A vent is formed from vent panels that can be formed by injection molding or vacuum forming. The first sidewall faces shorter first sides of the lower and upper substrates. The first electrodes are connected to the second electrodes by wirings formed over the main surface of the first semiconductor chip, a side surface of the second semiconductor chip and the main surface of the second semiconductor chip. If an overload condition exists, the depth value of the parent workload group is increased. Such a group of 139 sync blocks is obtained by dividing 2224 sync blocks contained in sixteen tracks by sixteen planes, 1668 sync blocks contained in twelve tracks by twelve planes, or 1112 sync blocks contained in eight tracks by eight planes. The cavity is vacuumed via the venting tube and mercury vapor and inert gas are then introduced into the cavity.