Category Archives: Screaming Circuits Blog

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

How Should You Mark Your Diodes?

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Current flows through a diode from the anode to the cathode – it will pass current only when the potential on the anode is greater than the potential on the cathode. This is mostly true, but not always.

For the common barrier diode, or rectifier, it’s a pretty safe bet. However, with a zener diode, or  TVS, it’s not true. And, that is why marking a diode, on your PC board, with the plus sign (+) is not good practice.

Take a look at the schematic clip below.

 

 

 

 

 

 

Once you put this circuit on to a PC board, you could legitimately place a plus sign on the anodes of D3 and D4, and another on their cathodes. In the next schematic clip, you could legitimately place both a plus sign, and a minus sign on the anode of D9.

We don’t know what you had in mind, and, we don’t have the schematic. If you use the practice of marking diodes with a (+) on the anode, we don’t have any more information than if you didn’t mark it at all. The same holds for using a minus (-) sign. It really doesn’t give us any information.

So how should you mark your diodes? The best method is to put the diode symbol next to the footprint. on the PC board, as shown below. You can also use “K” to indicate the Cathode, of “A”, to indicate the Anode. “K” is used because “C” could be mistaken for “capacitor.”

D5, in the illustration on the right, would be the preferred method. D7 will work as well. If you don’t have enough room on the board due to spacing constraints, you can put the same information in an assembly drawing.

Ambiguity is the enemy of manufacturers everywhere. Read a bit more on the subject here, or here.

Duane Benson
Help stamp out and eliminate redundancy, and maybe ambiguity, or maybe not

Those Danged LEDs Again

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I was caught by one of my own favorite “simple” traps last week: the dreaded LED footprint mess.

I designed a board based on the Microchip PIC32 — it’s a ChipKIT Arduino-compatible board — that has a number of RGB LEDs on it. I used RGB LED part number LTST-C19HE1WT, from Lite-On. The datasheet is easy to find, and the footprint information is right up front, just the way we like it.


Almost all is well, but I somehow missed taking my own advice and I didn’t double-check the footprint.The footprint I used is more or less 180 degrees off from this one. The common anode is still on pin 4, but the numbering is different. It’s got pin one in the same place, then pin two is in the lower left. Pin 3 is on the same place, and pin 4 is on the upper right. That’s the conventional pin numbering order.

Fortunately, the fix won’t require any mod wires. If I rotate the LEDs 180 degrees, the anode will be in the right spot. All I’ll need to do is adjust my software for the correct R, G and B pin locations.

Duane Benson
I’m dizzy with rotation

http://blog.screamingcircuits.com/

No Need to Waste Parts

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We love parts on reels. Who doesn’t? But reels aren’t always practical — and it’s not just about cost. Cost is, of course, important, but there may be other factors to consider.

Say, for example, you need 20 2.2K Ohm, 5% 0805 resistors. You could buy a small strip of 25 from Digi-Key for $0.32. That gives the 20 you need, plus a few spares just in case.

Alternately, you could buy a digi-reel ( a custom quantity reel). On the reel, you’ll probably want more parts to keep the strip long enough for the feeder. Let’s go with 250 parts for $1.39. Digi-Key charges $7 extra to create a custom reel, so that’s a total of $8.39. Still peanuts.

For a third choice, you could just buy a full reel of 5,000 for $10.64. Still peanuts. If you’re gong to need the same part for a lot of designs, this might make sense. But, there’s more than just cost to consider. You need to store and ship it. Shipping two dozen reels gets pretty expense. Storing and inventorying several dozen reels can become a hassle too. 

The beauty of Digi-Key, Mouser and other places that sell cut strips is that they essentially become your parts warehouse. You pay the 32 cents and never have to worry about whether the part is in your inventory, how many are in your inventory, digging it out of wherever you stuffed the reel when you last needed it.

If you do buy and store the whole reel, you don’t need to ship the entire reel to us. Just cut a strip with the number you need, plus about 5% for that “just in case.”

Of course, if you need a few thousand of the parts go ahead and send us the reel. It would make sense then.

Duane Benson
Reel, reel your part
Solder it, solder it, solder it, solder it
Cost is but a factor

Packing Parts for Personal Manufacturing

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Manufacturing, especially small volume one-time-only builds (like a prototype) is hard. It’s not wise for most people to actively seek out chaos, but that’s what we do, and we do it wisely. That’s what we’ve been doing since 2003.

We do it because it’s hard and because it’s necessary.

A big part of quality manufacturing involves risk reduction. Prototyping and quick-turns inherently add in a lot of risk. While we’ve designed our processes and systems around turning that risk into a quality product, there are a few things that you, the customer, can do to help reduce risk even further.

One of the best things you can do to reduce risk is to prepare a well organized kit, as shown in this video:

You can send us your parts in short, cut strips, like you get from Digikey or Mouser, long continuous strips, full or partial reels, tubes or trays. We machine place from all of those types of packages. What’s important is clear labeling and organization.

Individual, or mixed/loose components are not good, though. Pins get bent, leads get contaminated, values get mixed… Leave them in the strip, even if it’s short. If you’ve got multiple short strips of the same part, we can still machine place. Don’t tape them together. We can deal with them as is.

Duane Benson
Peter Piper Picked a Peck of Pickeled Manufacturing

http://blog.screamingcircuits.com

Proper PCB Storage — The Top 3 Hazards

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It’s late. Do you know where your printed circuit boards are? Let me rephrase that: Can unused PCBs be stored for future use?

Yes, they can – if stored properly. Keep them wrapped up, or sealed in a bag. Anti-static isn’t necessary in this case, but it won’t hurt. Keep them in a cool, dark place. Keep them clean. Do your best to avoid dropping them on the floor and stepping on them.

The board in this photo was left out on a desk for a while, and then shoved into a desk drawer. The environment took its toll on the immersion sliver finish, making it very much unusable.

What can go wrong:

1. Fingerprints. The oils on your finger can etch fingerprints into ENIG or immersion silver board surfaces. If you plan on committing a crime go ahead and do this so we can catch you. If you aren’t going to start a life of crime be careful to not get your fingerprints on the board surface. Handle on the edges, or at least, don’t touch any exposed metal.

2. Moisture. Moisture is good for your skin but not for your PCBs. Over time, PCBs can absorb moisture, especially in a humid location, or the ocean. If thrown into a reflow oven they then might laminate. Store boards in a dry environment. If stored for a long time, you may want to pre-bake them prior to use.

3. Atmosphere. Sometimes dirty air can contribute to tarnish or corrosion on the exposed land pads. Dust can settle onto the boards as well. Tarnish and dust can usually be cleaned off, but corrosion can’t. Wrap up your boards for long-term storage.

Treat your boards well and you can likely use them at a later date. Don’t treat them well and you may need to replace them, wasting a bunch of money. Often, the damage isn’t as clear as in the above photo, but could still lead to poor solderability.

Duane Benson
Don’t surf on your silver

http://blog.screamingcircuits.com/

What is Personal Manufacturing?

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There’s a lot of buzz floating around these days, about “Personal Manufacturing.” Screaming Circuits has more than a decade of bringing personal manufacturing to engineers. We pretty much started the category in the electronics industry, so we’re quite familiar – but not everyone knows what personal manufacturing is. I’ll do my best to describe it, and what it can do for you.

The short answer, is that personal manufacturing is building your boards on your terms, not on the terms of some nameless, faceless factory.

The longer answer is probably more useful.

Traditional manufacturing is all about statistics and fractions of a penny. Those factors are important; especially if you’re manufacturing millions. But, when you just need a few boards, or a few hundred boards, those factors can make your job nearly impossible.

With personal manufacturing you can decide when you want or need assembled boards on your workbench. You won’t need to beg for time on a busy volume manufacturing line. In the case of Screaming Circuits, it’s cloud-based manufacturing so you can order online from your desktop, when you’re ready, rather than waiting for someone to pick up a telephone.

With personal manufacturing; you design it, get some prototypes, make a few mods, lather, rinse, repeat. Then, you’ll get a few dozen, few hundred, or few thousand, and start selling. You’ll get what your budget allows and don’t need to commit to minimum volumes, or long-term business. You can polish your design faster, with less hassle, and you can get to market faster, with less hassle. Faster to market and less hassle both mean more time and money for you.

NPI (new product introduction) has never been easier than it is with personal manufacturing. Years ago, I was a product manager at a start-up. The entire NPI process was a nightmare. Our engineers couldn’t get anything built without half a dozen support staff. Someone had to make the documentation usable. Someone had to hunt down sample quantities of parts. Someone had to make sure the board would fit on the volume manufacturers’ assembly line. It went on and on like that, taking up months of the design cycle. We were at the mercy or people who only cared about making their part of the process easier.

Rather than producing the quality product we wanted, our new products would be shipped to customers with mod wires. I recall one board that needed 64 mod operations before it could be shipped. Yes, that was on a released, shipping product.

With personal manufacturing, as Screaming Circuits provides, you can get a few prototypes built right away. If need be, you can modify, and get a few more built at your convenience. When the mode wires are gone, you can build up a hundred and get them out to customers without delay. It’s not about what works best for Screaming Circuits; it’s about what works best for you.

Duane Benson
Right now a personal pan pizza delivered to my desktop would work for me.

http://blog.screamingcircuits.com

Manufacturability Index in Practice

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My prior blog covered the Screaming Circuits Manufacturability Index. It’s something I’ll be using from time to time when discussing new components I run across. I’ve got a few examples to put the numbers into context.

On the low side of the index, we have:

1: Just about anyone could hand solder the part
Examples: Through-hole parts

The SN7400 quad NAND Gate, shown on the right, is a good example. It’s big, it’s through-hole, and if someone has trouble hand soldering it, they really need a few more classes. 

Closer to the other end, is a new chip I’ve run across. The Silego GPAK4 is a small FPGA-like mixed signal device. It’s got a number of analog peripherals, a bank of programmable logic, and the ability to configure it up the way you want. Take a look at it below:

This little thing is housed in a 2mm x 3mm QFN package. That’s pretty tiny by the standards of my giant fumble-fingers. I’ve given it a rating of 4.b, on the Screaming Circuits manufacturability index. The number ranking “4” means: “Needs advanced automated assembly technique“, and the letter suffix “b” means: “Typical level of challenge within the number rank.” In other words, right up our alley.

Unless you posses super-human abilities, and maybe lasers in your eyes, you won’t be hand soldering these. You’ll have them assembled by us (or someone with the same technical capabilities as us), where it will be a standard process.

If you do want to put one or more of these in your design, you will want to make (or find) a custom library footprint for your CAD software. Due to the variable length pads, a standard one-size-pad footprint might lead to solder joint reliability issues.

Duane Benson
The chips go marching one by one, hurrah, hurrah
The chips go marching one by one,
The little one stops to suck her thumb
Just to see if the solder is lead-free

http://blog.screamingcircuits.com/

Mark Those Diodes!

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MarcoPOLOLogo5Every now and then, I write about ambiguity with diode marking; like here, here, or here. It’s a pretty important subject to get right, but what does it have to do with Marco Polo, you ask? Well, that depends on whether you’re asking about the person or the game.

In the game, people try to find someone, without sufficient information. One person, designated “Marco” closes their eyes and periodically yells out “Marco.” The other people respond with “Polo”, and the Marco tries to find one of those other people with just the audible cue. For some critters, that’s an easy task, but for the average human it’s not always so easy — especially when the diode doesn’t audibly respond to “Marco.”

If you’re talking about the explorer, Marco Polo; well, he set off on an adventure, got lost, and either saw a bunch of cool stuff, or made up a bunch of cool stuff (depending on whom you ask).

Again, you ask … “What does this have to do with hamburgers in a handbag, or with diodes?”

It has to do with the fact that he didn’t know where he was going, and, that without clear marking, it’s not always possible to know which way to point the diode.

BlackPOLOSo, we’re celebrating Marco Polo month with our Screaming Circuits Marco Diodo Polo shirt.

If you place an order with Screaming Circuits during May, 2015, we’ll send you an email with instructions telling you how to get a free Marco Diodo Polo shirt after your next order (provided the order is placed between May 1, 2015 and on or before June 5, 2015). If you place an order between now and then, and promptly respond to the email, you can get one for free (a shirt. Not an order).

Duane Benson
Fifty-four fourty, or fight!

blog.screamingcircuits.com

Indicating Polarity on Diodes

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Everyone knows which way current flows through a diode. Right? Of course they do. Diodes only permit current to flow in one direction.

Well, sort of.

In the case of your garden variety rectifier, barrier diode, or LED, that’s true. That line of thinking leads a lot of people to assume that you can indicate diode polarity by putting a plus sign “+” next to the anode.

Here’s why you can’t.

Zener and TVS diodes have a breakdown voltage. They are put in the circuit with their cathode on the positive side. In that configuration, they don’t conduct unless the voltage rises above their breakdown point. Zeners and TVSs are used for regulation, transient suppression, and things of that sort.

But wait! There’s more!

Regular diodes can be pointed backwards too. Anytime an inductive load is switched, like a solenoid or motor, you need a flyback diode to protect the switching logic. A MOSFET switching a solenoid on and off is a good case to look at.

When the MOSFET turns off, the current in the solenoid coil starts to drop. As it starts to drop, the magnetic field generated by the current flow starts to collapse. The collapsing magnetic field generates an opposite current, referred to as flyback, or back EMF.

To save your silicon switching device, you put a flyback diode across the coil, or motor, terminals, pointing backwards from normal current flow – with the cathode pointed toward +V. Doind so shorts the flyback current back into the coil, preventing damage to the MOSFET. These are typically Schottky diodes, but can be ordinary rectifier diodes.

A “+” plus sign alone, doesn’t tell anyone anything. For more information on what to do, read this post. Just for fun, read this post too.

Duane Benson
Diodes. Not just for breakfast anymore

Using the Newest Gen Arm, Part III

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The continuing saga of the 0.4 mm pitch KL03 ARM microcontroller. If you haven’t yet done so, read part I and part II.

Today, I have a look at the good, the bad, and the ugly – or more accurately, the good, and the bad and ugly. As I expected, I was quite pleased with the job done here in house. The board is nice and clean, the parts are well centered, and the solder joints are solid. No surprise here.

Here’s a top view of one we did here in Screaming Circuits:

Next, I’ve got one that I did at home. It actually surprised me and came out better than I had expected. Here’s a top-down view of the one I did at home with home-grade tools (No, I didn’t intentionally make it look bad. The board surface is just a bit shinier than the one above.):

Of course, “better” is a relative term. I didn’t say good. I could call this both bad and ugly. I did manage to center the parts quite well — that took a lot of careful nudging with sharp tweezers and and an X-Acto knife blade.

All of those little round shiny spots are solder balls. That’s what happens when you get too much solder on the board, get solder off the pads, or have the wrong reflow profile. They might look harmless, but if there are too many under the chip, the connections could be shorted.

The fillets on the 0201 capacitor are a little lean on solder in the one I did, and there’s a solder ball on the right side, but, again, it looks better than I expected.

Next time, I’ll post the x-rays and show what’s under the hood.

Duane Benson
Carburetors, man.
That’s what life is all about

http://blog.screamingcircuits.com