What is reasoning behind the common wisdom of limiting the temperature rise of a PCB trace to 5/10/20°C?

Question

When deciding on the trace thickness required to carry a certain amount of current on a PCB, the answer depends on how much temperature rise you are willing to accept. This leads the designer into the difficult situation of trying to decide how much temperature rise is reasonable. Common rules of thumb are to allow no more than 5°C, 10°C or 20°C of temperature rise, depending on how conservative you want to be. These figures seem remarkably small compared to the maximum temperature rises of power transistors, ICs, power resistors, or other heat-dissipating components, which may be 60+°C. What is the reasoning behind these numbers?

Possible reasons I've thought of:

• Maximum temperature of PCB materials. For most FR4-type materials this is around 130°C. Even allowing for a very conservative ambient temperature (inside the chassic) of 65°C, this would still allow another 65°C temperature rise.
• Allowing for further temperature rises of components. If an SMT MOSFET was going to see a temperature rise of 80°C for example, you wouldn't want to start it out 40°C above ambient because of the temperature of the surrounding PCB. However, this seems far too situation-specific to make for a rule of thumb. In the case of a heat-sunk through-hole MOSFET, for example, the heat flow up the leads is a fraction of the heat flow out through the heat sink, so the PCB temperature should not be a major concern. Even with SMT parts, I could have a thin trace which dissapates a lot of heat for the majority of its length, but then widen that trace before it reaches the component.
• Thermal expansion of PCB materials. As the PCB heats up, the materials will expand. If different parts of the PCB are exposed to different amounts of heat, this could cause flexing of the board which could crack solder joints. However, given that PCBs are regularly exposed to higher temperature differentials than this due to power dissipation in the components mounted to them, this doesn't seem like the answer.
• Outdated standards. Perhaps the 5/10/20°C limits were thought up years ago and no longer apply to modern PCB materials, but everyone has kept on following them without thinking about it. For example, perhaps old phenolic board materials were less tolerant of heat than modern fiberglass.

To put the question another way, say I find that a 20°C temperature rise is too limiting for my design. If I decide instead to allow a 40°C temperature rise, am I likely to run into either short-term or long-term reliability problems?

Bonus points to anybody who can cite standards that give a reasoning for the numbers, or who have historical evidence for why those numbers were picked.

Answer 1

Many things go into designing the width of the PCB trace, including temperature rise for current. Others are voltage drop, impedance, PCB fab capability, cost, packing density.

However, temperature rise is rightly one of the 'do not exceed' specifications.

A rule of thumb is just that, something you should follow most of the time. You will always be able to find edge cases where are higher rise is permitted, if you do careful calculations.

Part of the benefit of a rule of thumb is that if you follow it, your calculations don't have to be too careful, there's already a big margin of error built into the rule.

A peculiarity of temperature rise is that it's proportional to current squared, not just to the current. This reduces the importance of choosing one specific value. The current that gives a 20C rise is not twice the 10C rise current, it's only 1.4x the 10C rise current. If we double the 10C rise current, we get a 40C rise, which is starting to feel uncomfortably warm.

Why run a board cool? All sorts of good reasons. Component cooling requires a low ambient. Component life drops very quickly as temperature rises. Margin for operating in warm places (inside a car cabin in bright sunlight) is good. Debugging, run your finger over the circuit to find toasty components, you'll get confused by hot traces.

There is no one killer reason to run a board cool, and no one reason to choose 10C rise versus 20C rise. However, few designers feel inhibited by following this 'rule'. It's rarely the thing that sets the limit. If we do find ourselves in some corner case where the specification cannot be achieved by sticking to some arbitrary temperature rise figure, then we calculate and test the heck out of everything, to see what effect on lifetime and cooling higher temperatures will cause.

Answer 2

laminar copper isn't pretested for resistivity as it develops different temperature ranges it will show a non linear resistivity course. such a temperatures are stated in order the max temperature achieves the final thermal adequation of the layers leaving the original pcb diagrams for the very long term use...