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When we opened the chassis on the keypad side, a heatsink covering the center portion of the mainboard drew our attention. It is probably used to transfer the heat generated from Intel Corp's Atom Z520 and the US15W chipset, which are arranged under the heatsink, to the area close to the vent hole.
A cooling fan, a 1.8-inch HDD and a battery storage space are located under the mainboard. The width of the D4 seems to be determined by the total width of these components.