Tuesday, September 11, 2012

PCB Faux Pas

If it sounds like I was overly nervous about something as simple as a PCB, it's because I've had some bad experiences in the past. Here are a couple of PCB fabrication errors I've run into.

Way back in 2002 I had a batch of PCBs made by a board house specializing in one-day, quick-turn prototypes. I believe I submitted the Gerber files on a Thursday morning and the boards arrived Saturday morning. Most of the boards looked great, but this one appears to have been over-etched in a small area.

To give you a sense of scale, these holes are sized for 4-40 machine screws; I think they're 0.122" in diameter.

My other example is from 2009. I'd started experimenting with CPLDs, and decided to use one to design a controller for some old NEC 4164 (64K x 1-bit) DRAM chips I had laying around. The CPLD I wanted to use comes only in surface-mount packages, so I made my first tentative steps towards using SMDs. I spent days trying to decide how big the pads should be, trying to balance my desire for wide pads against my utter lack of experience soldering anything tighter than 0.1" pitch, let alone a fifth that (~20 mils). Finally I settled on 12 mil wide pads as a good compromise.

Here's what I got. Notice that the 8 mil traces don't get any wider when they emerge from under the solder mask. The device leads are 8.7 mils wide, making them wider than the "pads" they're supposed to sit on. Since the "pads" are pre-tinned (hot air leveled solder) they're convex, and trying to align the device on the "pads" was a real pain. The leads kept sliding off into the open spaces between the "pads", which are quite wide enough for a lead to sit in.

When I called the board house, they protested that they'd made the boards exactly as I'd specified. Yet Eagle and every Gerber viewer I could find showed 12-mil pads. After far too much denial, their customer service person finally admitted that they'd left off the pads entirely. What really pissed me off, though, was that the best they'd offer to make things right was a discount on another order. Needless to say, I never placed another order with them.

So forgive me when I get excited about the high quality of the boards I just received. It's a welcome change.

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