Category Archives: Screaming Circuits Blog

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

Weekend Wondering: Is Anything Really New?

Posted on by

Last year, Screaming Circuits started closely following the Ti OMAP processor and it’s package on package (POP) form factor. The OMAP 35XX processors are very nice on their own, being a very hi-performance ARM jobby, but the package on package made it something for us to take real notice of. POP had been done before, but this seemed to be the first broad-audience application of the technique.

But is it really the first common package on package application? I’ve seen some unintentional package on package, like the capacitor-under-connector pictured on the right in this post. That’s not so fun, but it can keep your board size down — as long as you don’t want it to actually work. And then, many, many network cards used to have a chip placed right under their ROM socket. Would you call that package on package?

How about the old Ti SN754410 motor driver? It’s pretty common for robot buildersto stack a pair of them to get two amps of drive from a pair of one amp chips. That’s probably more a case of actual package on package than is the network card example. Maybe the network card should be called “package under package”. I know, the OMAP is a BGA, but that might actually make it easier to manufacture. With the 754410, all of those leads have to be hand soldered. The OMAP, we just put it on our machines and they do all the work for us.

The other “new, but really old” subject I’m thinking about is cloud computing. Yes, it’s the newest rage in the software and application world right now, but is it really that new? Or has just the name been changed so some pundit can claim to have invented a new concept? I learned software development in a cloud computing environment – back in the early 1980’s.

Duane Benson
The story you are about to hear is old; only the names have been changed to protect the egos.

http://blog.screamingcircuits.com/

PCB Edge Clearance

Posted on by

In my continuing saga of answering the question, What are the real limits?, I’ll spend a little time giving my thoughts on edge clearance. That makes the question for the day: How close can I put parts to the edge of a PCB?

It’s a good question but, regardless of what IPC says, surprisingly difficult to nail down.

The problem is that there are a number of valid answers, all governed by the phrase “it depends.” I’m not going to leave it at that, though. I could. But I won’t. Not today anyway. It really does depend on a number of factors, but I think it can be nailed down a little tighter than that. Start by considering a few things:

1. First, our old friend IPC-7351A does specify a keep-out area (aka Courtyard Manufacturing Zone) on the part footprint. Keep stuff out of that area. This includes the board edge. Don’t have any part of the component’s keep-out area hanging over the edge of the PCB.
2. Second, look at your PCB mounting arrangement. Make sure the keep-out area does not interfere with any mounting screws, and that includes any washers you may be needing. If your PCB mounts with slots or rails, make sure the keep-out area doesn’t interfere with any of the rails or slots or case edges.
3. Check with your manufacturer about its specific line limits. Many manufacturers have specific edge clearance limits based on their assembly line.

Now, take your application and consider those three items. Whichever gives you the biggest number is the edge clearance you need to follow. Based on your mounting scheme, you may have different clearance requirements on different parts of the board. Still, make sure that for each section of the board, you use the biggest number from above.

Now, in the world of prototypes, things are a little different. At Screaming Circuits, we sometimes build up boards with parts right up to, and in some cases, over the PCB edge. You just have to ask yourself two questions for your prototype:

1. Is there enough copper to give a good electrical and mechanical connection to the PCB?
2. Am I likely to knock the part off the board with my handling?

If the answer the #1 is yes and #2 is no, then go for it. Of course, in the prototype world, you can always accept the risk yourself and go for it whatever the answers are. That’s up to you.

Duane Benson
No worries. The green patches will burn off in the reflow oven.

http://blog.screamingcircuits.com/

IPC Says What?

Posted on by

I recently wrote about spacing, IPC standards and such. James M. commented on the post with a question:

I run into this issue all the time. Assembly houses love to say that they can make anything that meets “IPC Standards for Placement” but when pressed no one can provide the exact number that gives this information. I know of IPC7351A, in fact I own a copy. And the keepout is clear there. But that doesn’t influence things like component placement grids, how close things can get to the edge of a board, etc…. Are there other specific IPC standards that do?

After reading James’ comment and rereading my post, I realized that there isn’t a lot of actual content in that blog post. Certainly not much actionable data. And, upon further thought, I think perhaps, all of us assembly folks kind of use that phrase “can make anything that meets IPC Standards for Placement” as a bit of a cop out. Not totally — we do have to set some limits on what we can build, but yikes! I’ve been trying to navigate the morass of different standard numbers (Maybe I should say “plethora” instead of “morass”) and I don’t know how anyone that doesn’t specifically live the standards for a living could easily find those kind of answers. But we manufacturers really do owe it to our customers to come as close as we can to giving definitive limits that don’t take a week of reading to interpret. I think I have to do this in stages.

First, we have three things: IPC-7351A for land patterns, and IPC-A-600, covering the PC board workmanship. Then, we have IPC-A-610, covering our workmanship when we build and inspect the board assemblies. Those are the key standards that we live by.

There are a few other questions, such as edge clearances and things like that. I’ll dig some more into that one later, but one thing to note is that for the Screaming Circuits prototype service, we don’t require any edge clearance, nor do we require panels, rails or fiducials on our full-proto service. I hope this helps.

Duane Benson
http://blog.screamingcircuits.com/

Using Package on Package (PoP)

Posted on by

Back in August of 2009, we at Screaming Circuits assembled our first package on package chip set. We did a number of test components first to tune the process and then built up some Beagleboards. We’ve done a few more since then. It’s not yet a high-demand item, but it is getting more popular.

At the ESC (Embedded Systems Conference) last week, we had a number of folks stop by our booth and ask about how to use the part. The OMAP processor from Ti that comes in a PoP form factor is a great high-performance part, but I think a lot of designers are still intimidated by it. Really, though, there’s nothing special about designing with PoP.

It’s a 0.4 mm pitch BGA and that gives some challenges with escape routing and PCB masking, but those are standard BGA-type issues. For escape routing, go to beagleboard.org, download the beagleboard reference manual. They have their version of the escape routing in the book. And, check out this post for some advice on the BGA footprint.

That’s pretty much all there is to it. The memory chip just plops on top of it, so as a designer, you don’t have to worry about that. Just do a good 0.4 mm pitch BGA layout and your PoP will come out just fine.

Duane Benson
Does Kellogg make Raspberry flavored Package on Package Tarts?

http://blog.screamingcircuits.com/

Circuit Design ECOsystem

Posted on by

Years and years ago, I was a product manager at In Focus, the projector manufacturer. It was a great time to be in the display industry. New technology was being invented left and right (and center and back, and some over in that far corner too). Competition was still reasonably light and we were ahead of most of it.

It was always interesting to take one of the early overhead projector-style displays through airport security. Laptops were rare at the time, let alone a big clear display that looked like a see-through touch-pad computer, but without the computer. But that’s not the point.

Back in our engineering department, we had the electronics engineers, a few folks to work on firmware, a layout specialist, documentation specialists to deal with all the documentation (duh), purchasing people to buy the parts and PCBs, technicians build up the prototypes, manufacturing people to get the pre-production and production going. And here,s the contrast today. Quite a few engineers I talk to these days have to do all of those jobs except final production. That wouldn’t be too much of a problem except that while all of those jobs were being assigned to the engineer, everything got more difficult. Parts got smaller, timelines shrank, competition got more fierce, clock speed increased and a lot of formerly company functions, got out-sourced. It’s a lot of work and a lot of ground for that engineer to navigate.

A handful of companies — Digi-Key, NXP, National Instruments, Sunstone Circuits and Screaming Circuits (my company) — have gotten together to form the Circuit Design ECOsystem; a cross-company organization designed to help that design engineer get a design from inside the brain to the market.

NXP makes components and is creating library components for the CAD software made by National Instruments and Sunstone. Sunstone allows quoting and ordering of Screaming Circuits assembly service on their website and Screaming Circuits does the same with Sunstone PCB fab. Digi-Key is working to improve the data-flow to Sunstone’s PCB123 CAD and streamline the parts procurement process to Screaming Circuits.

It’s still early in the process, but the idea is to take the, now fragmented, design to manufacture process and make it easier for the electrical engineer to get through – to remove roadblocks, add in new services and improve communications to make it easier to produce a quality product.

Controlling the Uncontrolled

Posted on by

A nice coincidence. Recently, I wrote a bit about choosing a microcontroller and some issues that crop up when people not used to microcontroller design are tasked with automating systems.

My supposition is that, traditionally, most folks in the industry concentrate on designing and choosing microcontrollers and tool sets from the perspective of an expert in embedded design. However, the new world has a lot of people tasked with microcontroller hardware and software design that are not electronics or software engineers. Mechanical engineers are tasked with integrating electronic controls into their systems. Pure digital engineers are being tasked with adding analog sections into their designs. Hardware engineers are having to learn microcontroller firmware programming. That changes the ground rules.

Last week, I signed into a virtual conference on motor control (I started writing this post as I was listening to the virtual conference, but didn’t get around to finishing it until today). I signed in late to start listening to the keynote address by John Hanks of National Instruments, who, at that moment, discussing this very subject. As he described it, domain experts in such fields as solar, wind and other areas are being asked to add additional automation into those systems. As domain experts, they may know more about their field than an EE or SE, but they likely have not been trained in the application of hardware, firmware and software development.

Interestingly, this group has a lot in common with the electronics hobbyist community. In both cases, the concepts and the tools are frequently quite new to them. In both cases, the budget for training and tools is frequently pretty minimal. In both cases, we have smart people who many not be trained in our field.

Those of us that create tools and offer services in this industry need to keep this trend in mind if we want to fully serve the new engineering audience.

Duane Benson
See us at ESC, booth 827

http://blog.screamingcircuits.com/

Castellated/Half-Vias, Parts 2 – 255

Posted on by

I’ve written about the castellated/half-via mounting system a couple of times in the past. Now I can’t seem to avoid them. I’m seeing them everywhere. The GPS I recently wrote about, low noise amplifiers, POL power modules, frequency synthesizers, VCOs …

When did this become the package of the day? I haven’t even seen what IPC has to say about it yet but these things are all over the place. Our engineers and assembly folks are cool with it. It’s not that tough to build, fortunately. But, as I saw with the ublox part I wrote about, there are new design issues to contend with.

I can certainly see the advantages of the package. The half via can allow for a good solid fillet providing good mechanical connection. They’re typically a PCB-type substrate so the coefficient of expansion and flex strengths should be similar to the underlying PCB. On the other hand, like with an LGA, the low profile after soldering will tend to exacerbate and expansion or flex risks.

Duane Benson
Once in a dream
Far beyond these castellated walls…

http://blog.screamingcircuits.com/

Who Are Your Tool Sets Made For?

Posted on by

I’ve been thinking a lot lately about who’s using microcontrollers and why these days. There’s a lot at stake with this question. And, not just in terms of which microcontrollers are and will be most popular. There’s an element of the Toyota question in here too.

Traditionally, I suspect that electronics component manufacturers, hardware EDA tool vendors and software tool vendors assume that their customers have been trained in EE, CS or similar discipline. I think to a point, that serves the industry well. But change is afoot in our industry. Because of a number of factors — too many to list here — virtually everything is getting some level of electronic control now. Years ago, that would have resulted in the hiring of a lot of electronics and software engineers. But not today.

The tried and true EE, accustomed to designing with logic and letting someone else worry about firmware, is now often tasked with designing in a microcontroller and then producing the firmware as well. Or a mechanical engineer is tasked with the same thing; something he or she never trained for. From what I can see, all sorts of technical folks that don’t have programming experience, or any electronics design experience, are now being given that task. Schematic designers are now responsible for the board layout. Pure digital folks are often being required to add in a few RF sections.

What happens if all of the software tools (CAD packages, compilers & tool changes) are designed for well trained experts, but intelligent but untrained, in that field, folks need to use them?

When cars suddenly accelerate, MRI machines over-radiate or satellites fail, it’s all good to look for tin whiskers, cosmic rays, manufacturing defects, software bugs and causes of that sort. But, what if the root cause is simply that someone trained and practiced in pure digital design was tasked with the “simple” function of adding in a few analog sensors and a tiny microcontroller. What if that designer had to learn a new discipline, a new tool set and still make budget and a tight deadline?

Maybe 20 years in digital design didn’t prepare that designer for the quirkiness that goes with analog signals from sensors, or for the challenges involved in writing a small, but bullet proof SPI interface code. Maybe the designer is used to determining spring strength and durability but now has to design a small electronic circuit to replace that spring. What does that do to quality and reliability? Food for thought.

Duane Benson
Thought is hungry today

http://blog.screamingcircuits.com/

Castellated / Half-Vias

Posted on by

Modules with castellated mounting holes are showing up more and more these days. I’ve also heard it called a “half-via” setup. Both seem to fit. The copper mounting pads on the bottom of the module’s little PCB wrap up the side of the PCB with a half-via. Thus, the name “half-via.” If looked at from the base, the edge does look a bit like the outline of the top of a castle wall, so that makes sense too.


Whatever. The name isn’t that important. There are a few things that are important though, such as the land pattern and the solder deposition. I hadn’t heard any special requirements for using this type of part until recently when I ran across a GPS receiver module from ublox. What they say is that you need more solder on the outside of the pad, than on the underside of the pad, so that it can properly wet up the half-via. That makes perfect sense. There are probably multiple ways of doing this, but you can see their take on it in the image here on the right that came from their data sheet.

Obviously, follow the datasheet that comes with your specific part, but if you don’t have any official guidance and can’t get word from your part manufacturer’s applications engineers, you might take a hint from this method. The copper land is just a standard rectangle, as is the solder mask. The paste layer, however, is not. It’s wider toward the outside of the part. This will help create a proper fillet up the via while reducing the chance of solder balls and other messy things that can happen when you have too much solder under a part.

Duane Benson

Crooked Components

Posted on by

I was looking at two different PCBs recently; one assembled here at Screaming Circuits, and one, a PC graphics card assembled someplace else in the world. In both cases, a crooked component caught my eye. The graphics card, at least, would have done a passable job of meeting IPC-A-610 Class II, but in both cases, the offset was enough to inspire me to take a closer look and write this blog post.

On the graphics card, the particular component was an SMT inductor. I’ve been having some issues with the card and when I had it out of the PC, I saw that inductor. With further examination, I found a lot of other crooked components. My off-the-cuff conclusion was that the manufacturing workmanship was poor, therefore I shouldn’t be surprised that the card was having issues, and perhaps I should look for a different brand for my next purchase. Again, everything would likely have met IPC Class II, but perception is close enough to reality and that manufacturer has probably lost a customer.

In the case here at Screaming Circuits, only the one component, an SMT SD card socket, was crooked. In addition to the signal pads, this socket has four large SMT pads, one on each corner, for mechanical connection to the PCB. One of those four pads on this PCB has a big hole in it. Not just a little 10 mil via in the pad, but a either a really, big via or a bolt hole. The unequal surface tension in the solder, caused by that pad being cut in half, caused the connector to twist a bit.

Yes, in a prototype world, we made it work, but it caught our attention here and if such a thing went into final manufacturing, it might very well catch a customer’s attention and elicit the same response I had to the graphics card. It will work okay and would probably stay secure on the board (maybe), but it’s not good practice and it looks wrong and wrong looks can cause customers to go elsewhere. Yes, function is key, but if form chases customers away, then function never gets a chance to matter.

Duane Benson
We’ll keep the robots in line
You keep the PCB in line

http://blog.screamingcircuits.com/