Calculating PCB temperature increase

Question

so I have created a PCB with a lot of components on it, its a rather large PCB, roughly 18cm x 19 cm. A lot of the components' datasheets (things like Resistors, Switches, capacitors, relays, CAN controller/ transceiver) don't mention the thermal resistance of the components, nor does it include the power dissipation. Does anyone have suggestions to work around this? or generalizations / rules to estimate how much a PCB will heat up.

If it helps, some more information about my board, Board lays horizontally, 1.6mm FR4 board

3oz/ft^2 thickness, 2 traces run 30A, 3 traces run 15A and 6 traces run 10A. They just run to a connector going off the board, only running through current sensor and relay. But I am more interested for learning purposes, how thermal changed can be estimated with limited data?Also, do diodes heat up a lot typically and is it a concern for board temperature? because I have 9 small diodes it says their Thermal resistance is like 1000C/W 100mW dissipation

Answer 1

The power dissipation of a resistor is totally dependent on your circuit. Similarly with diodes.

Your question asks about "how much the PCB will heat up" which is a different question from "how hot will my semiconductor's junction get above the PCB temperature".

You can treat the PCB as a flat sheet of material and if you know the total power consumption and other sources of dissipation on the PCB (like those hot traces) you can estimate the temperature rise above ambient assuming natural convection or forced convection or whatever your actual situation is. To get a really accurate number you might have to prototype it or build a mockup, including the enclosure.

Your heat sources like a diode junction will heat above that PCB temperature based on the thermal resistance from junction to case. That may be worth calculating in some cases.

Practically speaking, usually only a few sources account for the bulk of the heat dissipation and only a few of those are worth looking very closely at.

One way to get a handle on the dissipation of individual components is to run simulations, using a program such as (free) LTspice, which will show you the typical dissipation of a part even with a complex waveform. For example, a power rectifier that is passing "spikey" current will have a higher power dissipation than you might estimate from the average current because there's a resistive component to the diode.

If it's just DC like a series resistor you can easily calculate it by hand, or even roughly estimate it in your head, of course. A 10K pullup on an input dissipates only 2.5mW so it can usually be ignored for thermal design under normal atmospheric conditions.

Answer 2

You should know the supply voltage and you can measure the current. Then you can calculate the power dissipated by the board from P = VI.

Temperature will stabilise when heat lost to ambient = power into board.

Answer 3

Electrical insulators are often also Thermal insulators such as Epoxy, and FR4 and air.

Thermal Resistance of anything in a network is much like electrical resistance, so an understanding of Ohm's Law helps estimate temp rise for each medium interface to the outside air.

Average power consumption by design should not exceed 50% of it's max rated to avoid the 100 deg C temp rise of parts. Saturn PCB design.exe (free ) can tell you about conductor temp. rise.

Forced air velocity reduces thermal resistance according to surface area and air velocity at the hot surface. Convection free air can also reduce temp rise. A max of 85'C case temps are considered prudent at max. ambient temp. but for caps, reliability improves greatly with lower temp rise.