Bags, trays or racks: what each one actually protects against
· Sam
Bags, trays and racks get compared as though they are alternatives. They are not. Each one solves a different failure, and most production floors need at least two of them. Here is what each is actually for.
What an antistatic bag protects against
Static, and, if it is a moisture-barrier bag, moisture. The material standard is MIL-B-81705-B, and a good bag with a desiccant and an indicator card is the correct way to hold a moisture-sensitive assembly whose floor-life clock you want to stop.
What a bag does not do is hold a board away from another board. Bagged boards in a tote still press against each other every time the tote is lifted, and the bag does nothing to stop a connector on one board digging into the face of the next. A bag is packaging. It protects the board from the environment, not from its neighbours.
What a tray or tote protects against
Bulk movement, mostly. A tote gets a quantity of boards from one place to another in one trip, and a conductive tote keeps that quantity at the same potential. That is a real job and worth doing well.
Unless the tote is slotted, though, it is a container rather than a fixture, and what is inside it is a pile. The moment boards are populated, the pile is the problem.
What a rack protects against
Mechanical contact, and it is the only one of the three that does. Each board sits on its edges in its own slot, with air on both faces, carrying nothing but itself. Any board can be pulled without disturbing the others, which matters more than it sounds: most handling damage happens while somebody is digging for one specific board.
Racks also go places bags and totes cannot. A magazine rack with the right pitch feeds an SMT loader directly, and a rack with the right temperature rating goes into a bake with the boards still in it. Our series are rated 80 C, 120 C, 200 C and 300 C for exactly that reason, and the rating you need comes from the hottest process the rack will see rather than from the storeroom.
Choosing between them
A short way to decide. If the board is sealed and waiting, use a bag. If a quantity of bare boards is moving as a batch, a conductive tote is fine. If boards are populated, in process, handled repeatedly, or going anywhere near heat, they belong in slots.
Most floors end up with bags for incoming and outgoing and racks for everything happening in between. That is not redundancy, it is two different jobs.
Getting the fit right
If you are moving from totes to racks, the two numbers that decide everything are your largest board dimension and the height of your tallest component, because the second one sets the slot pitch you can use. Magazine racks run 10 mm between slots and the slotted-edge bench racks run wider at 15 to 16 mm. The sizing guide walks through measuring both, and the selection guide covers the adjustment mechanisms.