Category Archives: Screaming Circuits Blog

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

Chowing Down

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I don’t know how common the phrase “eating your own dog food” is. I know I’ve heard it before in some of those obnoxious business seminars. Not all business seminars are obnoxious. Some are quite helpful and actually, now that I think about it, I’m not really sure if I’ve heard the phrase in the obnoxious seminars or the useful ones. Maybe the so-so ones. Hmm.

Anyway, in case you haven’t heard the phrase (it may be a regional thing), it means to use your own product, or in our case, service. I’m not an engineer, but I play one on the Internet. Still, I design and build little things. Since generally what I build is hobby-related, I tend to solder them up myself, leaving our capacity here at Screaming Circuits for the paying customers. But right now, I’m doing something a little different.

SC Promo 042011 top layer I’ve got a little design that I’m going to use to help some folks better understand how things work around here. At first, I’ll just give it to some writers and editors (writers and editors, feel free to shoot me an email about it), but at some point, I hope to be able to have enough to send out to design engineers that want to get a feel for our process. It’s quite a simple board” a PIC microcontroller (18F25K20 SSOP), some switches, resistors, a bunch of 0603 LEDs and some bypass caps. One Schottky diode too. I’m putting together a sample kit just like the sort of kit we like to receive. The files will be on a Screaming Circuits USB drive. The PCB, fabbed at Sunstone.com, will be in there. All the parts, purchased from Digi-Key will be in individual bags; one per BoM line item.

The idea is for someone to take the kit as though it were theirs, create an account on our website, quote the job, place the order (no payment needed), upload the files and send in the kits. Along the way, that person will see what we like to see in a parts kit and how the whole register, quote and order process goes. Once they receive the working board back, all they have to do is decode the secret message it displays.

Back to the dog food. From my side of the Interpipes, it’s easy to say that things are easy. I sit back, drinking lattes and eating oatmeal while everyone else does the real work. But during this process, I’ll get a refresher course on what it’s really like to get a prototype built.

The other day, I sent the Gerber files off to Sunstone to get the PCBs fabbed and the parts order off to Digi-Key. Tomorrow, I’ve got to kit everything up. Stay tuned. Details as events warrant.

Duane Benson
Is this the kind that makes gravy when you pour water on it?

http://blog.screamingcircuits.com/

Picking Packages

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A long, long time ago, in a place pretty close to here, picking a form factor was easy. Your CPU came in a 40 pin DIP. Your logic came in 14 or 16 bit dips. You picked resistor sizes based on their current carrying needs. Transistors and other power components got a little more difficult, but not much. It was largely a matter of power dissipation requirements.

Different story now, though. First, there’s through-hole vs. SMT. Then there’s a plethora of options beyond that. So, what really matters? A specific resistor size may come in multiple wattages. Chips come in multiple packages — often from big DIPs all the way down to tiny QFN or BGA packages. Let’s look at a few examples.

Here’s a simple microcontroller: the PIC18F25K22. It’s a pretty typical 8-bit PIC. You can purchase it in four different packages:

  • DIP, $2.05 each, Qty 100, Tube
  • SSOP, $1.86 each, Qty 100, Tube
  • SSOP, $1.90 each, Qty 2,100, Tape & reel
  • QFN, $1.86 each, Qty 100, Tube
  • SOIC $1.89 each, Qty 1,600, Tube
  • SOIC $1.93 each, Qty 1,600, Tape & reel

(DigiKey prices as of the posting date. Some are non-stock items.) There’s also the part presentation to consider; e.g., reel, cut tape, tube.

Next, look at a 1K resistor that might be used as a pull-up. (As listed in DigiKey) through-hole resistors range from 1/20W up to multiple watt packages. SMT parts range from 1/32W up to lots. Simplifying a bit and just looking at 1/4W, you can purchase 0402, 0603, 0805 and 1206 packages. For high volumes, price will be a factor, but for lower volumes, the price difference can be trivial.

If you have plenty of space to work with and you need to build by hand or for some reason need a socketed part, your choice is the DIP. If space is a bit of an issue and you may or may not hand build, then an SOIC is probably your pick. Some people will hand build QFNs and SSOP packages, but that’s not realistic in anything but rare cases.

When size, speed, current or performance need to be at maximums, selection is still not that difficult. You’ll often have far fewer options to choose from at the performance edges. But when there’s headroom all over the place, how do you decide? Why an SOIC over an SSOP over an QFN? Why 0603 over 0402, 0805 or 1206?

Duane Benson
Peter Piper picked a peck of pickled PIC packages.

http://blog.screamingcircuits.com/

TI + National Semi

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Everyone else seems to be weighing in on the Texas Instruments-National Semiconductor acquisition, so I thought I’d better do the same.

Depending on whom you speak with, the ramifications could be quite large or not terribly noticeable. 7400 TH Myself, I’m going for pretty much not noticeable. First, we’ve never met a National Semiconductor part that we didn’t like. Second, we’ve never met a Texas Instruments part that we didn’t like. I’m guessing that we’ll never meet a Texas National Semiconductor Instruments part that we don’t like either.

I feel better about the fact that it’s one old-guard company buying up another old-guard than if it were a upstart doing so. That makes this look to be more of a “Boeing buying McDonnell Douglas” than an “AOL buying Time Warner.”

Duane Benson
I have met a blog post that I didn’t like

http://blog.screamingcircuits.com/

Which Surface is Best?

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RoHS has been in effect since, when, 2006? Pretty close to five years now. It’s been around long enough that there’s even talk of follow-on legislation. All of the PCB fabricators have pretty much figured out how to deal with RoHS. There are Pb-free versions of every PCB finish at this point. But, we still get questions about the best choice of PCB finish.

I don’t think industry has selected one PCB finish as the “standard” preferred choice. A lot depends on the application and the componentry being used. For large parts, HASL, leaded or lead-free, is a good choice. It’s inexpensive and works well. For leaded work, HASL still seems to be the most common finish. We don’t see quite so much Pb -free HASL, though.

If you’re working with small geometry parts, then you really need to go to immersion silver or ENIG. The consistently flat surface of those finishes will help keep the small parts on the pads where they belong. The disadvantage of silver is that it requires a little more care in handling and storage. It can oxidize which will make soldering more difficult. ENIG is more expensive, but it tends to work real well and is easier to store. Fingerprints can be a problem though. We’ve seen the oils from a fingerprint essentially etch the gold surface off. Weird.

OSP becomes a viable choice with high-volume, cost critical applications. It used to be very sensitive to storage and handling, but has gotten a lot better over the last few years. We don’t see immersion tin much at all. It apparently is harsher on the environment to produce than other finishes.

All that makes it more understandable that we don’t have one preferred finish. It seems confusing, but really it’s not that different than any other product. There certainly isn’t just one preferred style of tire for all motor vehicles.

Duane Benson
Gotta have those monster truck tires if you live in Kelso

http://blog.screamingcircuits.com/

Something Free at ESC

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My post title rhymes. I’m a poet and don’t even know it.

So, what about that free thing at ESC? And is there a catch? Of course there’s a catch. There’s always a catch. This is a good catch, though.

If you are an electrical design engineer and are at ESC this May, come to our booth and see if you make the cut. The first 50 qualified electronic design engineers will get one of these things.

Yes, I’m being non-specific about the details. That’s a technique. It’s supposed to make you more curious. That’s what they tell me anyway.

The details:

  • Embedded System Conference, Silicon Valley
  • McEnery Convention center
  • San Jose, CA
  • May 3-5

    • We’re in booth 823.

    Duane Benson
    Robots for world domination!

    http://blog.screamingcircuits.com/

    That Final Check

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    I’m not talking about the final check that you get from an employer laying you off due to outsourcing. That’s a bummer of a final check. The final check I’m talking about is a good thing. It always pays to do this kind of final check. Of course the other kind of final check pays too, but only once. This kind can pay off numerous times.

    Here’s the scenario: I have an MCU board that can take 5v power from either USB or from a dedicated power source. I want part of the board to receive power all the time and one small high-current section Schematic wrong pwr source to receive power only from the dedicated power source. I don’t want to suck too much current out of a poor little USB.

    My circuit has three different power busses: USB regulated 5V, on board regulated 5V, on board 9-12V. I even fabbed up some PCBs and built a first prototype. It needed a few mod wires, but I missed this problem. After my mods shown on the older posts, the circuit still worked, so I stopped looking for problems.

    Fortunately, I took one last look before sending off for v2 PCB. Two of my bypass caps went to the wrong supply (they were supposed to go the “BRD5V” instead of “5V”). Not a huge deal and in my test set-up, it didn’t prevent the circuit from working, but who knows what would have happened in real use. In any case, it would either resulted in another board spin or left the potential for intermittent problems when in use.

    Duane Benson
    Once again, time for oatmeal

    http://blog.screamingcircuits.com/

    Bouncing BGAs

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    I dropped my cellphone on the pavement the other day. That’s bad enough, but in my instinctive attempt to catch it, I actually hit it and increased its downward velocity. Luckily, everything still works. The odd thing is that I just assumed that it would still work. No real questions or doubts on that thought.

    That realization got me thinking. (It happens now and then.) What other devices do I have that I automatically expect to survive a drop onto concrete? I have a carpenter’s hammer. I expect that to survive a drop intact. I would not expect my camera to survive such a drop intact, and have empirically verified that fact. A little car GPS? Probably not. Laptop; uh … no.

    I’m sure there are some other devices that would easily survive. I just can’t think of any off the top of my head. I suspect that there are a lot of factors that go into making cellphones survivable. The case, the overall mass, the quality of solder joints.

    Along those lines, some folks use an underfill glueish type substance to hold BGAs more securely. Some designers use pick and placeable solid underfill. Some just rely on extra good soldering and some leave it to luck. Of course, not all BGA installations require much shock resistance. How do you secure your parts when shock or vibration are serious concerns?

    Duane Benson
    Quick, where’s Henry? I need an inductor.

    Virtual Questions

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    Here’s a question I received during my Virtual PCB chat session on March 8:

    • From Jack: “Here’s my default question (as a designer), what is your biggest headache from designers?”

    My answer: “Probably the most common difficulty has to do with CAD library footprints. That’s really a headache caused by the CAD software”

    • Jack: “Ha, well it seems like the majority of problems stem from incorrect library footprints (including mask, silk, etc.). Maybe we just need to get together and make a universal library for everyone, eh?”

    I’ve been hearing a lot of lamentations over the last year regarding CAD library footprints. It seems to be one of those issues that has been around long enough and is now reaching a critical mass of attention. There are a few partial solutions in the works. PCB123 is trying to make the most complete set of libraries possible. NXP has been supplying factory libraries to PCB123. There is the IPC-7351B land pattern generator. Some manufacturers give good footprint guidance at least (TI, Freescale). Sparkfun and Adafruit are supplying libraries for most of the components that they use and sell.

    All good things and all in the right direction, but still not a consolidated universal effort. There’s also talk floating around of crowdsourcing libraries. I can see that working for Eagle and a few other packages, but I question whether large companies using expensive CAD systems would rely on such a thing. I guess that all means that we don’t have a solution in sight, but if the problem is getting broad-based visibility, than maybe someone will come up with an actual complete answer.

    Duane Benson
    Esperanto for CAD libraries

    http://blog.screamingcircuits.com

    Who’s Right?

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    Jack commented on my prior post, An Unanswered Question. His point was that instead of just saying “check with the manufacturer’s datasheet”, like I so often suggest when talking about land patterns, I should give more credit to the IPC and understand that many datasheets are the result of less-than-thorough study. That’s a very good point.

    The challenge is that some manufacturers do a great job of figuring out how to use their packages, such as TI with its Package-on-Package (PoP) OMAP, or Freescale with some of its ZiBee chips. u-blox has done a good job of documenting paste mask requirement for its castellated mounting configuration, too. On the other hand, some other manufacturers seem to have cut-and-paste part of an old datasheet without even giving it a once-over. As Jack mentioned, with some of the newer packages, IPC doesn’t always have the data yet. I didn’t see that IPC-7351B covers 0.4mm pitch BGAs yet. It does do a good job of covering the need to segment the solder paste stencil over a QFN center pad, which I also have written about more than a few times.

    I guess my thinking is that the part manufacturer should be the best equipped to tell us how to use their components. To Jack’s point though, that would be in an ideal world. But, reality rarely holds up to the ideal. Some manufacturers do quite well and some seem to virtually forget that they even made the part once it’s out of the development labs. IPC does a very good job but isn’t necessarily the most current. Then, of course, some manufacturers don’t follow IPC guidelines. Board fab houses and stencil makers have a lot of good data too, but also aren’t always up to date (nor are assembly houses).

    I suspect that I get a little cynical on this subject in general because we see so many diversions from standard come through our shop. The designers, by and large, would much prefer to lay out their boards for greatest manufacturing success, but so many of them have a very difficult time finding the necessary data.

    In some ways, I think the environment is getting better. More people seem aware of the need for good standards and to follow those standards. IPC seems to be pretty quick in adding in newer packages. The IPC land pattern generator is a big help. But the proliferation of new parts in new form-factors negates a lot of that gain.

    Duane Benson
    I’m not convinced that in net, this post has any actual content.

    http://blog.screamingcircuits.com/

    An Unanswered Question

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    I’ve been reading through my Virtual PCB chat session transcript from Tuesday. It was a fun session and I have a much better idea of how the virtual shows work now. I think I may just be getting it.

    The chat session had a lot of interesting questions and dialog. I did notice, however, that I missed one question and thus didn’t answer it. Oops.

    Owen asked if I am of the opinion that all footprints should have rounded pads (probably stencil cutouts, too) to help with paste release. Sorry I missed your question.

    I’m not of that opinion. There are a lot of factors that come out of stencil decisions. Paste release is one of them. There are others, some more important. For example, the shape of a pad and stencil cut out can either encourage or discourage solder balls. The size of the opening can put too much or too little paste on the pad. Wide open cut-outs over heat slugs can cause float.Bad QFN paste w caption

    The pads themselves, should follow the part manufacturers recommendation for shape and size. Most are rectangular. BGAs have round pads. Unless you have a very good and very specific reason, I would not deviate far from the part manufacturer’s recommended footprint.

    Some of the factors that influence paste release are the stencil thickness, whether it’s polished or not, the angle of the cut, ratio of thickness to width and paste properties. How long the paste has been exposed to air as well as the room’s temperature and humidity can also have an impact. Lots of permutations.

    If you’re reading this Owen, sorry I missed your question in the chat. I hope this answers it for you.

    Duane Benson
    If it’s going to the EU, make sure it’s peanut butter-free.

    http://blog.screamingcircuits.com/