Category Archives: Screaming Circuits Blog

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

Pads on Ground Plane

Posted on by

Generally, small pads for passive parts are connected  with a single PCB trace of equal size to each pad. That’s the right way to do it.

Top pads solidly connected to copper pour

However, sometimes, circumstances dictate a little different approach. The illustration on the right shows something of a worst-case. This is for a snubber (resistor, capacitor pair) between two power planes.

A couple of things will likely happen. The power plane will act as a heat sink, preventing the solder paste on one side from melting, resulting in a poor connection. Or, the unequal melting could lead to surface tension pulling the part up, causing tombstoning.

Most designers are aware of that, but sometimes, thermals will be deliberately turned off to allow for better current capacity to and from the large power Mosfets (not shown).

Thermal pads on side connected to pour.

If that’s the case, make sure that you can turn the thermals (see figure at bottom right) on or off by the part, rather than just by the plane.

Duane Benson
The rain falls mostly on the ground plane due to static attraction

 

http://blog.screamingcircuits.com/

 

Geek Week on YouTube

Posted on by

In case you haven’t heard, it’s Geek Week on YouTube. In honor of that, here are the top ten most incredible pieces of trivia from ancient Geek Mythology. You can scroll down and read them here, or have the questions read to you, by me, over on our YouTube channel.

First question:

F: Everyone has heard the trite phrase: “There are 10 types of people; those who understand binary and those who don’t.” Who’s missing?

E: All your _____ are belong to us. Fill in the blank.

D: Who, in the late ‘70s correctly predicted that by the turn of the century, it would be possible to use our computers to find the answer to any question?

C: And, what name did he give his computer?

B: Bell is reputed to have said: “Watson, come here. I need you.” What was Charlie Klien’s equivalent statement in 1969?

A: Who caused the “Y2K” problem? And, no, that’s not it. You’re already wrong.

9: Who did business under the name “Traf-O-Data”, starting in 1971?

8: One particular semi-nautically named person wrote the first word processor for the Apple II while in prison. Who was that?

7: And, what was that word processor? Bonus points if you’ve actually used it.

6: Who, in 1995, while on his deathbed, claimed to be DB Cooper? Bonus points if you know why I might know this.

5: How many tubes did a standard superhetrodyne radio have?

4: Name them.

3: When did the first man go into space? Be advised that you’re probably wrong.

2: When someone refers to “scout water”, what are they referring to?

1:  How many instructions did the first CPU have?

Now, drum roll please… ,

0: What is the least known, yet probably the most significant law that enabled the personal computer revolution to happen?

And… This is a contest. The first five people, in North America, to get the correct answers will receive a T-Shirt from us – or the five closest to complete and correct. You have until next Monday (8-12-13), when we post the answers. If you choose to submit answers, send them to dbenson @ screamingcircuits . com with the subject line “Trivia answers”

Duane Benson
According the The Buggles, video killed the radio star
If so, then why do we still have radio telescopes? 

http://blog.screamingcircuits.com/

Super Small Via-in-Pad

Posted on by

Via-in-pad is an old issue that still pops up now and then. Our standard answer hasn’t changed: No open vias in pads. But one of the questions we get related to the subject is: “What if we make the vias really small?”

Logically, that makes sense. In fact, in some cases, the via is so small that it’s essentially closed. If it’s so small that it really is closed, then it’s not an open via. But look close — if it’s closed with solder, that solder may melt during reflow leading to an open via.

The images here show some pretty small vias. I believe they’re 0.3mm in diameter.

In the first picture, on the left, it appears that the vias are open. They aren’t though. This board (an unstuffed Beagleboard) uses solder mask on the back side of the PCB to close off the vias, as shown in the image on the right.

Our recommended method (see more detail here and here) is to plug the via with copper or epoxy and have it plated over at the board fabricator. Next, we’d recommend via caps on the component side. Finally, capping the back side with solder mask, like this example can work, but it comes with the risk of voids. The via caps and also pop open, leading to an open via.

Duane Benson
No more open vias-in-pad, I mean it!
Anybody want a peanit?

http://blog.screamingcircuits.com/

CAD Data Files

Posted on by

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 you 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

Posted on by

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.

The image at right is an actual unmodified clip from the datasheet.

I 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?

http://blog.screamingcircuits.com/

 

Intergalactic Geek Pride Day Quiz

Posted on by

There was a time when “Geek” was far from a badge of honor. Jr. High School (AKA middle school) was developed specifically for the purpose of making geeks miserable. We were told that lockers were  designed for holding books, lunches jackets, but in reality, the secret anti-geek coalition had lockers installed so geeks could be stuffed inside of them, or could have the doors slammed in their faces.

But, then something happened. While the world wasn’t looking, a geek became the richest man in the world. Pro-nerd and pro-geek movies became popular. It became cool to claim to be a geek or a nerd. The problem is that there’s a big difference between claiming the title “geek” and being given the title “geek.”

Well, May 25 is Geek Pride Day. In honor of Intergalactic Geek Pride Day, I’ve put together a little quiz on the subject.

Questions:

  1. Is it better to be considered a nerd, a geek or both?
  2. What’s the difference between a nerd and a geek?
  3. Does the outside world know the difference between “nerd” and “geek” and thus does it matter which one you’re called?
  4. If you’ve never actually been called a geek, but claim to be a geek anyway, are you really a geek?
  5. If a geek talks in the woods, but there’s no one there to hear, did the geek actually speak?
  6. If you can explain what you do for a living (or hobby) to a random stranger and have more than about one in fifty understand, can you still claim to be a geek?
  7. If you don’t have enough cables laying around the houses to connect just about any two pieces of computer / electronic equipment together, can you really claim to be a geek?
  8. If you can’t assemble a spare PC from parts you have around the house in about an hour, can you still claim to be a geek?
  9. If you can’t count in more than one base, can you still claim to be a geek?
  10. If you don’t love songs by Tom Lehrer, can you still call yourself a geek? (If you don’t know who he is, quick: Youtube)

Bonus question:

  1.  Tesla or Edison?

Answers:

If you’re a true geek, you already know the answers so I don’t need to list them.

Duane Benson
The best revenge is not violence or deviousness
The best revenge is to be happier
…and to build robots for world domination

http://blog.screamingcircuits.com/

The Value of Open Source

Posted on by

There were a number of interesting sessions and debates at the DesignWest show. One of the more passion filled, was on the value of Open Source hardware. Some people think it will save the world,if only everyone will do everything open source. Some people think it’s a stupid waste of time without a real business model.

Personally, I see more value in it than either of those extremes. What I see is that open source hardware (and software) has lowered the barriers to entry for people who want to create, design, build and sell. Yes, big companies innovate, but a vast amount of innovation and employment comes from small companies that start out without anything more than ideas.

A decade ago, before open source hardware became well know, it was pretty difficult and expensive to start a hardware company. In fact, I recall quite a few predictions and discussions about the death of hardware as an industry in the country. Open source hardware has lowered the barriers to entry and raised the level of awareness of hardware to the point that now, in my opinion, the environment is as ripe for start-up companies and innovation as was the late 1970s and 1980s.

Open source hardware has given us that, and that will be far more positive economically than whether an specific business can find a way to make money with open source hardware. These new businesses may or may not sell open source hardware. Again, that’s far less important than the fact that open source hardware has really enabled so many more people to create.

Duane Benson

http://blog.screamingcircuits.com/

LEDs: Seeing Double

Posted on by

Like I do so often, I’m being a bit redundant. While I’m all for stamping out and eliminating redundancy, this is redundancy with a purpose (not a porpoise). Not long ago, in a galaxy not far away, I blogged about annoyances in surface mount diode polarity markings. You can read that here.

I’ll wait.

Messy isn’t it? Well, after reading that blog, someone asked me about dual diodes. For some reason, I can’t seem to find the page covering dual diodes in my IPC book, but that’s not the important part. What is important is the way the diodes are marked on the PC board.

We do ask for centroid data which, in theory, contains the component rotation. That would be cool except that we find that far too often, the zero degree orientation (and the rotation from that) differs from the standard. That, and there are seemingly half a dozen or so standards.

Since LEDs don’t work too well backwards, we really would like to see everything marked in a non-ambiguous way in silk screen (or in an assembly drawing if you don’t have silk screen). A “cathode bar” won’t work because it could be a bar indicating the cathode or negative. The cathode isn’t always negative, especially when looking at TVS or Zener diodes.

Mimicking the diode markation pattern printed on the part may not be secure either. Read that article I linked to right at the start of this blog. What if you put silkscreen down to match one of those LEDs but ended up buying the other one? That’s exactly what I did myself. Trust me. It just leads to disappointment and possible soldering iron induced finger burns.

So what is the answer, and why am I talking about single LEDs and TVS diodes when the blog is about dual LEDs? Well, the answer is the same. The best way to communicate the desired polarity of an LED or any kind of diode is with a mini version of the schematic symbol. It doesn’t matter if it’s a single LED, dual LED, Schottky, Zener or whatever kind of diode. The schematic symbol is the clearest way to go.

Led marking

The diode footprint has the manufacturer’s polarity marking, but I don’t care. I still put the diode schematic symbol next to it. If you don’t have room for silk screen, put it in an assembly drawing. You won’t regret it.

Duane Benson
And they called him Flipper…

http://blog.screamingcircuits.com/

Will a Via Fit Between?

Posted on by

I don’t know that it would be accurate to say that BGAs have ever been easy, but with 0.4mm pitch being common and 0.3mm pitch showing up, some of the older size, like a WHOLE millimeter pitch seem 0.5mm pitch padspositively spacious. With 1mm and larger ball pitch, putting a via between the pads (not in the pads) is a no-brainer.

IPC-7095B classifies 0.8mm and smaller pitch as fine-pitch. It really starts to get complicated at around that point. For example, take a 0.5mm pitch BGA. Since we’re looking to put a via between the pads, the diagonal pitch is the critical measurement. In this case, it’s 0.71mm (17 mil). It might immediately seem like that’s plenty of room for a 6 mil via, but upon closer examination, not so much.

0.5mm pitch pads viasIPC states that a 0.5mm pitch BGA will have a nominal pad diameter of 0.3 mm. It should be a non-soldermask defined pad, which will add about 0.07 mm to the pad diameter. That gives 0.44 mm total pad diameter. The radius is 0.22 mm (8 mil). Take that out of the 0.35 mm (14 mil) you have to work with and you’re not left with much space.

If your fab house can do 3 mil trace and space, you will end up with enough room for a 0.06mm (5 mil) via, including annular ring. That’s not much space. Most designers, at that point, will seriously consider putting the via in the land pad and having it filled and plated over. You can’t leave the via open or un plated.

Duane Benson
All was in chaos, ’till Euclid arose and made order

http://blog.screamingcircuits.com/

More Cautionary Tails

Posted on by

I recently wrote about the horrors of LED marking variations. Unfortunately, LEDs aren’t the only place to find inconsistencies in our world. Another part to keep a close eye on is the ubiquitous three-terminal voltage regulator. For just short of a million years, pretty much all three-terminal voltage regulators followed the 78XX convention. Lm7805 convention

It is not completely universal, though. (Is saying “completely universal” repetitive and redundant?) There are some regulators that divert from convention in thru-hole and in SMT form-factors. Despite the overwhelming prevalence of the 74XX pin-out, you may find some parts that dispense with convention and can bite.

Take the LM1085, low drop out (LDO) regulator, for example. It looks, for all intents and purposes, to be a standard TO-220 or TO-263 three-pin regulator. You’d look at it and assume that it’s a direct replacement for any old 75XX series. But, rather than In-Out-Ground, it’s pinned as Ground-Out-In. The LM1117T is the same.

Mismatched SOT-223You might think: “Of course, it’s different, the part numbering doesn’t follow the 74XX number scheme.” That sounds logical until you look at the LM2940. It follows the 74XX pin convention, as does the MIC39100. It’s not the LDO specification that justifies change the pin-out either. The LM2940 is also an LDO.

Unlike the LED polarity issue, this one isn’t as likely to bite you during assembly. The SMT regulators can only go onto the board one way. If your CAD library footprint is correct, it will be assembled correctly. The through-hole can be easily reversed though if your silkscreen isn’t clear. Marking pin 1 on the board (and checking the CAD footprint) is the recommended approach.

Duane Benson
In the land of the insane, only the sane are crazy.

http://blog.screamingcircuits.com/