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Materials and finishing for electronic board assembly

September 29, 2010

My question is about the choice of materials and finishing of hardware (screws, washers, standoff, etc.) used to mount electronic printed boards into metal cases.

Let's explain the context and application.
My company manufactures electronic equipment for railway rolling stocks, to be placed on the inside, so in a relatively protected environment, but not really controlled. The targeted useful lifespan is 20 years at minimum.
In general our electronic boards are housed into a metal case, that may be Aluminum with Alodine chromate or pre-galvanized steel, depending on application, size and weight constraints. In rare cases we use also stainless steel.
The purpose of the fixing hardware, normally using M3 threads, is not only to fix in place the electronic boards against vibration, but also to provide a ground path to the train metal structure for safety and EMC control.

Standard practice is to have standoffs or studs clinched on the metal sheet, the electronic board resting on them, and secured with crinkle washers and screws. The PCB supporting holes are metallized, tin plated over copper substrate.

So at the end we have several different metal in electrical contact:
- the case, aluminum or galvanized steel
- the clinched standoff or stud
- the PCB finish, tin over copper
- the washer
- the screw

Now I wonder what may be the best choice for the fixing hardware to minimize the corrosion danger.
From a practical point of view we have the following options:
- standoff or stud: steel zinc plated, stainless steel or aluminum
- crinkle washers: stainless steel or beryllium copper
- screws: steel zinc plated or stainless steel

What do you suggest as best compromise based in your knowledge and experience?

Note that in any case the electrical path to ground is directly from the PCB pads to the standoff to the metal case, without necessarily passing through the fixing screw.

We have already an historical experience with the set: aluminum case - steel zinc plated standoff - stainless steel screw and washer, and examining units with more than 10 years of actual service on trains we found no sign of corrosion.
However we have no real experience for steel cases, and after all our previous experience is limited to trains operating in a mild and favorable climate (Italy), while now we are facing harsher environments, like Russia and China, so we need to be more accurate in all our choices.

Thank you very much for your valuable suggestions, regards

Raimondo Caprio
Original Equipment Manufacturer - Concorezzo MI, Italy

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