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PCB Surface Finish for Switch Contact

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EPAIII:
I am working on a design where two PCBs will move in relation to each other in order to form a switch. This will be a change in distance apart only: there will be no sideways motion so no wiping action. Just contact and no contact. My goal here is to have a switch which will possess a high mechanical accuracy (+/- 0.0003" or better)(for the metric types here, that's +/- 0.0076 mm or better) over a long period of use. As far as I can find, there are no reasonably priced, commercially available switches with that degree of accuracy. But if you know of one for under US$ 1.00, please let me know.

I am considering other designs, but for this question the following applies. The contact between the boards would be a 3mm, stainless steel ball which will be constrained at one location relative to the PCBs by an insulator. The contact force will be as small as I can make it in order to prevent distortion of the PCB traces or the stainless steel ball. Hopefully this will be only a few grams. The stainless steel ball will also be constrained against any rotation by a friction fit in that insulator. I am considering stainless steel because of it's resistance to corrosion and the ready availability of high precision balls of that material at a very reasonable price.

My question is about the advisability of using an ENIG (Electroless Nickel/Immersion Gold) finish on the areas of the PCB where contact will be made. Of the finishes listed by most PCB fabrication houses, that one seems to be the best choice. But, will it work when used with stainless steel and in the long run?

The other ideas that I am looking at are the use of two of these stainless steel balls, one on each PCB. The big problem there is attaching stainless steel to a PCB pad. Acid flux is one choice but it would require extensive cleaning afterwards. Another I have looked at is a conductive adhesive. This would have more resistance, but my circuit can tolerate that. But I do not know about the long term reliability of such a bond. A third possibility would be a friction fit in a plated hole. But hole plating on a PCB is very thin and the specs for the finished diameter are not very tight. So getting a good contact that lasts for a long time (years) seems to be a very unlikely thing. Eyelets perhaps?

The positional accuracy of the balls in any of these alternatives would be ensured by a second PCB, with no traces, attached to the rear side of the primary PCB. The hole in the main PCB would be a bit larger than the ball size so it would rest on that back board while being installed.

I am open to any other suggestions.

mikeselectricstuff:
ENIG is extremely thin - you probably need a hard gold plating as used for edge connectors.

SeanB:
Make a larger hole, that fits the ball so it is at 60% of diameter, and use the conductive epoxy from the rear. Hole is plated through, and you would probably want to have a few vias to complement the big plated through hole anyway. Solder mask pulled well back at the rear, and a large plane, probably double the diameter of the ball, and the board under ENIG plated.  The vias untented, and 1mm diameter, so that the epoxy can also flow though them as well to provide extra mechanical anchor. Should hold the ball well, and the forces will tend to hold them in position. Prep for the balls would be a rough grind on the rear side, to provide a keying surface, and a clean in a solvent for both the ball and board before insertion with the rough side to the rear.

moffy:
PCB laminate is not the right material to build precision switches, +/-0.0003" repeatability, because it warps and is not flat. When using CNC to try and mill traces, a probing operation is generally necessary first to create a height profile so that the milling depth would be the same over the whole board, the displacements can be quite significant.

mikeselectricstuff:

--- Quote from: moffy on April 11, 2024, 08:14:51 am ---PCB laminate is not the right material to build precision switches, +/-0.0003" repeatability, because it warps and is not flat. When using CNC to try and mill traces, a probing operation is generally necessary first to create a height profile so that the milling depth would be the same over the whole board, the displacements can be quite significant.

--- End quote ---
Aluminium PCB might be a better bet

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