A test apparatus includes a tray including at least a first region and a second region, and a cap disposed over the tray. The cap includes a cap body, and at least a first magnet and a second magnet disposed over the cap body. The first magnet is configured to provide a first magnetic field to the first region of the tray, and the second magnet is configured to provide a second magnetic field to the second region of the tray. A strength of the first magnetic field is different from a strength of the second magnetic field.