This is my first time tearing down a laptop to replace a defective mobo.
If it makes any difference it's a toshiba L305D S5869.
I've got the motherboard out and sandwiched in between the heat sink portion for the gpu there's this square black material covering the chipset and in the center of that is a square of what looks like some sort of membrane made out of what I'm assuming is some sort of thermal pad.
Can this be re-used on the replacement mobo? If not, where do I get another one that is going to be the same size and thickness?
As I've never seen a thermal pad before, I'm a little confused. When you say to replace it with thermal paste are you talking about just the center square area inside the bigger black square of material? It has what looks like a more solid version of thermal paste stretched over the square hole.
Thermal pads do have their place in certain applications.
If a cpu/gpu doesn't perfectly contact the heat sink, simply using thermal paste and calling it a day can create less than ideal cooling situations. Personally, what I do is, if I don't plan to use the thermal pad in favor of some good quality thermal paste, I will apply a thin coat to the cpu/gpu and test fit it. If I need to spread it thick so that it can touch the heat sink surface completely, then I will opt for a thermal pad (reusing the original if it's in perfect shape, or using a new one if it's not).
On top of that, laptops will flex during handling, and a thermal pad may work better since most don't harden up.
Keep this in mind when deciding on whether to use paste or a thermal pad and make the decision based on the individual application.