QFN/BTC Pad Configurations for Solder Stencil Use

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Hello SMTA, We currently use Solder Stencils for installati... - Apr 27, 2019 by SMTA-Joe SMTA

When you say "....is there a way to easily (or at all) refer... - Apr 29, 2019 by Steve Thomas

I wish I had that much power! They're always going to selec... - Apr 30, 2019 by SMTA-Joe SMTA

Well to some extent, but with QFNs, device manufacturers see... - Apr 30, 2019 by Spoiltforchoice

That's a good summary and the only thing I would add to that... - Apr 30, 2019 by Steve Thomas

Thanks for the suggestions. This seems like the way I'll hav... - Apr 30, 2019 by SMTA-Joe SMTA

SMTA-Joe

#82458

QFN/BTC Pad Configuration Commonality for Solder Stencils | 27 April, 2019

Hello SMTA,

We currently use Solder Stencils for installation/rework of QFNs/BTCs, and have tremendous success with them.

While they aren't overly expensive considering their high quality it does seem like a waste of money to purchase more screens than needed if this can be avoided.

Are pad dimensions/configurations of QFNs/BTCs completely unique, or are there some common? We currently have screens for ~10 different components, all with unique dimensions/configurations, and this number will be growing.

Is this something we have to live with, or is there a way to easily (or at all) reference parts with the same specs so we can pull double duty with our screens?

Thanks!

This message was posted via the Electronics Forum @ SMTA SMTA

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Steve Thomas

#82464

QFN/BTC Pad Configuration Commonality for Solder Stencils | 29 April, 2019

When you say "....is there a way to easily (or at all) reference parts with the same specs..." do you mean when ordering stencils, or are you trying to drive your designers to choose a package that you already have in play when they have package options for new designs?

Boy, if you can drive designs based on your stencils on hand you're a lucky guy! Being an EMS, we just live with what we're handed.

In a lot of cases we just apply paste manually under a scope, especially for some of those odd shaped BTC's. It's just not cost effective for us to buy a mini/micro stencil for a part with a usage of 200/year.

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SMTA-Joe

#82468

QFN/BTC Pad Configuration Commonality for Solder Stencils | 30 April, 2019

I wish I had that much power!

They're always going to select components according to their needs, but I want to know if there are standard pad layouts (complete size/shape) that are shared between components so I can re-use some screens for multiple parts.

We currently have screens for around 10 components, all with their own unique dimensions (according to their datasheets).

I just wonder if there are a limited number of these sizes that would allow me to re-use some screens.

This message was posted via the Electronics Forum @ SMTA SMTA

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Spoiltforchoice

#82469

QFN/BTC Pad Configuration Commonality for Solder Stencils | 30 April, 2019

Well to some extent, but with QFNs, device manufacturers seem to have really gone to town with inventing new shapes and sizes. So while you could certainly generalize a bit with QFPs with common shapes like a TQPFP100 for instance; thats harder with QFNs. I have two 20 pin QFNs in use here that both have 0.5mm pitch, you'd think they'd be the same but no; one is in a bigger case with a larger central pad. Take a look at TIs footprint list on their website- they have lots and they give them each a TI drawing number. Some of those might match other companies footprints, some will not. On the Pick and place side I name all the footprints using the device manufacturers names, generic QFN20 type names are simply not helpful.

The best advice I can offer is when you encounter a new shape and get a stencil for it, you keep its vitals on a list somewhere, pitch, pins, pin width, pin length case size etc. the you can at least look that up before ordering an identical stencil to one you already have.

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Steve Thomas

#82471

QFN/BTC Pad Configuration Commonality for Solder Stencils | 30 April, 2019

That's a good summary and the only thing I would add to that is maybe to include an image of the land pattern in your collection.

Some of these asymmetrical parts are just about impossible to provide all of the dimensions for. For instance, who wants to measure this?

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