This is the drilled and etched PCB for the brassboard of the Hall effect current control LED Blinky Light:
The big rectangle at the top will heatsink the main p-MOSFET power switch, which shouldn’t dissipate much power at all. The two lower rectangles heatsink the n-MOSFETs, although I think they may each require a stand-up metal tab to handle the dissipation during high-duty-cycle blinkiness.
The silver line around the edge is soldered copper foil tape connecting the top and bottom ground planes; a dozen or so additional Z wires will connect the planes at high current nodes. It’ll get a bunch of flying signal wires, too, because I’m not a fanatic gotta-embed-all-the-wires kind of guy on stuff like this.
The PCB is 30 mil FR4, which (once again, I make this mistake over and over again) seems a bit bendy for surface-mount parts; I must print a simple nest to stabilize the poor thing. Some of the drilled holes look white, because I hadn’t rinsed out the remains of the silver plating powder; the surfaces are a lot more silvery in person.
Before I etched the back side, I realized I’d made a classic layout blunder: the high-current return path from the center MOSFET flows around the Hall effect sensor near the center of the board. So I filled in the grid pour with a fat point black Sharpie to get more copper in that area:
I think it probably wouldn’t matter either way, but nothing exceeds like excess. FWIW, I use the grid pattern as a way to verify the end of the board etching: when all the holes in the ground pour come clear, all the traces are done, too.
The etched backside came out OK, although with a few etched squares sprinkled in the Sharpie masking:
The three leads for the Hall effect sensor are just to the right of the center, with the middle lead connected to the ground pour. Given the millivolt-level signals, this isn’t a good place for ground bounce…
It’s etched with ferric chloride, rubbed with a sponge, and took under ten minutes on each side. That’s less than the usual time, which suggests the PCB is plated with half-ounce copper (that’s 0.5 ounce / ft2), rather than the one-ounce copper on the other boards I’ve done recently; just one of the hazards of buying surplus PCB stock. Doesn’t really matter, as the peak currents will be under half an amp and now I know not to use this batch of raw board stock for high current circuits.
All in all, it looks good enough. Now, for some component soldering.
The Smell of Molten Projects in the Morning 