Screaming Circuits: Need help with Printed Circuit Assembly?


Need help with Printed Circuit Assembly?

First, a question: Is "printed circuit assembly" a noun or a verb? Anyone... Anyone...

We tend to use the phrase "PCB assembly" more often than just "PCA" or "printed circuit assembly". Though, I have been told that "PCA" is a more appropriate noun used to describe our finished product. Regardless of what you call it, pcb assembly, or the printed circuit assembly process or prototype assembly; it's getting a lot more difficult.

I've been paging through a catalog from Practical Components - a manufacturer of dummy components, test boards and training aids. Page 17 shows a dual row MicroLeadFrame package (MLF and MicroLeadFrame are registered trademarks of Amkor Technology). It's basically a QFN with two rows of signal connections and the big heat slug pad in the middle. The signal pads are on a .5mm pitch and this particular line ranges in size from a 9mm square with 108 leads up to a 12mm square with 164 leads. Page 11 shows a CVBGA package that can have 432 balls on a .4mm pitch squeezed into a 13mm square. Lmx9820a_bluetooth_14116bga_150They have dummy components for just about an size package you might run into.

When I was designing boards, most parts were available in SMT and thru-hole, but you could build just about anything with a smattering of different thru-hole DIP sizes. Not any more. ZigBee radios pretty much only come in QFN packages. Some of the newest embedded processors only come in .4mm BGA packages. I could go on.

You just can't hand assembly these bad boys anymore. That may seem to make things more difficult, but not really. If you simply can't do it, you have to send it out and you can get back to designing. At least that's the way I like to look at it, biased, though I may be.

Duane Benson

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Comment on Ward's comment below
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Well, I think it's hard, but as I said, I'm biased. :-)

If you are interested in trying the do-it-yourself route, you might want to check out Spark Fun Electronics. They have a couple of nice tutorials on hand placement and manual reflow using toaster ovens, hot plates and hot air guns. They even have a PIC micro-based temperature control kit to turn a common toaster oven into a regulated reflow oven.

Stencils Unlimited also makes small stencils sized for hand screening if you want to try that out.

Myself, I'm not up to dealing with QFNs and BGAs by hand. I've hand soldered 0603 LEDs and a few SSOPs for my robot hobby but I really don't like to go below 0805 and SOIC packages more than once in a great while.

Duane Benson

I agree, but only with respect to BGAs, and I'm not so sure even about that. The tools I use to hand-assemble with QFN packages are:

Stereo Microscope
Hot air gun
Tweezers
Solder paste dispenser
A steady hand

The last item can only come through practice. The first four, though, should run about $1500...less than the NRE for a single assembly build. They're significantly less if you're not averse to buying used, or using a syringe rather than a paste dispenser.

For micro-BGAs and dual-row MLF, things get harder. Neither one can be easily visually inspected. I've had pretty good luck mounting Xilinx CSP BGAs with just a quick flux dip and a shot of the heat gun. I've never tried a dual-row MLF, but I can imagine that paste volume control would be a big problem. A stencil might help here.

One trick I found with the micro-BGAs was to always route traces at 45-degree angles off the corner of the packages. At assembly time, these traces can be used to visually align the package, not unlike fiducials for machine placement. Using copper eliminates error due to silkscreen misalignment, and is also compatible with the dirt-cheap copper-only processes out there.

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