Category Archives: Screaming Circuits Blog

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

QFN Solder Paste Layer

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LBDCminiI’ve got the fab order placed with Sunstone.com for my next demo project. The little board is represented here at pretty close to actual size on screen – provided you have a 22″ monitor set at 1680 pixel horizontal resolution. Given that, you might want to click on it to pop up a bigger representation of it. That makes it about 4X life size.

When you do that, take note of the QFN/DFN parts: The processor in the middle, the LiPoly battery charger right between the upper two mounting holes and the RS232 driver in the lower left. I’ve followed my paste layer advice and segmented the paste stencil layer to reduce the chance for float or major voids.

I found a footprint in the library for the big processor in the middle. I just had to modify the paste layer, as shown here. I made the footprint for the charger and RS232 chips from scratch. Neither had anything close enough in the library.

The DFN has a slightly different approach to segmenting the stencil layer. Little squares like I used on the other two chips work just as well, but this is effective as well.

Another thing to take note of is the marking on the LEDs. The original footprint for the 0402 LEDs does have a polarity mark, but it’s one of the types that can easily be misinterpreted or can be difficult to see. The diode symbol put down in silkscreen removes any possibility of ambiguity.

Duane Benson
I’m happy I live in a split level head.

blog.screamingcircuits.com

Particle Update

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I’ve been ignoring my Geiger counter for awhile now, but I picked it back up and made some progress again. For some reason, I just wasn’t getting the 555 based HV power supply to generate a high enough voltage. In frustration, I bypassed the 555 and fed a PWM signal in from a microcontroller board that I have laying around.

That fixed the problem. I still don’t know why I wasn’t able to the the 555 doing what it was supposed to do. I’ll have to spend some more time on that some other day, but for now, I’ve prototyped it out and I’m happily detecting particles. I whipped out the new layout and will send off to Sunstone.com for another set of PCBs.

I’ve also replaced the Atmel chip with a PIC. I don’t have anything against Atmel. I’m just more familiar with PICs. Once I’ve built a few of these, I’ll change to really small packages — QFNs or BGAs for the chips — to make the board a little more fitting with our assembly capabilities. The SOIC chips are fine, but our machines don’t even come close to breaking a sweat with things that big.

Duane Benson
We treat agoraphobia for PC boards

 

blog.screamingcircuits.com

Who Are You?

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A lot of events are preceded with a “meet and greet” session. It gives you something to do for an hour or so before the real activity takes place. I’m not much of a schmoozer myself, but if the crowd is right, there is value in the activity. It’s good to get to know folks with similar interests or vocations.

On the Internet, Facebook is kind of known as the place for that sort of thing. The problem with it though is the low signal-to-noise ratio. Too much drivel to sort through to find the valuable nuggets. But don’t despair. All is not lost. Over at EEWeb, they have something pretty close in their “Featured Engineer” series. As of this writing, they have well north of 100 profiles. Peruse through and get to know some or the people making things happen in the world these days.

If you look close, you can find yours truly in the list. And the first person to name the science fiction movie I’m thinking about gets a free Screaming Circuits polo shirt. The first person who can correctly identify the lighthouse gets one too. (The lighthouse is small in the photo, but if you’ve been there, you’ll recognize the area.) Only in the US though. Sorry, but customs gets me down so I’ll only ship to a US address. It might not be the same shirt I’m wearing, but close.

Duane Benson
In life I was your partner, Jacob Marley.

http://blog.screamingcircuits.com/

Is Your BoM a Smart BoM?

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Please be aware that there is no “B” at the end of these BoMs. Still, without the “B”, you can have a smart BoM or a dumb BoM. It is important to note, however, that a dumb BoM may not be a bad thing. It just depends on what you want to do with it.

No matter what you’re doing, there are a few necessities. Let’s start at the very minimum, for someone designing something to be self-built from mostly already owned parts; maybe just a few from a dealer.

Reference designator: R1, R2, R3, C1, C2, U1… You have to have this information.

Quantity is important so know know how many to pull. Although, with a small garage-built project, you can probably just as easily count how many you need for a given value. And what about the value? Actually, the value isn’t always all that necessary if you have the correct part number information. A line item number is hand for big bills to keep things straight.

This is actually too basic and kind of pointless, so I’m going to jump ahead. Take an assembly house like Screaming Circuits. Screaming Circuits will either build your boards from your kit of parts or purchase them from your BoM (or a combination thereof).

Once you have the item number, quantity and reference designator, you need to tell your assembler or purchaser what it is. If you already have the parts kit, just add in the manufacturer’s part number and a description / value. That should do it. Some assemblers, like Screaming Circuits, will take part numbers from a distributor in place of or in addition to the manufacturer’s part number (e.g., Element14, Digikey, etc.).

If the assembly house is going to buy the parts, then add in the manufacturer and double check that the part numbers are accurate with all suffixes and things of that sort. The distributor part number can be added, but when the assembler is going to build the boards, you really should include the manufacturer and manufacturer’s part number to cover all basis.

BoM Sample

That’s cool, but your circumstances might require just a little more. You might need to list an approved substitute or two for parts that come in and out of stock frequently. You could also list multiple distributor part numbers for the same specific component, again, in case of lead-time or stock issues.

Sometimes I get myself into a bit of trouble by not specifying some part values at design time. I might just throw in things like bypass caps, RS232 driver charge pump caps or LED current limiting resistors assuming that I just know what the value is. It’s not a big issue, but it would probably be less work to just do it at the start.

Duane Benson
What the Bureau of Meteorology has to do with your parts kit, I’m not sure.

http://blog.screamingcircuits.com/

Let’s Get Small – 0.3mm Pitch BGA

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I recently got an email from Practical Components about its new 0.3mm pitch evaluation board and dummy 0.3mm pitch BGA. Now, we’ve been assembling 0.5mm and 0.4mm pitch BGAs for years. Those sizes are kind of not really anything special anymore. We’ve even been putting together PoP (package-on-package) for a couple of years. But we’ve yet to see anything smaller.

Shrinking BGA pitchJust looking at the numbers, 0.3 may not look all that much smaller than 0.4, but that’s 25% down. Thinking of it in those terms makes it much more intimidating. I haven’t found the pad dimensions yet, but just using rough estimates, a 0.003″ trace would have about 0.0015″ on either side for a between the pads trace. That’s getting pretty dangerous. Likely, you’d have to do every thing with filled and plated-over vias in the pads. (No Open Vias! Not one. Don’t do it.)

I can see a lot of good future use for this size in miniature devices; more processing power in hearing aids and embedded medical devices, for a start. I don’t know how necessary 0.3mm pitch will be for phones. They seem to have stabilized in size and the trend is more toward system in chip than it is toward more shrinking. Regardless, I would expect that in a year, we’ll be seeing mainstream parts in this form factor.

Duane Benson

Go ask Alice
I think she’ll know
How to run your escape routing

blog.screamingcircuits.com

 

Top 10 Things To Do In 2012

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Unless you’ve managed to live off the grid for the last five years, you know that the Mayan calendar has predicted the end of the world on Dec. 21, 2012. Ignoring all conventional wisdom, centuries of scientific knowledge and the fact that the Mayan community doesn’t even believe it, I feel compelled to list key electronics things that everyone should see or do in this last year before the end of the world. Because, you know, just in case…

Here are the top 10 things to look into or do in 2012 before the end of the world on December 21, 2012:

0b0000.0000: Build something open source. It’s amazing what is available. Back in the heyday of hobby electronics, it was easy to find projects in magazines such as “Popular Electronics.” You could get kits from Heathkit of RadioShack. But, for the most part, all of those things went away. With Open Source, you can build an almost unlimited variety of gadgets. What’s more, you can taylor your build to your experience level. If you want to do it all, just get the files, fab your boards, buy your parts and solder it up. If you’re less ambitious, buy a complete kit. If you’re even more ambitious, mod the design and post it up for the community.

0b0000.0001: While you’re out fiddling with Open Source, get ahold of a Beaglebone. It’s Ti’s second take on open source hardware. The original Beaglebard was and is a great way to get to know their OMAP processors, both in therms of programming it and in terms of designing a PCB for it. The Beaglebone is an easier to use, easier to expand, but not quite as powerful adjunct to the Beagleboard.

0b0000.0010: Try out some new CAD software. Sunstone released PCB123 version 4 in 2011 and Element14 released EagleCAD version 6. Check them out and see if they do what you need. Both are good economical ways to get into circuit design and layout. PCB123 doesn’t cost anything initially. It sends your boards through Sunstone and they earn their rent that way. Eagle has a tiered pricing model, starting at a small non-commercial version for free and stepping up to a full-feature professional system.

0b0000.0011: Try out some newer technology. I’f you’ve always been intimidated by QFN or micro BGA packages, go ahead and give it a try. They’re more difficult at first, but once you’ve got the tricks down, you get access to a slew of new components that only come out in those form factors. Check out some guidelines on QFN use.

0b0000.0100: Look at space pictures. There are a number of probes up running around our solar system these days, and one on the verge of leaving it. Hop on over the nasa.gov or space.com and see what’s going on. There’s a probe orbiting an asteroid, new ones off to Jupiter and Mars, one orbiting Mercury, some new space telescopes and more. There’s just a lot going on out there right now. You can even leave your computer, go outside and look up at the sky for real.

0b0000.0101: Try and make something really, really small. For my part, I’m taking a little two-motor robot brain I’ve built and an trying to see just how small I can make it. You have to think differently when size is a prime consideration. Factors that didn’t matter much suddenly become design critical. It might be an opportunity to freshen your brain up a bit (although, if the world is ending on 12-21-12, having a fresh brain may not be all that important).

0b0000.0110: Go back in time. Get a 2N2222 or 2N3907, or both. See if you remember how to build basic common base, common emitter and common collector amplifiers. No. Don’t go to Wikipedia. Try to do it from memory. I’m sure you built all of them waaaay back in your school days. Next try to build some basic logic gates with transistors. After you’ve done that, see if you can build up a RSIC processor and a 512 Mbyte RAM block using only discrete transistors and passives. As your final assignment, use the computer system you built to calculate the first 100,000 prime numbers.

0b0000.0111: Take your most recent resume and replace all of the letters, spaces, tabs and line ends with their hex values. Submit it in that form for your dream job. Then sit back and wait for the hiring manager to bow down to your superior skills. Since the world is ending, it really won’t matter that the first person to see the resume thought it was gibberish and round-filed it.

0b0000.1000: Introduce you kids (if you have them) to robots. Get them started down the technical path early; both boys and girls. And, if you have daughters, make sure they don’t get discouraged by peer pressure or whatever pressure. If they are interested in a technical career, don’t let the world around them pressure them out of it. If you don’t have kids, build some robots yourself and introduce your pets to them. See who ends up chasing whom around the house.

0b0000.1001: Finally, ignore all of this. Come on. Really? The Mayans knew when the world would end? Even I don’t know that. Although, there was that John Cusack movie. And I’ve read about it on the Internet and everyone knows that if you read it on the Internet, it must be true…

Duane Benson
See you all on the other side

http://blog.screamingcircuits.com/

Sardines Seven Miles High

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Being in the electronics industry means working on cool projects with cool people, but it also means other things. Like plane travel. I love flying. Not necessarily in an airplane though. I mean, I do prefer to be in an airplane when I’m flying. I’m more likely to have a subsequent flight if I first actually fly in an airplane rather than not in one. It’s just these “big” commercial airplanes that we get stuffed into these days. Most of the complaints I read about relative to commercial flying have to do with the TSA or being stuck on a runway for six hours with overflowing toilets, but I haven’t had those issues.

In point of fact, I don’t think I’ve run into a TSA person that hasn’t been polite. Especially in Portland. Still, even if they’re nice people, I really don’t like the idea of a choice between being nuked or groped. For the record, I chose to be nuked when I had to make the choice. I’m sure the government-sanctioned groper wouldn’t like it any more than I would, so I did us both a favor and stepped into the radiation chamber. I don’t feel any worse for the rays. Maybe they were nice rays.

So, I don’t have any complaints about the TSA. The crowds sometimes get me down, but all things considered, they aren’t all that bad. What does get me is the straight jackets that they call seats these days. I’m in a motel in Milwaukee right now. I’ll be heading home tomorrow. First in a mosquito plane to Chicago and then, probably, a 737 to Portland. I like the 737 in concept. It’s a good plane. I just like being able to breath a little. It’s natural to not have any room in the micro plane that I’ll take to Chicago. It’s not much bigger than my truck and will have twenty people crammed in it. You expect to be folded like a pretzel and spam crammed into one of those.

But a 737 is a big airplane. I can stand up in it and I’m not short. It’s not like a little micro car. It’s like my pickup truck (just not like it with 20 people in it). In the olden days, I preferred window seats for the view. Then I went for the aisle seats for easier access in and out. Never the middle seat. Now, though, they keep taking leg room out so I’m not so sure. The seats get smaller every time I get on a plane.

If I take the window, I’m stuck for the duration of the flight with my knees just about in my face. Leg room in the aisle seat is narrower so when I put my laptop bag down there I can’t even wedge my feet under it to steal an extra few inches. Now that they charge for checked baggage, everyone brings their luggage as carry on and there isn’t room to put both my suitcase and my laptop up in the overheads. I think they have the sky marshals throw you out the back window if you try to put both your carry-ons up top.

And so it goes. I’m just going to take whatever random seat I get and hope for the best. And I’ll feel like a king if they grant me the supreme luxury of an entire 12 oz. can of warm soda pop.

Duane Benson
Curse you Red Baron!

Through-Hole Parts

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Screaming Circuits uses machines to place surface mount parts; even if it’s just one board. Through-hole is a different story, though. Way back in the cobwebby section of the building, we do have a through-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 through-hole parts get hand-inserted at Screaming Circuits. We have three options for soldering the parts into your prototype. We can hand-solder all 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 through-hole parts, but how, you might wonder, do we know where to put the through-hole parts? The SMT has the centroid file to tell our machines where to put them. Through-hole locations being more of a manual process, we rely on visual data. If your silkscreen markings are readable, we can use 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 silkscreen 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?

http://blog.screamingcircuits.com/

Zzzzzzzap!!! Static be Bad

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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.

People can carry around a static charge anywhere from several thousand volts to more than 10,000V, 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 insidious 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.

http://blog.screamingcircuits.com/

QFN Stencil Gerber

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In the previous episode, 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 The 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 rejoin 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 to 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

http://blog.screamingcircuits.com