PCB Surface Finish for Switch Contact

[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]

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.

Aluminium PCB might be a better bet

[ajb]

Is initial accuracy important, or just repeatability?  That distinction is important.

There are solder-on contact pads meant to be used as more durable targets for spring contacts that could be useful.  Basically just little flat pads of plated metal that come on a reel.  Initial accuracy would depend on the soldering process of course. 

For making contact with the stainless ball, a PCB-mount spring contact could be a good option.  Lots of parts to choose from with different contact orientation, sizes.  Widely sold for battery contacts or to electrically connect a PCB to a housing for EMI control.  Should be possible to arrange two or more symmetrically to avoid spring pressure influencing position, if that's a concern. 

[EPAIII]

I think I have the warping covered. The boards will be relatively small, around 1.25"/35mm rectangles. The main and back boards will be combined with a good epoxy after roughing. This should produce a plywood like structure to resist warping. I think FR-4 may/will have some initial warp, but I don't think it will change over time. So once the device is assembled and calibrated it should be stable. Also the overall configuration of the device will help make any remaining warping in the swithch assembly irrelevant in the device's operation.

I would like to use PCBs for some of the parts due to their ready availability and relatively low price. I am trying to work around their undesirable characteristics. I will also be using Send-Cut-Send for some of the flat metal parts. Again they will be easy to acquire and relatively inexpensive.

In any case this is experimental. I will build several prototypes and use at least some of them for long term testing. I will see how well it works.

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.

[EPAIII]

Just repeatability. The initial accuracy will be established with a calibration procedure.

I have considered the spring type contacts. And I still am. But I am looking into several options.

I am not aware of the contact pads. They sound interesting. Can you provide any further details: who makes them, what is a good search term for them, etc?

Is initial accuracy important, or just repeatability?  That distinction is important.

There are solder-on contact pads meant to be used as more durable targets for spring contacts that could be useful.  Basically just little flat pads of plated metal that come on a reel.  Initial accuracy would depend on the soldering process of course. 

For making contact with the stainless ball, a PCB-mount spring contact could be a good option.  Lots of parts to choose from with different contact orientation, sizes.  Widely sold for battery contacts or to electrically connect a PCB to a housing for EMI control.  Should be possible to arrange two or more symmetrically to avoid spring pressure influencing position, if that's a concern.

[MarkT]

Not sure stainless is the right choice, it won't solder easily to say the least.

Why not extract the contacts from a cheap relay, they come conveniently mounted on brass or beryllium copper strip which will solder...

[EPAIII]

I do intend to price aluminum PCB as one option. I have not purchased any aluminum boards and have no idea of how they compare in price to FR-4. It seems to me that extra steps would be needed in their fabrication and that would mean a higher cost. Also the relative volume of the two types would come into the equation.

On top of all that, I am hoping to panel-ize the PCBs for this project in order to keep the price down. But one of them should be a heat insulator. Aluminum does not seem to meet that requirement. I would guess that using aluminum for some of the PCBs and FR-4 for the others would be an additional increase in the cost.

Again it is the repeatability that really matters, not any initial error that can be removed in the initial calibration. As I said, I am trying to work with the characteristics of the materials. And I am considering other designs. I am only asking about one aspect of one of those possible designs in this thread.

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.

Aluminium PCB might be a better bet

[EPAIII]

It is difficult to solder SS to copper traces! Wow, you think.

I knew that from square one and, in spite of that, have already already invested dozens of hours reading about it and well over $100 in experimenting with it. Yes, it can be done, but it is not easy. And an acid flux is probably absolutely necessary. I have made a preliminary judgment that it would take an inordinate amount of time and would probably leave some acid residue on the PCB. I probably will not be going that route.

As for salvaging relay contacts, I have thought of that. But here's the thing, relays are designed to close with a small wiping action of the contacts. That wiping is there to remove any corrosion on them. It would be difficult to install them in my device while keeping that wiping action.

Not sure stainless is the right choice, it won't solder easily to say the least.

Why not extract the contacts from a cheap relay, they come conveniently mounted on brass or beryllium copper strip which will solder...

[moffy]

I don't know what your constraints are but I would prefer a non contact method, though that adds to the complexity, specifically capacitive, which can be quite sensitive as its sensitivity increases as 1/d.
https://www.mtwmag.com/innovative-capacitive-sensors-for-industrial-use/
Not that you use their sensors, but the electronics is straightforward. It's just an example of resolution but the approach might be counter to your search for a simple low cost solution.

[EPAIII]

I had not mentioned it, but one of the biggest constraints is battery powered. With a low current LED as the indicator, I hope to use a single, 3V, coin cell, probably a CR2023. And I would like it to last for a long time, a year or more, between changes. I know that does not completely eliminate an active sensor, but it does make it difficult. It might even put in the realm of a custom IC.

I am going to check out your link.

I don't know what your constraints are but I would prefer a non contact method, though that adds to the complexity, specifically capacitive, which can be quite sensitive as its sensitivity increases as 1/d.
https://www.mtwmag.com/innovative-capacitive-sensors-for-industrial-use/
Not that you use their sensors, but the electronics is straightforward. It's just an example of resolution but the approach might be counter to your search for a simple low cost solution.

[EPAIII]

Well, I went down that rabbit hole. Never did find a single Micro-Epsilon product where it could be purchased so could not get even the slightest idea of cost. But their stuff looks really nifty ($ $ $ $ $ $).

Did a search at DigiKey for capacitive sensors. The least expensive one was $12.90. That's more than ten times what I want to spend. I looked at the DigiKey listing but could not find either an accuracy spec or a data sheet. I guess you buy one and play with it until it does something. Sounds like a super high price for what is probably a metal plate surrounded with a ground ring. Perhaps it has some built in electronics. I don't know. Oh, but it IS ROHS3 Compliant. Whoopee!

Their next sensor, by price costs $52. That's about what I would like to sell my entire device for. NOT MAKE IT FOR, SELL IT FOR! Perhaps they are flexible on the price. Or maybe they have coupons.

And don't tell me I am crazy. The Chinese make a micrometer that can measure to 0.00005" / 0.0013mm, ship it across half the world, and it still sells for around $40 with a profit for both the Chinese factory that made it and the importer who sells it here. They don't do that by buying parts from others at $12.90 each, allowing those other companies to make a tremendous profit. I am looking for the <= $1 "solution" (I hate that word but it fits).

I tried Mouser but they seemed fixated on turning the room lights on and off.

[moffy]

I had not mentioned it, but one of the biggest constraints is battery powered. With a low current LED as the indicator, I hope to use a single, 3V, coin cell, probably a CR2023. And I would like it to last for a long time, a year or more, between changes. I know that does not completely eliminate an active sensor, but it does make it difficult. It might even put in the realm of a custom IC.

I am going to check out your link.

I don't know what your constraints are but I would prefer a non contact method, though that adds to the complexity, specifically capacitive, which can be quite sensitive as its sensitivity increases as 1/d.
https://www.mtwmag.com/innovative-capacitive-sensors-for-industrial-use/
Not that you use their sensors, but the electronics is straightforward. It's just an example of resolution but the approach might be counter to your search for a simple low cost solution.

Unless you can accept a really low duty cycle, which doesn't make sense for such a high resolution sensor, I can't see an active sensor lasting a year on a CR2023 either.

[mikeselectricstuff]

I had not mentioned it, but one of the biggest constraints is battery powered. With a low current LED as the indicator, I hope to use a single, 3V, coin cell, probably a CR2023. And I would like it to last for a long time, a year or more, between changes. I know that does not completely eliminate an active sensor, but it does make it difficult. It might even put in the realm of a custom IC.

I am going to check out your link.

I don't know what your constraints are but I would prefer a non contact method, though that adds to the complexity, specifically capacitive, which can be quite sensitive as its sensitivity increases as 1/d.
https://www.mtwmag.com/innovative-capacitive-sensors-for-industrial-use/
Not that you use their sensors, but the electronics is straightforward. It's just an example of resolution but the approach might be counter to your search for a simple low cost solution.

Unless you can accept a really low duty cycle, which doesn't make sense for such a high resolution sensor, I can't see an active sensor lasting a year on a CR2023 either.

Depends entirely on duty cycle - one advantage of a physical contact sensor is you can have near-zero standby current if you only need to wake on a change of state, and near-instant (few microseconds) wakeup.

Even something that periodically wakes to do sensing is entirely doable on a 2032 if you don't need low latency - the biggest factor is how long you need to stay awake. There are  plenty of MCUs with suitably low standby current.

[mikeselectricstuff]

Might help  if we had more info about the application - I'm guessing a touch probe for cnc?
 
Inductive proximity sensing might be an option - inverse-square law helps here, and this can be done with extremely low power draw - ping a tuned circuit and count the number of rings. I used this many years ago on a PIC for sensing metal vanes in a water meter, this drew something like 5uA avarage waking a few times a second

May be doable with off-the-shelf SMD inductors.

If you can use the contact event as opposed to proximity, peizo may be worth a look. or an accelerometer.

For physical contact, gold on both sides is probably the  only  reliable option. all the cheap ball-based motion sensors I've looked at use gold contacts and a gold-plated ball

Also think about whether you can be clever about power saving - e.g. in the case of a CNC touch probe, you can use motion ( accelerometer) to go to sleep when it hasn't moved for a few mins.

[moffy]

One last thought, if you had the contact point as a piece of piezo material you can get reasonable voltage output for low levels of deflection. That way you wouldn't need an electrical contact, you would need a solid stop to prevent damage to the piezo material.

[mikeselectricstuff]

Looking back at the first post, .0076mm is getting into the world of surface electrostatic effects, thermal expansion etc. I can't see physical electrical contact working for this except maybe in combination with some kind of lever to amplify the movement.

Simple contact with a piezo may have issues with hysteresis ang ageing, but maybe a piezo which is actively driven at its resonant frequency, and sense damping when it makes contact.

[ajb]

I am not aware of the contact pads. They sound interesting. Can you provide any further details: who makes them, what is a good search term for them, etc?

They're often listed alongside pogo pins and other spring-loaded contacts, since they're meant to be used together. Hopefully this search link will work: https://www.digikey.com/en/products/filter/contacts/contacts-spring-loaded-pogo-pins-and-pressure/311?s=N4IgjCBcoKxgTFUBjKAzAhgGwM4FMAaEAeygG0QBma%2BANgAYQBdIgBwBcoQBldgJwCWAOwDmIAL6SgA

Pairing one of those with a solid domed pin (example) may provide a better-defined contact point vs two flat pads, at the expense of more deflection of the contact due to higher contact pressure. 

Looking back at the first post, .0076mm is getting into the world of surface electrostatic effects, thermal expansion etc. I can't see physical electrical contact working for this except maybe in combination with some kind of lever to amplify the movement.

 

Those and several other challenges have to be factored into the mechanism, materials, and manufacturing choices for sure.  OP had a previous thread about a CNC toolsetter where some of this was discussed.  But it is doable with a well-designed and well-made mechanism -- commercially made CNC probe systems (eg Renishaw) claim repeatability down to <1μm for mechanical contact systems. 

[Conrad Hoffman]

Non-wiping switch contacts seem like a reliability problem unless both contacts are gold plated. You can make a very accurate hall effect sensor with (duh!) a hall effect device and two neo magnets. Place the magnets side-by-side with the fields opposing. This creates a very narrow transition region so the hall effect device switches suddenly. Little neo magnets are cheap but I haven't priced simple hall effect devices lately.

[mikeselectricstuff]

commercially made CNC probe systems (eg Renishaw) claim repeatability down to <1μm for mechanical contact systems.

How do they work? Actual contacts or something else like piezo or inductive  ( shuffles off to ebay to look for a broken one to take apart....)

[ajb]

commercially made CNC probe systems (eg Renishaw) claim repeatability down to <1μm for mechanical contact systems.

How do they work? Actual contacts or something else like piezo or inductive  ( shuffles off to ebay to look for a broken one to take apart....)

There are probes used for surface scanning that use capacitive or strain gauge mechanism for analog outputs (more useful on CMMs than CNC machines), but general purpose toolsetter and work probes are almost always mechanical contacts. This sort of mechanism seems to be what everyone does, but there may be other solutions out there: https://youtu.be/vJdcbhGa7Jk https://youtu.be/LXLAGprt3Is

[thm_w]

So OP are you building a tool setter? Why not link the previous thread for context.

There are tons of discussions and testing related to sensing endstops:
https://www.cnczone.com/forums/mechanical-calculations-engineering-design/149985-forum.html
youtube.com/watch?v=il9bNWn66BY
youtube.com/watch?v=2o8woJfTEVc
youtube.com/watch?v=XLa5PICnxpg

If you want to do better than that, as mentioned, you'd want some kind of mechanical advantage linking into a switch or sensor (opto, hall, whatever).
You don't need to buy a $12 part from digikey, you can use a 50c microcontroller to do capacitive or inductive sensing.

[moffy]

According to the second video thm_w posted:
at the 11:43 time mark a simple inexpensive microswitch had a standard deviation for repeatability of about 1um, problem solved.