Zzzzzzzap!!! Static be Bad

Engineers these days have so many issues to worry about just in component handling alone:

  • Do my parts need baking to get the moisture out before reflow soldering?
  • Are my parts in stock?
  • Are my parts real or are they counterfeit or secretly remanufacturerd?
  • Are my parts really lead free?
  • Are my passive components small enough to make it out of the holes in my salt shaker so I can put them on the PCB?
  • Are my parts too small form my manufacturer to handle?
  • Are my parts too complex for my manufacturer to assemble?
  • Have my parts been zapped by static electricity either before or after assembly?

Static electricity is really something that no engineer should have to worry about these days. We know how it gets created. We know how to artificially create it and we know how to guard against it. There's really no excuse - especially from those that an engineer entrusts to build his or her designs.

Tesla_colorado_adjusted 500

People can carry around a static charge anywhere from several thousand volts to more than ten thousand volts; just by walking around. Joe Volta would be proud. Touching an electronic component or assembly the wrong way at the wrong time can discharge much of that through the electronics. Yes, most chips are better able to handle static electricity than the old 4000 series CMOS that could get zapped just by being looked at harshly, but pretty much any active component is susceptible to static damage to some degree. What makes it so Anti stat shoesinsidious is that the damage may be done in handling or in assembly but might not show up until the unit fails in the field.

The whole world knows how to keep electronics safe (that's an exaggeration, but at least most people in the Industry know how), and the whole industry understands the risks, so why would anti-static handling or packaging be an extra cost option? If it's you're own stuff, then fine. It's up to you. But someone you're paying? I don't get it.

Take a close look at the picture on the right. If you ever get a tour through Screaming Circuits, you'll see a lot of this. The floor is conductive. The bright green straps on the shoes are not a fashion statement. They're grounding straps. The blue jacket is conductive. Parts and PCBs are protected from static through these means and others all the way in and all the way back out to the customer. It's the right thing to do and the healthy way to do it and it doesn't cost extra. It shouldn't cost extra. Follow good static mitigation procedures yourself and make sure that whomever is assembling your parts does the same. That's my two cents worth.

Duane Benson
Frankenstein was grounded through his neck bolts, so he's okay.

 

Thanksgiving - Real or Manufactured?

A decade or so ago, PC (political correctness) was the phenomena of the day. People talked about it. People implemented it. People balked at it. The end result was good, but the period was kind of a mess as some people went too far, some refused and everything in between. Today, I think it's "AHC" or Alleged Historical Accuracy.

I say "alleged" because most of these things happened way before anyone alive today was around, and back when record keeping was spotty at best and often as much fiction as fact. In the Soviet Union, they did a lot of revisionist history. This isn't that, but it's along the same lines. With AHC, sometimes 640px-Embarkation_of_the_Pilgrimsthere is a lot of truth, sometimes not, and, again, everything in between. The latest victim of AHC is Thanksgiving.

Way back in grade school (those of us in the US) were taught that Thanksgiving originated with the pilgrims and was a celebration of a good harvest and friendship with the native population or something like that. Hollywood tells us that a pilgrim is a friend of John Wayne's, but that's completely unrelated. I've recently read that Lincoln started Thanksgiving, turkeys weren't part of the original one, the Spanish started thanksgiving and a bunch of other newly reveled possible facts.

Personally, I don't think it matters. A lot of things change over history with original meaning being obscured in the fog of the past. Bleach was once used for whitening teeth. Now we know that using it for that will destroy your teeth so we don't do it any more. Listerine has been used as a floor cleaner but now we put it our mouths. The Listerine, not the floor. (I hope)

What matters most to us though is what we use it for now. History does have its place and accuracy in history is very important, but unless something is inherently bad, there's no need to call out something as wrong just because its used differently now or our original view of its start has changed. Personally, I'm thankful for a great job doing work I'm proud of and for family and friends. There's lot's of other stuff too, but this is enough for the purposes of this post. I'd write more but I'm going go buy groceries and then write some test software for a robot board.

Duane Benson
And I'm thankful that most people don't like the nice crispy turkey wings as I do

Thru-Hole Parts

Screaming Circuits uses machines to place surface mount parts; even if it's just one board. Thru-hole are a different story though. Way back in the cobweby section of the building, we do have a thru-hole part sequencing and insertion machine. Our volume manufacturing division still uses it on occasion, but it's just not efficient for small quantities, which is why thru-hole parts get hand inserted at Screaming Circuits. We have three options for soldering the parts into your prototype. We can hand solder all of the parts, we can send the board through our selective solder machine or we can send it through the wave solder machine. We'll pick whichever route makes the most sense based on quantity and configuration.

It's good that we can solder the thru-hole parts, but how, you might wonder, do we know where to put the thru-hole parts? The SMT has the centroid file to tell our machines where to put them. Thru-hole Thru-hole locationsbeing more of a manual process, we rely on visual data. If your silk screen markings are readable, we can used that as a reference. If the parts will only fit one way into one footprint on the board, then  it's not much of a challenge. Regardless, make sure that the polarity is clear for any polarized components.

Sometimes, though, there isn't enough room on the PCB for clear silk screen and parts will fit in a number of different places. That's where the assembly drawing comes in. This illustrates an example of a suitable assembly drawing. It's got your web order number in the image and all of the parts are clearly pictured and their locations clearly identified. If any of the parts are polarized, make sure you include that information as well. Send the assembly drawing as a .JPG or PDF file format in your ZIP file with the BOM, Gerbers and Centroid.

Duane Benson
It just goes to the back side of the board. It's not a wormhole going to another galaxy.
Or is it?

QFN Stencil Gerber

In the previous epside, Wally’s attack on Dilbert’s kingdom prompts Ratbert to perfect an “N”-Ray, to be discharged from a powerful Nullitrion, to neutralize and render useless Wally’s power plant. Dilbert tells Dogbert the Nullitrion can best be directed against Wally’s palace from the Devil’s Dome, in the Land of QFN segmented stencilThe Dead. Wally learns of their plans, and his soldiers plant a powerful time bomb on the Devil’s Dome, but are promptly captured by Pointy Haired Men. Dogbert and his party land, unaware of the bomb and the Pointy Haired Men who are watching and….

As we re-join our intrepid heros, you can see, circled in red, what the custom QFN stencil layer, from the previous episode, will look like in the Gerber file. Obviously the stencil cut outs will look like this too. Except they won't be green. These format cut-outs will deposit the recommended 50 - 75% paste coverage in the center pad of the QFN leading to a good solid solder joint.

Stay tuned for next week's episode where Dogbert assists Dilbert in assaulting the manufacturing warehouse of Devil's Dome to recover the missing 0402 bypass capacitors.

Duane Benson
Azura, Queen of Mars, ordered the Russian Phobos-Grunt probe to be disabled by a ray-beam

40 Years of 4004

I'm assuming eight pages excerpted out of 142 qualifies as fair use. I had forgotten that I had this thing buried in a box in my garage. I have a lot of old junk hidden in boxes out there, but this one piece seems most appropriate today.

  4004 005 4004 006 4004 007 4004 008

4004 009 4004 011 4004 012 4004 013

 

 

 

 

 

 

 

 

 

 

 

 

This user's manual covers the 4004 and the chip set that went along with it. It also has some pretty detailed information about a couple of computers based on the 4004; such as the SIM4-03, MCB4-20 and Intellec 4. I didn't get this new. I found it in a garage sale back in the early 80's. I wish the unit had been there too.

Duane Benson
4 bit data bus, anyone?

Major Major and Standard Standard

We ask for your bill of materials, Gerber and centroid files to assemble your pcbs. All of those pieces of information are necessary to properly program our machines to place your parts. That's pretty standard stuff, but did you know that when the Gerber format reference book was first published, Jimmy Carter was President of the United States, Russia was the "Soviet Union" and Voyager 1 was well inside the Solar System? Use of the format has been going on even longer. Yeah. It's been around a while. For some reason, it has been very difficult to get everyone to agree to and use a standard file format. Gerbers really don't have enough information in them to do the job properly, but it is the standard. Hopefully not for too much longer. How many of you reading this were even born when Gerber was new?

XKCD 15th standard
(Drawing courtesy of XKCD) <- worth spending time on

There are a number of formats around that are better than gerber and Screaming Circuits will accept many of them. First, your CAD software probably will export an "ASCII CAD file". This is a good format. Some export ODB++, which is one of the newer formats, again a good choice. One of the newest standards is the IPC-2581. It's been around a few years and is now getting a lot of attention. If you happen to use Eagle CAD, you can also send us the Eagle ".brd" file.

IPC-2581 includes the best of ODB++ and GenCAM. It has all of the fab data, assembly data, netlist and BOM. Everything needed in one convenient file. My understanding of the format is that you can exclude portions of the data set that you consider proprietary. You can learn more about the format here. There's more background information on the subject over at Circuits Assembly magazine too.

Duane Benson
Where's Henry? I need an inductor.

QFN Custom Stencil Layer in Eagle

It's been said over and over that you don't want to leave the solder paste opening wide open for a QFN center pad. 50 - 75% paste coverage will get you bets results. With full coverage, your QFN can end up floating too high and not connecting with all of the pads due to their significantly smaller aperture.

But hCustom paste layer 1ow do you create a custom paste layer? In Eagle, it's not terribly obvious, but it is easy. Open the part that you want to customize in the Eagle Library editor. Open up the package for that component. Now, select "i" on the left side and click on the center pad. You might need to turn off the "tcream" layer in order to select the pad. Custom paste layer 2

In the Properties dialog box, un check the check box for cream. That will get rid of the standard stencil layer. Now you can use the rectangle tool to add in stencil cut-outs as you want the. Make sure you set the layer for the rectangle to be "tcream" and remember that you are drawing the cut-outs of the stencil, not the blocked part.

Obviously it will be different for every CAD package, but the concept is the same. As is the need to do so.

Duane Benson
The Internet is weird.
There's actually a website for paste eaters.

Happy binary Veteran’s Day - 11/11/11

010010000110000101110000011100000111100100100000
010101100110010101110100011001010111001001100001
011011100010011101110011001000000100010001100001
011110010010111000100000010100100110010101101101
011001010110110101100010011001010111001000100000
011000010110111001100100001000000110100001101111
011011100110111101110010001000000111010001101000
011001010010000001100110011000010110110001101100
011001010110111000101110001000000101001001100101
011011010110010101101101011000100110010101110010
001000000110000101101110011001000010000001101000
011011110110111001101111011100100010000001110100
011010000110010100100000011011000110100101110110
01101001011011100110011100101110

Thru-hole to SMT

Thanks to a comment from Michael yesterday, I think everything is now cool with my Geiger counter. I had left the AT2313 default fuse setting at clock/8. That dropped the RS232 speed from 9600 to 1200 and it made the clicking sound into more of a tone, which just didn't sound right for a geiger counter. I still need a good radiation source though. I think I've picked up just a few clicks of background radiation, but that could just be wishful thinking.

WishfDFN-8ul thinking or not, that's not the point. The point is that this was an example of migrating from thru-hole parts to SMT. I managed to get virtually everything into SMT. The connectors, the power switch, the buzzer, batter holder and fuse clips for the tube stayed thru-hole. Although I'm sure I could have all but the battery holder and fuse clips into SMT had I wanted to. I tend to keep switches and connectors that will get a lot of use as thru-hole just for the extra staying power. If they aren't used frequently, then SMT is just fine.

There are a number of things to consider when switching from thru-hole to SMT:

  1. Everything is smaller, so you can fit more in the same space or the same in less space. I took advantage of the extra board area to add in a RS232 line driver so I could connect directly to a serial port. I also added in a power-on LED.
  2. Everything is smaller so your layout is more critical. Most PCB houses will build 8mil trace and space as standard process these days. That gives you a lot of flexibility in squeezing your routing into tight areas, but it doesn't give complete freedom. You have to be core careful because you frequently do have to route a bunch of traces into a pretty small area. When you get into the really fine pitch parts, like .5 or .4 mm center to center, you have to be extra careful.
  3. Some parts are dimensioned in metric and some in SAE units. If all are one way or the other, it's easy. But when you've got both, you may have to tweak with your grid spacing off and on to make sure your traces are centered in the SMT pads they connect to. It usually isn't a horrible problem, but it can make even spacing more difficult and can make you more likely to violate a design rule.
  4. You don't have automatic "vias" on each component leg so routing can be more difficult. You'll likely have to spend more time tweaking the part locations and the trace routing to get a decent layout. A lot of times everything's too close so it's not practical to just plant a lot of vias all over.
  5. Hand soldering is less or not practical. Some people do hand solder some pretty tiny parts, but it's not practical in more than isolated cases. If you're a hobbyist or on a tight budget, this might limit you to thru-hole or some of the largest SMT parts. For commercial work though, SMT is the way to go.

Some things to think about. But what do you get in return? Typically lower cost - especially if you want your design to go into volume manufacturing. You also get access to the newest parts that only come in SMT packages. And, many designs are space constrained, so you can cram more in while still keeping your board size down.

Duane Benson
I shot a neutrino into the air
And where it landed I already knew

 

Geiger Counter without the Muller?

My Geiger counter project has been on holiday for a while. When I originally ordered all of the parts, I ordered fuse clips (to hold the tube) with solder lugs too big to fit in the holes and a trim pot (VR1) too small for the SMT pads. I moved on to other things for a while and just now got around to ordering the correct parts and soldering them in. I've verified that everything works except the tube. Apparently, S-13BG GM tubethe specific tube I bought (SI-3BG) is not very sensitive.

Other than not knowing if it will actually detect radiation, everything seems to work just five with Mighty Ohm's original Atmel code. The only difference from his instructions is that the RS232 is 1200 baud instead of 9600. I'm not entirely sure why that is. The source code specifies 9600 and I have an 8MHz resonator just like his kit. I'll worry about that later. At least it works.

It will beep if I touch my fingers to each lug of the geiger tube, but I haven't been able to detect any naturally occurring particles. There are a couple of possibilities.

  • I had to choose a different transistor for Q1 and different diode for D1 because I couldn't find those specific parts in SMT. Maybe the gain or some other performance spec is too far off.
  • The type of tube I bought is not sensitive enough so I just need to find a stronger radiation source.
  • I don't have VR1 set right to give a high enough voltage to trigger the tube.

I'm going to try a 100:1 voltage divider to a unity gain current amp to measure the voltage and go on the hunt for a hotter (but still safe - I hope) radiation source. I might go back to eBay and buy a different tube too. Lastly, I'm going to get out some data sheets and look at my subs again. Maybe try to find something even closer to the original. Once I've verified that it all works, I'll make the design files available as open source.

Duane Benson
If the Alpha is the tough guy, why can it be stopped by a single sheet of paper?

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