To solve this problem, Toshiba employed a "honeycomb rib" structure and a "honeycomb beam" structure and applied them to the chassis that faces the LCD display when the notebook PC is closed. The honeycomb beam and honeycomb rib structures were applied to the part that holds the keyboard and the back side of the part that functions as a palm rest, respectively.
The honeycomb beam structure means that the board used for the chassis has many hexagonal holes. Previously, Toshiba used a lattice structure. But, to improve the resistance to twist as mentioned above, the company employed the new structure so that many beams are parallel to the diagonal line to which the stress is applied.
The honeycomb rib structure means that the ribs used on the back side of the chassis have a honeycomb shape. Like the honeycomb beam structure, it enhances the resistance to twist.
The reason why Toshiba did not use the honeycomb rib structure for the part that holds the keyboard is that there is not enough space (height). Also, from the viewpoint of weight reduction, the honeycomb beam is more advantageous.
Moreover, to improve the twist resistance of components mounted by using ball grid arrays (BGAs) such as CPU and chipset, Toshiba used L-shaped plates to reinforce the parts that support the four corners of the BGAs. BGAs are less flexible than PCBs. Therefore, when a torsional stress is applied to the PCB, a strong stress is applied to the four corners of the BGA. To withstand such a stress, the company used the L-shaped plates.
Normally, BGAs are reinforced by using quadrangular backup plates that support their whole surfaces. But Toshiba used the L-shaped plates to reduce weight.