How to Maintain Good Shielding Contacts to an Aluminium Enclosure?

[niconiconi]

For good RF shielding, all authoritative EMI/EMC textbooks and references say a shielded connector ideally should have a tight, complete 360-degree bounding to the enclosure. Also, in some cases, it's desirable to tightly bound the circuit board and the enclosure together through grounding fingers at the edge of the board.

But I still don't understand how does one implement this properly in an aluminum enclosure. I read that aluminum oxide is not a good electrical conductor. It means that the shielding connection may degrade due to oxidation, with questionable long term reliability.

[bugrobotics]

You can treat the aluminum enclosure with alodine/chromate or nickel plating to avoid oxidation and maintain good continuity.

[profdc9]

Mechanical pressure can maintain a good electrical contact between two surfaces.  A "wiping" action with applied between two conductors creates a "wet contact" which has a region between the conductors that excludes oxygen and prevents oxidation.   This is the basis of connectors, for example, which use crimps or friction to join each other.  Alternatively, pressure from a fastener such as a screw can apply the force.

However, dissimilar metals eventually can have galvanic corrosion between them despite the initial wet contact being made with pressure.  For example, copper in contact with aluminum is notorious for aluminum oxidation at the interface.  A plating may need to be applied to the copper first, for example zinc or tin, so that the actual surfaces in contact have a smaller electrode potential difference.  In addition, some oxides such as tin oxide, or zinc tin oxide, are partially conductive, so that some conductivity is maintained even after an oxide is formed between the metals.

Rather than using fingers, if you are using external coax connectors, an external bulkhead connector can be bolted to the side of the case.  These will often already be nickel plated, simplifying the bonding to the enclosure.  You can also ground the PCB to the case through metal standoffs as well.

[mag_therm]

A method was used on power transformers in Australia where Al windings and Al busbars are common.

Just before bolting apply a grease to the contact surfaces then abraid the surface with a s/s fine wire brush through the grease.
Then clean off the grease and apply a fresh thin coat and bolt tight.  There were standard bolt patterns based on current.

The high current joints were inspected by temperature rise on samples.

There was a proprietary grease, green colored, but I think vaseline etc would be OK.

[thm_w]

Rather than using fingers, if you are using external coax connectors, an external bulkhead connector can be bolted to the side of the case.  These will often already be nickel plated, simplifying the bonding to the enclosure.  You can also ground the PCB to the case through metal standoffs as well.

And the connector may come with, or you can buy, a star washer, which will bite into the aluminum to provide better contact.
If the aluminum was painted, you'll have to mask off that area during painting.

https://www.pacrad.com/pan-pacific-bnc-3193-75.html

[David Hess]

I have worked on old test instruments that used a metal gasket between aluminum surfaces that was like very sharp finger stock.