Thermal conductivity calculations and dimensions usually end up being pretty precise. However, it is important to remember that materials will always have some tolerance in the manufacturing process. Thickness does impact thermal conductivity to a degree (pun half intended).
Example Part Number
1944-1050-ND (62-08-0912-G974) is a thermal pad that measures 228.6mm by 304.8mm and the nominal thickness is 2.032mm. The base material is silicone and the overall thermal conductivity is 5W/mK. The manufacturer has informed us the expected range for the thickness is between 1.829mm and 2.235mm. This is a +/-10% tolerance, approximately. This might seem like a lot, but in terms of conduction, this is minimal.
Thermal Conduction Calculation
Let’s say we cut down the pad size to an area of 0.9mm2. If we assume 10W is the amount of energy passing through the material and the hot side temperature measures 30°C, the other side will be about 25.484°C according to the equation below. This result is based on the nominal thickness of 2.032mm.
If you change the thickness to the minimum value of 1.829mm, the temperature increases to 25.936°C. If you increase the thickness to the maximum value of 2.235mm, the temperature decreases to 25.033°C. Factors like tolerance may want to be addressed, but usually the difference is miniscule. It all depends on the application. Very rarely are tolerances called out on thermal pad datasheets due to the minimal change in conductivity.
Brief Excel Data and Resources
If you want to check the calculations, I have attached the Excel sheet below. I ran three different wattages for an example.
ConductionExample.xlsx (9.5 KB)
This site helped me figure out how to format the calculations.