I have conducted a test where I already found out the temperature rise of the pcb traces in question. The product has a relay output accessible by the user on an output terminal block. The relay contacts are routed from the actual PCB pad (where the through hole relay is soldered) to the terminal block via PCB trace or leadwires. The test I conducted was done at starting temperature of ~27°C (room temperature). Results of test

The problem I have is deciding the allowable temperature rise for PCB traces. From UL 508, there is table 43.1 that states the max temperature rises of different components/materials. However, it says that table 43.1 is for devices with max operating temp of 40 °C. enter image description here

Our product's stated operating temperature range is -10 to 50 °C for reference. 1st question: Is the rated temperature of our product that maximum value on its operating range (50 °C)?

Going back to the UL file, there is a section saying: enter image description here

So using rated temperature of our product as 50 °C (assuming this is correct, see 1st question above), then I would use the formula in 43.6. I get the following:


Using table 43.1 and the row for printed wiring boards, I go to comment 'n'.

enter image description here 2nd question: What is the operating temperature of the board? Is it based on material (FR-4) or based on the rated temperature of the product as a whole (50 °C)? Using 50°C, I get Tt = 50-40 = 10°C.

Now I only need Tm (Elevated ambient temperature marked on equipment). The pdf file points to 62.1.1. enter image description here It appears that Tm = 50 °C in my case? (max operating temp. of product) So finally, the formula becomes: Tr=Tt-(Tm-40)

Tr=10-(50-40) = 0, telling me my allowable temperature rise is 0°C. I believe I messed up somewhere on calculation (or maybe wrong interpretation of variables). Can anyone help me out?

Table 43.1 enter image description here

  • \$\begingroup\$ My first correction would be on my 2nd question. I was thinking that the maximum operating temperature of the board is based on FR-4 material OR based on the components soldered into the board? If this is the case, I get non-zero temperature rise on the equation since the variable Tt becomes much higher in the formula. Say I use 110 °C (FR4) for max temperature of board, then Tt = 110 - 40 = 70. Then the final formula becomes: Tr = Tt- (Tm-40) = 70 - (50-40) = 60°C allowable temperature rise. \$\endgroup\$
    – user139731
    Commented Dec 6, 2021 at 0:27

1 Answer 1


If the permitted trace temp rise is 20'C above 40'C and your Ta= 50'C then your design must decrease temp. rise in the conductors <10'C. Consider thermal coupling on adjacent traces as local heating, which increases the internal ambient and factor thermal resistance of the enclosure as well.

Therefore you must redo your design to accommodate much wider and thicker traces.

Stovetop relay PCB designs often flood the top surface of the PCB conductor area with thick solder as a cheap solution over a large copper area.

Relay MTBF drops significantly with internal ambient and also from arc energy or temp rise on the contacts on opening from oxidation.


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