Electrical Resistance Change Of Aluminum Over Time?

[CaptainCarp]

Hello,

Looking for some help in understanding an issue that we are having which I think may be materials related.

We have an automotive module which is used to drive some relatively high current loads (5 - 20 A).
The module is roughly the size of a thick paperback book with a base made of A360 aluminum.
We are using the aluminum base as the return path / "ground" for the module's power feed.

The electrical / mechanical interface between the module circuitry's "ground" and the aluminum base consists of four contact points, one at each corner of the module's pc board.
Each contact point consists of a "sandwich" of the pc board copper "ground" traces, a ring of Sn/Ag/Cu solder, and the aluminum base.
Each "sandwich" is secured with a steel screw which is not part of the electrical interface.

Our issue is that when the modules are first assembled, the electrical resistance between the circuitry's "ground" and the base is typically < 1 mOhm.
But after some time (inconsistent), the resistance appears to increase dramatically.
What is extremely puzzling is that this new "resistance" appears to be non-linear.

We measure the resistance by applying a constant current of 10 A between the circuitry's "ground" and the aluminum base, and then measure the applied voltage (R = V/I = ~ 8 mV / 10 A = ~ 0.8 mOhm).
When the module is first assembled, the measured applied voltage is directly proportional to the applied current (= constant resistance).
But when the "issue" is present, the measured applied voltage does not vary greatly, pretty much staying in the 300 to 500 mV range, even when the applied current is varied from 5 to 20 A, and implying a much higher (30 - 50 mOhm), unacceptable contact resistance.
Other "odd" electrical behaviors have also been observed. Sometimes giving the module a rap will "fix" it (at least for the time being). Quickly disconnecting / reconnecting the applied current drops the resistance to a fairly "good" value for a while and then it starts climbing again.

Are the listed materials compatible with each other as far as providing a consistent, low electrical resistance contact?

Thank you,

 

[dgallup]

Pretty sure you are getting a buildup of aluminum oxide in your contact points. This is a well know phenomenon and why aluminum electrical wiring is banned after burning down numerous homes.

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[IRstuff]

Sorry, but using chassis as a return is typically verboten, because of that, and because the possibility of introducing noise into the active circuitry from other noise coupling to the chassis.

A distant possibility is that there's some electromigration because the drive transistor is undersized.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[3DDave]

Bad combination. Aluminum oxide buildup will raise the resistance. Personal experience with a similar situation where corrosion took an ignition module off line. Very annoying to have to refurbish the distributor before driving home; especially the first time when the ignition cut off while driving in rush hour traffic.

I would not be surprised if there is local heating during the later test that causes the parts to deform and give varying results.

You might plate the contact points on the aluminum to avoid corrosion or find a conductive oxidation barrier material, but be aware that an oxide film could form before the barrier material is applied and require removal at assembly.

 

[EdStainless]

Use a ground strap.
Combine the oxidation of the Al and differential thermal expansion and this has a low probability of being reliable.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Compositepro]

Any electrical connection to aluminum should be protected with an anti oxidant grease.

https://www.sanchem.com/electrical-contact-lubricant.html

This link has some interesting discussion:

https://www.w8ji.com/dielectric_grease_vs_conductive_grease.htm

 

[CaptainCarp]

Digging a little deeper, it seems that there will always be problems using aluminum as a conductor unless special care ($$$) is taken.

Found these related links when searching for "aluminum oxide contact resistance" which I thought were interesting:

https://electronics.stackexchange.c...ance-ground-contacts-to-an-aluminum-enclosure

https://physics.stackexchange.com/q...ricity-travel-through-the-surface-oxide-layer

Thanks for all the info.

 

[IRstuff]

One option that used to be done in military systems is some sort of conversion coating that prevents the oxide from building up and maintains a more consistent electrical conductivity:

https://www.cybershieldinc.com/aluminum-finishing/

Nevertheless, issues with durability and wear resistance remain, so a grounding strap (typically for EMI, not running current) is usually preferred, since the contact tends to be reasonably gas-tight, and the washer tends to dig into the surface oxide and makes intimate contact with the aluminum.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[WKTaylor]

Copies of the following Aluminum Association documents may be useful, this subject...

AA TH56 ALUMINUM ELECTRICAL CONDUCTOR HANDBOOK

AA SSCC STRESS-STRAIN-CREEP-CURVES FOR ALUMINUM OVERHEAD ELECTRICAL CONDUCTORS

Adding to comment by IRstuff, RE Electrical grounding to a chassis... this is usually for SAFETY only... to prevent arc/spark and facilitate bleed off static.

A 'clean' return grounding wire-pathway is vital for sensitive/critical systems to function properly.

Question.
With a turn to low-power digital circuits [and LEDs, etc], in homes and transportation, etc... anyone foresee resurgence-of insulated aluminum wiring... or is aluminum as a conductor always going to be a 'hard-sell'?

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[3DDave]

It might, but only if copper prices go way up. The adverse ability to form an insulating layer is a big problem and, while transmission line electricians are very reliable, the shear number of residential electricians is too many to believe they are all reliable - including that many appliances are not installed by residential electricians - thank you very much furnace guys who used the wrong connection to wire in some aluminum wire and burned the insulation back 4 inches from the junction. Nothing I'd like better than the entire heating system of my house bumped to 120 Volts or the wiring burning up in the conduit.

Same type who wired in the new electrical panel for code compliance failed to make the house connection correctly, leading to random loss of power that dropped one side of the 220 for up to a minute at a time. Yeah. I love resetting electronic clocks and the alarm. The power company sent out a competent guy who used the correct anti-oxide grease on the aluminum connection.

It was interesting - the guy spotted the problem from the street. He'd seen it enough to identify the change in the exact shade of black between the electrical wrap and the carbonized electrical wrap.