Under normal conditions operating on the arms of a multirotor, ESC temperature should be well-controlled by the airflow passing over them from the propeller exhaust. It isn't a good sign that your ESCs are reaching dramatically higher temperatures like 70° C, given that you should be expecting to see ~30-40° C on a regular basis. One of three things is likely to be happening here:
- The ESCs are undersized for the motors/propellers you have them driving
- The ESCs are broken and showing signs of wear
- The design of the ESCs is bad
- Your PID/filter tuning settings are bad
Without knowing the exact details of your multirotor setup, it could be the first case due to the ease of accidentally picking too small an ESC to pair with power-thirsty motors and propellers. If this is true, then you should replace the ESCs with properly sized units, because there is a possibility that the overworked ESCs could catch fire or spontaneously die on you. (not to mention the fact that the efficency of the ESC will be decreased at higher temperatures)
However, the likely causes change because you're running a smaller multirotor, as @QuadMcFly said in his comment below: (emphasis mine)
I would add for a 5" inch drone it is extremely unlikely that the issue is related to the ESC. I have run even tiny 20A ESCs on some ridiculously overpowered setups and they come down cool when tuned properly. In the vast majority of cases, hot ESCs come from bad tuning or bad hardware, either in the FC or the ESC. The first place I would look is at flight logs of the gyro and motor output data. That will tell you right away if it's a tuning issue.
Mitigation of ESC Related Thermal Issues
The heat you see generated by the ESCs is likely related to the power losses in the MOSFETs (metal–oxide–semiconductor field-effect transistors) used by most all ESCs to switch the power and off between the three phases of the BLDC motor. There are calculable power losses that occur while the MOSFET is conducting electricity and when it switches state between on and off, both of which contribute to heat generation.
The most direct solution to the issue is to add a decently-sized heatsink to the top surface of the MOSFET transistors and capacitors on the ESCs. You'd likely have to remove the plastic insulation for the best heat transfer between the MOSFET casings and heatsink.
Something like this, although with the insulation removed:
When you re-affix the ESC to the frame arm, it would be best if you also put down some kind of electrical insulation like electrical tape on the surface of the arm between it and the ESC so that there isn't a risk of the frame shorting out an ESC component.