Category Archives: Laying It Out

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

Hand to Machine

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It’s getting very difficult to hand solder many parts these days. Some people give it a try, but in general, if you’re dealing with the really tiny parts or leadless parts, it’s just not possible, or at least not  practical.

QFN worng part library Sometimes a designer will start out with the idea of hand soldering the board up and then either decide against it when first looking at the raw PCB, or will build one and then decide that it’s too much work. That’s not a bad thing. You can get more reliable assembly and it keeps me employed. But there are times when a layout designed with hand assembly in mind does not work for machine assembly.

Case in point, this image. Now there are two things wrong here. The first is that the land pattern is for  a smaller part than the actual component. Let’s pretend that problem doesn’t exist. The other problem is that big via hole in the middle of the pad. When hand soldering parts with a solder pad underneath, like QFNs or QFPs, folks will often put a large hole there. They’ll solder the outside connections first. Then, turn the PCB over and stick a soldering iron and some solder in that big via to solder up the pad.

That works more or less for hand soldering, but it’s a really bad thing to have a big open via like that when machine assembling parts. The solder will flow down and out the other side. You’ll get a mess on the bottom of the PCB and you may get little or no solder on the pad.

So, the moral of this story is that if you’ve designed your PCB for hand soldering and later send it out for automated assembly, go through the layout and make sure you remove things put in there for hand soldering that aren’t conducive to reliable machine assembly.

Duane Benson
Don’t fall in…

http://blog.screamingcircuits.com/

0.4mm Pitch BGA Redux

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I’ve written about it before, and again here.

When dealing with new technology parts, it’s really important to look up all of the manufacturer’s component information that is available. I’m going to quote from the Texas Instruments document “PCB Design Guidelines for 0.4mm Package-On-Package (PoP) Packages,” Section 10 (PDF page 8):

Industry reliability studies have revealed that NSMD-type pads are highly recommended for most 0.5mm pitch BGA applications. However, there is a problem with this approach at 0.4 mm pitch.

Real-world assembly experiments with the BeagleBoard and the OMAP35x EVM revealed a tendency for solder bridging between pads when NSMD were used. There was insufficient solder mask webbing between the pads to ward off bridging. Therefore, a SMD design was used which resulted in much better assembly yields with no solder bridging.

If you are using a 0.4 mm pitch BGA with the balls aligned in a grid (as opposed to staggered), read the design guidlines from the manufacturer before laying out the board.

In a presentation about the development of the Beagleboard, Gerald Coley, Beagleboard designer, notes that their first two runs had non soldermask defined pads, resulting in a 10% yield. After another run of PCBs where the pads on the PCB were the same size as the pads on the device and the PCB pads were soldermask defined, yields rose to 96%. And verify that your PCB house does in fact follow your instructions. Some will think they know better and will change the mask layout.

If you are still unsure or think your design will have different requirements, call an applications engineer at the component manufacturer and discuss your project and the layout.

Duane Benson
Trust but verify

http://blog.screamingcircuits.com/

Et Tu, Embedded Passives

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I don’t know if or when embedded passives will become the “next big thing” in PCB design, but they are on the way. We at Screaming Circuits have been asked about the use of embedded passives a few times.

Embedded passives
The purported advantages of the technology lie primarily in the ares of cost reduction and space reduction. You could potentially get your bypass caps much closer to where they need to be as well. The space parameter is pretty obviously an advantage, but the jury is still out on costs. I suspect that at this moment, it’s pretty difficult to find a board house that can fabricate a PCB with embedded passives.

If you’re not familiar with the concept, capacitors or resistors are built up on the inner copper layers of the substrate. There are a couple different methods used such as plating, printing or thin-film. As shown in the illustration, the resistors and capacitors inside the PCB negate the need to mount them on the outside. I can see rework being a problem if any of those embedded parts has issues.

In terms of assembly, we wouldn’t treat such a board any different than any other PCB. If your fab house notes that there are temperature or any other restrictions, let your assembly house know. Beyond that, all the standard rules apply.

Duane Benson
Note from Forbin: Colossus is watching

http://blog.screamingcircuits.com/

On Final For Landing on Runway 0805

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S part pad shift in oven process Keep out areas can be a problem when adapting a CAD component land pattern, but that’s not the only potential problem. Sometimes the part may be close, but the footprint is different enough to cause problems, as in the picture on the right.

You can also run into issues that don’t necessarily cause PCB assembly problems, but can be expensive nonetheless.

Say you are designing with a small microcontroller and the schematic symbol and land pattern don’t exist for the one you’re using, but something close does. Even though the two parts may look like pin for pin replacements, they may have a few differences.

The PIC family has a number of examples of this. For example, the PIC18F2321 and the PIC18F2455 have enough similarities that they look like pin for pin replacements. However, upon closer inspection, you’ll find that RC3 exists on the 18F2321, but doesn’t on the 18F2455. SCK/SCL and SDI/SDA are in differnt places on the two processors. You could end up with a bunch of jumpers and a PCB re-spin if you just used one land pattern for the other. It pays to check for those little details.

Duane Benson
Turn left at the big tree, and go until you see the creek.

http://blog.screamingcircuits.com/

Keep Out and Keep Off the Grass

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The part land patterns in your CAD package parts library should all come with a valid keep-out area defined. That’s the theory anyway. However, theory and practice don’t always match up so well. I’ve written a lot about such things as verifying that the actual footprint matches the physical part. That’s important. But it’s not all that’s important.

Sometimes the footprint will match up just fine, but the body of the part won’t. This might be because you’re using a different variant of the same part or perhaps a different part altogether that just happens to have the same footprint.

It’s common practice to modify an existing library component for a new part, but if you do that, make sure you consider the physical dimensions of the part body too. Sometimes you can get away with the keep-out area being different, but as shown in this image, not always.

Duane Benson

Via Caps

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The best way of dealing with via-in-pad requirements is to have the vias filled and plated over at the board house. There can still be some issues with that technique, but if done right, is very reliable and the best way to go. But, what do you do for a Klondike bar? I mean what do you do if you can’t afford to fill and plate over your vias, or don’t have the time? Well, if your vias are in a BGA pad and you don’t want to have them filled and plated over, than you can just go back to the drawing board because your board most likely won’t work. There may be some secret, prototype-only things that your assembly house can do, but avoid it at all costs.

If your vias are under a QFN or SOT-23 or something bigger like that, you have more options. Don’t leave them open. Worms will use the open vias as homes. You can usually use soldermask to cap your vias in cases like this.

C is not an option. A and B can work. D can work also, but should be your last choice. With the tight masking like in A, most manufacturers recommend that the soldermask cap be about 100 to 125 micron wider than the diameter of the via. B is self evident – just make it bigger.

The pitfalls with D are that solder will still get sucked down into the via which can lead to excess voiding. Or, the via cap could pop open which results in effectively a C. With all soldermask via caps, top ot bottom, you need to check carefully to ensure that the soldermask gives a complete seal. And there is always a risk of the cap breaking. Ugh. Now do you see why everyone recommends filling and plating?

Duane Benson

I’m a capitalist and I’m okay…

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I’m a capitalist and I’m okay
I work all night and I sleep all day

Well, I don’t work all night so much anymore. I used to.  I am a capitalist though. I think money is good (what some people do with it, not so much). And, I also think that when people make money, they should do so in such a way that others benefit as well. That’s one of the reasons I like Ti’s Beagleboard so much. Speaking of Ti, they have another microcontroller product that I’m excited about as well.

The MSP430 LaunchPad is a little development board designed for education in general, as well as familiarization with the MSP430 line of microcontrollers for experienced developers. (I hope they don’t mind that I’m using their picture. I don’t have one so I couldn’t take my own picture of it.)

A lot of companies have development boards for their chips. That’s nothing new. But what is really cool is that they have set a retail price for this of $4.30. Yes, the price of a 16 ounce latte.

Now I know that a latte is important. Some people have speculated that civilization would collapse without caffeine. But, here’s what you get in the place of that latte: (from the Ti website, again)

“For $4.30, the LaunchPad includes a development board, 2 programmable MSP430 microcontrollers, mini-USB cable, PCB connectors for expandability, external crystal for increased clock accuracy, and free & downloadable software integrated development environments (IDEs)”

Cool. I can explode one and still have another to finish the project with. I’m going to get me one of these and spend some time with it. I don’t have any personal experience with the MSPP430 line, so it will be filling it’s primary mission.

On the subject, I ran across an interesting website dedicated to the MSP430: www.43oh.com. If you’re already a 430er or are just intrigued by the chip, go check it out.

Duane Benson
Buttered scones, anyone?

Monsters Under the Bed

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TO-263 land with mistakes Monsters, metaphorically speaking, that is. Take a look at this little land pattern for a TO-263 part. Can you tell me the two main things wrong with this land pattern?

I’ll give you a hint. One of the problems is an absolute no-no. The other one could be justified with a low-current application. But then, wouldn’t you use a smaller package?
Duane Benson
Green Grow the Traces Ho

http://blog.screamingcircuits.com/

Open Source Mea Culpa or Backpedaling? You Decide

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Last week I wrote about “Ten electronics things to be thankful for in 2010.” If you’re reading this now, you probably don’t need the link because you probably read that article then, too. But that’s not really relevant. What is relevant is that in #6, about open source hardware, I wrote, in part, that in some ways open source seems a bit exploitative of the designers.

In the comments, “pt” asked if I could give an example of how open source is exploitative.

I got to thinking about my choice of words and came to the conclusion that “exploitative” doesn’t quite cover what I was trying to say. Although, in some cases, I think it does. It’s possible that there are some aspects of the open source movement that I just don’t get. Or it’s possible that I have the capacity to pick a black cloud out of anything. If that’s the case, I like to think that I can also pick a silver lining out of anything as well. That combination becomes a problem with recursion.

I’m a capitalist so I believe that (a) it’s important to have profit as an ultimate goal of any commercial endeavor. I still have a bit of idealism left so I also believe that (b) when making that profit, we should be like the Boy Scouts and leave the planet a little better off than we found it. I get sick to my stomach when I read about executives making massive millions of dollars when their employees struggle to adequately feed and clothe their kids. (Is this post turning into one of those “I believe …” manifestos?) Following up that last point, I believe that (c) if someone does good work, they should get something in return for it. It’s a trade. You give me something valuable and I’ll give you something valuable in return. Not always money, but something of value.

That’s where the mushiness comes in for me. Here’s the good side. Ti is a big company that, with the Beagleboard, is giving something of great value to the electronics design community. As far as I know, the people at TI working on the project are paid. My guess is that the ultimate motivation of TI is it to help sell chips, but the project has given a whole lot of people access to a level of performance whom would not have had access it otherwise. That endeavor meets my abc conditions. Companies like Adafruit, Sparkfun and DIYdrones have built successful small (and growing) businesses with the help of open source hardware and software. People are making a living (I assume) from those organizations. Both companies give a lot back to the community and both companies make it very clear that they benefit from and really appreciate the efforts of open source designers. They give the folks recognition and support. They and companies like them meet my abc.

The other side of open source, and where I smell the exploitation, is when big companies use open source, make large profits and don’t return anything. I mean, sure, the license allows them to and I suppose that by reducing their costs, they can be more competitive and stay in business, keeping their employees employed. But when a software company buys the remains of another company or two that allegedly “own” some open source code and then tries to make a business of suing people that use that open source software; I consider that to pretty exploitative of all of the people that voluntarily gave their time to the project.

When a large muti-national company that sells server farms uses an open source OS and doesn’t return anything to the designers, I find that also to be exploitative. I don’t know what the answer is. I mean it’s cool that Linux, for example, is used in so many places. The fact that big corporations put so much weight on it certainly validates the legitimacy of it. But I can’t help but envision open source developers out there, that could really use a bit more money in the bank, looking at those big corporations that are profiting off of their backs, feeling a little used.

So, am I missing something? Do I not get it?

By the way, this piece has a lot of personal opinion in it, but I do believe that my company works hard to meets my abc so I don’t have a problem posting this on my work blog. The two times in my career that I did work for companies not meeting my abc, both ended badly for me. Fortunately, I believe in this one.

So, help me out here. If I’m not getting a part of this, feel free to chime in.

Duane “Does idealism hold up in the face of reality?” Benson

http://blog.screamingcircuits.com/

0.4mm Pitch BGA Land Patterns

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We’ve been getting more questions about laying out the 0.4mm pitch Ti OMAP BGA, as is used in the Beagleboard. As I’ve written before, some of the rules change at these tiny geometries. The Beagleboard folks discovered that non soldermask defined pads (NSMD) can lead to bridging and poor yields and therefor they recommend soldermask defined (SMD) pads. Check out page 10 in their design guide. If in doubt, or if you’re concerned that your set up might be different enough to warrant NSMD pads, I’d suggest you give a Ti Applications engineer a call.

And speaking of the Beagleboard, they just recently reduced the price on their pre-built Beagleboards. Like $125 for the original and $149 for the new xM version. Very nice.

If you’ve got a 0.4mm BGA part from a different manufacturer, check with that particular part manufacturer for the final say. Some 0.4mm pitch parts have a staggered arrangement and in that case, there is enough room center to center to successfully use NSMD pads.

Duane Benson
Joe Cool here.

http://blog.screamingcircuits.com/