Blog - Screaming Circuits


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

Building boards for the Intel Edison

I've recently spent some time getting familiar with the Intel Edison. The Edison has a dual-core 500 MHZ 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 5 volt and 3 volt 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.

Stacked Edison

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.

SCmultiboard layout

After getting the files ready and placing a turn-key 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.

DSC_0007

In one of our MyData My500 solder paste printers

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

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The parts plate in the machine.

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With most of the components placed

DSC_0036

 Through the reflow oven, prior to final inspection

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The final product, top view

DSC_0002 (3)

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

Duane Benson
Happy birthday (month) Nikola Tesla

Seeing Gravity Waves

Here at Screaming Circuits, we assemble PCBs for a lot of interesting organizations. Perhaps one of the most interesting is LIGO (Laser Interferometer Gravity Wave Observatory). In case you've been busy with Pokemon GO for the last few months, and missed the announcement, in February, LIGO announced the first 20160414_130935ever detection of gravity waves.

Back in April, I visited the LIGO lab at MIT, and in July, I went to South Central Washington and toured one of the detector sites. Over the years, we've built electronics for both locations. The other two locations are another lab at CalTech, and another detector installation in Louisiana. The image on the right shows the Squeezed vacuum experiment being developed at the MIT lab.

Gravity waves were predicted by Einstein 100 years ago, and confirmed with this detection. Since the first announcement, LIGO has seen two more events. One has been confirmed as another gravity wave, the other, they're still not quite sure. In the words of my tour guide at LIGO Hanford: "It walks like a duck, talks like a duck, but doesn't quite smell like a duck."

Here's a view in the control room at the Hanford site:

LIGO control room

One of the detector arms. This isn't the actual detector tube. You're seeing the concrete tunnel protecting the tube. The light path goes through a steel tube kept at a near perfect vacuum (one trillionth of an atmosphere). Their tumbleweed mitigation equipment is in the foreground.

DSC_0028

Duane Benson
No, Doc. Nothing happened to the earth's gravitational field

Where to Put Panel Tabs

Many small quantity PC boards 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 of all, don't try and create a panel in your CAD software. Just lay it out as a single board and have the FAB house put it in a panel. You'll get the most efficient use of PCB space that way, and the FAB house 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 over hanging parts.

This blog article here, gives some background on the connectors.

If you have overhanging components, make sure that the panel tabs are not placed where the over hanging components are. Some components, such as the connector in the link above, have protrusions that will keep it 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:

Overhanging connector On tab 800

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

Overhanging connector Off tab 800

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.

Overhaning part keepout

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

 

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

 

Happy Memorial Day

Happy Memorial Day to everyone here in the USA. We will be closed on Monday for the holiday.

POP with dime

 

Fiducials and Odd PC Boards

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 PC board, 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 mock up in this image. Rigid flex mockup

The boards didn't have any fiducial marks. Normally, what we do, is find a via hole, thru-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

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?

Arduinos Spotted

ESC2016 bage proto 640I've given a few talks at the various Embedded Systems Conferences on the subject of the Arduino as a real prototyping tool. I've designed and built a number of custom Arduino-compatible boards myself. I've become an advocate for the Arduino as a legitimate tool for the commercial engineering world. But sometimes advocating isn't enough. You need to see it in action to believe it.

Here at Screaming Circuits, we build a lot of different types of boards for a lot of different companies and organizations. The are boards being used for R&D, consumer products, education, medical devices, military things, space craft, Kickstarter companies, aviation... You name it, we've probably built it; up into space, down underwater, and anything in between. What's the engineering joke? "Civil Engineers build targets, mechanical engineers build weapons to destroy them." We build the electronics for both.

But, I've gotten off target. The point I wanted to make is that, if we're building it, it's most likely a professional/commercial caliber product. The other day, amongst the many other designs being assembled out on our shop floor, I spotted a shield, full size for the Arduino Mega. I don't see anything close to every job that we build, but it's gotten pretty common for me to see something Arduino compatible - either in form-factor, code compatible, or both, out on our shop floor.

One such example is the electronic badge we built for the upcoming Embedded Systems Conference in Boston. Our partner, Sunstone Circuits provided the PC boards, Synapse-Wireless designed it and provided the wireless modules. We bought the rest of the parts and built it.

With as many as I'm seeing these days, I can only conclude that the Arduino has entered the commercial design world.

Duane Benson
Open source the pod bay doors, HAL

Advanced Technology now available

CCB - Concrete Circuit Board: Rumored for years, scoffed at for even more years, and now available to you - Concrete substrate for circuit boards!
 
The US NCA (National Circuit Agency) recently declassified this new technology, and granted exclusive assembly rights to Screaming CCB layersCircuits, due in large part to our prior work with the revolutionary Cordwood technology.

CCB has many advantages, such as a thermal expansion coefficient of 17, and a density of 18.98 g/cm3, almost as much as depleted uranium, allowing for solid grounding without bolts.

Prior to now, CCB had only been used in the most secret aviation and space applications, such as the still classified KHD-18 spy satellite and the LB3 Keystone light bomber. It's used all over Area-51 and has been identified as the cause of 42% of UFO sightings. (I can't remember if I'm allowed to say that or not)

For the civilian world, CCB is ideal for mobile* and IOT (Internet of things) devices

* CCB circuit board substrate comes in a minimum 1.2" thickness to prevent cracking. Applications subject to thermal variance or vibration may require the addition of rebar. Titanium and beryllium rebar are not available.
 
Duane Benson