RE: XPS-15 L502X Well-Known Problems
I see what you are referring to, there are thermal pads on the heat sink that attaches to other chips on the mother board. I believe the chip near the CPU is the northbridge controller and the four chips surrounding the GPU are the VRAM modules (the white pads cover these).
Those thermal pads are important, so I hope you placed them back.
However, thinking back I should have replaced those pads when I replaced my thermal compound on the CPU and GPU