ESD clamping voltage in SIMCard interface too high

Question

I planned to use SIM868 and what makes me confused all this time while choosing ESD protection is the voltage after clamping the transient voltage, it is still way too high especially when looking at ESD clamping characteristics graph. But first time I wanna tell you what have I done to solve this issue (indeed it still not solved yet).
According to SIM868 hardware design v1.07 page 37 they also include the recommendation part number for ESD protection, which is ESDA6V1-5W6 from ST or SMF05C from Onsemi.
After looking both datasheets, here is some basic information :

ESDA6V1-5W6 : Vrwm 3V, Vbr min 6.1V, Vcl @ 8A is around 12V

SMF05C : Vrwm 5V, Vbr min 6.2V, Vcl @ 8A is around 12.5V

Unfortunately only ESDA6V1-5W6 that has ESD response graph or maybe I can call it ESD clamping characteristics, the test is using IEC61000-4-2 air discharge +15 kV. The chart is in page 5 of ESDA6V1-5W6 datasheet, here is the screenshot :

I am pretty sure it is 5V/div so it makes around 15V at the beginning of pulse and then stay at around 6V.
In that case, the voltage that will SIMCard interface see is 15V max and I think it is too way high for the interface.
Later on I also looking for absolute maximum rating for SIM868 and found it in hardware design v1.07 page 58 but there is no information about the maximum voltage that allowed to come in to SIMCard interface.

After that I looking for internal ESD protection of SIM868 and found it in hardware design v1.07 page 60, and again, there is no information about maximum voltage for SIMCard interface.

I think this is little bit the same with other data line ESD protection, maybe like I2C or USB but they need more small parasitic capacitance, so I use the search bar in Electronics StackExchange and found some answer.

Using some series resistor, it will limit the current when overvoltage is occurs
Using bypass capacitor to eat the remaining energy after clamping
Using additional zener diode and series resistor

I believe all the advice above are working, but I think when it used to protect the data line I need to consider more about signal integrity, and I believe the best way for keeping the signal integrity is only using one protection.
So after all I wanna ask, is that OK if in such short time the SIMCard interface is exposed with more than its data line voltage level? (around 3V for SIMCard and exposed by more than 10V while ESD occurs)
For another ESD protection candidate that has lower clamping voltage I will choose PUSB3FR4 from Nexperia, but I little bit not sure about snapback voltage that can be as low as 1.5V, what do you think?
If you have any resource like article or some similar question, you can post it here, that will be a Great new information for me.
Please correct me if I wrong, Thank You.

mkeith · Answer

ESD diodes will dramatically reduce the effect of ESD pulses. You are right that the clamping voltage is still pretty high, but that is under specific test conditions that may not apply to your design.
What you should do is put in the ESD diodes, as recommended. For any data lines, you can add some series resistance (start with 0 Ohms) and maybe a capacitor, too right at the input to your IC, and then build and test.
If you pass ESD testing with no capacitor and a zero ohm resistors, great. But if you don't pass, then you have the option to put in the additional filter elements, and hopefully signal integrity will still be OK.
For the RF input and output signals, do not improvise. Do it EXACTLY the way the manufacturer recommends. No extra components, and no substitutions. The reason for this is that RF inputs are very sensitive to any change in impedance. You have to implement that part of the circuit as recommended, or you risk seriously degrading the performance of the GNSS system.

ESD clamping voltage in SIMCard interface too high

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