A question for the hardware engineers.

I have a design for a tiny PCB 6mm x 16mm, with a Microchip PIC microcontroller on it.

I would like to program the chip in-place and was thinking of using a cheap bed-of-nails setup, something like this:

http://img-europe.electrocomponents.com/largeimages/W434324-01.jpg

However, this particular item needs a custom-made jig AND doesn't provide any way to holde the piece in the jig as far as I can tell. I could just use three probes and hold them in-place during the programming, but that seems a bit hit-and-miss, mechanically.

The design has components on all edges and they can't be moved, so an edge connector is out of the question. Also, a header is out of the question for cost reasons. Because the circuit is at the design stage, the pads can be placed pretty much anywhere except at the edges of the board.

Does anyone know of a solution that:
* is really inexpensive
* would be quick to set-up
* will hold the piece in place (remember the small size)

Can you photograph the piece and post it here?

Solder three lengths of wire to the pads, and put a connector on the end of them.

6mm x 16mm can't be standalone so there must be some sort of connector off the board to other circuits. Can 3 pins be added to the connector? i.e. basically what Mark has said.

Beds of nails really need locating holes too. If you have enough space for the holes, that's the way.

If cost is so sensitive that a small connector is unfeasible, then I'm assuming high volume. High volume would exclude soldering wires to the board.

From the sort of vague description it would appear a bed of nails is the only option, possibly with a guide outside the board if no space for guid holes is available.

You say there's no room for a connector on the edge but the track going to LED8 could easily be moved and you could have the pads on the side of the board. It doesn't appear the track layout will have any impact.

I'd just have 3 0.1" spaced pads on layer 2 ala an old PC ISA card and then make a 3 way connector to "plug" the boards into for programming.

Or have the boards already been made? If so, none of that helps

I'm assuming every component is visible. Only half of the board needs to go in the connector. The end with the LEDs could just hang out.

It's either a custom jig or a custom probe. If your pcb size is consistent then a custom jig shouldn't be too hard to make, just fiddly. Find some acetal or similar and carve away. I'd suggest a cheap chopping block as a source of suitable plastic. Cut a base plate, drill for your pins, insert and glue. Build up an outer perimeter until you get to the level of your pcb. I'd suggest that the final layer (at pcb level) consists of 2 U-shaped pieces to make removal of the pcbs easier. Glue the whole lot together.

You could probably secure the pcbs within the fixture with nothing more than a rubber band wrapped around the whole thing. A couple of grooves in the top 'U's will help locate this band.

I've whacked together a quick CAD model using 3 layers of 10mm thick acetal, assuming that the pins are 2mm tapering to .2mm thick and 25mm long. Each pad in this model has a plated through hole (0.5mm). That should help with the registration, allowing you to ease up on the tolerance of the pcb outline.

Unfortunately I had to screenshot the images so they are in that nasty Word .doc format, rather than a bunch of nice jpegs. But hopefully it should give you an understanding of what I mean. Get your dremel out!

The only alternative that I can see is to make a single probe containing all the pins. This would be much easier to man handle than multiple probes. Use a stock pin header as the probe head - this gives you a predetermined pitch for your pads which allows you to put some registration holes in. (Make sure they are slightly smaller than the pin diameter obviously.)

bedofnails.doc

BTW, not sure if you need one or not, but you don't appear to have any decoupling caps.

Thanks to everyone that posted suggestions.

I liked Shonky's connector idea but couldn't see a way to get it to work with my design that has components alone every edge. I'm probably going to go with genixia's chopping board suggestion and use the Farnell probes I posted at the top of this thread. I have a Dremel, so it should be fun making a little jig. Plus, a chopping board will give me enough material to make a few attempts if the drill wanders and I don't get the holes for the probes just right first time.

I did think about using an old smart-card connector (the sort you find under the battery in mobile 'phones), so that might be my backup plan.

I'll let you know how I get on. Thanks again.