More Cautionary Tails

I recently wrote about the horrors of LED marking variations. Unfortunately, LEDs aren’t the only place to find inconsistencies in our world. Another part to keep a close eye on is the ubiquitous three-terminal voltage regulator. For just short of a million years, pretty much all three-terminal voltage regulators followed the 78XX convention. Lm7805 convention

It is not completely universal, though. (Is saying “completely universal” repetitive and redundant?) There are some regulators that divert from convention in thru-hole and in SMT form-factors. Despite the overwhelming prevalence of the 74XX pin-out, you may find some parts that dispense with convention and can bite.

Take the LM1085, low drop out (LDO) regulator, for example. It looks, for all intents and purposes, to be a standard TO-220 or TO-263 three-pin regulator. You’d look at it and assume that it’s a direct replacement for any old 75XX series. But, rather than In-Out-Ground, it’s pinned as Ground-Out-In. The LM1117T is the same.

Mismatched SOT-223You might think: “Of course, it’s different, the part numbering doesn’t follow the 74XX number scheme.” That sounds logical until you look at the LM2940. It follows the 74XX pin convention, as does the MIC39100. It’s not the LDO specification that justifies change the pin-out either. The LM2940 is also an LDO.

Unlike the LED polarity issue, this one isn’t as likely to bite you during assembly. The SMT regulators can only go onto the board one way. If your CAD library footprint is correct, it will be assembled correctly. The through-hole can be easily reversed though if your silkscreen isn’t clear. Marking pin 1 on the board (and checking the CAD footprint) is the recommended approach.

Duane Benson
In the land of the insane, only the sane are crazy.

0.4mm Pitch BGA Redux

I’ve written about it before, and again here.

When dealing with new technology parts, it’s really important to look up all of the manufacturer’s component information that is available. I’m going to quote from the Texas Instruments document “PCB Design Guidelines for 0.4mm Package-On-Package (PoP) Packages,” Section 10 (PDF page 8):

Industry reliability studies have revealed that NSMD-type pads are highly recommended for most 0.5mm pitch BGA applications. However, there is a problem with this approach at 0.4 mm pitch.

Real-world assembly experiments with the BeagleBoard and the OMAP35x EVM revealed a tendency for solder bridging between pads when NSMD were used. There was insufficient solder mask webbing between the pads to ward off bridging. Therefore, a SMD design was used which resulted in much better assembly yields with no solder bridging.

If you are using a 0.4 mm pitch BGA with the balls aligned in a grid (as opposed to staggered), read the design guidlines from the manufacturer before laying out the board.

In a presentation about the development of the Beagleboard, Gerald Coley, Beagleboard designer, notes that their first two runs had non soldermask defined pads, resulting in a 10% yield. After another run of PCBs where the pads on the PCB were the same size as the pads on the device and the PCB pads were soldermask defined, yields rose to 96%. And verify that your PCB house does in fact follow your instructions. Some will think they know better and will change the mask layout.

If you are still unsure or think your design will have different requirements, call an applications engineer at the component manufacturer and discuss your project and the layout.

Duane Benson
Trust but verify

AT Tiny is Tiny

ATTINY44A-MMH I just spotted a note on Twitter, from SiliconFarmer, referring to the ATtiny44A coming in a 0.45 mm pitch QFN as well as a 0.5mm pitch MLF package. (In practice, an MLF is the same as a QFN, by the way.) (Just in case you actually care, we’re on Twitter at “pcbassembly.”)

I’ve run across a number of 0.4 mm BGA packaged parts, but this is the first sub-0.5 mm QFN I’ve seen. Interesting that they have two different sizes of QFN package, one at 4 x 4 mm and the other at 3 x 3mm. If you’re that tight on space, that little 7 square mm of extra open area can make a difference.

Screaming Circuits won’t care on the assembly floor. We do plenty of 0.4 mm parts so a 0.45 isn’t anything new. The most important thing to remember is to use the right footprint. It’s easy enough to accidentally use a QFP footprint when you have a QFN (like here). I could see it being even easier to swap for the wrong footprint with this part. Doing so would be bad, most certainly. You might get one or two contacts per side on the right footprint, but that’s pretty much as good as none.

Duane Benson
It’s like Ice-9. The same, only different.

Mismasked PCB

Stencil w mask

Personally, I think the PCB and stencil above is prettier with the green showing through. It breaks up all of the boring silver color. It adds some life in …

… Unless you are a chip wanting to be soldered down. If that’s the case, then it doesn’t look so attractive.

Whomever really, truly and universally solves the library problem should get a Nobel Prize or a Pulitzer. Maybe a free latte. Something tells me the problem won’t be solved in my lifetime, though.

It really shouldn’t be that difficult. How many different package form-factors are there? Yes, a lot, but a manageable lot. The problem comes in when you have to match those footprints to the millions of schematic symbols. Maybe there could be a way to decouple the specific footprint from the schematic symbol.

The schematic could have its pins defined to an abstraction layer and then that abstraction layer could be automatically connected by the layout CAD software pin to pin on the specific footprint selected. Maybe. We can dream, can’t we?

Duane Benson

I’m happy I live in a split-level head

Parts Substitution Gone Big

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.

But 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, 50V. 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…