Grounded in Reality?

LED scroller 006 thin trimmed
Looking back, I think I could have done a few things different. For example, I don't really need a separate power-on LED. I could either just not populate it, or rev the schematic a little to make it a software controlled indicator rather than hard linked to the power switch. That change would allow me to put the device to sleep. The PIC18F25K20 draws somewhere around a tenth of a microamp while in sleep mode. That being the case, I might just discard the power switch all together. Otherwise, it's cool.

I could have given each LED its own via to the ground plane too. That's supposed to reduce noise a bit. Although, this will probably never be clocked high enough to make much difference, nor used in an environment where it matters.

Duane Benson
Only half of 10 the types of people will understand

Done and Done.

LED scroller 005 trimmed And... Drum roll, please... It works. I put in a couple of batteries, programmed the MCU and turned it on. It works. I'm always surprised when something I design works on the first try. This being such a simple design, I probably shouldn't be surprised though. I should at least give myself a little more credit.

The unpopulated lands on the board in the photo are supposed to be unpopulated. I left a few things out because they aren't needed for what I'm doing with this piece now and leaving them off keeps the cost down.

So, what did I learn from the process?

  • If you have a lot of different parts laying around, it's pretty easy to grab the wrong one
  • I ran into some variability in the "zero rotaion" position in the CAD library land patterns
  • The whole process is pretty easy, but start to finish, there are quiet a few steps
  • It's a nerve wracking wait after sending a box of parts off
  • Good communications between designer and assembler are very important
  • Clear documentation from the designer is very important
  • This was a WHOLE LOT easier than hand soldering all the SMT parts (I've done that before)

That's a good set of educational results. Next time, I think it will be easier.

Note the large diode polarity indicators on either side of the long row of LEDs and by LED D25. D1, the Schottky on the upper right has the same polarity indicator, but it's in between the pads, under the part. In case you're interested, I have a 3 volt supply. The LEDs drop 1.8 volts and I've got a 150 ohm resistor for each. That gives me a theoretical 8 mA per port for a total maximum of 176 mA with all 22 lit up. That's within spec in the -40C to 85C temperature range but too much when above 85C. I'm not sticking this in an engine compartment or anything, so no worries there.

Duane Benson
0x45 0x53 0x43 0x20 0x62 0x6F 0x6F 0x74
0x68 0x20 0x38 0x32 0x33 0x20 0x20 0x20

My Mistake - Naturally

I received my PCBs for this project from Sunstone yesterday at about 10:10. I quickly dropped them into my box of parts and delivered it to the receiving pile-of-boxes in our shipping.receiving department. At  11:40am yesterday, I received an email from our auditing department informing me of a BOM mismatch.

BOM mismatch Yes. I had made a mistake in my bill of materials. The board has a bunch of yellow LEDs and one red LED. I had mistakenly only packed up yellows. Our audit department caught my mistake and sent me a quick email. If I hadn't have responded yesterday, I would have received another email at midnight. I could have just told the Downsized_0421110938a manufacturing folks to put a yellow LED in that spot, but the yellows are for a display and the red is a power-on indicator so I got my red LEDs and delivered them to receiving.

This morning I got my assembled boards all nicely wrapped in anti-static bags along with all of my leftover parts in their original packaging. Next step, get some batteries and power them up.

Duane Benson
Thor, Dog of Thunder, is not allowed

Short Cuts Don't Always Make Long Delays

The saga continues. I have my parts kit. The PCBs should be here from Sunstone tomorrow. When I placed the order on our website, I estimated that I'd have the parts and PCBs today, but I wasn't really sure. I knew there would be a chance that it would be tomorrow, but I thought it would be good to see how our communications goes when something is late. Obviously, an assembler can't start building until the parts have arrived, so the Industry standard is to start the turn-time once everything is in the shop.

If a box is late and the sender doesn't know it, unintended delays can be added into the process. Knowing this, we recently did a lot of work to improve our communications, on such issues as late parts, to help reduce delays. Sure enough, I dropped on over to the website and right on the top of the home page is a note that I have an issue (late parts) with my job. Tonight at midnight, I should receive an email telling me the same thing too.

On the subject of the PCBs, I sent Gerbers to Sunstone. That works just fine, but I'm always a bit nervous about the accuracy of my layer mapping. They double check, so I've never had problems, but I still get nervous.

If I'd waited a few days, like until today, I could have taken a short cut by just sending in my CAD board file - they just started accepting native CAD files. You can still use Gerbers, but if you use Altium, Eagle, OrCAD, National Instruments Circuit Design Suite, Ivex Winboard or PCB123, you can just send in the board file and save some time and hassle.

When I get the boards tomorrow, I'll pack everything up and deliver it to the receiving folks. Then I'll see how the rest of the build process goes from the other side of the fence, and I'll see how we deal with extra parts. I did that on purpose also. With a couple of parts, I'm delivering several hundred more than I need. With a few other, just the requisite 5% over. It will be interesting to see just how I get the extras back.

Yes. I know. I work here, so I shouldn't have any doubt about how all of this stuff works. I do know how it goes, but it's always a good thing to, every now and then, check and see how well practice matches up with theory.

Duane Benson
Grip, Fang, Wolf! Guard the mushrooms!

Centroid / XYRLS / Pick and Place

Call it what you may, but surface mount assembly robots need this magic file to determine where to place your components and how to orient them. We call it a Centroid. Others may call it something else, but it's all basically the same. In our case, the basic format is comma delimited, in mils:

Ref designator,     Layer,     LocationX,     LocationY,     Rotation
    C1 ,                       Top ,           0.5750  ,       2.1000  ,           90

That's not too difficult. Most CAD programs will automatically create this file for you. Eagle doesn't natively, but we have a ULP to do it for you in Eagle (Downloaded here). Again, no problems here. Mostly...

I say mostly because, at this point, you are at the mercy of the person who created the CAD library part. Provided they center the origin and follow the IPC for orientation, everything should come out just fine. Unfortunately, we do find parts that don't follow those rules. We'll do our best to catch and correct such things here, but for maiximum reliability, check you library components to make sure. We find the problem crops up most commonly with passives.

IPC says that zero orientation for two pin passives is horizontal, with pin one on the left. For polarized capacitors, pin one is (+). For diodes, pin one is the cathode. They note that pin one is always the polarity mark pin or cathode. Pin one is also on the left for resistors, inductors and non-polarized capacitors, but left vs right doesn't matter so much with non-polarized things. The most common orientation error we se is to have the "zero rotaion" 270 degrees off from the IPC standard.

Every now and then we'll find that someone assumes that since usually the anode on a diode tends to be on the positive side, that the anode should be pin one. Nope. Nope. Nope.

Duane Benson
Is it pulling electrons or pushing holes?

Cat Chow

I recently wrote a bit about eating my own dog food relative to a small design I've put together. Today, I'm moving along with that process and kitting it all up, so I thought I'd pass on some hints on making a good parts kit. I'm looking at this from the perspective of Screaming Circuits, but my guess is that it would fit for just about anyone assembling your prototypes. Even George's PCB Assembly and Dry Goods Emporium down past the railroad tracks.

If you're sending all of the parts, you can leave them in the original package. Just be sure to clearly mark the packages with your reference designators. If you've got to cut your strips down, you may need to Downsized_0418110808 re-package the parts, as I am. I got these little handy dandy anti static bags from Digi-Key (part number 16-1032-ND) for less than ten cents each in a pack of 100. You can use the little pink anti-static bags too.

I've labeled each bag with the reference designator, the component manufacturer and the manufacturer's part number. The more ambiguity that you can remove without adding excess clutter, the better. Making the labels was easy. I used Avery #5366 labels and mail merged from my BOM spreadsheet.

You can also put the component value on the label as well, if you can do it without clutter. Maybe line 1, reference designator; line 2, manufacturer and manufacturer part number; line 3, component value. Once you've got the bags labeled, go ahead and fill them up with the parts needed for your assembly. Add in 5% extra just in case (50% extra for 0201 passives).

If any of your parts are moisture sensitive either leave them in the original moisture barrier packaging or let us know that they need to be baked prior to assembly. That will prevent popcorn in the reflow oven.

Duane Benson
You'll like it better or my name isn't Orville Partenbacher

 

Dog Chow

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 over 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 will be needed), upload the files and send the kits in. 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 up.

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?

Picking Packages

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 lttle more difficult, but not much. It was largely a matter of power disipation requirements.

Different story now, though. First, there's thru-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) Thru-hole resistors range from 1/20th Watt up to multiple Watt packages. SMT parts range from 1/32 Watt up to lots. Simplifying a bit and just looking at 1/4 Watt, 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 n SSOP over an QFN? Why 0603 over 0402, 0805 or 1206?

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

 

Texas Instruments plus National Semiconductor

Everyone else seems to be weighing in on the Ti + NS 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 then if it were a new 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

Top-Five Ways April Fools Is Good For You

April first isn't quite here yet, unless you're reading this a month from now. Then it's long come and gone. The concept of "April Fools" goes back many years. In some circles, the first April Fools Joke is said to have been played on November 13th in the year 1959 by Duchess Gloriana XII of Grand Fenwick. Others pin the first joke several centuries earlier.

Conventional thinking is that the jokes played on that day can be humorous, annoying, disruptive or downright dangerous. It's the disruptive and dangerous jokes that have caused the formation of a movement to ban all jokes on April first. Despite that, history has shown that the levity of the day can have positive effects as well. That being the case, here are my top-five reasons that April Fool's Day can be good for you:

  1. It can be good for your peace of mind. If your co-worker plays a joke on you by secretly changing the calibration on your scope so the ranges will be off, you can have a day of feeling good about your OP Amp circuit, thinking the noise level is down to just where you want it.
  2. April Fools jokes can lower your blood pressure. For example, say a co-worker replaces your pepper with a bunch of 1mm x 1.3mm 6 bump chip scale BGAs. You then put generous helpings of salt and "pepper" on your chicken sandwich. While probably not at all healthy, the little chips probably aren't fatally bad to consume but would taste bad so you'd spit the first bite out and not eat the rest of your sandwich, thus not consuming all of that blood-pressure-raising salt.
  3. It can make solder selection easier. As it is, your lead-free vs. leaded decision (for stuff not going to Europe) has to be made based on the BGA. Mixing leaded solder with lead-free BGAs and vice verse is not a good thing. If someone in materials plays an April Fool's joke on you by scraping all of the solder balls off of your BGA, you may at first feel despair. But then you realize that without the pesky solder balls, you can use it like an LGA and pick whatever kind of solder you want.
  4. It can help with recreation. When you arrive to your cubicle loaded with 20,000 ping pong balls, you may be dismayed at first. But, take heart in the fact that you now have a lifetime supply of ping pong balls. You can now learn the sport without fear of losing your supply of ping pong balls due to explosion or crushing. And, be glad that they didn't used golf balls.
  5. You can get a promotion and a big raise. This is your opportunity to shine. Play a humiliating and very public prank on your boss, or perhaps your boss's boss. By bringing humor into his or her life, you'll not only be noticed, but will also be greatly appreciated for raising moral though public humiliation of management. They always appreciate that.

Duane Benson
Ever get that sinking feeling - you had a very tiny part out so you could use the part number in a humor blog post referring to consuming that part and when you're about halfway through the snack you've been eating while writing the post, you discover that the tiny little part is gone? At least it's a lead-free part.

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