Keith's Electronics Blog

Design

Don’t make assumptions about the fit of the case. Inspect for obstructions and measure where the PCB will actually fit. Beware of three-dimensional interference where other parts of the case may interact in unexpected ways when closing it.

Print the silk-screen and board outline onto paper, cut them out, and test-fit inside the case.

Plan carefully for bulky components that may not fit the mounting scheme. Particularly with single-sided boards, it may not be possible to hide them on the back.

PCB Layout

Make the pads much larger than the default when laying out a board for iron-on transfer, to make sure there’s some copper all the way around the holes even if alignment is imperfect.

Copy IC orientation markings from the silkscreen layer to the copper layer so they’re still present on homebrew boards with no silkscreen.

CNC Drilling

Check that drill file mirroring corresponds with which side of the board is up on the drill.

Drill with the copper side up. It leaves much cleaner holes.

Double-check CNC drill height settings. Also set a Z-axis feed rate appropriate for the bit being used.

Stock up on drill bits. I like the 25-piece #72-#64 wire-size set from Drill Bit City.

If you run out of small drill bits, don’t bump up to larger ones; wait for more of the right size bit. Large holes make it more difficult to get a good iron-on transfer, and large unplated holes are harder to solder well.

If you’re drilling all the holes with the same size bit, make sure the machine knows that. It may optimize the drill path better.

Eliminate duplicate holes. The machine may not be smart enough to do so on its own.

Make sure the feed rates are calibrated appropriately for the motors and their drivers. Losing steps == holes not drilled where you want them.

Iron-On Etch Resist

Export/print the bottom copper layer unmirrored in order to make a transfer that will iron on in the correct orientation.

Use a thin pad (kitchen paper towel worked well) between the iron and the toner transfer sheet to help distribute pressure through any surface irregularies (like dimples around drill holes).

Etching

Plastic etching tanks are all around you, if you shed your preconceptions about what a tank looks like.

Use lots of PVC cement to get a tight joint in an unusual configuration like a PVC pipe cut in half.

Heat the etchant to at least 100°F to make it etch much faster.

Be careful when pouring etchant. Comet bathroom cleaner does remove etchant stains from porcelain.

Soldering

Lesson: Check boards for shorts before soldering on components, to find any areas that may not have etched completely through.

Coat a board with Tinnit to make it take solder very well.

If not tinning a board, polish the copper with wet 600-grit sandpaper before soldering. Even a few hours in the open air is long enough for oxidation to impact the copper’s ability to take solder.