To what temperature should the PCB be limitted?

[EngIntoHW]

I got a voltage divider that comprises a thermistor and connects to an ADC, and by that I measure the temperature of the PCB.

When the PCB gets hot, it means that too much current is flowing through it, and at that point, I'd like to switch the relay in order to stop the current from flowing through the PCB.

This PCB is for commercial use (not industrial use) - meaning it will be used in residents and offices.

What should be the max PCB's temperature allowed?

I read in Wiki: "Commercial grade: 0 °C to 70°C (sometimes −10 °C to 70 °C) "

Does it mean that max PCB's temperature allowed is 70°C?

 

[crutschow]

Generally the maximum temperature refers to the ambient or case temperature of the devices on the board. For operating margin and good board life, you should probably not allow the board temperature to go above about 60°C.

 

[EngIntoHW]

Thanks Crutschow!

Where did you get the required info about the temperature?

I was wondering how I should have looked for that.

 

[colin55]

Obviously try to provide the lowest temperature possible. I have had half-watt (high temp) resistors destroy the fabric of the board in less than 3 months. If you cannot hold your finger on the board, it is getting too hot. Many high temp components use the tracks of the board as heatsinks. This is why the board gets charred.

 

[gelbschnee]

Usually in electronics 60C is the peak for safe operating temperature, however, that is very undesireable. If you can keep your board below 40C you will have pretty good life.

Do you know what your temps are currently?

 

[ronv]

https://www.electro-tech-online.com/custompdfs/2010/08/Phenolic_G10_FR4.pdf

They quote 284 F. That is for the board material.
If you are talking about the parts on the board they will have various maximum specs. For example a power resistor will be higher than an op amp. If you are making a product you get to specify operating temperature. Usually you do this before you design the hardware so you can choose the correct components for that temperature. What are you protecting against? Ambient temperature? Short circuit?

 

[EngIntoHW]

Usually in electronics 60C is the peak for safe operating temperature, however, that is very undesireable. If you can keep your board below 40C you will have pretty good life.

Do you know what your temps are currently?

Hey,

Currently, temperature doesn't reach over 45 Celsius (113 Fahrenheit) degrees.

Where did you get the 60 (or 70) Celsius degrees number from?
I'd really like to understand where it comes from.

Thank you

 

[ronv]

You shouldn't back into it that way. Lets say you have a TO220 device dissipating 2 watts. With a thermal resistance of 62C per watt junction to ambient at 45C you have a junction temperature of 124 plus 45 or 169C. At 70C ambient you have 194C, which is out of spec for most components. The 45 C may be ok, but you should check the temperature of the components to make sure they are not to hot or going to be to hot if the outside ambient goes up.