Category Archives: Laying It Out

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

Thoughts on DesignCon

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Back from DesignCon, and thankful I was able to avoid the horrific weather that delayed many traveling to and from this event.

It was the first time in years I attended DesignCon, held at the Santa Clara Convention Center and under the new management of United Business Media (UBM). (As an aside, UBM was our parent company for a few years — small world!)

Our primary focus at the event was to launch printedcircuituniversity.com (PCU), a new online, e-learning and training site for the industry – and a longtime dream of UPMG President (and now Chancellor) Pete Waddell. In a nutshell, the reaction to the site was outstanding. Mere mention of our having built it on the bethesignal e-learning platform was enough to capture attendees’ attention. Add in sharing a booth with Bogatin Enterprises and the presence of the well-known Signal Integrity Evangelist (aka Eric Bogatin) — and we had plenty of enthusiastic eyeballs. (Fortunately, Eric was not seen lobbing candy from the booth at attendees as he has often been known to do during some of his seminars!!)

Mentor Graphics is our first PCU Design Excellence Curriculum sponsor and held a drawing at DesignCon for an annual scholarship to printedcircuituniversity. The winner to be announced shortly.

Show floor traffic was solid, steady, and quite often heavy during both days of the event as attendees wandered in and out of technical sessions. Comments I heard included being pleased with the traffic, but unhappy with disorganization of some technical sessions, and some negative comments on exhibit set up. I’d also say – a very “sparse” evening reception — unlike the usual PCB West style!! So, we’ll see what direction the event takes next year under this new management.

One show down … on to IPC Expo/Apex in April.

–Frances Stewart

BGA Woes

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Quite a few of the new chips I see coming out stick to the BGA or QFN form-factor. Sometimes they’ll be referred to as WSP (wafer scale package) or CSP (chip scale package), but those are still just little BGAs. Some do show up in larger packages, but many of the really new designs seem to stick to these form-factors.

A few years back, we tended to see a lot of design problems related to regular, big BGAs (0.8 mm or greater pitch). Things like black pad,  microvoids and via in pad cropped up to cause proto-headaches. While those problems still show up from time to time, they have become much less frequent. No, we’re seeing issues with the tiny ones — 0.5 and 0.4 mm BGAs, CSPs and WSPs.

With a big BGA, you can route to vias in between the pads. That’s easy. With the small ones, especially 0.4mm, you can’t. You have to put the vias in the pads. Of course, you have to fill and plate over the vias. Big BGAs tend to prefer non-soldermask defined pads (NSMD) while some of the 0.4 mm BGAs require soldermask-defined (SMD) pads. A really flat surface is more important for the tiny parts too. Don’t fear extra small parts, but you may need to do a bit more homework and relearn a few old rules-of-thumb.

Duane Benson
I’m solderin, I’m solderin, I’m solderin for you

http://blog.screamingcircuits.com/

PCU Goes Live!

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As “Dean Pete” (I think Professor Pete sounds better) intimated earlier today, we have just gone live with Printed Circuit University, the industry’s first online e-learning and training resource for professionals involved in the engineering and design of printed circuit boards and related technologies.

Printed Circuit University is built on the robust, established and time-proven beTheSignal e-learning platform, and features instruction by SI guru Dr. Eric Bogatin.

Pete and VP of Sales and Marketing Frances Stewart are demonstrating Printed Circuit University this week at DesignCon in the Santa Clara. Be sure to stop by and take a look at what promises to be the future of printed circuit board design education and training.

The 15 Contest!

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Or would that be the 125 contest? Maybe the 22B contest or the 1053 contest. Ah — the 555Contest. That has a ring to it. Come one, come all who want to have fun with the venerable 555 timer chip. I mentioned this contest a little while ago, but it’s even cooler now. They’ve announced the contest prizes. Check them out! Go. Check it out. It’s cool. I might enter myself. Maybe.

Jeri (twitter.com/jeriellsworth) suggested a design contest centered around the old stalwart 555 timer. Chris (twitter.com/Chris_Gammell) caught on to the idea and they put together this entire contest through Twitter. It was an amazing demonstration of one of the capabilities Twitter has that I had no idea of.

The entry deadline is March 1, 2011. Entries will be accepted beginning Feb. 21. Go for it. Have fun and maybe win fabulous prizes. I’m not in any way affiliated with the contest, so I can enter myself and I can order you to enter it. I insist.

On a more serious note, there are lessons to be learned in this event. First, while a lot of us may not yet understand Twitter, thinking it mostly a vehicle for passing on messages such as: “I 8 a apple.” This contest came from an idea and a couple of creative folks. They Tweeted about it. Other people picked it up and retweeted, and it went from there. They have a website, sponsors, prizes and judges all in the spanse of about two weeks.

One of the other lessons may have to wait until either you get out your 555 or the contest is over. We may just find some new life in an old chip that is thought more of as a hobby and low-end device chip. WHo knows. Maybe there will be a few legitimate new uses for the part that we collectively hadn’t thought about before.

Duane Benson
Dead bug is okay, I presume, if the dead bug isn’t dead

http://blog.screamingcircuits.com/

Rework Foibles

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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 (above), check your land patterns.

I couldn’t find my wire-wrap wire late last night, so instead, I used the leads from old though-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 frommyself, 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 I did this rework 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

http://blog.screamingcircuits.com/

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/