Tired of all those small parts? Can't figure out how to route traces to all 1,900 balls on that hot new FPGA? If 0201 passives have you running scared and the possibility of 01005 parts coming soon has you on the floor, Screaming Circuits has the answer.
Everyone knows which way current flows through a diode. Right? Of course they do. Diodes only allow current to flow in one direction.
Well, sort of.
In the case of your garden variety rectifier, barrier diode, or LED, that's true. That line of thinking leads a lot of people to assume that you can indicate diode polarity by putting a plus sign "+" next to the anode.
Here's why you can't.
Zener and TVS diodes have a breakdown voltage. They are put in the circuit with their cathode on the positive side. In that configuration, they don't conduct unless the voltage rises above their breakdown point. Zeners and TVSs are used for regulation, transient suppression, and things of that sort.
But wait! There's more!
Regular diodes can be pointed backwards too. Anytime an inductive load is switched, like a solenoid or motor, you need a flyback diode to protect the switching logic. A MOSFET switching a solenoid on and off is a good case to look at.
When the MOSFET turns off, the current in the solenoid coil starts to drop. As it starts to drop, the magnetic field generated by the current flow starts to collapse. The collapsing magnetic field generates an opposite current, referred to as flyback, or back EMF.
To save your silicon switching device, you put a flyback diode across the coil, or motor, terminals, pointing backwards from normal current flow - with the cathode pointed toward +V. Doind so shorts the flyback current back into the coil, preventing damage to the MOSFET. These are typically Schottky diodes, but can be ordinary rectifier diodes.
Diodes. Not just for breakfast anymore
Today, I have a look at the good, the bad, and the ugly - or more accurately, the good, and the bad and ugly. As I expected, I was quite pleased with the job done here in house. The board is nice and clean, the parts are well centered, and the solder joints are solid. No surprise here.
Here's a top-view of one we did here in Screaming Circuits:
Next, I've got one that I did at home. It actually surprised me and came out better than I had expected. Here's a top-down view of the one I did at home with home-grade tools (No, I didn't intentionally make it look bad. The board surface is just a bit shinier than the one above.):
Of course, "better" is a relative term. I didn't say good. I could call this both bad and ugly. I did manage to center the parts quite well - that took a lot of careful nudging with sharp tweezers and and an X-Acto knife blade.
All of those little round shiny spots are solder balls. That's what happens when you get too much solder on the board, get solder off the pads, or have the wrong reflow profile. They might look harmless, but if there are too many under the chip, the connections could be shorted.
The fillets on the 0201 capacitor are a little lean on solder in the one I did, and there's a solder ball on the right side, but, again, it looks better than I expected.
Next time, I'll post the X-rays and show what's under the hood.
That's what life is all about
I'm a bit behind in my blog work - well, way behind, actually. I started this series back in January with the intro post.
Here's where I am right now:
- I have three different sets of PC boards.
- One set, I took home to see if it's possible to solder a micro BGA at home. (As someone working at a car manufacturer might want to see if they could balance a crankshaft at home, for fun)
- Two sets, from our partner, Sunstone Circuits, are here in my desk with parts, ready to go through our machines.
After I've got all three sets built, I'll have them X-rayed to see how they look under the hood. Finally, I'll solder thru-hole headers on and fire up the chips to see if the shared escape system works.
Here's one of the boards without access to the inner pads:
And, here's the shared escape:
The main concern I have is that Reset is on one of the inside pins (B4). I'm not sure if I can get the chip to a state where it will operate properly without unobstructed access to reset.
The routing I've chosen is probably the only possible option for reset. Pin A4, right above, is used for the single-wire debug (SWD) clock. I'm assuming that can't be shared. B5 is Vdd, so that's out. It might be possible to go down. C4 defaults to one of the crystal pins, and D4 defaults to a disabled state.
In the route I've chosen, B3 is an ADC input, so it should start out high-impedance, and therefore not interfere. A3 defaults disabled, so it won't get in the way.
Next step: solder time!
One other thing - The images above show non-solder mask defined (NSMD) pads. Those are standard for BGAs 0.5mm pitch and higher. This part is 0.4mm pitch. Some manufacturers recommend solder mask defined pads (SMD) for 0.4mm and smaller. I'm actually testing several pad styles: SMD, NSMD and solder mask opening = copper.
Run it up the flag pole and see who solders
I’m not sure exactly when the term “Internet of Things” (IOT), was coined, but it’s become one of the hottest topics in the electronics industry. The IOT is all about connected devices, most small and independent; many from makers and new start-ups.
In the IOT of the near future, virtually every household, office, and personal device will be remotely controllable to some degree. And, it’s not just about control. Most of those devices will also sense conditions, respond, and communicate appropriately.
If you were to take a tour of our factory floor today, and compare it to a tour of a few years ago, you’d, of course, see more large boards loaded with complex components. You’d also see a lot more super small boards crammed with microcontrollers, wireless communications, sensors, and tiny parts. Many of them are no larger than a US quarter. Those are Internet of Things devices.
The number of different devices being churned out is staggering, yet is a trivial number compared to what we’ll see in the next few years. Scoff, if you must, but there will come a time when your favorite ball-point pen can let you know just who stole it off of your desk and where they've hidden it.
In honor of the spirit of innovation brought forth with the IOT, Screaming Circuits has declared April 2015 to be Internet of Things month.
You too may be able to join in the celebration by placing an assembly order during March 2015 and requesting our “Internet of Things Gone Bad” poster: a contrarian view into a possible dystopian world where humans have to argue with their clothes, coffee pots, and cars, before leaving the house.
If you places an order with us in March 2015, will get an email asking if you want out Internet of Things poster. Just reply in the affirmative, and we'll send it out to you.
All things on the Internet are relative
All my relatives are things
My relatives took all of my things
I've noticed that a lot of CAD library footprints for two-pin polarized parts have pin one pointed up as zero degree rotation. According to IPC, pin 1 pointed to the left is zero degree rotation.
Why is this such a common error? I can't be certain, but I have a pretty good idea.
Surface mount parts, as everyone knows, generally come in reels of tape. It stands to reason, that the parts would be placed into the tape at a standard zero-degree rotation. They generally do. Before putting a perplexed look on your face, take a look at the image below.
When looking at the tape, it's a pretty natural thing to pull it out and hold it horizontally - with pin 1 up - perpendicular to our angle of vision. Makes sense. It's not a stretch to look at this strip of tape and end up assuming that pin one is up at zero rotation.
However - the machines are the ones being spoken to. Not humans. The machines get the parts in line with their line of vision. That puts pin one on the left.
For more to the part rotation story, tune your browser dial to here. Or just scroll down a little bit. It's right below.
The long and winding reel leads to your pc board. Not your door.
Before we (or any old assembly house) go about putting surface mount parts on your board, we need to program our assembly robots. I'm oversimplifying, but essentially, the machine program needs to know the X / Y coordinates, relative to the board origin (which is the lower left-hand corner), the part rotation, and the side of the board.
In years past, we needed a Centroid file (AKA pick-and-place file) containing all of that information. In some cases, we still need the Centroid, but not always. Today, we can get the same information from ASCII CAD files, ODB++ CAD files or Eagle .brd files. You only need a Centroid if you send us your board files in Gerber format.
If you do send us a Centroid file, you no longer need to worry about rotation. The IPC has defined the zero degree orientation, as well as proper rotation direction, but too many part footprints set the zero degree at different angles. We can't rely on the data.
While we have to ignore rotation and figure it out with other means, we still do strongly recommend that you follow IPC standards when you make your own footprints. I've got some illustrations below, showing how footprints are supposed to be oriented.
There's no earthly way of knowing
which direction we are going
There's no knowing where we're rowing
Do you need PCB Assembly Services, or do you not? That is the question. Well, it's A question. Just one of many, I suppose.
One of many, but it is a question just about every electronics developer needs to answer at some point. The answer isn't always yes, nor is it always no. The answer is quite often "It depends." I work here and I don't always have a clear answer to the question. I've sent some board through our plant, and have hand built a few.
For me, it comes down to a few options:
Use Screaming Circuits PCB Assembly Services:
- Does it need to be done right?
- Is time a consideration?
- Are there too may placements for me to deal with?
- Are there more than one or two boards?
- Are the parts too small?
- Are there any BGA packaged chips?
- Will it be monotonous?
Build it myself if:
- It's a no-hurry project.
- The parts big enough.
- It be fun.
- It will be a valuable learning experience.
I can enjoy building up a board myself in the same way that someone working for a car manufacturer might rebuild cars at home as a hobby. 0805 passives aren't a problem for me to hand solder. I don't mind a small number of 0603's. I'll hand solder 0402's in a pinch. I've tried a few 0201's with poor results.
Forchips, I don't have a problem with SOIC's. I'm not bad with a TSSOP. QFN parts are a challenge, but some types have enough exposed metal on the side to solder. I really can't place BGA's, but I'm experimenting to see if I can find a way to solder small ones in my toaster oven.
With the impending advent of desktop pick and place machines, there will be a few more options, but the basic question will remain the same as it is with "build vs. buy" in any industry: "Which do I have more of, time or money?"
Let's get small!
I've written a few times about the new Freescale KL03 ARM Cortex M0+ microcontroller. This particular part comes only in very small packages, with the smallest being a 1.6mm x 2mm WLCSP (wafer level, chip scale package) 0.4mm pitch, 20 bump, BGA. That's a mouthful - albeit a very tiny mouthful. Maybe just a toothful.
On the left, here, I've got a pair of them on a US postage stamp.
For us, it's not a particularly difficult part to assembly; just a garden variety 0.4 mm pitch BGA, as far as we're concerned. We place loads of them. But, it can be a very different story for a designer. Conventional wisdom says that a PCB designer has two choices with a part like this: a very expensive PC board, or don't use the part.
Escape routing becomes very difficult (read: expensive) at 0.4 mm pitch. This part only has six connections that need to be escaped, but that can still be a problem. You can't fit vias between the pads to escape out the back side. You can't put vias IN the pads, unless you have them filled and plated over at the board house. That's expensive in small quantities.
This blog post series is going to examine some possible ways to use these parts with more of a standard fab, such as Sunstone quickturn. I've got three different process blank PC boards, each with four different land patterns.
I've been asked about home reflow too, so as a bonus, I've done my best to duplicate hobbyist conditions for one of the board sets.
"Screaming Reflowster" not sold here
We have a number of manufacturing engineers running around here at Screaming Circuits. They're very good at what they do, as are our operators and technicians. They are not, however, electrical engineers. Our parent company has a big group of electrical engineers, but they're at a different location
What that means is, though we endeavor to be experts at building things, we often don't know what the circuits and components do in your specific application. People tend to send us their difficult projects so we've probably seen just about everything possible go through our plant. But, every now and then we see something unfamiliar. It doesn't happen very often, but it does happen.
Sometimes it's an exotic new package (like the 0.3mm pitch wafer scale BGAs now
showing up). Other times, it's something a bit older, but just not clear. Rather than put a job at risk, if we aren't sure, we'll always hunt down the designer and ask.
Okay. That was a long winded intro.
We recently ran across just such an unknown; a "polarized" inductor, without an accompanying "polarity" mark on the PC board. Not only that, but the markings on the inductor were a bit ambiguous. One half is black and the other half is green. The datasheet is in black and white, so there's more room for interpretation than we're comfortable with.
At first glance, you might wonder why polarity / direction matters in an inductor. I did. It's just wire. Right?
Almost: it's not just wire, it's coiled wire. In most cases, the direction doesn't matter, but in cases with multiple inductors, or with super high speeds, it can matter due to the fact that the coil winding direction has an influence on the flux and the actual induction.
I won't go into all of the theory, but think of walking. In most cases, it doesn't matter whether you start with your left foot or your right. However, if you're marching in a coordinated group, you want everyone to start with the same foot.
Look at the two sets of air-core inductors above. When set like this, directionality starts to make a bit of sense. Imagine the electrons being pushed around in theses things and try to picture the resulting lines of flux.
The moral of the story: eliminate ambiguity. If the part is polarized, either mark the board, or make it the direction clear to your manufacturer in build documentation. Do this even if the polarity doesn't matter to you, 'cause we don't know that.
After photographing these, I ended up recalling this bit of knowledge. It's just so rarely needed that it had vanished in to the fog. I put a few more photos after my signature.
Which way did he go?
Which way did he go?