Screaming Circuits: July 2011


Via Shifting

Here's an example of what via in pad can do for a small passive component. Other things can happen too, like tomstoning or twisting. But take a close look at this photo. In doing so, you'll note that both sides of  Small fillet passive via in pad the part are soldered down. Sure, it's shifted, but who really cares? It's electrically connected. Right?

In this case, much of the solder on the lower pad flowed into the via. This led to an imbalance in surface tension between the two pads which shifted the part. Some logic might say that since both ends of the part are soldered in and there aren't any shorts, it's all cool.

It is all cool because it's been out of the reflow oven for quite a while, but it's not cool because it's not good workmanship. The IPC created standard IPC-A-610 for just such an issue. Class I is the loosest. This might pass that. I'm not sure though because we don't do anything with Class I here at Screaming Circuits except reject it. Class II is the typical commercial type standard and this shall not pass that standard. Nor would this pass Class III, an even tighter workmanship standard for higher-reliability requirements.

That's the real issue: reliability. With a good, symmetrical solder joint, you not only have a good electrical connection, but you also have a reliable mechanical connection. It will resist flexing and thermal expansion stress. This one may not. Give it some good thermal cycles or bounce it around in a race car engine computer and you may find yourself sidelined.

The moral of the story is to keep those vias out of your pads; even with passive components. Or, put the vias there but fill and copper plate them at the board house.

Duane Benson
Balrogs in pad are bad too

Fun With Electrolytics

I was fiddling with one of my robot boards the other day - popping some passives on and off and checking out subs and alternate values. I was doing this on a couple of boards at the same time. Everything was going along fine until I started to do a power-on test. The first board was fine. The second one would briefly light the power indicator LED. It would start a full brightness and then fairly quickly fade out.

My first thought was that I had been too agressive with my soldering iron and had burnt something out. (who has already guessed what really happened?). Turns out, that wasn't the case. I put it aside and came back to it a few days later. This time, I gave it the finger test and discovered that my regulator was hot. Darn. Next, I found a hot tantalum cap. Nothing looked out of the ordinary/ I stared at it for a while. The + side was on the left in both parts and... The plus side was on the left in both parts. One was supposed to be on the right. Oops. The cap had a high enough voltage rating that it didn't blow up. It just pulled down the supply until the over-current protection in the regulator shut it down.

I've heard a number of folks recommend that you try and keep all of your polarized parts facing the same way. It's not always possible, but it can certainly reduce opportunities for errors like I made here.

Duane Benson
Left, right. Left, right. Left, right. Left, left. Left, right...

So Long Old Friend

Just watching the final shuttle launch and pondering a few questions.

A significant number of innovations came out of the Mercury, Gemini and Apollo programs and filtered down to public life. Some were in materials, some were in electronics, some in software algorithms  and some in other technology areas. It was pretty much all new back then. When the shuttle was first being developed back in the 1970's, innovation in materials and other areas came about as well, though it did use a fair amount of recycled technology in the beginning.

But since that time, have there been any major breakthroughs directly from the shuttle to filter down? Though it never lived up to the "one launch a week" billing, it did, in a sense, become the space "truck." Sort of an old pick-up truck. Not much new. The occasional upgrade. The occasional breakdown. But mostly just there hauling stuff around.

When the next manned launch vehicle comes out, will it deliver a wealth of innovation as did the first decade of manned space flight? Or will it be designed with primarily off-the shelf or near off-the-shelf technology?

In the 1960's, private industry benefited greatly from the research that went on in the space program. I suspect that the next time around, whether it's a NASA design or a commercial design, it will be the other way around and the space vehicle will benefit from research paid for by commercial activities.

Duane Benson
Thanks for all the fish

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