Category Archives: Screaming Circuits Blog

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

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/

Pour Or Not — Just What Is My Opinion?

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I posed a question about using copper pours (AKA flood) a not long ago. The premise was a simple microcontroller board with a 20MHz clock and no special requirements.

I had a couple of different comments on the post with some very good insight. Myself, I generally don’t use copper pours. My only reason is that I think it usually looks better without (although I do like the look of the cross-hatch pour on the Arduino). A well done flood can be pretty cool, but still my inclination is to only use it if it’s needed. If it’s a shop doing the PCB, the metal will be recovered and recycled, so the conservationist in me is pleased.

If it’s a home-etched deal, then a pour is probably a better idea because it will reduce the amount of etchant needed. Although you do need to be careful to keep plenty of space between things to prevent solder bridges. Solder bridging isn’t such a big deal on a PCB with a good solder mask, but it certainly is on a board with no mask or thin mask.

If there is a good reason, I will. Like a high-current motor driver — I use the pour to keep the current capacity up and the kelvons mellow. Heat-sinking is a good reason for a pour. High-speed stuff usually benefits from a flooded plane of some sort too and in four-layer boards, using the innerplanes for power and or ground is nice and convenient. But you all know that. I’m just rambling now.

Duane Benson
Does high speed stuff on a flooded plane require a speed boat?
Will too much heat sink it?

http://blog.screamingcircuits.com/

On My Via-In-Pad Soapbox (Again)

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There’s never enough time. There’s never enough money. There’s never enough room.

I certainly say those things often enough, and sometimes it’s actually true. But other times, I’m just not looking in the right places. Here’s a board that is pretty much plumb out of room. Everything is so tight that many of the vias have to be put in the pads. Well, maybe.

Take the IC footprint (above). It needs a via to take a couple of connected pads to the other side of the PCB, but there isn’t enough room between the IC and the part just below it. Naturally, the logical thing seemed to be to put the via in the pads. Unfortunately, doing so will make it difficult to get a good solder joint. The big open hole will wick solder down to the other side of the board.

At first glance there doesn’t seem to be anything to do. But upon closer examination, there is some unused space here. I’d just slide the part up a little, as in the illustration below.

Then move the via south a bit and connect it to the pads with a trace just long enough to accept some solder mask. The solder mask will stop the solder from chasing the via off the pad and getting sucked down.

Duane Benson
Some solder suckers sit South of Sunday

http://blog.screamingcircuits.com

Diode Silk Screen Markings

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It still happens. In fact, it just happened last night. We had a PCB with plus (+) mark to indicate the polarity of diode. Unfortunately, that doesn’t tell us which way to put the diode. (Read why here.)

You just can’t always tell. If it’s a barrier diode or a zener, the cathode might very well be the positive side. Or, it could be the negative side. An LED will usually have the anode positive, but again, there may be a few scenarios where it’s not. The bottom line is that a plus (+) or minus (-) sign doesn’t give us enough information to orient the diode.

We prefer that you use the actual diode symbol, or an industry standard anode or cathode indicator. “A” orGood markation “C” for anode or cathode can also work. Just make sure you also put the reference designator (D1, D2…) so we know it’s not a capacitor.

In the job last night, the build instructions were conflicting so we called and with the help of the designer, figured it all out, but it’s always best to do it right the first time. So be clear with your silk screen, the PCB you save may be your own.

Duane Benson

Spider or worm?

http://blog.screamingcircuits.com