PS We just did an exercise where a PCB was screwed to a large heatsink
with lots of differing thickness heat pads.
We used a paste height measuring system to show that the board was being
bent by incorrectly specified heat pad thicknesses.
Rex
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Rex Waygood
Sent: 19 April 2010 16:30
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.

RL 

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