Category Archives: Screaming Circuits Blog

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

How NOT to Mark a Diode

Posted on by

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:

Marking 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 up and the negative side down.

Now for comparison, look at the LED image below:

Do not mark diode like this

I did not alter this image in any way except to crop it. Look at it. Now look closely. Look at it again. The schematic symbol on the right has cathode up and anode down. 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.

Now here’s one final thing to look at. I have two nearly but not quite identical 0805 SMT LEDs in the following photo. Wait for the punchline:

Backwards markings

The punchline is that the  cathode is on the left on both of these LEDs. 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. One of the LEDs is represented in the data sheet excerpt above. The other has a nearly identical datasheet with the same markings, except the mark is called an “anode mark” and the schematic symbol is reversed in orientation.

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

http://blog.screamingcircuits.com/

 

More Beagle CAD Paws

Posted on by

Continuing on from my last post …

As I said, I do everything I can to avoid reusing the package footprint when adding the the parts library in Eagle CAD. The schematic symbol can be a different story though. It still takes a lot of caution, but it’s less risky (in my opinion) than reusing the package footprint.

Eagle v.  6 made some improvements in the way copy and paste works. It’s still a little different from your typical word processor, but it’s not that difficult.

Eagle footprint menu bar 3 buttonsBut before I get to that, I want to mention one item that caused me a fair amount of confusion early on. And that’s the way all of this fits together. There are three buttons you will need to worry about. From left to right in the green oval are; the device, the package footprint, and the schematic symbol. In my last post, I pointed out the package footprint and today I’m talking about the schematic symbol.

Really, you only build the footprint and the schematic symbol. Then you connect the two up to create the devices. And, you can build the footprint or schematic symbol in either order, but you have to have them both before the last step (the icon in the green oval with four little AND gates).

If you’re using a chip that comes in a couple of different packages (e.g., DIP28, SOIC28, TSSOP28) you most likely only need to make one schematic symbol. You can make the multiple footprints and connect them up in the device section as different variants of the same part.

There are a few exceptions though. Sometimes QFN, QFP or BGA parts will have a few extra pins. In those cases, it’s generally better to create a different schematic symbol.

Duane Benson
This solder paste stencil glows blue when goblins are around

http://blog.screamingcircuits.com/

Beagle CAD Paw Prints

Posted on by

Unfortunately, I can’t generically hand out Eagle CAD QFN footprints without knowing the specific part, but I can illustrate the areas I initially had difficulty with. All of the traps that used to get me seem blindingly obvious now, but they weren’t when I first tried to make my own library parts.

The very first thing I would recommend is to make your own library file. When I started in with my own parts, I would just add them to an existing library. For example, I’d put a new Microchip PIC processor into the “microchip.lbr” library. It seemed the logical choice because there are other similar parts to start with. But, when it’s time to upgrade, migration of those custom parts becomes a nightmare. So, now all of my custom parts go into “dfb-parts.lbr.” Eagle footprint menu bar.

Speaking of modifying existing parts, another recommendation I have is, except for parts where the package footprint is exactly the same, start from scratch with the package footprint. The schematic symbol is easier to reuse – just make sure you have the right pins in the right place – but subtle differences in the copper footprint can have a big difference at the assembly stage. Datasheet footprint page land pattern.

I also don’t try to hand size and hand position the pads on the silk screen. Start by just putting a pad in the footprint area. The use the Properties/Info button (the big “i”) and use the dimensions given in the data sheet to enter the size and position by number. Look for the “recommended land pattern” or similar diagram toward the end of the component datasheet. Entering the numbers in the Properties/Info box will bypass any position precision issues. Just make sure that you use the right units (i.e., metric to metric).

Stay tuned for the next installment.

Duane Benson
World to end at 9:30. Details at 11:00

 

The ESD Habbit, or An Unexpected Shock

Posted on by

Excitement is building here. In a little over two weeks from today, The Hobbit movie will be released to theaters. I’m sure everyone reading here knows the story, but in case you don’t I’ll spoil it for you.

It’s a story about Biblio who is, according to Spock, the bravest little hobbit of them all (google that if you don’t get the reference. You’ll be glad you did). Biblio is minding his own open source robotics business when the Wizard of Menlo Park (in CA, not NJ) invites 12 MCU designers over for a meal and discussion about the merits of hardware peripherals vs. bit-banged peripherals. The MCU guys convince Biblio to go with them to The Lonely Mountain Chip Fab and help them kill a terrible ESD Spike problem. Actually, it’s the Wizard that convinces the MCU guys that Biblio could help. The next day the MCU folks left early and Biblio ran out to catch up with them without even an ESD smock.

The ESD problem came from the North because it’s more humid up North and that tends to dissipate ESD. Our Terrible Spike didn’t like the idea of being dissipated without having first destroyed a few gold interconnect wires. The MCU guys need those gold interconnects to remain intact, so they brought a secret encryption key and enlisted help from the technician Biblio.

First though, they had to get past the TO-92 packaged parts that wanted to squash them into jelly or tacky flux. Fortunately, despite the bumbling of technician Biblio, the Wizard bought solder with no-clean flux which made the TO-92 parts stop moving once applied. After the TO-92 parts stopped working in daylight, they made a brief stop to inspect the last Homely Chip Fab in the Silicon Valley and see where the light sensitivity came from.

Passing over the Siskiyou Mountains on the way to Oregon and The Lonely Mountain Chip Fab, it started raining so they went into a cave and ate porc for dinner. Biblio ate so much that he fell asleep in the corner behind a chair where no one could see him and his buttons popped off. The missing buttons didn’t bother him too much because those ones had a de-bounce problem anyway. Luckily, the weren’t Grayhill switches or they would have hates Biblio forever, even if he used an achient gold Tolkien-ring network to bypass more porcs.

Biblio wasn’t the most skilled technician and he caught his pine cones on fire while trying to solder new switches into place, but the wizard was able to re-layout the board using Eagle CAD and an FPGA that could take many forms and would satisfactorily control the machinery and bears at the local honey production facility. But the FPGA brought them all into the murky world of Verilog and VHDL. That would have been fine except that the search engine spiders hadn’t crawled the eleven Wikipedia pages they needed to properly map out the clock routing.

The MCU guys got hungry and wouldn’t wait for Biblio to come back with pi so they rushed in causing so much in rush current that the lights went out with a snap. After eleven clock cycles in his new hall-effect switch, Biblio knew that the de-bounce problem would be gone except when he plugged the barrel jack into his Apple computer. But with no static guards to wine too, he had no choice but to use the Apple barrel jacks to get power to MCUs and switch open the flip flop made from a streaming-transistor logic gate.

Annoyingly, they split the story in two and the movie will end at this point. We’ll have to wait another year to see if Silicon Oakensubstrate is robust enough to kill the terrible ESD spike and pass final QC.

Duane Benson
One oven to reflow them all

http://blog.screamingcircuits.com/

BoM Process

Posted on by

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 3 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:

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 will usually fill the BoM for those items when I put them in the schematic.

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 10V 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.

http://blog.screamingcircuits.com/

Speaking of Reference Designators …

Posted on by

In my prior post about BOMs, I gave a few examples of reference designator formats in the BOM. BOMs are a common item that have standards but no standards as are reference designators. There are actually a number of standards covering reference designators, but I still find people referring to documents published in the 1970s!

Some aspects are pretty obvious. They are a code letter followed by a sequential number. Each and every placement on the PCB has to have a unique reference designator. The code letters are somewhat standardized, in practice. Some vary based on the particular user. Pretty much everyone uses “R” for resistor and “C” for capacitor. The mostly standard designator for an integrated circuit chip is “U”, although I’ve seen “IC” used enough times. Crystals and oscillators are supposed to be “Y”, but I’ve also seen “X”, “Q” and “U” used. Check this page over at Mentor Graphics for its recommendations.

Things start to get sticky when people have more than 10 of a given type of component or when putting together a family panel (several different designs on the same PCB panel). Let’s say you have 15 resistors. You could designate them as R1, R2, R3 – R15. But maybe you’re a little OCD and you want them to all have two digits. In that case, you might have R01, R02, R03 – R15. To a human, “R1” and “R01” might very well be exactly the same thing. But to a surface mount robot, they are two different things. The robot would be happy with R01, R02, R3, R03, R4… but that could cause problems for a human reworking or maintaining the circuit later. It’s best to be consistent. Basically, the assembly systems see reference designators as text items, not numerics.

Let’s take the example of a family panel. One board has C1 and C2 are a 10uf, 24V tantalum cap. The other board has C1 as a .01uf, 50V ceramic and C2 as a 220uf 24V metal can electrolytic. If you were having them built separately, there wouldn’t be any problems, but the two of those on a surface mount machine in a family panel and you will have bad news.

First, you could avoid running your boards as family panels. That’s not always practical, though. Second, you could just start numbering the second design where the first one stops: design one: R1, R2, R3, R4. Design two: R5, R6, R7, R8. That makes a lot of sense for a family panel. Just treat it all like one big design. That can get confusing though if you later run them individually or need to do some rework. Some poor tech could go crazy looking for R1 on design two. Even worse would be: design on: R1, R3, R7. Design two: R2, R4, R5, R8. Again, fine as a family but darn confusing when separated.

Personally, I would probably go with something like: design one: R101, R102, R103. Design two: R201, R202, R203.

Duane Benson
You know the nearer your designator, the more you’re silk screening away

http://blog.screamingcircuits.com/

BOMS Away

Posted on by

Yes, I’m talking about BoMs (bills of materials), not bombs. That would be silly and irrelevant. At least mostly irrelevant. If you make bombs, it wouldn’t be, but it would probably be all secret so we couldn’t talk about it.

The question of the day is: “What makes a good BoM?” There are a lot of BoM formats in use. It’s one area that the standards train more or less left behind. Well, there are standards. For example, IPC-2581 covers not only BoM standards, but a replacement for Gerbers and the whole manufacturing data package. One of these days, we’ll all be using the IPC-2581 formats for our data and life will be beautiful all of the time.

However, those standards aren’t really in common use today. And, they are complex enough that they can’t really be used in spreadsheet form. There’s a lot of nesting and hierarchy that makes it more difficult to deal with without a BoM management software package. Still there is good data in there. A lot of good data. So much good data that my head is still swimming.

But until that day, there is a set of data and data labels that will help ensure accuracy. The headers are important too. If this seems quite rudimentary, that’s because it is. But it’s important.

BOM snippet

  • “BomItem” or “Item #”: This is just the line number. Each type of part gets an item line, not each part. If the pat number is the same, you just put it down once and give the quantity.
  • “quantity” or “Qty”: How many of this specific part you need per board
  • “RefDes”: The reference designators used by the parts on the PCB silk screen. All of the same part number should be in the same excel spreadsheet cell: i.e., “R3, R4, R5, R6”. You can also indicate a contiguous range with a dash: “R3-R6” or “R3-R6, R10, R15”
  • “Manufacturer” or “Manf”: The name of the component manufacturer. It’s best to spell out the full name, e.g., “Texas Instruments”, but common abbreviations such as “TI” generally work too. The less ambiguity, the better.
  • “Mfg Part #” or “Manufacturer Part #”: The part number that you would use if you were buying this exact part from the manufacturer or a distributor. All of the suffixes are important too. For example, “PIC16F88” is not enough when you really need a “PIC16F88-I/P”.
  • “Dist. Part #” or “Distributor Part #”:Not strictly necessary, but can help in cases with a bit of ambiguity. Again, this would need to be the exact part numer as you would order it from that distributor.
  • “Description”or “Desc”: This is the component description as given by the manufacturer. Again, this isn’t strictly required, just a good idea.
  • “Package”: This is the standard package type, e.g., “SOT-23”, “TO-92”, “0201”. Again, not strictly necessary but can be a good redundant check.
  • “Type”: Optional indicator of the generic type. e.g., “fine pitch”, “smt”, “thru-hole”, “Leadless”. Not required but can help with assembly quoting.

That’s not IPC-2581, but it is a good set of usual requirements. It’s also best to put your final BoM on the first tab in your excel spreadsheet. That will make it easier for buyers to know exactly what you want.

Duane Benson
So long mom, I’m off to drop the bill of materials
So, don’t wait up for me

http://blog.screamingcircuits.com/

Let’s Get Small, as in 0.3mm

Posted on by

Not long ago, I wrote about a 0.3mm pitch wafer-scale BGA we received and were asked to place. The gist of that article was that those parts are very small and we d0n’t yet have a process that we feel will give the quality, reliability and consistency that we want to deliver. That means officially, we don’t, at the moment, support that form factor.

However, as it turned out, we went ahead and built it and the x-rays all said it looked good. Whew! We still don’t officially support it, but we’re working on it. If you have one of these things, you can always give us a call and see if it’s something our manufacturing engineers are comfortable with. If they say “sure, send it in,” it will be a non-standard, essentially, experimental, operation so our normal guarantees won’t apply. It will be “we’ll do our best.”

But that’s not the point. The point is that there are still a number of unanswered questions with 0.4mm pitch, and now we have a smaller one??!!

I’ve only seen 0.3mm pitch in two places: some data from Amkor, and the datasheet for this part.The part in questions is a Maxim MAX98304 Mono 3.2 Watt Class D amplifier. The entire package is just 1 x 1mm.

There is still a lot of difference of opinion on solder mask defined (SMD) vs. non solder mask defined (NSMD) at super small pitch like this. For BGAs 0.5mm and lager, the general consensus and IPC recommendation is NSMD. At 0.4mm, the Beabgleboard folks at Ti recommend SMD to reduce bridging. But I’ve had other folks say they get good results with NSMD. For 0.4mm, we’ve had best results with SMD. It’s more than just that though, you also need to religiously follow the manufacturer’s recommended pad sizes and such.

Shrinking BGA pitchFor this part, the datasheet shows the pad size (0.18mm), but doesn’t cover the SMD vs. NSMD question. Instead, it refers to a Maxim app note (#1891) for that bit of information.

Of course, this is where it gets sticky. That app note, as of this writing, shows 0.5mm and 0.4mm, but no 0.3mm. It does reference IPC-7351, which is a very good thing, but I don’t think IPC-7351 has 0.3mm pitch covered yet. Ugh. The 0.3mm part we placed used SMD pads.

Duane Benson
It’s not just Facebook where you can designate something: “It’s complicated.”

It (0.3mm) Finally Happened

Posted on by

Back in January of 2012, I wrote about the possibility of 0.3mm pitch BGAs being used here and there. I predicted that in a year, we’d see some 0.3mm pitch BGAs showing up. I was about three months off. Almost to the day.

I delivered a session at PCB West last month and asked if anyone had used a part with that pitch yet. One hand went up. That actually surprised me. What surprised me even more was when one of them (a 0.3mm pitch BGA, not a hand) arrived on our shipping dock in a parts kit earlier this week.

0.3mm pitch trimFor comparison, the land pattern for an 0402 passive component is about one millimeter long. This specific part is just shy of a millimeter square. Even as small as it is, this part can supply 750 mA continuous. The olden days are so very long gone.

We do many, many complex parts and PCBs. We’ve put 5,000 parts on a single PC board. We’ve built boards to be shot up in rockets and dunked way down in the ocean. Some very crazy stuff has come though our shop, but we don’t do everything. We don’t do 01005 passive components at the moment. Our machines have the technical capability, but we don’t rework them, which has to go along with the assembly capability, so we don’t support that form factor for now. The 0.3mm pitch components pretty much fall into that camp. Our machines can physically pick up and place the component, but until we’ve developed to process to assemble those parts with the quality people expect from us, we won’t be supporting them.

I expect we’ll be getting more and more requests for the form factor, so we’ll be looking at it. Keep checking back. One of these days, we’ll have the process down and reliable.

Duane Benson
It’s (Huey mm, Dewey mm, and Louie mm)/10

 

http://blog.screamingcircuits.com/

Missing Mars Probes

Posted on by

Back in ancient times when multi-legged beasts ruled the earth, there were a lot more standards. Or maybe there were just fewer total things resulting in fewer total variations, which looks like more standards.

In any case, if you got a 7408 IC from one manufacturer, it was pretty much equal to a 7408 from any other manufacturer. Even connectors were more or less standard. If you plugged in one PCB mount DB25, you could plug in just about any PCB mount DB25. There were variations, just not as many as now. Today, though, there are a very large number of variations to a standard footprint. For example, while the pin footprint on most Ethernet jacks matches, I’ve probably seen a dozen different arrangements of mounting and alignment pins.

Another area that can throw monkey wrenches all over is the dreaded metric v. SAE units. This seems to pop up most often with connectors, as in this image, but it occasionally shows up on other types of parts as well. The footprint here is for a 0.1″ (2.54mm) pitch connector. The connector has 2.5mm pitch. It would be fine for three pins, maybe four or five. But beyond that, it’s just not going to fit.

I don’t really understand the logic in 2.5mm pitch. If 0.1″, which equals 2.54mm weren’t such a ubiquitous standard, 2.5mm would make sense, but as it is, it’s just too close. It’s close, but they aren’t the same. 2.5 != 2.54.

Duane Benson
It doesn’t seem like much difference in mm, but in beard-seconds, it’s 4,000* units off

*By some definitions, including the Google converter, it would be 8,000 units off