)

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URL

#38334

Fine Pitch Stencil Design | 9 December, 2005

Sorry Rob, just what the guys tell me.

Yes, on the zipper question.

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Rob

#38335

Fine Pitch Stencil Design | 9 December, 2005

I'll rephrase that - you've got rather a lot forward!

Thanks on the zipper - wasn't sure if there was another trick to learn there. We used to use it on crap free issue boards with no solder mask between the pads, and on ultra fine pitch before we got decent placement machines.

Cheers,

Rob.

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#38443

Fine Pitch Stencil Design | 14 December, 2005

some of th IPC design guidelines have helped my factory understand the importance of stencil, (before me they used 9 year old guidelines) carzy i know, if you are interested in stencil design then listen to what everyone else has been saying however, find yourself a good stencil manufacturer its there job really just tell them what machine you have and they should do the rest.

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URL

#38450

Fine Pitch Stencil Design | 14 December, 2005

"find yourself a good stencil manufacturer its there job really just tell them what machine you have and they should do the rest."

I have to dissagree with this last statement on so many levels. There's a reason why it's called engineering. Plus, when your boss is asking for your arse cause you have 40% fallout for solder shorts, will you really have your stencil maker come in and figure it out for you?

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#38452

Fine Pitch Stencil Design | 14 December, 2005

the original question was: Please give me some guidelines on designing Stencils for Fine Pitch. which i did however, 40% fallout for solder shorts this could be caused by many other processes having gone wrong before, i.e. type of solder paste and oven temperature profiles, stencils would be the latter of these things that i would check for faults,,,, or maybe i am just lucky and have a great stencil manufacturer who checks my stencils for problems, how ever small they maybe.... latest problem i have had was heat sync under SMT comp. and too much solder paste resulting in tin balls and weak solder joints from the component legs, this was solved by my stencil manufacturer who spotted something i had missed.

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URL

#38453

Fine Pitch Stencil Design | 14 December, 2005

Yup, I saw the original question too. I just can't see how any one would let a stencil manufacturer dictate their processes. Maybe if your designs are small and straight forward, but are you really going to let a stencil maker tell you what products will be either double sided reflow or wave soldered?

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#38454

Fine Pitch Stencil Design | 14 December, 2005

Aperatures are often best decided by the stencil manufacturer principally because they would know what is easiest and best for there laser cutting machines, why would i dictate to them something that would not work well for there machines. but are you really going to let a stencil maker tell you what products will be either double sided reflow or wave soldered? dont be silly ;) however i work for Enics.com in the NPI Engineering dept. and most is industrial electronics, so my experience is limited to that. But generally double reflow is the norm here...with press-fit comp.

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#38455

Fine Pitch Stencil Design | 14 December, 2005

Also we have just made a prototype for Wave soldered SMT components D-packs + 0603 + TSSOP�s on the bottom side with THT comp. on the top, the stencil manufacturer gave his personal library of glue stencil openings and adviced on stencil thickness which was over 0.2 mm thick which was new for us, info they gave to us was useful to the project. Thats what i meant not engineering with the PCB my apologise if you didnt understand my writing.

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Champ Kind

#38456

Fine Pitch Stencil Design | 14 December, 2005

Snaggletooth,

I spent a good portion of my career fighting solder shorts on BGA's and QFP's, tombstones on 0402's, solderballs on passives, etc.. I've solved just about all those issues, and didn't solely rely on my stencil manufacturer...

Your stencil manufacturer, true, can provide general guidelines and techniques like homeplate design, oval design, recommendations on %-reduction, etc., but ultimately, it's up to you to know your process and board designs.... here's a few examples:

Process - how your paste behaves, release characteristics when you're at the "hairy edge" of violating aspect ratio, your profiling strategy, etc...process capability of your printers...

Designs - do your designers use correct pad geometries ont their passives, or are they too far apart - if they're too far apart, your your parts won't touch the homeplate "tips". Do you designers use overly long pads on QFP's; if so, there's room to reduce on all 4 sides....do your designers violate BGA-to-via spacings and/or not enough resist in-between... this is the type of stuff your stencil manufacturer DOESN'T know.

A good Engineer will know these things, have a feel for his processes, use CAD tools to design his apertures, and do "what if" scenarios with his aperture designs. He'll look at the design, and think about all the things above...

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#38457

Fine Pitch Stencil Design | 14 December, 2005

good points but to answer your questions... Although i am aware of most of those potentail problems we havent had most of them, fingers crossed. Unfortunately we have 2 sub-contractors doing the PCB palnning and design which mean i have to solve there problems with stencils design. but the original Q was about fine pitch stencil design not pad design. I have had to transfer half of the stencils ihave designed to oher factories with different solder printing machines and have had no complaints... so far working with-in the limits of many different print machine is difficult, I generally follow IPC 7525 and it works for mostly everything so far. and didn't solely rely on my stencil manufacturer... and i believe i said helped not solved thats what i am paid to do ;)

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trek fan

#38462

Fine Pitch Stencil Design | 14 December, 2005

Way to go! Now my favorite "deep space nine" website has turned into a "URL" fanclub. You should know better than to post that HERE!

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