Screaming Circuits: January 2011


The 15 contest!

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 1st, 2011. Entries will be accepted beginning February 21st. 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 re-tweeted, 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

 

 

Cute Wiring

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 (on the right here), check your land patterns.

I couldn't find my wire-wrap wire late last night, so instead, I used the leads from old thru-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 off of myself, 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 this is rework I did 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

Lesson Learned... Or Not

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, left) does not. This isn't the first time I've used a non-standard regulator, so I really don't have an excuLD1117A pin outse.

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 two-fold. One, if you're re-purposing a land pattern from a part that's close, but not exact, double check your work. Get both data sheets out and compare the pin-out.

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

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

Twitter, What is it Good For? Absolutely 555contest

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

A number of web sites 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 partial-voyeur 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!

A Bit More On the LGA

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 (eg 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 mfgrs 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 being said, you can use vias to internal and back-side planes to increase heat dissipation. Ideally, you would have Lot's 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 their 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?

What about the LGA?

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

Quick summary: 0.5mm 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 LGA 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?

What's Missing?

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 1GB 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 importantly, 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 he or she was 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.

0 or 1?

I've been running around telling people "Happy New Decade." Generally, such has been received with a blank stare or some comment related to me being sarcastic.

Back when we turned from 1999 to 2000 and again from 2000 to 2001, I heard a lot of controversy related to which year actually started the new century. Oddly, I haven't heard any of that this transition, even though the question is just as relevant.

I can certainly understand the two schools of thought on this. 01, 02 ... 09 all don't have a leading digit so they sound and look much different from something that does have a leading digit, e.g., 11, 12, 13... But, look closer at my first list. You don't see "00" in there. Certainly we did have a year "2000", but did we actually have a year "0000"? I've heard of the year one BC and the year 1 AD, but never zero BC or zero AD.

According to Wikipedia, which is never wrong, the Gregorian and Julian calendars do not have a year zero. Astronomical year numbering apparently designates 1 BC as the year zero. But "astronomical year numbering" sounds like a science thing and how many of the average population actually like to deal with "science things"?

If you're a digital person, maybe. Most arrays start with 0. Although some languages do start their arrays with 1.

But here's where I have decided to sit. I was one once. No one has ever said I was zero. I was also ten once and ten equals a decade. Although, decade counters do count from zero to 9. Again, in a purist digital world, 10 would start the new decade. However, for the most part, while machines operate in a digital world, people do not. On the other hand, as soon as I turned one, I had been on this earth for one year, so had the calendar been in existence for one year when year one started? If so, we would have had a year zero. People actually do have a year zero. Extra young folks are called one week old, one month old, etc. In that there is an implied zero and one month old. That is not the case with the calendar. Therefore, in my mind, 2001 - 2010 is the first decade of this millennium and 2011 is the start of the new decade.

So, happy new decade!

Duane Benson
Let's count it in binary next time

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