Author Archives: Duane

About Duane

Duane is the Web Marketing Manager for Screaming Circuits, an EMS company based in Canby, Oregon. He blogs regularly on matters ranging from circuit board design and assembly to general industry observations.

What Do You Do If You Can’t Have Reference Designators?

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The first answer to that question is probably going to be along the lines of, “Put them on the board.”

But, sometimes you can’t have reference designators on your board. Maybe it’s too densely populated and there isn’t room. Maybe, for aesthetic reasons, you’ve chosen to leave them off. With some products, like development boards, it’s sometimes necessary to use the space for instruction or functional identification and reference designators would confuse your customers. 

It’s always best to put reference designators as close to the part as possible, and on the same side as the part, but if that’s not possible, you can still create an assembly drawing. When laying out the board, put the reference designators in a different layer than the text you want in silk screen. Then, create a PDF that has all the component outlines in their place, with reference designators. Make one for the top and one for the bottom. Call this document “assembly drawing” and include it in the files sent in to be manufactured.

Figure 1 shows a good assembly drawing format. It has reference designators and polarity marks.

You might ask why reference designators are needed when all the surface-mount parts are machine-assembled. First, any through-hole parts are hand-assembled. Their locations and board side needs to be clear for the people stuffing them.

Second, CAD systems don’t always have 100% accurate information. If the center point of the footprint is off, surface mount machines (ours and anyone else’s) will center the part where file says to put it, which, in the case, would be the wrong spot.

The reference designators are also part of quality control. It’s another opportunity to remove ambiguity. Ambiguity bad. Certainty good.

Duane Benson
Car 54, where are you?

http://blog.screamingcircuits.com/

What Route Do You Take?

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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.

Routing 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 autorouters, 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 above 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

blog.screamingcircuits.com

Building Boards for the Intel Edison

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I’ve recently spent some time getting familiar with the Intel Edison. The Edison has a dual-core 500MHZ Intel Atom processor, with built-in Wi-Fi and Bluetooth. It comes with 1GB of RAM, 4GB of eMMC internal storage, and a USB 2.0 OTG controller. It doesn’t bring any of the connectors (power or signal) out in a usable form. Rather, it’s designed to be plugged onto another board through a 70-pin high density connector from Hirose.

I designed a small board with I2C (both 5V and 3V connectors) and a micro-SD card slot. My board still doesn’t have the power or console connectors. For that, I’m using a base board from Sparkfun.

Figure 1

Step one of the assembly process, is, of course, to design and layout the board. Using the Sparkfun open source designs as a jumping off point, I ended up with the nice, compact layout (1.2″ x 1.75″) shown below in Figure 2.

Figure 2

After getting the files ready and placing a turnkey order on our website, I followed the board through with my camera. Here it is after offline setup, with the parts ready for robot pick-and-place:

Figure 3

In one of our Mydata My500 solder paste printers:

Figure 4

On the pick-and-place machine, with solder paste, but before any components are placed:

Figure 5

The parts plate in the machine:

Figure 6

 

With most of the components placed:

Figure 7

Through the reflow oven, prior to final inspection:

Figure 8

The final product, top view:

Figure 9

I abbreviated the process a bit, but those are the major process steps along the way.

Duane Benson
Happy birthday (month) Nikola Tesla

Where to Put Panel Tabs

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Many small quantity PCBs are ordered individually cut. They come to us as a set of unconnected boards. For small quantities of reasonable size boards, it makes the most sense to order them this way. However, for really small boards, and larger quantities (50 or more), purchasing boards in a panel (also called an array) is more appropriate. It reduces errors and assembly time.

There are a few additional factors to consider with panelized boards.

  • First, don’t try to create a panel in your CAD software. Just lay it out as a single board and have the fabricator put it in a panel. You’ll get the most efficient use of PCB space that way, and the fabricator will create the files in the format that the assembly shop (Screaming Circuits) needs.
  • Avoid family panels. A family panel is when several different boards are put onto the same panel. The boards in family panels often repeat reference designators, which causes problems at assembly. See this blog article on how to properly assign reference designators on a family panel.
  • If you have overhanging parts, like the increasingly common micro USB connector, make sure that the panel tabs aren’t placed near the overhanging them.

This blog article gives some background on the connectors.

Some components, such as the connector in the link above, have protrusions that will keep them from laying flat on a panel tab. In all cases, even without the protrusions, the operation of separating the panels with a component on the tab can weaken the component solder joints, or even pop it off the board completely.

How not to do it:

Figure 1

Instead, make sure that the tabs don’t end up under your overhanging component. Have the tab moved like this:

Figure 2

You can put this instruction in the document layer of your CAD file, or in a separate document covering fab instructions. In the CAD image below, the overhanging component has a keepout area. The document layer has instructions to keep panel tabs out of the area.

Figure 3

If in doubt, don’t hesitate to contact us or Sunstone Circuits directly to ensure that your instructions are clearly communicated.

Duane Benson
Wood paneling as a wall covering is really out of style

http://blog.screamingcircuits.com/

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

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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 to 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, it 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 PCBs, and they are all laid out together, as though they are one design. If the board isn’t routed between the 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 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.

5. 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 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, 10V capacitor, I can grab a 16V, 25CV, 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 website 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

blog.screamingcircuits.com

USB Type-C Connectors

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It wasn’t terribly long ago that pretty much every cellphone came 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.

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 throughput, and it’s spec’d to carry up to 3A. Further, it has alternate modes for other standards, such as DisplayPort and Thunderbolt.

The connectors are larger than the micro-B, as you can see in Figure 1, micro-B, Type-C with only surface mount connections, and Type-C with both surface mount and through-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.

Figure 1. Micro-B, Type-C with only surface mount connections, and Type-C with both surface mount and through-hole wiring, and a US dime. Figure 2. Micro-B connector with tabs formed from the same sheet metal as the shell.

All three of the surface mount connectors shown above have through-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 PCB. This can lead to some deception when soldering. Without the tabs protruding, it’s easy to believe there’s not enough solder in the connection. If more solder is fed in, it will likely wick along the tab, and end up inside the receptacle, preventing the cable from being plugged in. If hand soldering or reworking these type of connectors, keep a close watch on the amount of solder used.

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

http://blog.screamingcircuits.com

Fiducials and Odd Boards

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One of the handy aspects of getting boards assembled at Screaming Circuits is that we don’t require fiducial marks for standard process boards. I would say that we build far more boards without fiducials than with. That’s cool, but there are sometimes when fiducials really are a good idea. In fact, if you’ve got room on the PCB, they’re always a good idea (just because something isn’t required doesn’t mean that it’s not a good idea).

Some boards are more in need of the marks than others. For example, not long ago, we got a rigid flex board in. It had three separate rigid boards connected by flex, designed to be folded into a stack. It looked pretty similar to the mockup in this image:

The boards didn’t have any fiducial marks. Normally, what we do, is find a via hole, through-hole pin hole, or some similar feature to use as a fiducial. That usually works, but not always. In this case, the length of the flex varied slightly from board to board. The PCB color was also very low contrast, which made it difficult for the machine to consistently recognize any mark we picked.

That meant our machines had a hard time finding the “home” spot, and we had to reset for each of the connected boards. Finding a spot on one board did not guarantee that we’d know where to place parts on the other two boards in the set.

In this case, it would have been far better if the boards were a consistent distance apart, and if each of the three boards had a set of fiducial marks.

What makes a good fidicual?

Most CAD packages have fiducial marks in their components library. Basically, it needs to be a metal dot surrounded by an area without any copper or solder mask. More than one is best. It should be an asymmetrical pattern that can only be oriented one way.

I’ve got some more details in this article here.

Duane Benson
Routed up like a fiducial
Another rigid flex in the night

http://blog.screamingcircuits.com

Happy St. Patrick’s Day!

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And some trivia. You may have noticed that the soldermask used on most PC boards is green, as is the paint used on most steel truss bridges. Why is that? And what do the two things have in common? Why green PCBs and why green bridges?

To answer, I brought in color expert expert Patty O’Patrick O’Dell, who stated: “Many bridges and PCBs are green because they absorb red and blue light, only reflecting the green.”

That wasn’t quite what I was getting at, but close enough. The important thing, is that, generally, in commercial products, the PC boards are hidden, so the color doesn’t matter that much. With prototypes and a lot of the hobby or development boards, that is not the case, so many companies have chosen to use a different color as a part of their identity.

Arduino products are blue, as are most boards from Adafruit. SparkFun makes theirs red. Ti Launchpads are red as well. The Beaglebone uses color, essentially, as a model number; Beaglebone black, Beaglebone green. This is possible because major PC board fab houses have made different colors more economical than they used to be.

I’ve been asked if the color makes any difference electrically. In general, no. If you’re dealing with super high speeds, RF, or other exotic conditions, it’s always best to ask your board house. In those fringe areas, a lot of things have the potential to make a difference. Other than that, if you can afford it, and want to make a statement, go for it. You can often get different color silk screen legend too. Just make sure there’s contrast between the two. White silkscreen on white soldermask would not be the best choice.

Duane Benson
Beware the monsters from Id

blog.screamingcircuits.com

Electronic Swarms — Overhangs

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As I’ve stated many times before, we see many, many different jobs go through our shop. In those jobs, we see some of the absolute newest components and packages; some not yet available to the public; some are so R&D that they never will be available outside of a lab. We see the best of the best in terms of design practices and complexity, and we see many that aren’t so much in that arena.

Given that, it would seem logical that the design problems we see would be pretty much scattered all over the map. By some measures they are, but on a day to day basis, they tend to cluster. For a few months we’ll see a lot of QFN footprint issues. In a different few months, we’ll see a lot of via in pad issues, etc. I don’t know why. It just works that way — problems come in swarms, or storms.

The latest swarm relates to panelized boards and components that stick over the edge of the board. We build things like that all the time. The problem comes in when the panel tabs come out right where the component overhangs. If the component overhangs in the cut out area, it’s usually not a problem. However, if the component is on the connection tabs, we can’t place that part without first depaneling.

Probably the most common example is the surface mount USB Micro-B receptacle. It over hangs the board by a small amount, and that overhanging part is actually bent down. If it’s at the tab, it won’t even mount flush. Take a close look at the images along the right. That connector won’t mount as it’s sitting on a tab.

So, what do you do about it?

You can have your boards made as individuals. Although if you want short-run production, or if the boards are really small, that might not be possible or practical. You can also talk to your fab house about it. They may be able to place the tabs in a spot that won’t get in the way of the overhanging part, of they might be able to tell you where the tabs will be, allowing you to keep clear in your layout.

Duane Benson
Anyone ever drink Tab Clear?

http://blog.screamingcircuits.com

An Electronic Business Card Holder

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I design and build electronics at home, late at night when the spiders are out, and by day, I put my hours into Screaming Circuits. My job here doesn’t involve building things. I’m the marketing department, but I like to keep as much manufacturing smeared all over me as is possible. Here’s one way I do that.

Business cards are a bit of an anachronism today. I don’t give out many, this being the 21st century and all, but I still need some on my desk – I guess to look businessy or something. No one’s ever given me a cheap card holder with their logo on it, and I don’t want to just scatter cards around. So, why not combine my need to display business cards on my desk with my compulsion to create electronic things? With that thought in mind, I decided to build an electronic business card holder. Of course, I first had to decide just what an electronic business card holder would be.

Here’s what I came up with:

  • It should be small, about the size of a business card
  • It should have a lot of blinky lights
  • It should do something when a card is removed
  • It should have a long battery life
  • It should use tiny parts to show off our manufacturing capability a bit
  • It should be 100% buildable within our electronics manufacturing process (meaning it should be just electronics; no bolts or case)

That’s not a long list, but does involve a few decisions. I’m pretty familiar with Microchip PIC processors, so that would be a logical choice to drive the thing. Arduino compatibility would be cool, but I’d have more trouble with battery life, and the PIC microcontrollers come in some pretty inexpensive forms.

I’d recently been using a variant of the PIC18F46k22 on another project. I comes in a 5 x 5mm QFN package and can be purchased for less than $3 in small quantities. it has plenty of I/O and can be set to a very low power sleep mode. I settled on that MCU and a CR2032 coin cell battery for power.

Rather than add any extra hardware to hold the cards, I came up with an arrangement of pin headers and small push-button switches. (as in the photo below right). One header is the six-pin Microchip in-circuit programming (ICSP) header, and the other is a six-pin I2C/SPI header. Not that I need I2C or SPI, but with that, you could turn this into a robot business card holder or something.

I considered a light sensor to detect when a card is being picked up, but that would require leaving the A to D powered up, and it would be less reliable due to changes in lighting. I looked around my junk box at home, and found a Freescale MMA8452 3 axis accelerometer in a 3 x 3mm QFN package. It also has a decent low power mode, and can be talked to over I2C.

Some 19 GPIO pins remained open, so naturally, I had to put in 19 LEDs.

Stay tuned for my next installment, where I’ll go through some of the design decisions. At the end of this series, I’ll be giving out 10 of these, so stay tuned to see how you might be able to get one.

Duane Benson
If you dreamed you saw the silver spaceships flying
That’s a okay. They’re RoHS compliant

http://blog.screamingcircuits.com/