Discussion What's the maximum usable thickness for thermal pads? (Before they become an insulator)

[MrNintend0]

So a little bit of info for context. I [somewhat] recently refurbished a slim PS3 and modded it (custom firmware, 1TB SSD, etc.). During the refurbishing I replaced the thermal paste and such for the GPU (RSX) and CPU, and replaced the thermal pads on the VRMs. While doing so I noticed that only the VRMs for the CPU had thermal pads, the GPU had none whatsoever.

The VRMs sink the heat into the steel EMI shield for the mainboard, which isn't a great conductor, but it's at least something. The problem is that when I went to add some to the GPU VRMs, the EMI shield has a large (~9mm) gap between one of the VRMs. I stacked three 3mm pads to close the gap, but now I'm wondering if it's more harmful than it seems.

Software (from the mods) only shows CPU and GPU temps, so unless I purchase and install a thermocouple on them, I have no idea what temps they're running at.

So now I'm wondering if this stack of thermal pads is actually working as an insulator instead of a conductor since the thermal energy will have to travel through that much silicone as well as any air gaps between the pads, which forms the question; What's the max thickness of a thermal pad for it to actually work as a thermal conductor?

I imagine the simple answer is "It depends", and there's more to it (such as airflow over the VRMs (with pads or not), ambient temp, etc.).

I'd like to hear your thoughts whether it's from experience, knowledge, or what have you.

 

[tinyitx]

I will take the first crack.
If it is between having nothing (leaving the 9mm gap empty) or using three 3mm thick thermal pads, I will bet on the latter.

A thermal pad is not a heatsink itself but it just transfers the heat across. My bet is your 3x3mm pad solution will transfer more heat to the EMI shield than just doing nothing.
Of course, thicker is not better. That is perhaps why pads usually come in thickness of 3mm or less.
A thermal sensor is not that expensive and you could probably have one lying in your spare parts box.
(eg )

One possible improvement may be:- 0.5mm pad--thick copper shim--0.5mm pad ?

 

[riba2233]

If you ask EVGA, 9mm is totally normal...

 

[msystems]

The thermal resistance increases, but, like someone else suggested, you can mitigate it with shims.

I did some experiments a while ago and used stacked pure copper shims in between a sandwich of pads and paste on the other side and it successfully transferred heat from the buttom of a cpu socket into an aluminum chassis. That distance is somethin like 6mm afaik (whatever the standoff height is). Its in my build log somewhere

Edit- pic

(above is what I tried - it worked but not that well)

Instead of these "layers", you'd want a thermal pad, then the thickest possible shim, and then thermal paste. Or, just a single high performance thermal pad (these blue ones are NOT the high performance kind). These blue pads are like "6w/mK" and there are expensive versions that are "35w/mK" in larger thickness

 

[bigmeanie]

Copper wire mesh works pretty decent as well, and if you get arts-n-crafty you can use it and paste to fill in very large gaps. I'll use thermal epoxy to glue down the edges of the mesh to my heat sink material, and fill the insides of it up with paste. I use to use this to glue heat sinks in place, the glue we had at the time really really sucked when it came to how thermally conductive it was.

 

[SFFMunkee]

Copper wire mesh works pretty decent as well, and if you get arts-n-crafty you can use it and paste to fill in very large gaps. I'll use thermal epoxy to glue down the edges of the mesh to my heat sink material, and fill the insides of it up with paste. I use to use this to glue heat sinks in place, the glue we had at the time really really sucked when it came to how thermally conductive it was.

That sounds.... risky...?

 

[Mr Whippy]

A more important variable for this application is probably how much energy the EMI shield can hold and disippitate.

It might be perfect for the CPU, which might also need some help cooling VRM.

The addition of the GPU heat to the EMI shield might now compromise the CPU VRM temperatures.

Given the CPU has VRM heat sinking, and the GPU didn't, it suggests CPU VRM temps were a concern, and the GPU ones weren't.


Just looking here:

PlayStation 3 Slim Teardown

A first look inside Sony's dramatically redesigned, cooler, sleeker PlayStation 3 Slim! Also check out our PS3 Slim teardown video on YouTube!

www.ifixit.com

Is the fan sucking air in over the mainboard, and then blowing it out over that heatsink?

If so, maybe some small copper heatsinks stuck to the VRM will give a better boost?

 

[AlexTSG]

K5 Pro thermal paste, which is not a replacement for thermal paste, but rather for thermal pads may be an option worth looking at.

I'm still waiting for some that I ordered a while ago. I plan to use it for repasting a few graphics cards.

I'll still be using traditional paste and pads for the GPU die and memory, but the K5 Pro looks like a good option for areas like the VRMs, especially if a suitable pad thickness is not available.