Pitch Switching

I recently started reading the magazine Chip Scale Review. It's a different take on things than I'm used to. Most of what I read for work is in the engineering and assembly realm, but this one goes back to the component packaging. I think it will be a good one in terms of keeping up on what sorts of things we'll need to be assembling in the future.

So far, I haven't seen anything really scary in it. There is talk of .3mm pitch BGAs, but those aren't totally new. I'm not sure if we've done any .3mm pitch before, but we've been doing .5mm for years and have done plenty of .4mm pitch as well, even in package-on-package (POP) forms.

Pitch switching adapter Speaking of really fine pitch BGAs and CSP type things, one topic I found interesting has to do with pitch switching adapters. It's basically a small PCB platform that has an underside footprint of a 1mm or 1.27mm pitch BGA and a land pattern on top for a fine pitch BGA. It has solder balls on the bottom, so once sandwiched together, it's treated just like a big BGA for assembly purposes. [Credit where credit is due: The image I'm using came from the Aries Electric web site.]

Such a part can negate the need to re-spin the PCB if your big part is updated and replaced in a new fine-pitch form-factor. (Although, personally, I can only imagine that if the chip is rev'ed, there will be some other change that has to be made to go along with it). The theory is, that if you've got a really expensive design, this might be a viable option allowing you to upgrade without a relayout.

Certainly though, at the very least, this could allow you use some newer fancy chips without having to resort to filled micro vias and tiny trace & space advanced (expensive) pcbs. Could be quite handy and same some money.

Duane Benson
Platform shoes are back!

Parts Substitution Gone Big

Cap too big tant I've mentioned some cautions with parts substitutions before; wrong V values on barrier or flyback diodes, counterfeit parts and things like that. Here's another example of something to watch out for if your supply is tight and choices are limited.

One of the things that I've run across a couple times, especially when hunting down capacitors, is the package size issue. Say, I need a 16uf, 10V cap on one of my boards. It's not a critical app. I don't particularly care about ESR, temperature or even much about tolerance. I just need a little head room in case of minor spikes or power line ripple. I'm not expecting a lot. I just want that safety margin.

Cap too big electBut when I run over to my parts supplier, the specific cap I picked two weeks ago, when I started the design, is out of stock or jumped in price. I want to get building, so I just look through my parts drawers for something close. There it is, a 22uf, 50volt cap. It'll still work just fine. The problem is, of course, that I neglected to realize that the part  jumped up a notch in size. Bummer days.

I've run across the same problem, not due to a sloppy sub, but also due to picking the wrong footprint in my CAD package. I find that particularly easy to do with SMT electrolytic caps.

The other thing in these examples to watch out for is the open vias next to the pads. Granted, they aren't in the pads, but they are close and without any kind of a break in the metal before the via. In the left pad of the yellow tantalum cap, I added in an example of a little solder mask dam between the pad and the via. That's the way you should do it. Even though the vias are off pad, solder can still wick away and down the via - especially with leaded solder. Bad news if that happens.

Duane Benson
Have no fear, Underdog is here...

Tented QFN/QFP Via in pad

Tented vias in padHere's a pretty decent example of mask-tented vias in the thermal pad of a QFP. Most manufacturers recommend no more then 100 - 125 um 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 fab house to put the mask down 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

Pin BGA Interconnects

My post "Speaking of Art in the Process" 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 what type of pins those are 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 digikey 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."

Module assembly from side Module assembly several This part came with the pins already on so I don't have a specific part number for the interconnect pins. I have some underside photos here 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 on DigiKey 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. DigiKey has lots like that. Here's one example of some thru-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

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 -Mask and stencil issues 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


Solder Paste Stencil Opening

Stencil openings too bigHere'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

Microcontroller In The Middle

I've written about Open Source hardware before, such as the beagleboard and Arduino. Those are both great options for folks needing to get moving on embedded microcontroller development. The Arduino is Mbed-microcontroller-angledpretty low-end and the Beagleboard is pretty high-end. I think I've run across a good step right in the middle.

A while back at the ESC show, we spoke with a gentleman from ARM about a project that would include an online IDE, and now, here it is. It's not exactly the same as open source, but it solves many of the same problems that open source solves. Mainly, it's a quick and easy way to get up and running with an ARM processor. Well, it looks easy, anyway. I haven't tried it yet. I think I'll see if I can get one and give it a shot.

By the way, we did not build this board. We have built some Beagleboards, but not this particular product. It certainly wouldn't be a problem for us, but that's not why I'm writing about it. It just looks like a great half-way point between something like the Atmel-based Arduino (or the PIC microcontrollers that I use) and the Beaglboard which uses the super-speedy ARM Cortex-A8.

If I can run one down and find the time to fiddle with it, I'll let you know what I think of it in actual use.

Duane Benson
Stay tuned. Bulletins as event warrant.
Maybe...

Speaking of Art in the Process

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.

PTH05060 on boardI 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...

A Part Without a Footprint. Very Sad

Trim pot wrong footprintPoor little trim pot. All it wants is a nice cozy little place to put it's feet. It can see it, so close, yet not close enough. Bummer. This sad state of affairs, could, of course had been easily prevented by double checking to match up the CAD footprint and the actual part footprint.

Maybe we could dead-bug this. Just put some glue under it and hand wire some wire-wrap wire between the legs and the pads. Nah. Probably not. Better to get a new form-factor part. The wire-wrap wire thingy would work though. People do that sometimes and in many prototype situations, it will work just fine. If I remember correctly, though, this one was to be used in a shockey (as in  bumps, not coulombs) environment.

Speaking of wire-wrap, does anybody still do that? I know the wire is still around. It has lot's of good uses beyond it's original purpose, but I'm not sure anyone still does wire-wrap. It's never been my favorite prototyping technique, but I've always thought it's one of the coolest looking methods, at least when the wire is routed with care. A nightmare to debug though.

Duane Benson
Which wire?
The blue one...

Unequal Pad Sizes, Take 2

I wrote a post not long ago about via-in-pad and unequal pad sizes as a cause of tombstoning and I received a question from Aaron about unequal pad sizes:

"In my designs, I have ground pours that directly connect to pads. So as an example, a cap that goes from vcc to ground would end up with one pad attached to the ground pour. Because of the solder mask expansion, the ground side pad will be noticeable larger."

This is a pretty common practice. I do it myself sometimes. The risk of tombstoning or other solderability issues relates to a couple of things. First, the smaller the part, the more critical all of this is and the more likely it will be for problems to crawl up. I'm assuming we're talking about SMT parts. Thru-hole parts can have some issues with copper pours, but not near as many as can SMT.

Pour-with thermal Pour-no thermalThe first issue is that the copper pour on one side will act as a big heat sink and may lead to tombstoning, or at least a poor solder joint on the pour side. If high current isn't needed, then use thermal pads. That will help. I would guess that with a cap like you're describing, high current isn't a requirement.

The other issue Aaron mentioned was the soldermask expanding on the copper pour and making the aperture size smaller then on the other pad. You can try to make soldermask defined pads on both sides. Then, in theory, the mask expansion should be close to the same on both sides. You can also make a larger mask opening on the side that goes on the pour so that after expansion, it will approximate the other pad size. The problem with this approach is that if you change board houses, you may not have the same amount of expansion. The best option might be a call to your PCB fab house for a chat about control of the mask expansion.

Also, if you do use thermal pads, as in the illustration, that might just solve the expansion problem right there. Most CAD packages have either a global setting to make all pads in pours thermal pads or a properties setting for each part that would make thermal pads as illustrated on the right above.

Duane Benson
Pour poor pitiful me into a picture of a pitcher