When sizing PCB traces for current carrying capacity, designers are often asked to choose an allowable temperature rise, commonly 5°C, 10°C, or 20°C.
What is the actual engineering basis for these commonly used limits? If a design allows a 40°C trace temperature rise instead of 20°C, what reliability concerns would you expect in practice?
The amount of allowed temperture rise will be related to what the intended operating temperture range is. Generally it is considered that the product starts out at room temperture of ~20C. How far you allow the temperture to rise will be based on circuit board materials, product intended operating range, and component operating temperture operating ranges.
If you allow it to get too hot, components can operate beyond specified limits or operational failures, solder connection failures, or PCB material failures which in turn causes the product to fail. PCB material failures can include cracked copper or solder due to excessive Z expansion, copper blown off the board, burned dielectrics, or delamination.
You want to avoid excessive temperture rise as that only leads to undesired results.
One thing worth noting is that trace temperature rise scales approximately with the square of the current. The difference between a 10°C and 20°C design target is smaller than it first appears — the allowable current only increases by about √2, not 2×.
That may be one reason these limits are treated as design guidelines rather than hard thresholds. Choosing a lower temperature rise gives additional margin for manufacturing variation, ambient temperature changes, and assumptions in the thermal model, without requiring dramatically wider traces in many cases.