Surface Mount, But Not Really

Sometimes parts labeled as surface mount aren't quite ready for prime time. I've written about this subject before (read here), and I'm going to write about it again - whether you like it or not. This time, however, I'm not talking about components that aren't up to thermal par. Today, it's about components that can take the heat, but aren't set up to be machine assembled.

Surface mount machines need a flat surface to pick on. They use small vacuum nozzles that need to seat on that flat spot. Chips, of course, are flat on top, as are most other components. Connectors, however, are often not flat on top. That doesn't leave any place for the "pick and place" machine to pick.

Single row header with pick and place padGenerally, manufacturers will place a small tab of Kapton® tape or a small snap-in plastic pad on top of the connector, giving the machine a surface to work with. You can see that in the photo on the left. Once the board has been fully assembled, the tape or plastic pad is simply removed.

Every now and then, we'll see connectors come in without that flat pick and place surface (like on the right). That means the machine can't place it, so it will have to be placed by hand. 1.25mm-Wafer-SMT-Connector

When buying your surface mount connectors, if you have a choice between a part with the tape and one without, you're better off picking the one with the tape. No offence intended to all of you humans, but machine assembly is generally preferred over human assembly.

Duane Benson
Only three more days until Mitten Tree Day!

How Do You Know?

This isn't a Thanksgiving blog. It is Thanksgiving day, and if it were a Thanksgiving blog, I would have to be working today, but we're shut down for the holiday, so I'm not working. I just woke up pondering what it would be like to do business with us (or anyone like us) and decided that I wanted to hear myself speak (metaphorically) for a bit. A word of warning though; I'm in a long-winded rambling mood today.

Take an example; the Beagleboard. I use that because it's a complex board that's open source, so I can freely talk about it. It was originally put together by Gerald Coley and Jason Kridner. I don't know how Beagleboard face onlong they spent designing it, but according to a UBM study, a typical product design cycle is about a year.

So, what we're really talking about is a year of a couple of engineer's lives. It can be a lot of cash money too. When ordered in large quantities, the Beagleboard and it's progeny are inexpensive enough to be sold for quite a decent price. However, when purchased in small quantities - say five - it can cost several thousand dollars.

When the Beagleboard was new, we built a few just to kind of show off. We took the open source files and ordered all of the parts. We tried to get some PCBs fabbed, but in that quantity, they would have cost us $1,200. Instead, I posted a request on the Beagleboard.org forum and found someone with some bare Beagleboard fabs.

I got those boards and the parts and ran them through our system. Had a customer quoted the build, it would have cost somewhere (if my memory serves correctly) around $800 per board for assembly. That would be $10,000 for a set of prototypes. That may seem like a lot for a board that retails for $150.00, but that's the difference between ordering hundreds of thousands and ordering five.

That cost comparison isn't the point. If you're in this business you know that getting small quantities of complex stuff in short notice is expensive in direct dollars, but more than worth it in time and effort saved. The point is that, while we build a lot of sub-$1,000 orders, we are frequently given orders that are valued at $10,000 or more. Sometimes CONSIDERABLY more. We've seen projects where parts alone are tens of thousands of dollars. I've seen a single FPGA cost several thousand dollars alone. Yikes!

You've spent a year of hard labor on a design. You hit "Save" for the last time. If you're like me, you want nothing more than to get a working board into your hands. The gap between that save and a fully built board is painful for me. But the prospect of shelling out $20,000 to some unknown company for the purpose of turning that year of my life into a physical product is positively terrifying.

Well, if you don't already do business with us, we are that "some unknown company." That makes me wonder how this all happens. I design boards myself - not the big ones, but I do design a fair number of them. Right now, I have four boards I'm actively working on and about that many that I've shelved for a few months. I understand a bit of the fear of handing a design off. Of course, I have an unfair advantage. I can just send some boards through our shop and get them done just about any time.

It's easy for me to trust us. I got a job here and I know that I take the stewardship of that big check and year of your life very seriously. I treat it like it were my own. I also know that I don't work for companies that don't share that philosophy. I've tried, out of necessity, twice in my career, working for companies that didn't treat customers they way I would and I ended up pushing my agenda so hard that I got fired. It wasn't pretty.

I've established that I (as in me) trust us. How do you get to the point that you can give us (or anyone else) the same trust? The Beagleboard guys didn't know us enough to do so. We built some of their boards on our own. Plenty of people do know us well or are somehow willing to make that leap. We quite literally* have built things that have gone up into space, down into the ocean and everywhere in between. It's pretty fun to look through our customer list and see so many names of companies doing really cool stuff.

All of the marketing mumbo-jumbo I spit out is designed to somehow convince you to let us take care of your design. But those are just words. Words are meaningless without the deeds. It's what all of the other people in my company do that really counts. I spill out glurge. They do their best to treat your project with the same respect and care that you do. I'm thankful for that, because if they didn't do that, I wouldn't want to work here. If they didn't do that, my job would be meaningless and stupid. Hey - this did turn out to be a Thanksgiving post!

Happy Thanksgiving!

Duane Benson
* The word "literally" is terribly misused these days, but I'm actually using it by the correct definition. Well, okay, the "everything in between" isn't quite literal, but "space" and "under water" are. And it's comprehensive a representative sample that I'm in the spirit of "literal."

More Fun File Facts: ODB++

In my last post, I wrote about the up and coming IPC-2581 PCB manufacturing file format. While IPC-2581 may be looked at by PCB fabricators and assemblers as a holy grail of sorts, it's not yet widely adopted by CAD software. But, that doesn't mean that Gerbers are the only option.

ODB++ was developed by Valor in the waning years of the last century as an improved method for getting manufacturing data into their CAM systems. Valor and, hence, ODB++ was purchased by Mentor Graphics in 2010. ODB++ is still widely available, however there's concern in some circles that it's not truly open. That concern is where IPC-2581 came from. In fact, IPC-2581 is somewhat derivative of ODB++.

I can see how a CAD software developer might fear the use of something owned by a rival. However, my understanding is that Mentor does it's best to treat it like an open standard and has made it available more or less as though it is open.

The history isn't really important. What is important is that ODB++ is a more complete format than the Gerber and is widely supported. Pretty much everything good that I said about IPC-2581 in my prior post also applies to ODB++.

The bottom line is that, regardless of whether Screaming Circuits is your fab (through our partner Sunstone) and assembly (through our factory right here) provider, ODB++ is a good thing. It makes the job easier and more accurate than does use of Gerber files. Both "easier" and "more accurate" help keep costs down and keep ambiguities to a minimum. As you know, ambiguity is the bitter enemy of both accuracy and quality.

Unfortunately, for all of you Eagle users, Eagle does not yet support ODB++. If anyone out there is really good with Eagle ULP scripting, you might want to create a on ODB++ and/or IPC-2581 creation ULP.

Duane Benson
I was ionized, but I'm better now. 

Fun Facts About Manufacturing Files

Circuit boards live and die by their manufacturing files. Without complete and accurate information, the board fab house can't fab the boards, the assembly house can't assemble your boards and nobody can buy the parts.

Our old standard, the Gerber file, has been around since about the time King Arthur pulled the inductor out of the solder pot. It's old. We all use it because it's familiar, but it's day is done. It's time to pass the torch.

IPC-2581 is the new standard in manufacturing files. It hasn't been fully adopted, but it's showing up in more and more CAD packages. The IPC-2581 format is much more advanced and has the complete data set in one file. While we still work with Gerbers every day, we can also accept IPC-2581 manufacturing files.

I've been called the champion of bad analogies, but I'll try one out anyway.

Imagine, if you will, a map of the city. All of the streets are there. All of the houses are there. What's missing are all of the street names. No street names, no numbers and no landmarks of any sort are labeled.

Given that information, find John Smith, at 1620 SW 14th Avenue. There is a house at 1620 SW 14th Avenue. There are a dozen or so houses at 1620 something. You just don't know where 14th is, or which direction 14th runs, or where the street numbering starts.

You can physically walk each and every street until you find John's name on his mailbox, but it's not an easy nor error-safe process. And, hopefully, the town only has one John Smith. That's a Gerber file.

IPC-2581, on the other hand, is an electronic map, with everything clearly labeled, and a GPS guiding you. Which would give you more confidence?

Duane Benson
IPC-2581 is like shatter-proof glasses for Henry Bemis

The Dangers of ESD

Question:

EsdWhat do a conductive floor, foot grounding straps, conductive work smocks, wrist ground straps, foot grounding testers, ESD training, bench-top grounding monitors, anti-static bags, anti-static boxes, grounded carts, anti-static attitudes, conductive desk mats and grounded tools have in common?

Answer:

They are some of the things that Screaming Circuits uses to protect components and circuit boards from the dangers of electrostatic discharge.

Ideally, those are things that everyone handling electronic components and circuit boards would use. This is the real world, though, so there are likely companies that don't use such tools or follow good ESD control procedures. Some companies might even charge extra for what is essentially a basic right. Bad news.

Just the act of getting up from a chair can cause an in body potential of 10 kV. The human threshold for feeling a shock is around 25 kV. Silicon chips can sometimes be damaged at significantly less than that. One of the worst things about ESD damage is that sometimes the failure mode doesn't show up until the device is out in the field.

One of our many missions here at Screaming Circuits is to keep the dreaded ESD monster away from your boards. Your PCBs and your trust are very important to us.

 

Duane Benson
"Zero potential" is bad when when coming from
your parents talking about career prospects.
But it's good when evading ESD.

PCB Assembly Parts Kit

Watch and see what's important when putting together your parts kit

 

Duane Benson
Who's on first?
I don't know.
PCB Assembly 

 

 

CAD Data Files

I've spent a fair amount of time researching and writing about the centroid file and about CAD library footprints. One of the challenges in this industry is that somethings that are "standard" really aren't all that standard. That's why we emphasize following IPC guidelines when creating library components.

Well, a few things have changed since we started doing this a decade ago. For one, some of the enhanced manufacturing file formats (as opposed to the 1970's vintage Gerber format) have become more prevelent. Those new formats are a very good thing.

Most CAD packages can now output either ASCII formatted CAD data or ODB++ format data. Those file formats have all of the data that would otherwise be found in the centroid and Gerber files. They also have more accurate data. If you can get one of those formats out, go ahead and send it to us. We can also take plain old Eagle CAD .brd files. If in doubt send one of these newer files along with the centroid and Gerbers. We'll use the file with the best data and, we may be able to simplify the file preparation Centroid snippet rot optyou have to do with future jobs.

And speaking of the Centroid, don't worry so much about the rotation column in the Centroid file. You can consider rotation to be optional now. You don't need to check the rotation, nor do you need to remove it.

Duane Benson
Who will win? Godzilla or Centroid? Maybe the Smog Monster?

 

Push-me Pull-you LEDs

I may never get tired of talking about LED and diode polarities. It's so much fun. Not long ago, I wrote about two LEDs from the same manufacturer, marked with opposite polarities. I recently ran into another one, but at least this one tells you on the same datasheet. This image is an actual unmodified clip from the datasheet.

LED confusing polarityI can't for the life of me understand why this would be done on purpose. I could maybe understand is one was designed in a different building, but it couldn't have been too hard for someone to say: "Hey - wait a minute..." before sending these things off to manufacturing.

Of course, maybe they built a million before noticing and then just decided it would be easier to change the datasheet. Regardless, it's kind of nuts in my opinion.

The other thing here is that, while you can generally get away with the indicators "+/-" on an LED, you can't with all diodes. Thin Zener and TVS.

Duane Benson
Matter + antimatter makes what?
Does it really matter?
Does anybody really know what time it is?

How NOT to mark a diode

A while back, I wrote about ambiguity in the markings on electrolytic capacitors. In doing that, I cobbled together a little image to illustrate how surface mount electrolytics are marked. Take a look at the image below:

Capacitors

Note how I have illustrations showing how tantalum and metal can electrolytic capacitors are marked. Further note, that I have the capacitor schematic symbol there too. Finally, note that all three are oriented in the same direction. I have the plus side on the left and the negative side on the right.

Now for comparison, I have two nearly but not quite identical 0805 SMT LEDs in the following photo. Look at the photo of the two LEDs below. I did not alter this image in any way. The mark on the LED image could be interpreted either way. The bump could be seen as pointing toward the cathode, since it is the cathode mark. Or, The line could be on the side of the cathode. That would make sense because the line on the schematic symbol represents the cathode.

There's one final thing to look at - wait for the punchline:

Backwards markings

The punchline is that the  cathode is on the left on both of these LEDs in the photo. I have empirically determined that to be the case, both by putting them on a board and by subjecting them to a diode checker. Punchline number two is that both are correct according to their respective datasheets. The following excerpts from their respective data sheets shows the problem.Reverse marked LEDs

And, drum-roll please ... The third punchline is that both of these parts are from the same manufacturer!

If your board uses SMT LEDs, send the datasheet with your assembly order. Include it as a PDF in your files set. It would also behoove you to double check your CAD library footprint against your specific part number datasheet. IPC says the cathode is pin-one and pin-one zero degree orientation is with pin-one to the left.

Duane Benson

Forward, the LED pick and place
Was there a machine dismayed?
Not tho' the engineers knew
Someone had blundered
Cathodes to right of them
Cathodes to left of them
Cathodes behind them
And I cannot reason why

BOM process

All of this talk about BOMs these days, (all of my talk, that is), kind of begs the question of how BOMs are put together. An Excel spreadsheet seems to be the most common "BOM management" tool in use today. Large organizations often have more formal systems, but for everyone else, it seems to be largely a manual spreadsheet process.

When I'm putting together a new design, I first look at the big parts, like MCUs and other "big" chips. Sometimes big is 3mm X 3mm, so "big" is a relative term. Then I'll put in active discreet components and connectors. Next will be specific passives (like for a crystal that requires a certain value of capacitor) and finally, the more generic components like pull-up resistors and bypass caps. This may not be the best, or even a typical process, but it seems to work for me and it more or less follows my circuit design process. There are some traps that I leave myself open to.

I pick the big components based on what the board is going to do and how much real estate I have. The next set of components are largely used to make the big components work. The specific passives attach to parts with specific requirements and the generics just get thrown in as needed. Often I don't even define the generics until after the design is complete. Take a look at this schematic clip:

Non-specific components
U3, the MCP73833 battery charger and U4, the ADM3101E RS232 line driver will both get specific part numbers right off the bat. I may change packages as I get into the layout, but I still will usually fill the BOM for those items when I put them in the schematic.

IC part of BOM

I've put U3, the charger chip in my BOM twice, each with a different package, because my preferred package isn't available at the moment but might be soon. It's not best-practice, but you can do that as long as the reference designator differs in some way and the part is labeled "DNS" (Do Not Stuff). I simply gave my alternate the part number "U3alt".

R6 and R8 get specific values because the battery charger chip calls out for specific values. Q1 will be a small P-channel MSOFET, but the specific part number can come later. All of the rest of the resistors and caps will also be defined later.

The biggest trap I have to watch for when filling out the BOM in this order is forgetting any design decisions or leaving fields blank when I go back and fill in the rest of the part number information.

Passive part of BOM
This is typical of a BOM of mine just after I've finished the layout. If I were building this by hand out of my own parts bins, this would be fine. But when sending it out for assembly, it's not. I personally know that C3 and C4 could be any of a dozen different part numbers. The only things that matter in that case are that it's .01uf, it's an 0402 and the voltage rating is 10 volts or higher. I know that R2, R3, R4 and R5 are just LED current limiting resistors and can be anything between 220 and 680 ohms for this particular circuit.

I know those things, but the assembly house doesn't know that those parts have pretty loose specs. It will just cause delays if I don't find an exact part number before sending it out. It may seem obvious, but just because it's obvious to me doesn't mean that it's obvious to anyone else. That ambiguity has to be gone before anyone else sees it.

Duane Benson
Knock three times
on the ceiling if you want p-channel.
Twice on the pipe If the answer is n-channel.