Just stick an 2 layer antistatic mat on the aluminum trays, it will make good contact with the boards and prevent any static from building up in the transport, as well will slowdown discharge. You can also refer ANSI/ESD S20. 20 (Chapter 8: ESDs item Handling). The ESD workpiece carriers reliably prevent the generation of electrostatic charges and discharge existing charges in a controlled manner. The rapid discharge can be harmful to some components, and could possible cause other issues if the Aluminum trays are used after test where capacitors may hold a charge and then short into other parts, cause sparks, etc. Even very small voltages that are not noticeable to humans can destroy microchips or circuit boards. These trays are made from materials that have conductive or dissipative properties, allowing them to safely dissipate any static electricity that may build up.
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