[ltp] Thermal pads and GPU cooling

[Richard Neill]

Dear All,

I think I may have got to the root of why my T60p has the fan on all the 
time, even when idle.

1. I've already replaced the fansink unit with a new one (fan bearings 
were failing), having cleaned the thermal bonding surfaces. The new 
fansink unit is identical to the old one:
   - it has a very thin layer of thermal paste between the CPU and the 
copper plate (with pressure applied by screws)
   - it uses a 1mm thick "thermal pad" onto the northbridge, (with 
slight pressure)
   - it uses a 2mm thick thermal pad onto the GPU, (with slight 
pressure), misused (imho) for gap-filling.

2. The result is that, when idling, the CPU (sensor 0) sits at 44 
degrees, but the GPU (sensor 3) sits at 78 degrees, requiring the fan.

3. Some specs for this type of thermal pad are here:
   http://www.farnell.com/datasheets/530848.pdf
Thermal conductivity is about 5 W/m/K.

The equation for heat-flow is:
   power_flow  =  thermal_conductivity * (area / thickness)  * delta_T

This means that with a pad 1cm x 1cm x 2mm, and a GPU that idles at 5W, 
the temperature drop across the thermal pad is 20 degrees! So the 
heatsink runs cold, while the GPU runs hot. I knew these things were 
bad, but that's terrible!

I shall bend the heatsink to be the right shape, and then use thermal 
paste...will report back.

Hope that's of interest.

Richard

P.S.  Normal thermal pads, as supplied on stock intel cpu coolers are 
decent enough - they are less messy than thermal grease, but work almost 
as well. However, they are usually about 0.1mm thick, not 2mm.