Via Current Capacity

Over on the Circuits Assembly blog, Michael asked a question about my Via in Pad Myth #5. He asked:

"I have a question about vias. I have seen charts on the current carrying capacity of traces, but what about vias?"

That's a good question. I've heard that you first need to know the thickness of the via wall. Then, once you know that, you can calculate the trace-width equivalent for the via by using the formula for the circumference (diameter X pi ). For whatever number that gives you, compare the closest smaller trace width.

Via cross section My related questions to all of you PCB fabrication gurus out there are:

Since vias are not created in the same way as the trace plating is, can that simple formula be used? While the trace copper is laminated onto a nice smooth PCB surface, the vias are typically created by deposition of copper dust in the via and then electroplating more copper. Then the surface finish is applied to all of the exposed metal. The via walls would generally be rougher than the flat substrate surface. Does that have an impact on the current capacity of a via?

Further, since airflow will be somewhat restricted in a via relative to a surface, should the via effective width be compared to an internal trace instead of an exposed surface trace? Should it be a compromise between the two?

If you look closely at this via cross I pulled from Wikipedia, you can see that the via wall looks to be thinner that the traces. You'll have to make sure that your board fab house can give you an accurate thickness of the via wall.

Duane Benson
If you know the via current capacity, can you calculate the past and future capacity?

Random Via-In-Pad Myth #5

Myth #5: When you need thermal vias, more is better, bigger is better

Hmmm. Logically, this would seem to be the case. There are limits though; especially if you want a reliably assembled product. Older parts with heat slugs easily accessible for bolting on heat sinks didn't have this issue. Just bolt on a piece of metal and maybe blow a fan across it. It's different with a lot of the new, Padinvia smaller surface mount packages. Many have a heat slug on the bottom which requires carefully placed thermal vias to a copper pad on the underside of the board.

An extreme case of flooding the land with vias can be seen in this illustration here on Padinvia_alt the left. In terms of assembly, you can hack this together for a prototype, but it'll never fly in a production environment.It would be much better to use fewer smaller vias and have the center land covered with solder mask except where the metal on the chip is exposed, as in the illustration on the right.

Duane Benson
Place one carrot seed in each via and cover it with planting soil

Loopy Ground Loops

A while back, I posed a question about using flood fill (AKA copper pours). I've been reading a lot about ground loops lately which brought me back to that original question.

LED scroll ground plane Some people suggest segmenting your ground plane between analog and digital sections. Some people suggest segmenting the ground plane for individual critical ground return paths. The follow on to my original question is: On non-exotic designs does segmenting ground planes really help? There's actually two questions, with the second being: At what clock speed does it make sense to start worrying about issues caused by ground return paths / ground loops? There are probably more questions. Those are just the two rattling around in my head at the moment.

Interestingly, though, when I wrote the original post, there didn't seem to be a clear "most common" between pour and no pour PCBs. Today, I'd have to say that the majority of designs we see here at Screaming Circuits do use flood-fill ground planes, either internal or external.

Duane Benson
You can solve ground and noise problems by just not hooking up power

CAD This or CAD That

I use Eagle CAD a lot. I can get away with the "Light" version, because the designs I create are small and non-commercial. I do use them sometimes to illustrate points here on my blog, but I think that still meets the qualifications of their free version. It's a good program and the multiple license levels from the free version up to the full professional version add a lot of flexibility to have the software grow with you.

Our partner, Sunstone, builds most of our PCBs here, which is a nice segue into an alternative CAD package. There are a lot of reasons to pick one CAD package over another. I won't go into that here because those reasons tend to be specific to the application. Most CAD packages are sold as a lump-sum purchase up front. A lot of them also have yearly license renewal fees. That works sometimes, but there are other times where up front costs are more important. The model that Sunstone uses for PCB123 is to provide the software at no charge and just add a little tiny bit of the software cost onto the PCB board purchase.

PCB123 isn't the only package that follows this business model and is tied into a specific PCB vendor. But, as far as I can tell, PCB123 is the only package of its sort that has enough capability to be a viable replacement for more traditional pay-first CAD packages.

I recently downloaded V4.1.11 and have started to run it through my own personal "can I use this for my stuff" test. I know it's a good package because we, here at Screaming Circuits get boards of all sorts designed with PCB123 to assemble from all manner of company. But, something can be a good package and still not fit an individuals specific requirements. Hence my personal tests.

I do find it odd, but not really an issue, that it starts you off in the layout editor instead of the schematic editor for a new design. Oh well. One click and I'm in the schematic where I can search for my parts. I use PIC chips and it's pretty rare that I find the exact chip. I always seem to have to find something close and then modify it, which just adds more opportunities for error. I know there's a jillion 28-SOIC,M28B_sml varieties, but once in a while it would be nice to just find the actual part.

Fortunately, today I'm looking for PIC18F2320 in an SOIC package. Fortunately, because it's actually there! I hit the "Insert" menu and choose "Add Part". Then I put "PIC18F23" in the search box, and there it is, but not on the computer. It was in their online labraries. (In the cloud?) It took all of about 15 seconds to automatically download the library footprint though, so first test = passed.

And the really cool thing is that once I have that part in there (for the parts found pre-made in the library), I just select the "Bill of Materials" tab down on the bottom and I can see if DigiKey has the part in stock and how much it costs.

Duane Benson
If it's in Oregon, the "cloud" is probably a rain cloud

How Many Spins?

The other day I wrote about my failure to follow my own advice. Obviously, advice is only for someone else. Just like the best standards are double. Right?

Hmmm. It got me to thinking about board spins. Years ago, I remember products produced by the company I worked for often coming out with double-digits worth of mod wires in production PC boards. I think with the ability to turn PCBs in a day or a few, that rarely happens anymore. But what about in the prototype stage?

Here at Screaming Circuits, surprisingly few repeats show up other than from people using us for small-lot production. We do see a lot of layout issues here, but likely we see a lot because we see a lot times a big multiplier of different designs here.

For my little dohickeys, I seem to need about one board spin due to design or layout problems for each five designs. Of course, mine are pretty simple. Most of my boards spins are due to me coming up with better ideas after using the thing for a while.

If my supposition is true that mods are required less often now, is it because designers are better now, tools are better now or components are better now? How many times do you typically re-spin a PCB due to design or layout problems?

Duane Benson
Four

Nightmare on BGA Street

I seem to be in a bit of a BGA mood lately. I do that sometimes - pick a subject and talk it to death before moving on. Well, maybe not quite talk it to death, but at least talk it to the pain.

9x13 via in pad BGA land Take a look at this land pattern for a bluetooth module. Anyone see anything odd? Yeah. All of those really big open vias. I know what the designer was trying to do. A good number of the vias are ground connections of one sort or another that need to be connected to an internal ground plane layer.

Given that is is a 1mm pitch BGA, there is plenty of room to put the vias between the pads and not cause any trouble. That would be one recommended approach. The other would be to have the vias filled and plated over at the board house. No matter what you do, though, the vias can't be left wide open like this. It's a real bummer.

Duane Benson
B.V.O.U.S.'s? BGA Vias Of Unusual Size. I don't think they exist.

Cute Wiring

Yesterday, I wrote about my foibles in ignoring my own advice. As SiliconFarmer pointed outRework 002 cropped over on Twitter, it's not just something you need to do when you're re-purposing a close land pattern. Sometimes even the "correct" pattern can have the wrong drill size or a few mixed up pins.

The bottom line is that if you want to reduce the chance of scrapping some expensive PCBs, or having spots that look like what I did (on the right here), check your land patterns.

I couldn't find my wire-wrap wire late last night, so instead, I used the leads from old thru-hole resistors. It's kind of a mess, but I do like the hatch-markish look that I gave it.

Not to shift any blame off of myself, but I do find it quite annoying when a part falls into such a common standard configuration, as in three-terminal regulator, but the manufacturer picks a different pin-out.

[Note that this is rework I did myself at home. The folks here at Screaming Circuits do  much, much higher quality work.]

Duane Benson
The problem with unwritten rules is that they're unwritten

A Bit More On the LGA

After my last post about LGA land patterns, I received a couple of questions asking for more detail in a few areas.

"The LinearTech  LGA apnote (LTM46xx series) shows planes on the mounting layer interconnecting pads that are solder mask defined. This is supposed to be for heat dissipation. Will smaller copper defined pads and vias to full internal copper ground and power planes provide adequate cooling?

What about using LGAs on the same layer as BGAs? BGAs have copper defined pads? We've been sending 1:1 soldermask gerbers to the fab house so they can adjust per their process. Can this be done selectively so the SMD LGA pads don't grow bigger? What kind of Fab Note should be in the "Readme" file?

Also, please warn LGA users to be careful using wizards (eg Pads Layout) to generate the pad numbering. Linear Tech's LGA does NOT follow the standard BGA alpha numeric numbering. I don't know about other LGA mfgrs numbering systems but ... Double check the pad numbering and avoid this nasty snake bite!"

First, as far as cooling goes, the answer, unfortunately is "it depends on how closely to the limits you are driving to part." You will get best results with more surface copper. That being said, you can use vias to internal and back-side planes to increase heat dissipation. Ideally, you would have Lot's of surface copper and vias to the internal and back side planes, but that's not always possible. The vias that are not under the LGA pads can be left open. Any vias in an area to be soldered must not be left open. Ideally, you would have them filled with a thermally conductive material and plated over. You do have some flexibility to reduce the surface copper and replace it with vias to other planes, but ultimately, the final answer will only come from your design testing.

You can have NSMD and SMD pads on the same PCB. How to do it is the big question here. Many fab shops will make their own decision on what is "best" for your PCB in this regard. I would speak with the board house and get their recommendations on how best to specify what you need in terms of NSMD and SMD mixed. You'll probably have to follow a slightly different procedure for each different fab shop.

I would double echo the comment about using caution when using wizards to create a land pattern. Not all manufacturers follow the same numbering scheme. You could get bitten badly with this one.

Duane Benson
Who was that soldermask defined man?

What about the LGA?

I've written a bit about soldermask defined (SMD) vs. non soldermask defined (NSMD) pads for BGAs.

Quick summary: 0.5mm pitch or wider spacing, go with NSMD pads. 0.4mm pitch seem to need SMD pads to prevent bridging (unless the pads are staggered. Then NSMD is fine)

But what about the LGA (Land Grid Array)? It's different due to not having the solder balls. Does that make LGA a difference? According to Freescale and a few other manufacturers, in most cases, you should treat an LGA just like a BGA and use NSMD pads. However, if you need extra strength holding the pad on to the PCB, you may want to consider using SMD pads. As always, consult the data sheet for your specific part for the final word.

Duane Benson
Checkers anyone?

What's Missing?

LED cathode There is something about this PCB that will likely cause trouble for anyone assembling it. The first three people to correctly identify the issue get a Screaming Circuits 1GB USB drive.

I know, this day and age, 1 GB doesn't sound like much. But you can still put a bunch of pictures on it. More importantly, you can use it to back up your CAD files just in case disaster strikes your workstation.

This issue is, unfortunately all too common . It's probably a case where the designer knew exactly what he or she was intending and could easily build it up without any problems. However, when sending it out to a third party for assembly, that "in the head" knowledge doesn't help much. If you've got a PCB that you've been assembling yourself and later send out for assembly, make sure you aren't assuming that the assemblers can derive what's hidden in your brain.

Duane Benson
Beware the monsters from Id.