Plugged vias in SMD pads

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I am having a board made and talking with pcbcart about plugged vias. They've tried to help me, but I'm still not really clear on what is possible or normal.

I have several QFN/MLF devices and I want to put vias in the central pad to improve grounding and dissipate heat. To prevent solder paste from draining away during reflow, I want to plug those vias.

But when I ask pcbcart how to do this they describe what looks to me like plugged and tented vias. They want me to have the solder mask over the vias. In other words, my "stop-mask" over the central QFN pad would have holes in it.

But I don't really want that since the solder mask would prevent the solder paste from adhering to the via's annulus (ring) during reflow. I believe what I want is a normal via (which has a solder mask opening) that is just filled with something to prevent draining.

Does this make sense or am I asking for something unusual?

-Brad

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Put the solder mask over the via's on the underside of the board only, and leave the tinned VIA under your chip, it will prevent liquified solder paste from running off through the vias...

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UNiXWHoRe wrote:
Put the solder mask over the via's on the underside of the board only, and leave the tinned VIA under your chip, it will prevent liquified solder paste from running off through the vias...

And prevent the unwanted shorts during the re-flow process at via's that are very close together.

We've been recommending that our customers let us mask via's on the bottom side of PCB's that we assemble for them, as the major manufacturing defect that we see are shorts at very close via's.

So solve several potential manufacturing problems with one solution.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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Thanks for the suggestions. I found this document which mentions various plugging techniques:
http://www.atmel.com/dyn/resourc...

After reading that doc, I wonder if a underside solder masked via would still suck up too much solder. To reduce costs my vias will be fairly "large". They are only 0.3mm, but the entire central pad of the QFN20 part is just 1.6mm square.

-Brad

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Even if the VIA's fill up, I believe simple capillary action will keep enough solder between the chip and the pad to make a solid link. What thickness of board are you building?

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0.8mm boards

-Brad

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Is this a production board that will be assembled by machine or a prototype/one off that will be assembled by hand?

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Hand made prototypes, but I'll be making more than one. Maybe in the range of 20 depending how difficult it is.

Maybe with vias plugged from the back I could manually fill the tops with paste to prevent any additional wicking. I'm probably thinking about this too much... I just don't want them to fall apart during a demo.

-Brad

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I've done a number of board for large scale manufacturing with boards that have vias in the thermal pads. Manufacturing never complained and there was NO problem with either insufficient heat transfer or parts poorly soldered.

I also never heard comments about needing extra paste, etc. We worked pretty closely with the board fab folks and would have heard complaints if there were any. On the other hand, these were not the first such boards the company had done and they may have worked out their own process for dealing with it.

My hunch is that solder in those vias improves heat transfer, so that may be a Good Thing.

For what ever its worth...
Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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ka7ehk wrote:
I've done a number of board for large scale manufacturing with boards that have vias in the thermal pads. Manufacturing never complained and there was NO problem with either insufficient heat transfer or parts poorly soldered.

I also never heard comments about needing extra paste, etc. We worked pretty closely with the board fab folks and would have heard complaints if there were any. On the other hand, these were not the first such boards the company had done and they may have worked out their own process for dealing with it.

My hunch is that solder in those vias improves heat transfer, so that may be a Good Thing.

For what ever its worth...
Jim


Actually, my understanding is that you want *less* paste on the center pad than you want on the outside pads. The company I use for stuffing recently posted about exactly this: http://blog.screamingcircuits.co...

So I suspect just covering the whole pad with paste will be just fine if you have a via or two on it.

But I'd also run that by your assembly house if you don't plan on stuffing the boards yourself.

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schickb wrote:
Hand made prototypes, but I'll be making more than one. Maybe in the range of 20 depending how difficult it is.

Maybe with vias plugged from the back I could manually fill the tops with paste to prevent any additional wicking. I'm probably thinking about this too much... I just don't want them to fall apart during a demo.


Plugging vias is quite expensive for low quantity runs like this. You'd be looking at something along the lines of $10+ extra per board, at least from my experience. You could try developing some technique to plug them by hand... but that seems like a bad idea.

I think you'll be fine.

How do you plan on soldering these?

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If you are soldering prototypes by hand, simply feed solder into the vias whilst heating the pad with the soldering iron. Use large vias. It works for me.

Leon

Leon Heller G1HSM

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nleahcim wrote:
Actually, my understanding is that you want *less* paste on the center pad than you want on the outside pads. The company I use for stuffing recently posted about exactly this: http://blog.screamingcircuits.co...
Yes, my stencils look exactly like that. But the issue is that I want most of that solder to remain, not drain out through vias. Most of what I've read suggests that a solid block of paste in the center can cause varios problems (with or without vias).

-Brad

Last Edited: Mon. Feb 25, 2008 - 11:49 PM
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ka7ehk wrote:
I've done a number of board for large scale manufacturing with boards that have vias in the thermal pads. Manufacturing never complained and there was NO problem with either insufficient heat transfer or parts poorly soldered.

I also never heard comments about needing extra paste, etc. We worked pretty closely with the board fab folks and would have heard complaints if there were any. On the other hand, these were not the first such boards the company had done and they may have worked out their own process for dealing with it.

My hunch is that solder in those vias improves heat transfer, so that may be a Good Thing.


Thanks for the feedback. I assume you mean the vias you have done that worked have been just standard non-plugged non-tented vias?

-Brad

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Considering all this good feedback, I think I'll have the board house plug and tent the vias from the back-side. Seems like a good compromise between doing nothing and trying to stop all solder drainage.

Surprisingly, I asked the board house and they said there is no extra cost. Maybe that is because these are smaller runs at higher costs already.

I'll reply again when I try to make my first boards in a few weeks. Although I don't have an x-ray machine to check to resulting solder quality ;)

-Brad

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The vias we used were normal (at the small end of the standard drill size range).

No special handling, plugging, etc, was used.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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schickb wrote:
Considering all this good feedback, I think I'll have the board house plug and tent the vias from the back-side. Seems like a good compromise between doing nothing and trying to stop all solder drainage.

Surprisingly, I asked the board house and they said there is no extra cost. Maybe that is because these are smaller runs at higher costs already.

I'll reply again when I try to make my first boards in a few weeks. Although I don't have an x-ray machine to check to resulting solder quality ;)


Who is your board house? All the board houses I've talked to about plugging have said that they have to ship the boards out to another company to have them plugged.

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Quote:

Who is your board house? All the board houses I've talked to about plugging have said that they have to ship the boards out to another company to have them plugged.
pcbcart.com

-Brad

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Quote:

Who is your board house? All the board houses I've talked to about plugging have said that they have to ship the boards out to another company to have them plugged.

So the customer service person at pcbcart told me there was no extra cost. Then after submitting and paying for the order they asked for an extra $5 USD per board for the plugged vias. I just wrote back, so I'm not sure if they'll do this one for free or just make them normal open vias.

If they end up as normal vias, does anyone know where to get the "ink fill" that they plug them with? Maybe I'll try to do it myself for the fun of it.

-Brad

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schickb wrote:
So the customer service person at pcbcart told me there was no extra cost. Then after submitting and paying for the order they asked for an extra $5 USD per board for the plugged vias. I just wrote back, so I'm not sure if they'll do this one for free or just make them normal open vias.

If they end up as normal vias, does anyone know where to get the "ink fill" that they plug them with? Maybe I'll try to do it myself for the fun of it.


In my experiences with Chinese board houses - they'll quote you one price and then charge you another - always. After getting this a couple times I decided to start supporting honest business practices and have been using Advanced Circuits ever since.

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nleahcim wrote:
In my experiences with Chinese board houses - they'll quote you one price and then charge you another - always. After getting this a couple times I decided to start supporting honest business practices and have been using Advanced Circuits ever since.

In pcbcart's defense, I should mention that they understood that this was their mistake and are doing the order without charging extra. The next time they will charge for plugged vias, however. They did indicate that plugging from the bottom would be more than plugging from the top due to their process. I'll post back when I get more details.

I considered Advanced Circuits, but there are two things I didn't like about them.

1. Their price is literally 4-5 times that of pcbcart depending on the board, and they don't even test the circuits for that price (which pcbcart does). I assume the lead times, quality, and service would be better but 4x or more without even testing is just too much for me.

2. I really don't like Advanced Circuits hard-sell approach. Since submitting an online quote I've been getting very spammy email with stuff like this one: "Let Us Take YOU Out for Dinner! Get a $50 Dinner Card with every 5 new Production part numbers...PLUS FREE Tooling!. And don't forget all the phone calls about 10% of this, $75 back if you order now, blah blah blah. How about just ditching all the promo junk... and reducing costs a bit from the start. ;)

-Brad

Last Edited: Mon. Mar 3, 2008 - 09:36 PM
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I still don't see why you need plugged vias for QFN package pads. Does the chip manufacturer recommend them? What chips are they?

Leon

Leon Heller G1HSM

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leon_heller wrote:
I still don't see why you need plugged vias for QFN package pads. Does the chip manufacturer recommend them? What chips are they?

One part is an AT90USB162 and the other is an SiLabs radio. These parts are so small that it's actually quite crowded around them, and the best spot for vias is in the central pad. I'm using the vias to shorten ground distances. Whether they need to be plugged or not seems debatable... opinions vary widely.

In the future I may remove them entirely or do as you suggested and fill them by hand somehow. And if these boards ever get made in large volumes, I'll have to re-figure this all again.

-Brad