Would You Like Parts With Those Frys?

In days of yore (last month), the PCB design process would more or less start with the schematic. (I'm ignoring all of the work that has to take place before the schematic) The bill of materials would typically come together during the schematic phase and get refined during PCB layout.

After that, it would be time to enter your bill of materials, a line at a time, into a parts website, upload your Gerbers to Sunstone Circuits for boards and then (hopefully) send the boards, parts and files to Screaming Circuits for assembly. If you're hand soldering, you might skip the last step. We hope you don't, but certainly understand if you do.

But here's something new:

At the Screaming Circuits website, you can now quote and order all three in real time.

1. Quote the assembly labor
2. Quote the cost of Sunstone PCBs
3. Upload your BOM and quote parts

If it looks good to you, you can place the order right then and there. Of course, we still have folks to talk things through on the phone if you have questions or need non-standard services, but if you like what you see, it's a real quote and real order. No waiting for an email quote back. No going to three websites.

As always, we’ll happily build as few as one board or up into the thousands (or hundreds of thousands in our EMS division). Once we have your parts and boards, we can assemble it all in as little as one day.

 

Sharing With Sunstone

Sharing is what our partner Sunstone Circuits would like today - sharing your stories. We've been partners with Sunstone Circuits for a very long time. When we purchase boards turn-key, we go to them first. They even build a fair number of the boards that we don't personally order. We can tell by the labels on the boxes on our receiving dock.

Naturally, when I had some projects to build, (like this one) I went to Sunstone for PCBs. I've run a few other projects with their boards as well. The reason I bring this up is that they are running a design story contest right now (stories.sunstone.com). It looks to be a fun way to run a contest and you can make a game of it. They want you to submit a story involving their PCBs, share it and get your friends to vote for it. The contest ends December 16 of this year, so if it's the year 2015 and you're reading this in the archives; too late.

Personally, I would recommend that everyone vote for my story, except that I haven't submitted it yet. And, if I do end up submitting mine, I'll be like a third party candidate and steal votes from one of the true contenders. Being a Sunstone partner, I wouldn't be eligible for a prize, but I could still upset the whole balance of promotion by grabbing votes that would otherwise go to a more worthy candidate. So, if I ever do get my story uploaded, don't vote for me.

Duane Benson
I'm not red or blue. I'm sepia

It (.3 mm) Finally Happened

Back in January of 2012, I wrote about the possibility of 0.3 mm pitch BGAs being used here and there. I predicted that in a year, we'd see some 0.3 mm pitch BGAs showing up. I was about three month's off. Almost to the day.

I delivered a session at PCBWest last month and asked if anyone had used a part with that pitch yet. One hand went up. That actually surprised me. What surprised me even more was when one of them (a .3mm pitch BGA, not a hand) arrived on our shipping dock in a parts kit earlier this week.

For comparison, the land pattern for an 0402 passive component is about one millimeter long. This specific part is just shy of a millimeter square. Even as small as it is, this part can supply 750 mA continuous. The olden days are so very long gone.

We do many, many complex parts and PCBs. We've put 5,000 parts on a single PC board. We've built boards to be shot up in rockets and dunked way down in the ocean. Some very crazy stuff has come though our shop, but we don't do everything. We don't do 01005 passive components at the moment. Our machines have the technical capability, but we don't rework them, which has to go along with the assembly capability, so we don't support that form factor for now. 0.3mm pitch components pretty much fall into that camp. Our machines can physically pick up and place the component, but until we've developed to process to assemble those parts with the quality people expect from us, we won't be supporting them.

I expect we'll be getting more and more requests for the form factor, so we'll be looking at it. Keep checking back. One of these days, we'll have the process down and reliable.

Duane Benson
It's (Huey mm, Dewey mm, and Louie mm)/10

Missing Mars Probes

Back in ancient times when multi-legged beasts ruled the earth, there were a lot more standards. Or maybe there were just fewer total things resulting in fewer total variations, which looks like more standards.

In any case, if you got a 7408 IC from one manufacturer, it was pretty much equal to a 7408 from any other manufacturer. Even connectors were more or less standard. If you plugged in one PCB mount DB25, you could plug in just about any PCB mount DB25. There were variations, just not as many as now. Today, though, there are a very large number of variations to a standard footprint. For example, while the pin footprint on most Ethernet jacks matches, I've probably seen a dozen different arrangements of mounting and alignment pins.

Another area that can throw monkey wrenches all over is the dreaded metric v. SAE units.

This seems to pop up most often with connectors, as in this image, but it occasionally shows up on other types of parts as well. The footprint here is for a .1" (2.54mm) pitch connector. The connector has 2.5mm pitch. It would be fine for three pins, maybe four or five. But beyond that, it's just not going to fit.

I don't really understand the logic in 2.5mm pitch. If .1", which equals 2.54mm weren't such a ubiquitous standard, 2.5mm would make sense, but as it is, it's just too close. It's close, but they aren't the same. 2.5 != 2.54.

Duane Benson
It doesn't seem like much difference in mm, but in beard-seconds, it's 4,000* units off

*By some definitions, including the Google converter, it would be 8,000 units off

 

Reliable Assembly

For those of you at my PCB West session on the 27th, thanks for attending. Here's the final presentation as delivered:

Download PCBwest2012 DuaneBenson ReliableManf

Duane Benson

Coming Soon!

If  you happened by our booth at DesignEast, you may have gotten a personal preview of our new automated parts quoting system. If you didn't get to see it, you will shortly. It's in the final stages of beta.

 

This sample shows what you might see when you order Screaming Circuits assembly along with Sunstone PCBs, and components from our website.

In the meantime, you can still quote your assembly and PCB prices online here, and you can have us quote your parts offline.

 

 

 

 

Duane Benson
Maybe not today. Maybe not tomorrow, but soon and for the rest of your life.
You won't regret it.

Via in Big Pads

The answer to the question: "is it ever okay to put open vias in BGA pads?" is simply No. It's no, no, no, no, not ever!!! That makes it easy. No technique to worry about. No tolerances. Nothing. Just don't put an exposed via in a BGA pad. The only option is between the pads, with a complete soldermask dam between the pad and via, or have the vias filled and plated over at the board house. Nothing but metal is allowed on the BGA pad.

Now, other components give you more flexibility and thus require some choices and guidelines. Andy B. asked about large components, such as voltage regulators where the manufacturer has recommended vias to connect the thermal pad to the ground plane, or to additional thermal area on the back side of the PCB.

The easy answer is to just treat it like a QFN and read our various suggestions surrounding that form factor. Here's some. Having the extra room does allow for additional flexibility, but if the vias are open, they still run the risk of sucking solder to the other side of your PCB. You can sometimes get away with really tiny vias, as in here. But it's not best-practice.

It's really a matter of trade-offs. I have seem opinions stating that you should never fill or cap the via because doing so might impede the thermal transfer. Well, power chip manufacturers, you shouldn't rely on unbuildable design to meet product specs. You can fill the vias with thermally conductive material. You can cap the via with solder mask, as in the link I just gave you. Just make the via cap as small as possible - 100 to 125 microns larger than the via.

Finally, segment your paste stencil layer. If you put solder paste on top of an open via or even on top of a masked via, you can be asking for trouble. In this image, the six vias (which will be capped) are put between the openings of the stencil.

Duane Benson
Tesla says what?

Via in Pad x 8

Here's an interesting via in pad case. On the one hand, the footprint is very symmetrical and clean looking. On the other hand, it has open vias in the pads.

At first glance, I thought this was a DIP footprint with extra long pads, but it's not. It's for an SMT part. Personally, I would have put mask between the pads. Looking at the rest of the board (not shown), the spacing between pads and mask is pretty wide, so there may be a good reason. I'm not sure though.

Definitely, though, I would not put the vias in the pads like that. Those open vias will cause solder to flow down to the other side of the board, make a mess there and leave the chips without sufficient solder.

Duane Benson
Sucking solder through a straw - or via

More Thermal Examples

Speaking of thermal relief... Here's an interesting example I ran across the other day.

Thermal relief can be a pain. If you've got a high current device, you may want more than just the thin little connections, one per side, that you get with thermals. You might feel the need for greater current capacity or you may need all the copper to distribute heat. You might have one pad, like in this image, that lands on a plane but not the other one.

In this particular board, the designer just made a few parallel traces coming out of the pad rather than one thick one.

The other side of this passive part sits right on the ground plane and has the standard thermals so the other reason this might have been done is to keep the amount of copper trace coming into the pad to be equal to that on the other side. It doesn't have exactly the same amount of copper going into both pads, but it's much close than if just one thin trace had been connected on the left pad.

Duane Benson
One thin trace rides away

Passively annoying

Passive components can be kind of offensive sometimes. I can understand them in analog circuits or charge pumps. But the fact that we need to put them all over our digital logic is just rude. Technically, I understand why they have to be there, but philosophically, they violate my basic principles of life.

Back in the early days of personal computers, there allegedly was a company that had it's engineers remove bypass caps one by one until the motherboard stopped working. Then they'd add the last one back in and smile about the short-term cost savings. Well, that was a bad idea. The reality is that we need them.

I've written about some of the problems that can show up because of passives (or other small two lead parts like LEDs and other diodes). Like here, here and here. That last example has popped up recently and I have some more thoughts on it. Essentially, I'm talking about multiple two-lead components that have one lead tied together. That's a pretty common scenario with bypass caps or LEDs (or the LED current limit resistor).

There are a couple of ways to do this. Some error prone and some not. First, the general rule of thumb for two lead passives is, if at all possible, to have the same amount of copper going into both sides. That means that if you have one 8 mil trace going to one pad, have one 8 mil trace going to the other. Also make sure that you have solder mask stopping the solder from going off pad.

Method A here is bad. It might just barely meet IPC standards, but it still is really not manufacturable. First, there are no thermals. That makes the solder melt much slower on the right side which can lead to unreliable solder joints or tombstoning.

Second, even though the theoretical solder mask openings don't touch and the keep-out (it's not shown but is just a hair narrower than the mask area) areas don't touch, they are close enough that you might not have any mask between the parts on the thermal pad. That can lead to components shorting.

Method B is also bad. You have your thermals in there so that's good, but the parts are still so close together that you might not get any mask between them, leaving a path of bare copper between the parts that can cause them to drift around and mess things up.

Method B1, on the right here has the same issue. Likely no solder mask between the parts and a bare copper path between the parts.

 

Method C here is fine. The parts are still at risk of not having mask between them, but there isn't bare copper running straight between them. There will be mask between the parts and the pad so there isn't any way for solder to bridge or the parts to drift.

 

 

 

Method D here is also okay. You do need more room to spread the parts apart. That's a bummer, but sometimes "bummer" is the cost of reliability. Here, there will be solder mask between the parts and there are thermals. Everything is happy.

Use method C if you have a little side to side room to play with or method D if you have a little top to bottom spare room.

 

 

Duane Benson
Prevent flanking maneuvers.
Don't be like the Solders at Thermopylae