ESD Blues, need suggestion please

Four general ESD immunity improvement techniques:

1. Insulate wherever possible.
2. Provide alternative low-inductance, low-resistance paths for ESD discharge
3. RC and LC filtering to attenuate the fast-risetime ESD events.
4. Tranzorb, MOV, gas tube, transil, and other non-linear breakover devices to limit peak voltages.

For LCD modules, there are wire grid and thinfilm overlays to use with technique #2. For low-speed signals, there are many opportunities to use combinations of series resistance, inductance, and shunt capacitance to reduce the amplitudes of otherwise disruptive spikes. Combinations of all 4 techniques will likely be necessary at 28KV. #2 is normally the most effective and most often neglected. Paul Mathews

Jacques St-Pierre wrote:

1. Use a metallic enclosure !

Graham

Think about it: #5 is redundant with #2, just one type of implementation Paul

I think he said there was some conductive coating but maybe not a very good one ?

You can get 'spray on nickel' coatings IIRC. That's about as good as it gets before you're into vacuum depostion.

I think his cable is also unshielded.

Talk about asking for trouble !

Graham

A matter of the plastic case. Make it conductive on the inside.

All wires going in and out need caps to short pulses that are not intended. Meaning 100nF 1206 over the power cable, 1nF to 10nF (1206 or so) from the communication wires to GND.

Rene

The unshield cable was not my idea, I always been again it, but you know, those who pay often change thing ;(

Anyway, We are working on a unit to correct some mistake, the new cable will be braid shielded, both section of the case will be attatch with a copper braid, the shield will be tie to frame conductive paint coat and we will add an filter at the end of the cable before getting into the system casing.

If this did not do it, next step will be changing case for a matel one.

Bye Jacques

I would be very interested in your results, if you would be so kind as to post them.

Thanks, John

We replace the unshielded cable by a Belden 8424 and make sure shield is well tag to case inside paint. We use the shield to connect the case to earth ground.

We discover that the conductive paint did not work well in corner. Case as plastic stud for screws, but paint crack at the base of the stud and conductivity his poor, 30? in some case. So we make sure our grounds are tag to case point, not the stud.

We add a ground wire between both case sections, this preventing the connection via the poor stud resistance.

This make a major difference, we are no longer able to crash the unit via the case and cable up to 27Kv.

We add a 1/16" clear plastic plate between the LCD and case, to make sure any zap can not reach the LCD metal frame. So this way, the zap ends up on the case paint instead of the LCD frame. This resolves the situation with the LCD.

We are still working on a solution for the switches. For now we can blow the unit zapping in the switches. We try different switches without success. We are waiting for a new PCB to support the switch on witch I add a ground plane and high pass in the hope of resolving the problem. More to come.

We try a membrane keypad instead of the switch. It works fine if we protect the side of the keypad. This approach will require a different case to work, but it's a proven avenue. It will work; we just have to make sure the keypad edges are inside the case to be sure the zap end up on the frame instead of the keypad trace. We can not zap the plastic keypad, just its edge. It's not a favored approach because of the high cost in low production rate. And we will need many different keypad, we have to get it in different language, not very practical.

Bye

Jacques

I'm always interested in the solutions to problems. Thanks for the report, Jacques.

Cheers, John

We finally receive and assemble the new PCB for the switches/LEDs. The new PCB as better frame ground since the top side is now a ground plane. I also add high pass on each data line just before going out of the PCB.

Results was not what anticipated, it help a bit, using that new PCB we have not been able to blow up the switches circuit, but we still able to blow the LEDs drivers. So this was a partial success.

So the next action as been to add a metal plate, punches to fit the switches perfectly and protect them from the zap. We install that plate on top of the case instead of the normal layout. We make sure that the plate was well attached to frame ground. This resolve the problem, using that plate, protect the switches and LEDs and we not have been able to blow any drivers. The unit is now stable even with zap up to 27Kv.

In the mean time, we also try to put a plastic sheet over the switches, like a skin. Even if we use a plastic not very thick, we have to operate the switches trough it, it protect enough to prevent blowing drivers.

So this complete our tests, we will now implement all the modifications, next will be field tests.

Bye

Jacques