(IMHO: True. I have done similar and found this is an excellent technique. This is because it evaluates the actual efficiency of the stack (and so includes transmission across the interfaces) by a direct measurement of the junction temperature of the device, which is what you want to know/control. Bulk conductivity of a TIM material is only part of the story and so is not necessarily an indicator of actual performance in use. If you just look at the TIM's conductivity, you ignore completely transmission from the TIM interfaces in actual applications. Regards Mike -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Rex Waygood Sent: Monday, April 19, 2010 4:30 PM To: [log in to unmask] Subject: Re: [TN] Best way to measure "actual" themal pad compression To me the gap is not the important issue unless it is HV related. The requirement is to determine the thermal resistance of the device, pad and heat sink. I have used this method in the past. Calibrate the forward volt drop of a diode somewhere on the device mV/degC by sticking it in an oven and driving it with a constant current. Arrange a circuit such that the power is dissipated in the device but for a brief period the power is removed and the forward volt drop can be measured as per your calibration method. Mount the device on the heatsink with pad. Pick a suitable mark space ratio to get the power in and your measurement made. This will give you Tj very accurately provided you have the right equipment and do you arithmetic right. You can plot Tj vs. power to make sure everything is OK. This allows you to do your reliability predictions using Tj with a bit of confidence. I learnt this technique a long time ago for a military job where the heatsink was a nice constant 120C. Thermal resistance was quite important! Regards Rex Rex Waygood Technical Manager Hansatech EMS provides value manufacturing through engineering and quality Hansatech EMS Limited Benson Road Nuffield Industrial Estate Poole Dorset BH17 0RY +44 (0)1202 338200 -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Technet Legg Sent: 19 April 2010 13:58 To: [log in to unmask] Subject: Re: [TN] Best way to measure "actual" themal pad compression Probably too late to be relevant to the original poster, but; If the issue is distance between surfaces of the thermal interface, then a capacitive measurement is probably the simplest method. This often avoids the question of real dissipator surface area as well. 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