Category Archives: Laying It Out

Off-grid thoughts from a burned-out designer turned editor.

Who’s Right?

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Jack commented on my prior post, An Unanswered Question. His point was that instead of just saying “check with the manufacturer’s datasheet”, like I so often suggest when talking about land patterns, I should give more credit to the IPC and understand that many datasheets are the result of less-than-thorough study. That’s a very good point.

The challenge is that some manufacturers do a great job of figuring out how to use their packages, such as TI with its Package-on-Package (PoP) OMAP, or Freescale with some of its ZiBee chips. u-blox has done a good job of documenting paste mask requirement for its castellated mounting configuration, too. On the other hand, some other manufacturers seem to have cut-and-paste part of an old datasheet without even giving it a once-over. As Jack mentioned, with some of the newer packages, IPC doesn’t always have the data yet. I didn’t see that IPC-7351B covers 0.4mm pitch BGAs yet. It does do a good job of covering the need to segment the solder paste stencil over a QFN center pad, which I also have written about more than a few times.

I guess my thinking is that the part manufacturer should be the best equipped to tell us how to use their components. To Jack’s point though, that would be in an ideal world. But, reality rarely holds up to the ideal. Some manufacturers do quite well and some seem to virtually forget that they even made the part once it’s out of the development labs. IPC does a very good job but isn’t necessarily the most current. Then, of course, some manufacturers don’t follow IPC guidelines. Board fab houses and stencil makers have a lot of good data too, but also aren’t always up to date (nor are assembly houses).

I suspect that I get a little cynical on this subject in general because we see so many diversions from standard come through our shop. The designers, by and large, would much prefer to lay out their boards for greatest manufacturing success, but so many of them have a very difficult time finding the necessary data.

In some ways, I think the environment is getting better. More people seem aware of the need for good standards and to follow those standards. IPC seems to be pretty quick in adding in newer packages. The IPC land pattern generator is a big help. But the proliferation of new parts in new form-factors negates a lot of that gain.

Duane Benson
I’m not convinced that in net, this post has any actual content.

http://blog.screamingcircuits.com/

An Unanswered Question

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I’ve been reading through my Virtual PCB chat session transcript from Tuesday. It was a fun session and I have a much better idea of how the virtual shows work now. I think I may just be getting it.

The chat session had a lot of interesting questions and dialog. I did notice, however, that I missed one question and thus didn’t answer it. Oops.

Owen asked if I am of the opinion that all footprints should have rounded pads (probably stencil cutouts, too) to help with paste release. Sorry I missed your question.

I’m not of that opinion. There are a lot of factors that come out of stencil decisions. Paste release is one of them. There are others, some more important. For example, the shape of a pad and stencil cut out can either encourage or discourage solder balls. The size of the opening can put too much or too little paste on the pad. Wide open cut-outs over heat slugs can cause float.Bad QFN paste w caption

The pads themselves, should follow the part manufacturers recommendation for shape and size. Most are rectangular. BGAs have round pads. Unless you have a very good and very specific reason, I would not deviate far from the part manufacturer’s recommended footprint.

Some of the factors that influence paste release are the stencil thickness, whether it’s polished or not, the angle of the cut, ratio of thickness to width and paste properties. How long the paste has been exposed to air as well as the room’s temperature and humidity can also have an impact. Lots of permutations.

If you’re reading this Owen, sorry I missed your question in the chat. I hope this answers it for you.

Duane Benson
If it’s going to the EU, make sure it’s peanut butter-free.

http://blog.screamingcircuits.com/

Virtual PCB

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Drop by Virtual PCB today (March 8) to join into my group moderated chat session: “DFM: From the Assembler’s Perspective.”

Registration is free. The session is accessible by logging on to Virtual-PCB.com and selecting my chat room from the Scheduled Chats. It’s at 3:30 Eastern Standard time, 12:30 Pacific Standard time and lasts for 30 minutes.

Duane
Come for the information. Stay for the donuts.
Wait… No donuts (unless you supply them)

http://blog.screamingcircuits.com/

March Madness and Printed Circuits

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March is traditionally known for the promise of spring and for the madness that accompanies college basketball. But in the world of printed circuit boards March means the Virtual PCB event. Yes, it’s already that time again. Virtual PCB is live next week, March 8 and 9 on a computer near you.

If you have not visited the show in the past, this year you owe it to yourself to check it out. It’s easy, you can look at things at your own pace, and you never even have to get on a plane or worry about parking. All the travel is through the wonders of the internet.

I’m not going to get into all of the things that will be going on at Virtual PCB, Mike Buetow’s post on here does a good job of that. But I want to make everyone aware that Virtual PCB is live March 9 and 9 and that we all hope to “see” you there.

Stay in touch,
Pete

Hot Time in the Small Chips Tonight

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Years ago, when I worked for a local projector company, we were introducing a relatively compact projector lighted by a halogen lamp. Cooling such bulb in a big, wide open projector wasn’t a problem, but we barely had a few cubic inches of open space around the 400W “heating element” in our projector. Our engineers had to dig up old and nearly lost information about cooling high-powered vacuum tubes in constrained places.

We don’t have to worry about cooling glass devices, and big processors, big regulators and other big power components have needed heatsinks and fans for quite a while, so cooling isn’t really a new science. But the science of cooling is changing. We’re seeing more and more tiny components needing advanced power dissipation techniques.

With our projector bulbs, just sticking a fan next to the bulb wasn’t good enough. These new tiny power components, like the MCP1726 regulator, have a similar issue. You can’t just stick a heat sink on them and call it good. You need to engineer the cooling system with thermal planes, thermal vias and other layout considerations. Some, like the CMLDM7484, a dual MOSFET from Central Semiconductor, in a 1.7 x 1.7mm package, ask for aluminum or ceramic core PCBs to survive its maximum power dissipation. Using the PCB for cooling can be a lot more complex than cooling with heatsinks and fans. Anyone remember how to cool a vacuum tube base area and PCB surrounding it?

Duane Benson
Kelvons have feet, but photons can fly.

http://blog.screamingcircuits.com/

Outsourced Out of a Future?

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Alert! Self-serving Content Ahead!

I suppose I’m supposed to be self-serving now and then. I mean, I don’t personally design and build things for a living. I do that for fun and dim hopes of robot world domination. These days, I tell people about things for a living so that makes it my job to be mostly self-serving (“self” being defined as “my company”). But self-serving isn’t always bad. I couldn’t get gas for my car here in Oregon without being self-serving. I could get hamburgers though, so I’ll have to conclude that it’s a 50-50 proposition.

We understand outsourcing here at Screaming Circuits. That’s what we do — take people’s outsourced prototype and short-run production assembly work. Being located in Canby, Oregon, USA, we see both sides of the outsourcing discussion. As I said, on the one hand, we do assembly for other people. On the other hand, being a North American manufacturing company, we’ve seen a lot of work go offshore.

Not that I’m against things not in the US. I firmly believe that most of this country’s success is due to the fact that we’re from here, there and all over the world. But, I do want to have a job and I want my friends and family to have a job. We have to be worth something in this country and the rampant pace of off-shoring sometimes makes me wonder if we’re just deluding ourselves in that regard.

The prompt for this post is this article in the Los Angeles Times about Boeing’s outsourcing in the 787 Dreamliner program. The quick summary of the article is to speculate that Boeing went way too far in their outsourcing and have put at risk not only the financial success of the program, but also the company’s future engineering prowess.

Now, here’s the self-serving part. The article outlines how they went wrong by over-outsourcing, but it also points to the value of specialty companies like Screaming Circuits:

That’s not to say that outsourcing never makes sense — it’s a good way to make use of the precision skills of specialty manufacturers, which would be costly to duplicate.

That’s us. We specialize in prototypes and small volumes. We specialize in new and difficult component package sizes. We see such a variety of different types of designs here in our shop that we get good at things like QFNs and micro BGAs sooner than anyone else. (Hyperbole, perhaps, but I do believe it none the less). We’ve built things that go under water, up into space and everywhere in between. We don’t specialize in one or a few specific vertical markets, like medical or consumer, we specialize in the prototype phase of the development program across virtually all market categories.

So, outsourcing: I’m in favor of intelligent outsourcing. My advice to you: Outsource where it make sense. Don’t outsource where it doesn’t. Look at the true cost of such decisions, not just the surface image. Keep some value add in your company and don’t just become a marketing shell.

Duane Benson
Time to make my oatmeal.
One

http://blog.screamingcircuits.com/

Drip. Drip. Drip. Drip. Drip …

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Okay, water doesn’t have to actually be dripping down on to your parts to be a problem. I don’t know if you’ve heard or not, but water also comes in vapor form. Weird. I wonder if there’s a way to harness water in that vapor stage and do something useful with it. Hmmm. Ponder material for another day.

Here is something not so useful about vaporous water: It can get into your parts and make them unhappy. I was recently asked about opening and resealing moisture barrier packages for moisture-sensitive parts. This old post has a link to the IPC and Jedec standards document (J-STD-033B.1) covering this subject. It can be a complicated subject and the document is worth the read.

In sort, the best thing to do is just leave the parts sealed in their original moisture barrier packaging. If that’s not practical, look at the MSD classification. There are eight levels with one being least sensitive and six being most. (Levels are 1, 2, 2A, 3, 4, 5, 5A and 6). Most parts seem to be level 3, which can be open and exposed for a cumulative time of 168 hr. The actual safe time may vary based on your local humidity.

If you want to open and reseal, you’ll need the humidity indicator card that came with the parts, desiccant and a thermal sealing gizmo. Open the package, take out the parts you need, put the remainders, desiccant and card back in the package and reseal it. Sealing it with tape won’t do the trick. Then you would count the time that the components were in the open air toward the cumulative open time.

If there’s any doubt, just let the assembly house know that the parts need to be baked. It will probably add some time to your job, but it’s better to add a bit of time than have bad parts.

Duane Benson
Add walnuts and chocolate chips.
Two

http://blog.screamingcircuits.com/

Hello… Any Good Part Shipping Ideas?

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Ever have one of those situations where there really aren’t any good answers? There may be right answers, but not necessarily any that fit well to the specific situation or are all that convenient.

The question at hand relates to packaging and shipping small quantities of large ICs to your prototype house. If you buy them in small quantities, go ahead and use that packaging to ship to Screaming Circuits. That’s not a problem. The problem (and this question) comes when you need to purchase a bunch of them but only need to use a few at a time. This is primarily an issue for parts that come in trays.

Let’s say you have a 23 x 23mm BGA or QFP that came in a tray with 60 parts. You need to ship five of the parts for an assembly order at Screaming Circuits. We tell you not to ship them loose. You don’t want to risk in-transit loss or damage to all 60 parts by shipping the full tray. What do you do?

For leadless parts like BGAs, LGAs and QFNs, you can carefully pack each one in a small individual anti-static bag and then wrap them in bubble wrap. You can’t do that with leaded parts like QFPs. Don’t do it. Bent leads don’t solder well. And, don’t put more than one BGA in an antic-static bag. Missing solder balls won’t solder either.

For all types of chips, you could take a band saw to a spare empty tray and cut it down to the size you need. Or, better yet, you can find someone that sells JEDEC Matrix IC Trays and see if they have any that meet your size and capacity requirements. www.topline.tv or www.practicalcomponents.com are good places to start. That won’t help much if it’s noon on Tuesday and you need to make a shipping deadline of 3 pm. But it will likely help for future projects.

If you do choose to send in your full tray, we will treat all of the parts with care, bake if needed, properly reseal and pack them for the return trip to you.

Duane Benson
Three

http://blog.screamingcircuits.com/

How Many Spins?

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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

http://blog.screamingcircuits.com/

Nightmare on BGA Street

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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 1 mm 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.

http://blog.screamingcircuits.com/