|Ask the Experts|
September 11, 2020
Aluminum Trays and Rapid Static Discharge
Occasionally we transport assembled circuit boards directly on aluminum trays. Some in our plant are concerned that the aluminum surface may discharge static charges too quickly.
I'm familiar with this concern, but wonder about all the assembly and test machinery we use where assembled circuit boards come in contact with various metal surfaces.
Should we be concerned when transporting assembled circuit boards that lie directly on the surface of aluminum trays?
|Expert Panel Responses|
Static is tricky. There is a lot of "tribal knowledge" spread around about it. Some is good and some is misleading. The best standard on the subject is the ESD Association ANSI/ESD S20.20-2007, "For the Development of an Electrostatic Discharge Control Program for - Protection of Electrical and Electronic Parts, Assemblies and Equipment." Long title, but easy and quick read. There may be a newer edition out but the rules of ESD have not changed very much. I highly recommend you buy a copy and read it through.
All the machines in your plant are connected to earth. There is nothing special about earth grounding except that the Earth is the one thing we ALL have in "common," in that if everything is electrically connect to earth, there would be no "potential difference" and thus no electrostatic discharge (ESD).
The problem is "insulators" that isolate objects (like your assemblies) from earth and thus they can build a charge as they are moved about. Plastic carriers can be insulators unless treated to be conductive. All conductive trays including aluminum are fine so long as they remains in contact with earth in some way. Typically your body and your conductive clothing, heel straps, etc are the means to connect the carrier to earth.
So as long as you practice proper ESD handling per the standard above you will keep yourself and your assemblies safe from ESD no matter what kind of trays you choose to use.
Senior Project Engineer
Electronic Controls Design Inc
I would recommend static dissipative trays for transport. 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.
The assembly machinery generally holds the edge of the PWBs, or board supports may be used that only touch the solder mask. Even a full support block for Stencil Printing would only touch the surface when no parts are populated. The SMT line, Flying Probe, Routers, etc. would not normally short 2 or more leads across a conductive surface.
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
In order to be in compliance with an ESD program all operators should wear wrist straps or ESD shoes when on the shop floor. This should also apply to visitors and all personnel should be tested whenever re-entering the shop floor. Antistatic mats should be used on all workstations if the work surfaces themselves are not antistatic.
Boards should also be handled using antistatic trays rather than aluminum trays since an antistatic tray will allow any static charges to bleed out in a controlled fashion, and will also not be as hard as an aluminum surface.
If this is a common practice for your organization, is the data telling you that you are causing problems with these trays? In general you do not see many aluminum trays out there being used for assemblies' transportation.
My question would be - are there any failures? Did you have any problems and a simple root-cause analysis assessment brought you to the conclusion that the trays are the culprit?
Engineering and Operations Management
Electric components should only be handled in an ESD Protected Area. Manufacturers must ensure that workstations are static-safe.
Aluminum is conductive, so yes it will discharge static charges too quickly, but this will again depend on resistance and charged board handling. 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).
There are two possible risks using conductive, metal trays. If your boards have components that can store energy, such as capacitors or batteries, or components with residual charge from testing, there's the risk that trays create a low resistance path for discharge. This can damage components if such a discharge occurs. Second, if a person is not grounded and touches your board while sits in a grounded metal tray, there could be a discharge event that damages components.Tom Solon, RH Murphy Co.
It's best to have dissipating material in contact with your boards to minimize these risks. The risk is real but how much risk depends on how good your protocols are and whether you have any charged devices on the boards.
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