ESD protection on keypad VS main PCB?

[ricko_uk]

Hi,
 am wondering where it would be best to place the ESD protection: on the main controller PCB or the keypad PCB. The keypad PCB contains tactile switches (exposed to the user) and a few other components. It is connected to the main PCB solely through a ribbon cable, which also provides power.

Normally I would place the protection at the source (keypad PCB). However, considering that the LEDs (behind a thin plastic film layer) on the keypad PCB can also be touched, there might be some induced "secondary" ESD. In that case, could it be more effective to place the protection on the main PCB instead?

From a purely ESD performance perspective, is it better to place the TVS diode on the keypad PCB (i.e., at the source) or on the main PCB?

Thank you

[thm_w]

If you already have SMD parts on the keypad PCB, to me it makes sense to put protection there. But its not clear if its usual membrane keypad or some kind of mixed PCB solution.

Is the enclosure metal or plastic?
If metal is there a connection from the keypad PCB to enclosure?

https://www.jrpanel.com/engineerhome/keyword/JRPanel_ESD_Protection_in_a_Membrane_Switch.html
https://lineproindia.com/blog/why-emi-esd-shielding-is-important-in-a-membrane-keyboard/

[ricko_uk]

Thank you thm_w,

Interesting links.

The enclosure is mostly metal but the keypad is mounted on a plastic support which then mounts onto the metal. Could you please elaborate on such setup?

Thank you

[Benta]

What's the protection supposed to protect?
In this case the controller, and in that case it should be placed there.
You're probably thinking about actuators (relays, motors, etc.), but what's placed there is not protection, but snubber devices.
Your controller would still need local protection on it's outputs (mostly inherent in the output drivers).

[srb1954]

Hi,
 am wondering where it would be best to place the ESD protection: on the main controller PCB or the keypad PCB. The keypad PCB contains tactile switches (exposed to the user) and a few other components. It is connected to the main PCB solely through a ribbon cable, which also provides power.

Normally I would place the protection at the source (keypad PCB). However, considering that the LEDs (behind a thin plastic film layer) on the keypad PCB can also be touched, there might be some induced "secondary" ESD. In that case, could it be more effective to place the protection on the main PCB instead?

From a purely ESD performance perspective, is it better to place the TVS diode on the keypad PCB (i.e., at the source) or on the main PCB?

Does the keypad have any ground connection apart from via the ribbon cable? If you only have the ribbon cable as the ground connection the effectiveness of any ESD protection placed on the keypad will be compromised by the inductance of the ribbon cable. This will develop excessive ground voltages due to the very fast rise time of the ESD current pulses. There would also be coupling within the ribbon cable producing spikes on other signal lines. In which case you would still need to provide transient protection on all lines onto the main PCB.

The most effective approach would be to have a connection direct from the keypad ESD protection devices to an earthed metal case so that ESD currents could be diverted away before they can get into the main PCB.

[Manul]

ESD protection should be placed near a device to be protected and share a low impedance path (usually ground) with that device.

In your example, ribbon cable has inductance, so main controller and keypad PCB does not share a common, low impedance ground. In the event of ESD on the keypad panel, all the potentials - signal lines, supply and "ground" will experience a voltage transient (relative to controller ground). So you clamped the ESD to "ground", but that "ground" itself might have shifted by 100V, thus rendering it not very effective from a controllers perspective. Better than nothing, but not optimal. ESD strikes are very fast events, nanosecond time scale. Even relatively low inductances present quite high impedance.

It's a perhaps a little similar to placing bypass capacitors. We have been told to place capacitors near supply pins of components. And the reason is that we need to minimize loop inductance. Bypass capacitor purpose is to present low impedance voltage supply point referenced to device ground. In case of fast current transients parasitic inductance must be low, otherwise it presents impedance which renders capacitors ineffective. I have seen people placing bypass capacitors like 5cm away from digital IC. In that case they could just remove it, it pretty much does nothing.

[ricko_uk]

Thank you all,
yes it makes sense to keep it on the main PCB.

Thank you

[xvr]

yes it makes sense to keep it on the main PCB.

As already said 'ESD protection should be placed near a device to be protected'. So it should be placed at main PCB, and, if keypad includes any ESD sensitive ICs it ALSO should be placed on keypad PCB

[ricko_uk]

Yes, the only other IC on the keypad already has its internal ESD protection.

Thank you