Issue link: https://iconnect007.uberflip.com/i/1143487
34 PCB007 MAGAZINE I JULY 2019 bled, which results in a gap between the plas- tic plate and riser card. The gap is -0.277 mm minimum and 0.697 mm maximum. When the system is in shock, only one end of the riser card is pressed; the other end would lift. Based on this gap issue, the new design was adopted. In the new design, foam is used instead of the plastic sheet to press the riser card; the foam is thicker and the damping area is larger. Figure 6 lists the difference between the cur- rent design and the new design. In the new de- sign, the tolerance analysis shows that the gap between the foam and riser card is -1.927 mm minimum and -0.953 mm maximum. There- fore, no gap exists between the foam and riser card. The two can be firmly pressed. The foam would be pressed tightly onto the top cover and would not lift the top cover due to its per- fect elasticity with the change (Figure 7). Third, although NIC/SSL is pressed by foam vertically, no protection is there horizontally. The jack that connects the riser card and NIC/ SSL is also seated horizontally. When NIC/SSL is in shock, they may slip from the jack on the riser card, resulting in poor contact. Adding motherboard locking mechanism can help pre- vent the disengagement of NIC/SSL card from the riser card during the vibration of the moth- erboard. Figure 8 illustrates two plastic holders used to fix the NIC to prevent PCB swaying in transportation. Figure 6: Tolerance analysis on a stack of mechanical parts. Figure 7: Damping material design and tolerance analysis. Figure 8: Board locking mechanism to restrain horizontal movement.