A position adjustment method includes forming a plurality of bores on a tray, forming a plurality of through holes in a mounting plate corresponding to the bores on the tray, installing a plurality of screws into the through holes, and threading a plurality of nuts onto the screws, so as to fasten the screws to the mounting plate. The method also includes threading the screws into the corresponding bores so as to fasten the mounting plate to the tray and adjusting depths of the screws in the bores so as to adjust the position of the mounting plate relative to the tray.