W/mK stands for Watts per meter-Kelvin. It’s also known as ‘k Value’. The comparison of thermal conductivity can be measured by the ‘k’ value. The k value, or Thermal Conductivity, specifies the rate of heat transfer in any homogeneous material. If a material has a k value of 1, it means a 1m cube of material will transfer heat at a rate of 1 watt for every degree of temperature difference between opposite faces. The k value is expressed as 1 W/mK. The lower this value is, the less heat the material will transfer.
This can be very helpful in selecting the thermal materials you might need when attaching a heatsink to a component and is a specification filter to help narrow the options for Thermal pads.
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is it about 1m cube or 1 meter thickness?
It’s defined as 1m cube for consistency (more surface area than 1 sqm or thinner than 1m and it will have better apparent thermal conductivity). Obviously there is no pad 1m thick so you will need to do some math for your exact size and thickness. Ex: A pad material has 21w/mK thermal conductivity. If it’s 1mm thick with an area of 100x100mm, .01sqm, It will take 210W of heat to cause a temp rise on the hot side of 1K or C (assuming an infinite heat sink attached, there’s other thermal resistances besides just the pad). W/mK is just the reciprocal of C/W (easier to calcuate temp rise with when adding up those other thermal resistances) so 21W/mK = 0.048 C/W = 0.0048C/W for the example pad. Higher end CPU heatsinks/fans may have around 0.1 C/W to add to that, need lower = either a bigger sink or have to upgrade to waterblocks or coldplates when that gets to be the cheaper option or size gets too big.
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