Category Archives: Screaming Circuits Blog

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

Using the Newest Gen Arm, Part II

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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 PCBs.
  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 through-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:

And, here’s the shared escape:

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.

Duane Benson

Run it up the flag pole and see who solders

http://blog.screamingcircuits.com

Component Footprint Rotation, Part II

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I’ve noticed that a lot of CAD library footprints for two-pin polarized parts have pin one pointed up as zero degree rotation. According to IPC, pin 1 pointed to the left is zero degree rotation.

Why is this such a common error? I can’t be certain, but I have a pretty good idea.

Surface mount parts, as everyone knows, generally come in reels of tape. It stands to reason, that the parts would be placed into the tape at a standard zero-degree rotation. They generally do. Before putting a perplexed look on your face, take a look at the image below.

20150220_143916
When looking at the tape, it’s a pretty natural thing to pull it out and hold it horizontally – with pin 1 up – perpendicular to our angle of vision. Makes sense. It’s not a stretch to look at this strip of tape and end up assuming that pin one is up at zero rotation.

However – the machines are the ones being spoken to. Not humans. The machines get the parts in line with their line of vision. That puts pin one on the left.

20150220_143650
Makes more sense when you look at it this way. Running into the machine, pin one, at zero rotation, is on your left.

For more to the part rotation story, tune your browser dial to here. Or just scroll down a little bit. It’s right below.

Duane Benson
The long and winding reel leads to your PC board. Not your door.

 http://blog.screamingcircuits.com/

Using the Newest Gen Arm Microcontrollers

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I’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 1.6 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 0.4mm 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 PCB, 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 to 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 PCBs, 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.

Duane Benson
“Screaming Reflowster” not sold here

http://blog.screamingcircuits.com

Polarized Non-Polarized Components: Inductors

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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.

Sometimes 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?

http://blog.screamingcircuits.com/

 

 

Do You Need that Part, or is It Just Habit?

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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 5V 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 5V 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.

My first PCB revision required a few hand-mods, but not many. Still, I decided to respin the board and remove the two mod wires. While doing so, it suddenly occurred to me — a blinding flash of the obvious — that most cellphones and other small devices are charged with a USB-connector 5V wall-bug power supply. Why then, would I also need a separate power supply and on-board 5V 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 PCB size, and thus cost, by about a third.

  1. I had the 5V 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)

Freescale KL03 and PCB123 at 0.4mm Pitch

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Small component packages seem to be a recurring theme with me. It’s understandable, I guess. Super tiny packages are becoming more and more common and we build a lot of product with them.

The smallest we’ve built is 0.3mm pitch. Those aren’t common enough to be considered standard — they’re still an experimental assembly — but not many chips use them yet. 0.4mm, on the other hand, is something we see on a pretty regular basis.

What’s so tough about that? The biggest challenge with these form-factors seems to be footprint design and escape routing. I can see why. There really isn’t room to follow any of the standard BGA practices. You can’t fit escape vias between the pads and you can’t put vias in the pads, unless they are filled and plated over at the board house. Filled and plated vias are the easiest way to go, but it can make for an expensive board fab.

KL03 WLCSP20 on a US Lincoln penny. One of my side projects involves trying to make the smallest possible motor driver. For this project, I’ve chosen the Allegro A3903 driver. It’s a 3 x 3mm DFN (dual flatpack no leads) with 0.5mm pitch pads and a thermal pad in the middle. The microcontroller will be the new Freescale KL03 32-bit ARM in a 1.6 x 2.0mm WLCSP (wafer level chip scale) package. It also comes in a 3 x 3 x 0.5mm pitch 16 pin QFN. Without an expensive PCB, that may be my only option.

Pick your CAD package. I’m using the newest version (5.1) of Sunstone Circuit’s CAD package, PCB123, but the principles here will apply to any CAD software. If you don’t already have a copy, download PCB123 V5.1 here.

If you’ve got fast Internet, you’re done now, so go ahead and install it. You’ll need the manual too, which you can get here.

I need to eat now, so stay tuned for Part 2.

Duane Benson
Nerfvana – It’s like Nerdvana, but with more foam darts.

http://blog.screamingcircuits.com/

QFN? QFP? QFWHAT?

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The QFN (quad flat pack, no leads) has become my favorite integrated circuit package. It’s very compact, yet is easier to use than a µBGA.

µBGAs of 0.5mm and smaller pitch become a bit more difficult and costly with more than two rows of pins. At those geometries, escape routing can involve plugged and plated vias, which add complexity and cost to the fabricated board. QFNs can be almost as small, but have all of the pins exposed around the edges, so there’s no need for escape routing.

One thing that’s important to note is that despite sharing the first two letters (Q and F), QFP and QFN footprints are not interchangeable. We do, from time to time, see boards laid out for one along with the other form packaged part.

Take a look at this PCB layout clip from the Arduino Leonardo. It has both footprints on the board. You can see how much bigger the QFP package is.

They put down both footprints because the Atmega32U4 chip used in the Leonardo sometimes has supply issues in one package or the other. This gives them the flexibility to use either without making changes on the board.

You might consider this as an option if there’s space for a QFP and you are concerned about the availability of one package variant or the other. If you do, there are some very important things to check out:

  • Make sure the pin-outs match. Some parts vary the pin-out a bit between packages or have extra pins on one or the other.
  • Make sure the extra space won’t cause noise problems. Generally, bypass caps should be as close as possible to the supply pins. This amount of extra space probably won’t be a problem when using a QFN, but in some designs it might.
  • Make sure the board won’t be in an environment where unsoldered pads will be a problem. Some harsh environments could attack the unsoldered pads. If that’s the case, consider conformal coat.

Duane Benson
We’re always being pushed and shoved by people trying to beat the clock
But we like it – it’s what we do

http://blog.screamingcircuits.com/

Remote ESD

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ESD, or electrostatic discharge, is of great concern to anyone who deals with electronics. That’s obvious. What’s not necessarily so obvious, is that some times, you don’t even need to be all that close to the circuit board or component to damage it.

This article by Douglas C. Smith illustrates why sometimes just a wrist strap isn’t enough.

That’s why we don’t only use wrist straps, but also have a grounded conductive floor and use ESD jackets and conductive foot straps to protect the boards and components out on our manufacturing floor.

Here’s a video showing the dreaded ESD monster and us protecting your gear from him:

 

https://www.youtube.com/watch?v=hWoyInZr2sU&list=PLZ6WZdSCHUOeuppvKHRdrOY9sDpsCbxJU

Duane Benson
Greased lightning is an interesting concept
Would it reduce power line transmission losses?

http://blog.screamingcircuits.com

 

Warped PCBs

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You just got a nice big PCB back from the fab shop. You set one on your desk to admire only to discover that it’s warped. What do you do?

There are two primary types of causes of board warping: process related at the fab or assembly shop, and layout-related issues. If it’s warped before assembly, it’s between fab and layout. If it’s flat before assembly and warped, after, it’s most likely between layout and assembly — although sometimes a fab problem won’t show up until a pass through the reflow oven at your assembly partner.

Determining the root cause is generally a bit of an iterative process. It’s tempting to start right off with your fab or assembly partner, but you need some information before giving them a call. You’ll need such things as the amount of warpage per inch, board size and thickness. With that, you need to take a good look at your design and consider copper pours, component size and component placement.

With that information in hand you can make your phone call. If the board is warped before assembly, call your fab shop. If it’s flat pre-assembly and warped post-assembly, call your assembly house.

The shop you call will want to talk over your design to help you pinpoint the cause. If you can rule out a design issue,then you need to talk with your partner to determine whether it’s a fab or assembly issue and next steps to take care of you.

 Here are a few design issues that could contribute to warping:

  • Uneven copper pour. Copper and FR-4 are a good match relative to thermal expansion, but they aren’t exact. A large pour on one side or corner of your board can lead to warping due to dissimilar expansion characteristics. This could cause warpage either at the fab shop or the assembly house.
  • Components with large thermal mass grouped together on the board. This would be more likely to cause problems during assembly than during fab. The thermal mass will act as a heat sink for that area on the board, which can lead to uneven expansion and uneven soldering.
  • A board that’s too thin for the size or number of components could lead to warping at any stage.
  • Odd shapes or large cutouts could also lead to warping at any point.

There may be other, more obscure causes, but those are the main design related causes. If it’s none of those talk with your partner.

Occasionally, design requirements lead to a board that is essentially non-manufacturable. Hopefully you never have this situation, but if you do, make sure that thickness, component location, pours, or cut outs really, really, really, need to be the way they are.

If you absolutely, positively can’t change anything, go back and try again. Then you can to look for heroic means to get the board fabbed and built.

Slight warpage might go away when the board is mounted. Just be careful with that. Some components may not stay securely soldered when you flatten it.

The board may need a special fixture during assembly to prevent warping. This will likely cost extra, but if you can’t change your design, and still need it built, it may be your best option.

Finally, if nothing works, you may need to look harder at the design, or look for a new fab or assembly house. We all like to think we can do just about anything, but every shop has its limits, and on rare occasion those limits can be difficult to spot.

Duane Benson
What if Godot was late because he was waiting for John Galt?

http://blog.screamingcircuits.com