Merifix

Here is an interesting collection of pictures showing the evolution of programming fixtures used during a long-running development project. The product is a small consumer electronics wireless device, and it has been through several generations over the course of a few years. It contains a microcontroller that needs to be programmed, many times a day during firmware development and testing.

The first photo shows the arrangement originally used. Wires are simply soldered on to the programming test pads and they go to a header that plugs in to the programmer box. This is simple and quick - it took about half an hour to prepare and solder the tiny wires and hook them up to the header. But it's not very robust; the arrangement doesn't really stand up to being moved around so it's risky to move it to the boardroom for a demo, for example. It's also not very scalable. It's fine for one board, but for putting the same firmware in five more prototypes for testing it's a bit of a pain to solder and unsolder wires.

The photos above show the approach that was used on the next generations of the product. A simple fixture was constructed with spring probes to make contact with the test pads, with tooling pins for alignment. The probes are wired to a header where the programmer plugs in. The board is simply held down by hand on the probes during programming.

The photo on the left shows a fixture where the probe plate was constructed from two pieces of FR4 PCB material, drilled by hand. This took a couple of hours to make, and the accuracy wasn't great. You frequently had to wiggle the board to get the probes to make proper contact. The version on the right (for a different variant of the product) shows the same sort of thing where the probe plate was made from Delrin, drilled with a CNC mill. This was more satisfactory.

With these fixtures it is much more convenient to program multiple prototypes, and because the boards don't have any wires hanging off them they can be handled as normal, and will fit in their enclosures. The fixtures are a bit ad hoc, with the base plates being constructed from scraps of aluminum drilled by hand to suit.

The final photo shows the fixture used during development of the most recent version of this product. It was constructed using a Merifix MF300 fixture kit.

This was quicker to put together than the ad hoc fixtures above. The probe plate had to be drilled of course, but there was no need to fabricate any other parts and assembly was just a matter of screwing together the parts that come in the kit. In use its is more reliable than the ad hoc fixtures, partly because of the good alignment provided by the locating probes and partly because the board is held down by the pressure plate rather than by hand.

It's also straightforward to make copies of this fixture, and this design has already been used to program a hundred pre-production boards.

Written by Matthew Kendall — September 05, 2012

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