ESD protection for signals routed through multiple boards

Question

I have a few questions regarding ESD protection:

1. If a particular signal is routed through multiple boards (through connectors), do we need to place ESD diodes at each port (connectors)? For example, if a keypad button is on a separate board and then the signal is routed through connector on the board to another board, do we need to add ESD protection at both the ports (Considering connectors also have certain current ratings)? Another example can be let us say that there is a sensor connected to the main controller board over I2C bus, does this need ESD protection on the sensor board and the main controller board (as both the boards are connected through connectors on both boards)?

2. Now, what if these connectors are not exposed to the user at all in the product, do we still need to add ESD protection (considering these ports will be handled and assembled during manufacturing and maybe sometimes very rarely to service). Does this change the answer for question 1?

3. What if the same signal lines are exposed through 2 different ports/connectors. Would a single ESD for each signal close to both connectors suffice?

4. Adding multiple ESD protections all increases capacitance on the lines, so need to check that the combined capacitance does not impact the signal (at a particular speed). Am I right?

5. Do we need to add ESD protection for antenna input as well. How important would this be compared to digital signals? The reason I am this question is because, I have seen some products which are CE, FCC certified without ESD protection on the antenna input (GPS, Cellular, BLE). I know that there are some very low capacitance ESD from Nexperia and other such companies for this purpose. In reality, how important is this?

Answer 1

1. It would be unreasonable to place ESD protection everywhere. Placing ESD protection on a connector to an internal board that does not expect any user interaction that is behind another board that already has ESD protection for its exposed controls/connectors, is like placing ESD protection around a chip in the middle of your PCB.

2. Do you expect manhandling during assembly? Hopefully not because technically everything is handled at some point but you can only do so much and those performing assembly should have ESD measures in place, but I've seen concerns before about untrained staff in the final stage of assembly connecting boards together. Or during servicing? I've heard of concerns about techs doing field maintenance where they do not have the ability or wherewithal for proper ESD measures.

3. This depends. You want to maximize the distance of the ESD protection from the vulnerable components. You also want to stop the ESD before it propagates too far since it can radiate along the way and cause issues.

4. Yes.

5. I don't know about antennas inputs. I imagine they are very sensitive so I don't know how ESD is dealt for them, if at all.