PCB – The sun is the thing

I finally got back on the PCB fabrication horse. I printed out v1.1 of the 74HC595 LCD backpack, and v1.0 of the MCP23008 version. I chopped up the transparencies and mounted them up on Saturday, then worked on getting the boards exposed and etched on Sunday.

Since it was a clear and sunny morning (a rarity in Seattle in early March), I decided to try exposing the boards in “natural light” instead of under a lamp (I’d tried both incandescent and “full spectrum” incandescent bulbs, at a variety of distances), which had been giving me difficult results (hot spots and weak exposures) and had long exposure times (on the order of 20 minutes).

So I set up the exposure frame, put it out in the sun for 20 minutes, and the whole board promptly wiped clean of resist within seconds of putting it in the developer. Um, oops. 🙂

Just to test the timing out, I took a board that I’d accidentally exposed, and which had been sitting around for a couple of weeks being exposed to whatever ambient light was around, popped that into the exposure frame, and tried 4 minutes of sunshine. It went into the developer, and started showing traces within the first 3 seconds, and was developed in well under a minute — wow! It was still actually a little overdeveloped (some of the finer stuff, like text, was gone), but the resist that was left was by far the darkest and sharpest I’d seen to date. Nice.

So I cut myself another new blank, got it all set up, and exposed it for 3 minutes. It came out really, really nice. Because it was so dark, I left it in the developer for long enough to get right down to nice clean copper.

Both the “overexposed” test board, and the “good” new board etched out pretty well. The “overexposed” one ended up having a little spare resist left in between the traces, so it took a lot longer to etch, and was a bit tougher to get real clean. I realized as I was etching that this is a big part of the problem I’ve been having all along — because the photoresist was underexposed, it was getting underdeveloped, and thus a bunch of resist was left over, and it made etching much more difficult.

The “good” board etched out in about 3 to 5 minutes (I didn’t really time it), and both boards immediately went brown when put into the etchant.

I moved on to other projects and didn’t get the boards sawn and drilled.

The takeaway from this is that the sun makes a great light source for exposures (like who didn’t see that coming), taking only a couple of minutes to give a nice, dark exposure. And, a good exposure is critical for the rest of the etching process to go smoothly. I am betting that I’d have been able to do a pretty good job with even vinegar and peroxide with such sharp resist.

Yet another hobby that’s easier to do with clear sky. Hmm.

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