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The shield - mounting the 2x4 headers:

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Next up are the 2x4 headers.

The general rule of electronic assembly is to do the components lowest to the board first, and the tallest ones last. That way you don't have to poke around trying to fit a small component in the space between two tall ones.

It's also a good idea to start in the center and work out, but we have a reason for breaking that rule in this case.

Drop the 2x4 headers into the holes and put a layer of tape over them to hold them in place. I use 2" painter's tape because one piece covers everything, and it's easier to remove than regular masking tape.

We can get away with installing the stacky headers first because they're exactly the same height as the 2x4 headers, and they make a convenient surface to fix the ends of the tape.

If you don't think the tape is necessary, try flipping the shield over and getting all eight pieces of 2x4 header into the holes by hand. It's possible, but it isn't fun.

As you can see in the full-size photo, the holes are larger than the narrow dimension of the pins. It makes placing the parts easier, but you can't rely on the holes for good alignment.

We'll start by soldering one pin of each header, then use those as pivots to swing the headers in line.

The observant among you may have noticed that the 'ET' stamp, indicating that the board passed its electrical test at the factory, has disappeared. Do not try to wash the rosin off your solder joints with alcohol. Apparently this soldermask dissolves in it.

Try to center the pivot pin reasonably well in its hole. Start by soldering the joint, then push the header's other pins to line things up before pulling the iron away.

The pivot pins will keep the headers from falling out of the board when you remove the tape, but the pins at the far end will still be able to swing slightly. The trick is to swing them so the headers line up with each other.

We do that with 1x4 header.

Plug a couple of pieces across the headers as shown: two pins in the first header and one in the second, then one in the first and two in the second.

The alignment headers will hold the 2x4 headers in place while you solder the remaining pins, but you can't flip that assembly over and hope it will be steady.

Plug four more pieces of 1x4 header into the ends of the stacky header to serve as supports.

Final tip: You still have a few pieces of 1x4 header left. Do both center rows at the same time, then do both outer rows.

The mental gymnastics of, "okay, I aligned the set on the left when the board was facing up, but now I flipped it over, so that means it's on the right, unless I rotated the board end for end" are more trouble than they're worth.

Symmetry is the antidote to Murphy's law.