This is not a subject I give much thought to.

For one, we here at Screaming Circuits don’t really care too much what convention you use for your components. We want them to match and be properly formatted when in your BoM, of course. But because we program our SMT machines electronically, we don’t really care if you mix things up; i.e., O instead of R for resistor, or F for capacitor. It’s not a good idea to do that, but we can still build it.

But, if we can build it, shouldn’t anybody be able to build it? And, if anyone can build it, why should it matter? Well, in theory, it shouldn’t matter at all. In practice though, people tend to be human and humans tend to be error-prone. That’s why we have standards, conventions and test procedures — to reduce the chances for errors. We also have conventions for the purpose of distributing bad, overpriced food and educational sessions, but that’s probably a different convention.

It would be kind of like if you drove into a small town and there was a sign at the city limits indicating that in this town, red means go and green means stop. You would have all of the information needed safely traverse the town, but you would still be very prone to go with the green light.

I just recently saw a design where the connectors were labeled U1, U2… Again, we can build this and we did. But, if it comes time to do any rework, or if you want to make some design mods in-house, of if someone else needs to work with the board, they’ll see “U something” and think you’re talking about an IC instead of some sort of connector.

There are some specific industry standard documents covering the reference designator conventions, but I bet it’s one of those things that most people just sort of know, but don’t have the official document to go with it. Wikipedia has a list and a lot of companies probably have their own conventions.

It is easy enough to find these lists of conventions, but it does leave me wondering how some of them came to be. I get “R” for resistor and “C” for capacitor. “Q” for transistor even makes sense, although it’s derived from a property of the device instead of the name as are R and C. But, why “U” for integrated circuit? It used to be “IC”, but that’s fallen out of favor now. Really weird is the inductor. It starts with “I”, it’s inductance value is measured in “henries” and henries are indicated by “L.” Go figure.

Duane Benson
U take the high road and L take the low road

http://blog.screamingcircuits.com

Myth #3: It’s okay to put vias in BGA pads because the extra solder (the solder ball) on a BGA will prevent any problems

Here’s an example of a via in pad situation we don’t like; a large number of vias in the BGA land pads. They’re pretty big via holes too. The primary reason we don’t want to see vias in pads is that when left open, those via holes act like little capillary straws and suck solder off of the pad. A couple of undesirable events can happen depending on the method used during board fab.

If your vias are left open, solder will tend to wick down into the via hole. The larger the diameter, the worse the wicking problem can be. You might end up without enough solder left to secure the component, or even a solder bump on the bottom side of the board which could interfere with other components or lead to shorts.

The extra solder in the BGA ball can just make a bigger mess on the back side of the board. And the extra space that BGAs have between the component and the PCB make opens even more likely. BGAs are more susceptible to this problem. Not less.

Some people will try to use soldermask to cap the bottom side of the via. However, the solder ball can still be wicked down into the via, leaving an open. Or the soldermask cap map pop off due to thermal expansion or outgassing, leading to a purely open via. Internal air bubbles can migrate up, leading to voids in your solder joint.

Duane Benson
Burma Shave

The Screaming Circuits blog celebrates its fourth year blogging today (Sept. 14, 2010). In that time, it has had 447 (448, including this one) posts and 250 reader comments (more if you count the spammy ones I deleted).

Popular subjects have been:

• The mighty QFN.
• The annoying via-in-pad.
• The uncertain “to pour or not to pour.”
• And the always fun parts-library oopsies.

A year ago, I noted that the blog had 311 posts, or about 100 per year. That gives 137 posts this last year, so apparently, I’ve picked up the pace a little bit. Some other likely non-interesting statistics: that’s about 140,000 words, a million key presses and 459 images. My fingers must be tired. Maybe bruised on the tips. Only four of them should be sore though. I skipped all the typing classes and came up with my own variety of modified two-finger hunt and peck using just the index and middle fingers of both hands.

For the most part, all of those images, key presses and words have been attempts at passing on useful information and some diversionary blathering with hopefully not too much self-serving drivel. I think I’ve avoided getting into any political discussions during that time. Nothing controversial. That’s probably best given that this blog is the voice of Screaming Circuits, not the voice of me. Maybe one or two of my top 10 lists may have come close though.

Duane Benson
Live long and preserve all raspberries

http://blog.screamingcircuits.com/