Screaming Circuits: July 2009


ESC Boston Ho!

Hey, y'all - we'll be at the Embedded Systems Conference in Boston on September 22 and 23rd.

Find us in booth 909. Check out the show floor map here. ESC_Boston 08 Ask us about our new service offerings. And, stay tuned for word about our show contest and special offers!

Duane Benson
01 if by land, 11 if by sea.

August Special

SunStone_logo_saturated 120  Let us make your job easier. You need PCBs anyway, so have us get them for you from our partner Sunstone.com. For any orders placed during the month of August, 2009, all you have to do is ask and we will get the boards from Sunstone for you and we'll just pass the normal Sunstone price along - there won't be any markup or service charge for providing this service.

This offer only applies to boards from Sunstone. PCBs from other board houses and parts will still have our normal turn-key service charges applied. Your order must be place via the website prior to midnight on August 31, 2009 or via phone prior to 5:00pm PST on August 31 and the boards must be purchased for an assembly order with us.

Q&A for this special:

Q: What if I normally use another board house?
A: Give Sunstone a try. They do great work and have great service.

Q: What if I just want to submit a kitted order but still want this special?
A: Just tell us that you want us to get your boards from Sunstone too. Put it in the special instructions on the web order or tell us if you're placing a phone order.

Q: What if I still want to use a different PCB supplier?
A: That's still okay with us. You just won't get the special.

Q: What if I already bought my boards from Sunstone?
A: I'm sorry that we can't give you any discounts on that order. You'll still get great service from both companies. Just stay tuned for our next special or maybe you'll need more during August.

Q: What if I'm looking for some kind of board for my house and not for electronics?
A: Home Depot carries a wide variety of boards of the non-printed-circuit variety. You might try there.

Q: I just placed an order today and I know you guys haven't started work on it. Can I get this special on that order?
A: As long as we haven't started work on it and you can accept that we'll probably have to add in some time to get the boards, then yes. Just give us a call and make sure we haven't started, tell us you want the special and accept the extra time. Cool!

Duane Benson

RoHS, REACH Updates and PCB finishes... Too much...

I've been doing a lot of reading lately on the continued efforts to ban all metals and all materials that actually contribute to society from electronic devices. This affects pcb finishes, solder, flux, board substrates, interconnects and components, obviously, but it also affects inventory, purchasing, disposal, handling, gophers, packaging, recycling, importing, exporting, merchandising... BBP, DBP, HBCDD, Bisphenol A, B, C and D, Lead, Cadmium, Chromium (of the hexavalent variety), Tin, Copper, Gold, Silver, Oxygen... RoHS, China RoHS, California SB 20, REACH, IPC 1752, WEEE...

I can't keep it straight. But, fortunately, I think I finally have found someone to help figure out exactly which materials are likely to be banned in the near future. Get rid of everything in his list and we should all be just fine.

Here's a guide set to music of everything we should be prepared to do business without:

Of course, as confusing as it all is, the intent is good and as long as we can keep it reasonable and not add in even more harmful unintended consequences, some of these regulations are a good thing. Read a bit from Dr Lasky over at Indium on that subject.

Duane Benson
So long mom, I'm off to drop the bomb...

QFN Solder Mask Issues

Here's another QFN oopsie. Presumably, in this case, the flag pad on the QFN is needed as a heat sink and those vias are designed to conduct heat to the other side of the board. That's all good. But, with the solder mask covering everything but the vias, it won't work. I'm guessing this was an accident.

QFN-reverse-mask The stencil looks okay. It's segmented to reduce the amount of paste. We here at Screaming Circuits recommend 50 - 75% paste stencil coverage, as do most QFN component manufacturers. But, again, on this board, it will all go to the wrong place. It will wick down the vias to the other side and may not connect the pad to the part at all.

The best option would be to have the vias filled with a thermally conductive material and plated over. Most fab houses can do this as a matter of course these days. The second option, would be to use slightly smaller via holes and reverse the mask so solder mask is capping the via holes and the rest of the pad is exposed for soldering.

Duane Benson
And keep moles out of your via holes too

Solder Fumes - Ugh

How funny (ironic?) is it that just about every time us human-types try to solve a problem, we create another. Artificial sweeteners are allegedly better for your teeth and waist, but they may do other things and may, in fact, promote over eating. Alcohol as a motor fuel may reduce dependence on long-deceased dinosaurs, but doubles the price of staple food in undeveloped parts of the world.

The debate along those lines for RoHS may never end, but here's another one I hadn't heard of before. The folks over at Weller (the soldering iron people) did a study about the effects of higher temperatures on fumes. According to their info, the added heat does bad things to the flux, resulting in a lot more bad junk getting into the air near a hand-soldering station.

I say bring on RoHS II.

Duane Benson
Crud. One more thing to worry about...

Moleish SMT Heatsinks

Little-moleWell, no one ever did give me a reasonable estimation at how deep a mole of moles would cover the  earth. But then, I didn't go back and dig up my old chemistry notes and see if I could find my calculations from nigh on eighty years ago either. Nor did I get out my old Handbook of Chemistry and Physics and try to recreate the calculations from scratch. Maybe I didn't give enough information or parameters for the question - like, just what kind of mole are we talking about here?

Anyway... back to something you might actually care about... D2Pak-w-heatsink-copperI ran across some writing about thermal vias and big parts the other day. Not that the material had anything to do with Avogadro - except maybe that sometimes it looks like a board will have a mole of via holes on the thermal pad in an attempt to get heat away from a power part. If done properly (filled and plated over), that can be a viable option.

For some chips there may be other options too though. A couple of companies make SMT placeable heat sinks for D2Pak and similar chips. I haven't personally used any of these, so I can't speak to the performance, but AAVID and Wakefield are two examples.

The basic idea is that you make the thermal pad a little bit wider, put a small strip of solder mask in between the part and the heatsink spot and you won't need an vias. Look at the heatsink data sheet specs to get the exact dimensions of everything. You'll end up with a part footprint that looks something like this one.

The cross hatch area represents bare copper (or whatever board surface you are using) and the rest would have solder mask. The thin layer of mask between the part and the heatsink will help to ensure that you have better control over the amount of solder under the part and the sink. It should also help prevent solder balls as well.

Duane Benson
How about gophers?

Happy - zap - Birthday - zap - Nikola - zap - Tesla

N_Tesla I don't normally celebrate the birthdays of people that I haven't personally met, unless, of course, I get a day off work because of them, but I was over on the RF Cafe website this morning and they noted today as being Mr. Tesla's birthday back in 1865. On the other hand, the "never in err" Wikipedia lists Tesla's birthday as July 10. Hmmm. In any case, we should all know that without Tesla, electrons would only go in one direction, and that would be boring.

RF Cafe also notes (Ok, I'm paraphrasing to the (n-2)th degree) that if you transpose the last two digits in the year of Tesla's birth, you get the deathday of Count Amedeo Avogadro. Without Mr Avogadro, millions of chemistry students would have been denied the pleasure of relieving boredom during long lectures by attempting to calculate the depth that a mole of moles would cover the earth. (Quick - who has the answer to that calculation?)

Duane Benson
Approximately 2.62 feet???

Very Compact Layout?

How do you do it? We've told you not to space things too close. We've told you not to put vias in pads - like a million times. We've told you to watch out for spurious vacuum tubes. Okay, the last one doesn't really count. Vacuum tubes put off a lot of heat and you won't find many with .5mm or less lead spacing so compactness isn't so much of an issue as it can be with mobile devices. Although, I haveTight tubes seen some pretty tightly packed tube designs from the '60s.

Anyway, say you're trying to put a little intelligence into a tiny little package and you just don't have space to route around here and there. Your parts are too close for anything more then maybe a single trace or two between parts.

Here is where the infamous via-in-pad, like Under-dog, can arrive to save the day. We ran across a project a while back with no visible traces on either surface of the board. That technique's actually showing up more and more often with dense-pack designs. All of the traces are on inner layers. It looks pretty weird, but it works very well - if done right.

Before assembly, there also weren't any visible vias. Not a one. All of the components, including Packed in tight passives did have vias - every connection was though a via (via a via???). However, all of the vias were properly filled and plated over at the board house.

So, if you need a 0.52" x 0.42" microcontroller / motor driver like this one, go ahead and give it a shot. Just make sure you fill and plate your vias, and you can use them liberally.

Also make sure you don't interfere with thermal requirements, though. I still have to move the parts out from under the thermal pad of the A3901 motor driver and the little 0201 out from under the thermal area under the MCP1726 regulator here. The PIC doesn't generate enough heat to worry about, so that underside area is fair game.

Duane Benson
Tiptoe through the 0201's with me

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