Category Archives: Screaming Circuits Blog

EMS provider Screaming Circuits offers tips and thoughts on design and manufacturing

Lesson Learned … or Not

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MC39100 pin out I’ve written quite a number of times about the perils of CAD software land patterns. Especially if you don’t have an exact match and need to adapt something close.

Recently, I was looking in my Eagle library for a low-drop out regulator, MC39100 is SOT223. It’s just a standard, run of the mill 7805 replacement. Nothing special. A million other parts share the same pin-out. Shouldn’t be a problem. Shouldn’t …

If I were to follow my own advice, it wouldn’t have been a problem. But did I follow my own advice? Well, not this part of it. I took for granted that all three terminal regulators follow the 78XX pin-out. Most do, but the LD1117A (below) does not. This isn’t the first time I’ve used a non-standard regulator, so I really don’t have an excuse.LD1117A pin out

Naturally, I assumed that the pin-out matched what I needed and I didn’t hunt down an LD1117A data sheet to verify their pin-out. Well, at least I didn’t do so until trying to get my new PCB to power up. Very sad.

So, is there a moral to this story? Probably. Most likely it would be twofold. One, if you’re repurposing a land pattern from a part that’s close, but not exact, double-check your work. Get both datasheets out and compare the pin-out.

The second part of the moral is, if you give advice … follow it yourself. Duh.

Duane Benson
Help! I’m blinded by the obvious.

http://blog.screamingcircuits.com/

Twitter, What is it Good For? Absolutely 555contest

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I follow a few people on Twitter. A few people follow me on Twitter.

A number of websites have engaged their readers in debates about the usefulness of Twitter (and other social media) to engineers. In general, these debates are talking about hardware engineers. A lot of software engineers pretty much live on the Internet and will embrace or invent any new thing.

The overwhelming majority of the responses that I’ve read put Twitter in the class of “a waste of time.” Not everyone feels that way, but there’s quite a few who do. I’m still in study mode. I can see how it can take up a lot of time if you don’t exercise some self-control and it has a weird, semi-voyeuristic aspect.

If I’m following, person A and person B, I’ll see any conversation they are having. Most discussions are undertaken with that in mind, but some start to lean toward the personal side. Now, if person C gets in the conversation with one or both of A and B, and I’m not following C, I end up 723px-NE555_Bloc_Diagram.svg seeing part of the conversation. It’s a bit like listening to someone talk on the phone and hearing only their side. It’s odd.

It can lead to interesting activity though. Recently, one tweeter, Jeri (twitter.com/jeriellsworth) suggested a design contest centered around the old stalwart 555 timer. Chris (twitter.com/Chris_Gammell) picked up the ball with her and in about three days, just over Twitter, they organized it, other tweeters chimed in, sponsors offered prize money and they’ve set up a website for it. Fascinating.

If you’re on Twitter, search for “555contest” to see the conversation in action. In any case, if you’re a fan of the now 368 year old 555 timer, you might want to dig into the recesses of your brain for entry ideas. You can also follow Screaming Circuits on Twitter at “twitter.com/pcbassembly” and see for yourself if it’s useful or too loaded with mindless drivel.

Duane Benson
Huh! Yeah!

http://blog.screamingcircuits.com/

A Bit More On the LGA

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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 (e.g., 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 manufacturers’ 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 said, you can use vias to internal and back-side planes to increase heat dissipation. Ideally, you would have lots 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 its 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?

http://blog.screamingcircuits.com/

What About the LGA?

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

Quick summary: 0.5 mm 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  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?

http://blog.screamingcircuits.com/

What’s Missing?

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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 1 GB 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 important, 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 they were 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.

http://blog.screamingcircuits.com/

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