Screaming Circuits: Circuit Design


What Route Do You Take?

There are a lot of polar opposites in the "what is my philosophy" world: Mac vs. PC, on shore vs. off shore manufacturing, Ford vs. Chevy, Atmel vs. Microchip (well, maybe not that one so much any more), auto router vs, hand route... Yes, I'm specifically avoiding political opposites.

OMAP escape routingRouting is what I'm really interested in today. The conventional debate is hand vs. auto route. CAD companies spend a lot of time and money on auto routers, but there's definitely a line of thought that says it's not ready for prime time yet. This shirt designed by Chris Gammel, on Teespring pretty much says it all.

But, it's more complex than that. Most auto routes end up requiring some hand work, either to finish routes that can't be found automatically, or to clean up a few less than efficient choices. There are differing techniques for complete hand-routing as well. 

I often find myself looking at a layout project a bit like a chess game. I don't just start at one end of the board and work my way to the other side. I tend to focus on specific parts or critical requirements first, like signal paths that need to be short, or sections with more critical grounding requirements. (The image on the right isn't mine. It's from the Beagleboard)

When it gets to the mass, I tend to try and think ahead, projecting moves out, as though it were a chess game. When I'm looking for the best route for signal path A, I try and think ahead to how it will impact B, D, D... as far ahead as I can go.

I'm not sure if doing it this way is easier, of if it would be better to just start routing and then re-route as I run into roadblacks. What about you? How do you approach a complex layout?

Duane Benson
Holy cow. I Googled "Trust no one" to get some ideas for my signature
Never do that. It's going to take a week to shake off all the negativity

Top 5 Things to Know When Moving from Hand Assembly to Robotic Assembly

A lot of factors go into the decision to hand build or outsource circuit boards. I hand build my own sometimes, simply because I enjoy the challenge. Of course most of the projects I design are for my own use, so timeliness isn't that important. When I do design something that will go out to a customer, like my electronic business card holder, I will send the board through our shop. In those cases, quality is important, as is delivery, and the quantity is often too high TI TPS62601 front and backto hand build. Machine building also allows me to use smaller and more complex parts.

That same decision - hand build or outsource - takes place in the heads of designers all over the country. When the decision is to outsource, there are a few important things to consider. Some things that work fine when hand soldering may stand in the way of quality, repeatability, and reliability when machine assembling.

Here are five of the most important considerations when changing from hand built to outsourced at a place like Screaming Circuits

1. Use solder mask and silk screen

A good solder joint needs the right amount of solder in the right place. Solder will tend to flow down bare copper, bleeding outside of the area it belongs, and down exposed copper traces and vias.

The main purpose of solder mask is to keep the solder where it belongs. It also protects the traces, but that's a longevity issue. Solder bleeding is a manufacturing and reliability issue. This isn't a problem when hand soldering. In fact, it can even work to your advantage when hand soldering really small parts. It gives you more room for your soldering iron to hit metal.

Not so with solder paste and machine assembly. Use solder mask.

2. Avoid the pseudo panel

Keeping small boards in a panel is the recommended best practice in the manufacturing industry. We appreciate it and, while not always necessary, can reduce your costs. We sometimes see what we call a "pseudo panel." This is a board where multiples of the board are put in the same PCB, like a panel, but unlike a panel, the boards don't have routing or V-score between them. Sometimes the designer will put a bunch of vias to outline the board, or just ask that we use a band saw to separate them.

That's a time consuming, expensive, and potentially damaging process. The vibration of the saw can crack solder joints, and, you're unlikely to get boards that are all the same size. Have small boards panelized by your board house.

3. family panel (pseudo or not)

Similar to the pseudo panel is the family panel. A family panel is a case where a project is made up of several different PC boards, and they are all laid out together, as though they are one design. If the board isn't routed between to designs, you'll have the pseudo panel problem described above.

The bigger problem, though, comes with reference designators. We typically see family panels with duplicate reference designators. Each design, for example, will have its own C1, R1, Q1, etc. We use the reference designators as position identifiers/ If you have three different parts labeled R5, our machine programmers will have a problem with it. It's even worse if the values differ; on one design, C1 is a 0.1uf capacitor, while on another design, it's a 22pf cap.

If you're making a family panel, give each and every placement a different reference designator. One way would be to us extra digits. For example on one design on the family panel could have C100, C101, C102... The next would be C200, C201, C203, and so on.

And - don't forget the routing or V-score between the designs.

4. QFN - hole  in the middle

A common technique in the hand soldering world, for soldering QFNs and other parts with thermal pads underneath is to put a big via in the middle of the center pad. By doing so, you can stick a soldering iron and some solder down through the hole and get a good solder connection on the bottom pad.

This doesn't work with machine assembly. the solder paste will flow down and out the hole in the reflow oven. You'll end up with a poor connection (or no connection) to the thermal pad, and solder slop on the back side of the board.

BOM line items 0055. Parts and the Bill of Materials (BOM)

When I build my hobby projects, I often get a bit carefree with the bill of materials. It's not good practice, but I do. I'll put a part in the BOM that I used before, and not check to see if it's still in stock. I'll put parts in the BOM with just the values and not any part numbers. Things of that sort require tribal knowledge, which only the designer has.

When building, sometimes I'll just grab a part that's close. If I need an 0805 1uf, 10 volt capacitor, I can grab a 16 volt, 25 volt, etc. I can even make an 0603 part work. You as the designer may know that something close will work, but an outside house can't know. You need to tell them exactly what the part is.

Before sending anything through our shop, I do clean up the BOM. In order for us, or any manufacturer, to build the boards, the BOM needs:

  • A unique reference designator for each part placement
  • The quantity of each part used on the board
  • The manufacturer
  • The manufacturer's part number
  • Digikey part numbers can be used as well

Here's our web site page explaining the BOM format in more detail.

The transition from hand building to outsourced machine building can be an intimidating one. But, with a few considerations, it can be an easy and rewarding transition.

Duane Benson
Put the right part in
Put the wrong part out
Put the right part in
But please don't shake it all about

 

USB Type-C Connectors

It wasn't terribly long ago that pretty much every cell phone came out with its own custom charging cable. It was a major step forward when they all (except Apple) standardized on the USB micro-B connector.

However, there are a number of limitations with the. First, it takes a minimum of three attempts to get the orientation right when trying to plug in a cable. Second, it's limited in maximum current carrying capacity.

Image70

Now, along comes the USB 3.1 Type-C cable and connector. It's similar in size, universally polarized (the connector and the cable can be plugged in any end to any end and in any orientation), it has much higher data thru-put, and it's spec'ed to carry up to 3 Amps. Further, it has alternate modes so other standards, such as DisplayPort and Thunderbolt.

SMT - TH uUSB with PCBThe connectors are larger than the micro-B, as you can see in the comparison photo above: micro-B, Type-C with only surface mount connections, and Type-C with both surface mount and thru-hole wiring, and a US dime. The size difference won't be an issue in most cases, but it could be in really small devices. My guess is that we'll be talking about a smaller, Type-D connector, not long from now.

All three of the shown surface mount connectors have thru-hole mounting tabs. That adds strength, but it does bring one caution with it. Looking at the micro-B connector in the image on the right, you can see that the tabs are formed out of the same sheet metal as the shell.

You can also see that the tabs don't stick all the way through the PC board. This can lead to some deception when soldering. Without the tabs protruding, it's easy to believe that you don't have enough solder in the connection. If you feed more solder in, it will likely wick along the tab, and end up inside the receptacle, preventing the cable from being plugged in. If you're hand soldering or reworking these type of connectors, keep a close watch on the amount of solder you're using.

Duane Benson
Fester Bester Tester is alive and well and living where?

Using the Newest gen ARM, Part III

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:

4mil top view 800

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.):

Home top view 800

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

USING THE NEWEST GEN ARM, Part II

I'm a bit behind in my blog work - well, way behind, actually. I started this series back in January with the intro post.

Here's where I am right now:

  1. I have three different sets of PC boards.
  2. One set, I took home to see if it's possible to solder a micro BGA at home. (As someone working at a car manufacturer might want to see if they could balance a crankshaft at home, for fun)
  3. Two sets, from our partner, Sunstone Circuits, are here in my desk with parts, ready to go through our machines.

After I've got all three sets built, I'll have them X-rayed to see how they look under the hood. Finally, I'll solder thru-hole headers on and fire up the chips to see if the shared escape system works.

Here's one of the boards without access to the inner pads:

KL03 SunstoneFF 4mil (2)-001

And, here's the shared escape:

KL03 SunstoneFF 4mil (3)-001

The main concern I have is that Reset is on one of the inside pins (B4). I'm not sure if I can get the chip to a state where it will operate properly without unobstructed access to reset.

The routing I've chosen is probably the only possible option for reset. Pin A4, right above, is used for the single-wire debug (SWD) clock. I'm assuming that can't be shared. B5 is Vdd, so that's out. It might be possible to go down. C4 defaults to one of the crystal pins, and D4 defaults to a disabled state.

In the route I've chosen, B3 is an ADC input, so it should start out high-impedance, and therefore not interfere. A3 defaults disabled, so it won't get in the way.

Next step: solder time!

One other thing - The images above show non-solder mask defined (NSMD) pads. Those are standard for BGAs 0.5mm pitch and higher. This part is 0.4mm pitch. Some manufacturers recommend solder mask defined pads (SMD) for 0.4mm and smaller. I'm actually testing several pad styles: SMD, NSMD and solder mask opening = copper.

KL03 footprint contenders

Duane Benson
Run it up the flag pole and see who solders

Using the Newest gen ARM Microcontrollers

KL03 on stampI've written a few times about the new Freescale KL03 ARM Cortex M0+ microcontroller. This particular part comes only in very small packages, with the smallest being a 1.6mm x 2mm WLCSP (wafer level, chip scale package) 0.4mm pitch, 20 bump, BGA. That's a mouthful - albeit a very tiny mouthful. Maybe just a toothful.

On the left, here, I've got a pair of them on a US postage stamp.

For us, it's not a particularly difficult part to assembly; just a garden variety 0.4 mm pitch BGA, as far as we're concerned. We place loads of them. But, it can be a very different story for a designer. Conventional wisdom says that a PCB designer has two choices with a part like this: a very expensive PC board, or don't use the part.

Escape routing becomes very difficult (read: expensive) at 0.4 mm pitch. This part only has six connections that need to be escaped, but that can still be a problem. You can't fit vias between the pads KL03 SunstoneFF 4mil 800to escape out the back side. You can't put vias IN the pads, unless you have them filled and plated over at the board house. That's expensive in small quantities.

This blog post series is going to examine some possible ways to use these parts with more of a standard fab, such as Sunstone quickturn. I've got three different process blank PC boards, each with four different land patterns.

I've been asked about home reflow too, so as a bonus, I've done my best to duplicate hobbyist conditions for one of the board sets.

Check back next week for the first set of results, and be sure to quote your assembly job at Screaming Circuits or your PC boards at Sunstone.com.

Duane Benson
"Screaming Reflowster" not sold here

Polarized non-polarized components: Inductors


We have a number of manufacturing engineers running around here at Screaming Circuits. They're very good at what they do, as are our operators and technicians. They are not, however, electrical engineers. Our parent company has a big group of electrical engineers, but they're at a different location

What that means is, though we endeavor to be experts at building things, we often don't know what the circuits and components do in your specific application. People tend to send us their difficult projects so we've probably seen just about everything possible go through our plant. But, every now and then we see something unfamiliar. It doesn't happen very often, but it does happen.

20150114_19440220150114_194410Sometimes it's an exotic new package (like the 0.3mm pitch wafer scale BGAs now
showing up). Other times, it's something a bit older, but just not clear. Rather than put a job at risk, if we aren't sure, we'll always hunt down the designer and ask.

Okay. That was a long winded intro.

We recently ran across just such an unknown; a "polarized" inductor, without an accompanying "polarity" mark on the PC board. Not only that, but the markings on the inductor were a bit ambiguous. One half is black and the other half is green. The datasheet is in black and white, so there's more room for interpretation than we're comfortable with.

At first glance, you might wonder why polarity / direction matters in an inductor. I did. It's just wire. Right? 

Almost: it's not just wire, it's coiled wire. In most cases, the direction doesn't matter, but in cases with multiple inductors, or with super high speeds, it can matter due to the fact that the coil winding direction has an influence on the flux and the actual induction.

I won't go into all of the theory, but think of walking. In most cases, it doesn't matter whether you start with your left foot or your right. However, if you're marching in a coordinated group, you want everyone to start with the same foot.

Look at the two sets of air-core inductors above. When set like this, directionality starts to make a bit of sense. Imagine the electrons being pushed around in theses things and try to picture the resulting lines of flux.

The moral of the story: eliminate ambiguity. If the part is polarized, either mark the board, or make it the direction clear to your manufacturer in build documentation. Do this even if the polarity doesn't matter to you, 'cause we don't know that.

After photographing these, I ended up recalling this bit of knowledge. It's just so rarely needed that it had vanished in to the fog. I put a few more photos after my signature.

Duane Benson
Which way did he go?
Which way did he go?

20150114_19431320150114_194348

 

Do you Need that Part? Or, is it Just Habit?

At the moment, I'm working on an Arduino compatible clock. Like most of my Arduino compatible boards, this one uses an Atmega32U4, with USB built in. With the Atmega32U4, I sacrifice a little in program memory and SRAM, but gain a bit in reduced parts count.

A USB capable Arduino-compatible is, of course, programmed via USB, and can be powered by the USB port. Most Arduino boards also have a 5 volt regulator to be used when being powered by a wall-bug power supply. Naturally, I put the USB connector on the clock board, as well as the 5 volt regulator. With the two different supplies, I also put in circuitry to auto switch sources and protect the USB host when both supplies are connected at the same time.

NeoPowerSupplyMy first PCB revision required a few hand-mods, but not many. Still, I decided to re-spin the board and remove the two mod wires. While doing so, it suddenly occurred to me - a blinding flash of the obvious - that most cell phones and other small devices are charged with a USB-connector 5 volt wall-bug power supply. Why then, would I also need a separate power supply and on-board 5 volt regulator?

By pulling the regulator off of the board, I could eliminate a few capacitors and the supply auto-select / protection circuitry. Not only did I save in component cost, but I was able to reduce the PC board size, and thus cost, by about a third.

  1. I had the 5 volt regulator in the design because Arduinos can be powered by either USB or a non-regulated power supply.
  2. The reverse power protection is necessary to prevent damage to the USB host if the other power is also connected.
  3. The auto-power switching circuit is necessary so that a user doesn't need to flip a switch or change a jumper when changing power sources.
  4. I had two extra LEDs to indicate which supply was powering the clock.

I questioned my original assumptions, found a "because it's always done that way" and eliminated it. Assumptions are meant to be challenged.

Duane Benson
Question authority!
And then get squashed
(or, squash extra space out of your PCB)

VTP - Very Tiny Parts

FreescaleKL03A while back, I wrote about a new ARM Cortex M0+ chip from Freescale. It's not the first M0+, but I do believe that it's the smallest. I've been checking stock off and on and finally found the smallest package to be in stock and available to ship.

I actually bought a couple of different types. First, there's the WLCSP 20. It's got 32K FLASH, 2K SRAM and an 8K bootloader. The real kicker is that the package is only 1.6mm X 2.0 mm. I also got a few in the QFM 16 package, which is a bit more workable at 3mm X 3mm.

Finally, I bought a Freedom development board with th 4mm X 4mm QFN 24 package. The dev board is hardware compatible with Arduino shields, so that will make for some interesting possibilities.

Anyway, here at Screaming Circuits, I'm most interested in that 1.6mm X 2.0mm package to see how easy (or difficult) it is to use - see if there are any particular layout challenges. The other stuff is just for after hours play time.

Duane Benson
I'm not a number. I'm a free development board!
(Free, as in named "Free...", not free as in "don't cost nothin")

Cost Reduction in Design - More Advice for Makers

As a Maker, you really need a decent price, with good quality, and good service. Contrary to what many people think, you don't need to look outside of North America for this. You can keep your gaze West of the Atlantic and East of the Pacific.

Like everything else in the modern world, design decisions can have a pretty big impact on your cost. So, lets take a look at some design decisions that can make your manufacturing more affordable.

  • Accept longer lead times

Lead times are one of the biggest factors in electronics manufacturing. Screaming Circuits can turn a kitted assembly job overnight, but it costs a lot of money to do that. Screaming Circuits also has a 20 day turn-around that is much, much more affordable. Accepting longer lead times on PCB fab will drop your cost as well.

  • Avoid leadless packages like QFNs and BGAs

We build tons of QFN and BGA boards - even down to 0.3 mm pitch micro BGAs. That's great if you need those packages. However, since all of the leads are underneath, we have to x-ray every part. That adds a bit of cost to the process. If you can, stick with TSSOPs and other parts with visible leads.

  • Use reels, or 12" or longer continuous strips

Tab routed multi panel 1024We will gladly assemble parts on strips of almost any size. But, to save costs, use full or partial reels or continuous strips of at least 12" long. It costs us less time to work with reels and continuous strips, and we pass those savings on.

  • Stick with surface mount

These days, thru-hole components tend to be hand soldered. That costs more than machine assembly, so use surface mount wherever possible. Surface mount components tend to be less expensive than thru-hole too. If you do need a few thru-hole parts, this is an opportunity to put in a little sweat equity by soldering the thru-hole yourself and save a bit of money.

  • Consider keeping your surface mount parts on one side

Putting surface mount parts on both sides of the pc board is a great way to better utilize space. However, if cost is more of a concern, and you only have a few parts to put on the back side, it may be more cost effective to move them to the top side.

If you've got a lot of parts, the additional cost for assembling both sides may be less than the cost for the extra PC board size, but with a small number of parts that's probably not the case. Quote it both ways and see which is less expensive

  • Panelize small boards

We can work with really tiny boards individually, but sticking with a larger size makes the job easier, and, again, we'll pass those saving on. If your PC board is smaller than 16 square inches, panelize it. We put in less labor and you get a price break.

  • Save on start-up costs with PCB123┬«

Just the act of starting our can pretty much break the bank. PCB123, from Sunstone Circuits®, is a full-featured PCB CAD system you can get free of cost.

By sticking with Screaming Circuits, you get the same care and quality that we give to boards going up into space, down into the ocean, and everywhere in between. By sticking with Screaming Circuits, you get a known turn-time; not an "about ..."

By following these guidelines, you get a decent price and really good quality and service.

Duane Benson
That would be telling