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36 SMT Magazine • January 2017 Shock Response on Fixture Shock impulses should be uniformly distrib- uted over the four boards on the drop table. The shock responses on the four fixtures are checked as follows. Fixtures are screwed to the drop ta- ble. No boards are mounted, only standoffs. One reference accelerometer keeps the same location on the table for each drop to ensure shock values repeat closely from drop to drop. A second accelerometer is placed on the table surface sequentially at the projected center of each test board location to measure the shock responses during successive drops. Accelerome- ter readings at the four board locations are plot- ted in Figure 6. Maximum acceleration values are listed in Table 2. The variation in maxima by location is 1.6% (907G ± 0.8%). Strain Measurement on Board Strain gages are mounted at designated sol- der mask opening locations on the assembled side of the board near the component corners, one gage on each of the four boards mount- ed. The strain gage locations for each board are shown in Figure 7. Because of a channel count limitation only strains at 45° are measured. Due to the design symmetry, these 45° strains are ex- pected to be the principal strains. Samples are Figure 5: Diagram for drop test apparatus and mounting scheme for assembled test board 11 . Figure 6: Accelerometer readings on the table fixture measured at the projection of each board center. Figure 7: (a) Solder mask opening for attaching strain gage, (b) four boards mounted on table with table accelerometer location indicated and (c) board location identifiers. Table 2: Peak table acceleration response by location. EFFECT OF SOLDER COMPOSITION, PCB SURFACE FINISH AND SOLDER JOINT VOLUME