Author Archives: Duane

About Duane

Duane is the Web Marketing Manager for Screaming Circuits, an EMS company based in Canby, Oregon. He blogs regularly on matters ranging from circuit board design and assembly to general industry observations.

Via-in-Pad with Passive Components

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Most of the via-in-pad writing I do concerns BGAs and QFNs. I do cover other parts from time to time, but the subject seems to come up most often with those packages. It is an important subject with passives too though. If you need to make your board smaller, putting vias in the pads of all of your passives may seem like a viable option to gain a lot of space. If you fill and plate over the vias, then, yes. It’s a good plan. If you leave the vias open, then no. It’s not.

Here are some via-in-pad guidelines:

Part type open via Solder mask
capped via		 6mil or smaller
open via		 filled and
plated over
BGA and LGA land pads Bad Bad Bad Good
QFN, TO-(power part) thermal pads Bad Acceptable Maybe Good
QFN signal pads Bad Bad Bad Good
Passive pads Bad Bad Bad Good

You can probably see a somewhat common theme in the table above.

All of these pictures show bad stuff. These are from the “don’t ever do this” camp. Open vias on passive parts can lead to tombstoning, poor mechanical connections, solder blobs on the back side of the board and crooked parts. Open vias on BGAs can also lead to the solder ball being sucked off of the the BGA. Bummer dude.

If you do use solder mask capped vias in a thermal pad, most manufacturers recommend the via cap be about 100 microns bigger then the via. This prior post here shows a decent example of using solder mask caps in the center thermal pad of a QFN (the rules from QFPs and DFNs are the same as for QFNs). And, I’m calling it a thermal pad in the center of the QFN, but the rules still apply of the pad is just for grounding and not for cooling.

Duane Benson
Where are we going? Planet ten
When are we leaving? Real soon

http://blog.screamingcircuits.com/

My MBED Is Up

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I wrote about the new mbed development board a while back and mine just arrived over the holiday weekend. I have to say, true to it’s promise, It was the easiest piece of development hardware that I’ve ever brought up:

1. Take it out of the shipping box
2. Plug in the USB cable to the board and my computer
3. Wait a minute for it to be recognized and open up like a USB thumb drive
4. Double click on the web shortcut in the drive
5. Register
6. Click the Compiler link
7. Pull up a code sample and modify it a bit (I didn’t need to modify it, but I did anyway)
8. Click the compile button
9. Save it to the mbed as though it were a USB thumb drive
10. Press the reset button on the mbed board.

That’s 10 steps, but it’s only 10 steps. There was nothing else to do. Nothing. The longest step was No. 7, which took me about two minutes. I programmed a “Knight Rider” sweeper with the four on-board LEDs. I made one of those for my Jack-o-lantern back at Halloween, so it was the first test program that popped into my head.


I built the Jack-o-lantern sweeper with eight LEDs and an 8-bit PIC16F819. The PIC I used came in an 18 thought-hole DIP package, costing $3.22 at Digi-Key, and I hand soldered it all on an old perf board. It runs at 20 MHz, has 16 GPIO, 3.5K program code space, 256 bytes of flash and 256 bytes of RAM.

The 32-bit NXP LPC1764 runs at 100 MHz in a 100-pin LQFP and costs $8.70 in a quantity of one at Digi-Key. (The dev board, of course, costs more then that) It has 512K of program flash and 64K of RAM. The dev board can have up to 25 GPIO (the chip can have up to 70 GPIO with your hardware) along with the standard assortment of peripherals that can be configured, including six hardware PWM channels. The mbed dev board is like a breakout board configured as a 40 pin 0.1″ DIP so it will be easy to prototype with.

The processor, being a fine-pitch package really isn’t hand-solderable like the PIC except for by the most adventurous of folks, but that’s where Screaming Circuits comes in. Why wait for your custom hardware before starting on the software. Get one of these mbed dev boards to work on your software while the EE folks are designing the custom hardware. Then, when they’re done, we’ll assemble up the prototypes and you can integrate it all together. Take some time out of your development schedule that way.

I’ve wanted to try out an ARM processor for quite a while, but prior to this, haven’t found the right way to do so while keeping within the limits of my time availability and skill set, but this looks like it could very well do the job.

Duane Benson
Robots rule!

http://www.screamingcircuits.com/

10 Electronic Things to be Thankful For in 2009

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It’s that time of year when we take stock of what’s good in our little worlds. Since I’m writing this on my work blog, I’ll keep my top 10 items focused on work-related thingys.

Number 10: Allocation!? Well, maybe. Nobody likes parts shortages and allocation, but maybe, just maybe, it means that we’re seeing the light at the end of the recession tunnel.

Number 9: The mighty QFN. Yes, I know the package can be a pain to layout properly, but the size reductions we can get with it are pretty cool. It used to take something like a TO-220 or D2Pak to drive an amp of current drain, but some of these new devices can do it in a little QFN (properly laid out, of course) form-factor.

Number 8: 99.47% on-time delivery in the last year. That’s less then one job late per month – and remember, if we’re one day late, the assembly is half off and if we’re two day’s late, the assembly is free.

Number 7: The Beagleboard being open source. It’s really opened up the world of high-end non-i86 embedded processors to a very large segment of the industry that just couldn’t quite get there before. Well done Beagleboard folks!

Number 6: The Internets. Back in the olden days when I was burning my fingers soldering up discrete transistors and plain TTL and such, I had a shelf of data books. I think I may still have an old purple National Semiconductor TTL data book buried in a box somewhere. It was always cool to page through those data books, and, of course, I didn’t need to be online in order to find what I needed, but heck, I can find it all now and even more without getting up and walking across the floor to my book shelf. In fact, I pretty much don’t have to move at all anymore thanks to the Intertubes.

Number 5: Google translator. Earlier today, I got an email written in German. Before online translators, I wouldn’t have been able to do anything with it and I would have missed a very big opportunity. The email was from a barrister in the tiny country of Togo. Apparently, he’s been looking for an heir to pass an inheritance to and can’t find one. He said that he went to the American embassy and they suggested me. If not for the Google translator, I would have missed out on this wonderful opportunity to get seven million dollars transferred right into my bank account.

Number 4: Level translators. It’s still a pain to deal with interfacing signals at different voltage levels; like a 5V I2C device to a 3V I2C bus to a 1.8V GPIO, but it was way more of a pain before easy to use level translator chips became widely available. Especially the bi-directional chips. Much more convenient.

Number 3: Better static protection built into chips. Yes, we still religiously use static ground straps. We have a conductive floor and wear foot straps and anti-static jackets and have anti-static stuff all over the place, but chips are so much more robust then they used to be. I can remember the old 4000 series CMOS chips. It almost seemed like if you breathed wrong, they’d get zapped.

Number 2: The LGA form-factor package. Just kidding. LGAs are annoying. Sure, there are some redeeming qualities: low profile, a RoHS part can go both leaded and unleaded, decent heat transfer. But, they also don’t flex as well as a BGA and the pads have the disdvantages of both BGA and QFN packages. Basically, they’re just annoying.

Number 1: And the number one electronic thing that I’m thankful for are these little Flash 8-bit microcontrollers like the PICs (that I use) and Atmels (like the Arduino uses). Holy mackerel, they make life a lot easier. All that GPIO, no support chips. And, self programmable flash. Ahhhh … Anybody out there still have a UV EPROM eraser?

Duane Benson
Embedded in my head

http://blog.screamingcircuits.com/

Tented QFN/QFP Via-in-Pad

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Below is a pretty decent example of mask-tented vias in the thermal pad of a QFP. Most manufacturers recommend no more then 100 – 125 microns wider than the via to minimize voiding and thermal insulation in cases like this. This is a reasonably inexpensive way to handle vias in the thermal pad. Sometimes though, the tents will pop open, allowing solder to wick down through the via.

The mask over the center via on the right looks a little thin, so you’d want to give it an extra look over after reflow to make sure it’s okay. (We’d do that here, of course.)

We’d rather not see this technique on really small parts because it gets difficult for the fabricator to put down the mask with enough precision. With small parts, filling and plating over the vias is the preferred technique. Well, that’s always the preferred method. It’s just more important with smaller parts and BGAs. This method is acceptable for most QFPs and larger QFNs, though.

Duane Benson
All your via are belong to us

http://blog.screamingcircuits.com/

Pin BGA Interconnects

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My “Speaking of Art in the Process” post used a photo of a point of load power module as an example. The specifics aren’t really relevant to this post though, but a commenter by the name of “Me” asked about the type of pins connecting the module to the main PCB.

“Do you know where to get those pins to attach two boards like that? I mean, do they sell just the pin for example on Digi-Key and give it a name, or is it just wire. Can’t see if they are pins with a lip to lift board to a set height.”

The part (above) came with the pins already on, so I don’t have a specific part number for the interconnect pins. I have some underside photos (below) that give a better view. They are basically solder-type terminal pins with a solder washer and BGA ball on one side (to attach to the main PCB) and either a press-fit or solder type side to affix to the module PCB.

I wasn’t able to find this exact part from Digi-Key or Mouser. Vector sells the solder washers and lots of interconnect pins of this sort, so they may be able to steer you to them with a phone call. This board uses the BGA style, but we’ve seen other POL modules of a similar type with thru-hole solder pins too. Digi-Key has lots like that. Here’s one example of some through-hole terminal pins from Mil-Max. You could use the solder-washers (like a T124 from Vector) to put some space between the module and the PCB.

Duane Benson

QFN Land Pattern Mix-Up

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Another QFN oopsie here. The QFN looks good and the stencil cut-outs look good. The mask, though, as shown in the middle, is not good. In general, you shouldn’t put mask on the QFN land pad except to cover up open vias.

The QFN center thermal pad for this part should be free of solder mask. The stencil does give a good amount of paste — roughly 50% coverage; maybe a little more.

That’s the way we like it. But, with the mask on the board, there will be too much solder and too much with nothing to stick to. It will either flow to the small metal areas and raise the QFN up so the sides don’t solder or the excess solder may turn into solder balls and short something out.

There are some cases where it’s okay to only expose the metal in small areas of the QFN thermal pad (see here), but if you do that, the stencil has to match the pattern. Otherwise, like in yesterday’s post, you’ll probably end up with a gloppy mess.

Duane Benson
That’s the way aha aha your pcbs like it aha aha

http://blog.screamingcircuits.com/

Solder Paste Stencil Opening

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Here’s a case of close, but no cigar with the stencil opening. The pads are, in fact, covered by the openings, but as you can see, the openings are too big.

This stencil would end up laying way too much paste down. Some of it would be on the solder mask which might bubble up and turn into solder balls. All in all, the use of this stencil might just lead to something of a gloppy mess.

When you’re making your paste layer in the library component (presumably, this was custom-made), it’s sometimes appropriate to make the paste opening the same size as the pad and it’s sometimes appropriate to make the opening smaller, but it’s never appropriate to make the opening bigger then the pad size.

After writing this, I for some reason got curious as to the origin of the phrase: “close, but no cigar.” I know it’s been around a long time, but I couldn’t come up with any plausible meaning for it. Then I remembered this thing called the Internet, so I looked it up. According to a couple of different sites, carnival booths, like the big hammer, would give out cigars as prizes so if you almost made it, the Carney, would say “close, but no cigar.” Huh. Interesting, but much less interesting then I had expected.

Duane Benson
Sorry. We don’t give out cigars if your stencil is good.

http://blog.screamingcircuits.com/

Speaking of Art in the Process

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I really like this picture. It’s nothing particularly special. Just a BGA-type attachment point of load (POL) power module in the middle of a big PCB, but the contrasting colors, the angle, the range of focus, the component positioning. It just looks cool to me.

, PCB

I know the function is supposed to be the most important thing, but I’ve always felt that there is a lot of art in PCB design. It’s been said that an airplane that looks good will fly good and I think there’s something in that phrase for electronics too.

No, I’m not advocating putting the visual appeal ahead of clock rise times, trace impedance and current capacity. It is first and foremost, an electronic device with an important function to deliver. But I think a visually attractive, while still superbly functional, product makes a statement about the designer’s overall attention to detail.

Duane Benson
Or, it might just be making a statement about how tight the development schedule is …

http://blog.screamingcircuits.com/

Via-In-Pad — Let It Slide?

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No.

Sometimes, you can get by with vias in your pads. Sometimes, but not very often. I wrote about this a while back here. The thing is, I was talking about big pads — like QFN or QFP thermal pads and stuff like that. We never like to see it and it’s always a manufacturing risk at some level, but as described in the earlier post, sometimes you can just roll with it.

Pretty much never with a BGA pad, though. The picture above shows just about a worst-case scenario. Very big. Very bad. Relatively very big holes anyway. (This is for a 0.5 mm pitch Bluetooth module BGA.)

The vias in the image below worked okay with a QFP because they’re really small — practically closed up — and it was Pb-free solder.

We still wouldn’t want to see a via, even that small, in a BGA pad though. Process variations leave enough opportunity for a few of the vias to be open all the way through and even if one BGA ball gets sucked off the BGA, you’re out of luck. Even if it’s just partially sucked off and still connected, it’s much more susceptible to cracking and things like that. (By the way, we did find a way to build the board on the left and make it work. We won’t guarantee that we can make something like this work though.)

A lot of fabricators will epoxy fill vias these days. Even microvias. And, yes, you should even have your microvias filled and plated. Especially with small BGAs. It’s just not worth all the risks that come along with it.

Duane Benson
We need little moles to fill those holes

http://blog.screamingcircuits.com/

That Dreaded ‘Allocation’ Word

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Here at the old Screaming Circuits homestead, we have a strict policy about parts substitution. We won’t do it unless you have approved the sub. If you ask us, we’ll look around for something close, but we’ll still get your approval before using the substitute part. No big deal: We always do it that way and we’ll keep doing it that way.

The reason I bring this up has to do with the blasted economy. Rumor has it that the industry is starting to heat back up a bit. That’s pretty cool, except that some of those parts suppliers that cut back this time last year are still in cut-back mode. We’re starting to see parts on allocation and with annoyingly long lead-times.

Interestingly, most of the long lead-times seem to be with passives at this point. Actives may end up that way too, but for now, it’s mostly in resistors and caps.

So, what do you do about it? Well, if you spec’ed the parts out a couple of weeks ago when starting the design, you might want to drop on over to Digi-Key (or equivalent) and check to see if the parts you specified are still available. If not, find a sub before sending it off for assembly. It helps for you to do this even if we’re buying the parts for you. If we can’t find what’s on your BoM, we’ll look around for a couple of options, but if you’ve already found a replacement that works with your design, that will save some time and back and forth email or phone call steps. It also prevents a last minute surprise and keeps you on top of the planning.

Anyone else out there seeing allocation and lead-time problems?

Duane Benson

http://blog.screamingcircuits.com/