Hi Mike,
While I agree with you in general about the reactivity of aluminum, in
specific cases it can be used successfully. One obvious example is aluminum
sheet metal cases housing circuit boards. It is necessary to maintain
electrical continuity from panel-to-panel and from panel to earth ground
(third wire) if you need to comply with IEC60950 and many other standards.
I have commonly used anodized or anodized and painted aluminum parts on
cases, and the anodizing is a nearly ideal insulator. To solve this
problem, we use star washers under the screw heads and the nuts are
self-broaching fasteners pressed in from behind. The ground wire terminal
is fastened with a nut onto a self-broaching stud pressed into the aluminum
chassis. I've seen some very old equipment come in from the field (chemical
laboratory environment) very grungy but without corrosion and still having
continuity.
My impression is that when you compress the star washer with enough force,
the projections on the washer that pierce the thin anodized layer form a
gas-tight connection to the aluminum substrate and therefore do not oxidize
or corrode. Yes, the fasteners and their finishes have to be compatible for
the environment, but this approach has been successful for many years.
Similarly, it is possible to fasten a PCB to an aluminum standoff with a
stainless steel screw, with or without a star washer. The screw threads
break through the anodized coating inside the threaded aluminum fastener and
the screw head makes good contact with the top side of the PCB. UL does not
accept this as a primary ground connection, though this has nothing to do
with corrosion. Since the aluminum spacer contacts the tin/lead (or tin)
coated pad on the bottom side of the board, I would be very careful to not
let anything get at that joint. Personally, I prefer to use steel standoffs
when possible. My first choice is a self-broaching threaded standoff
pressed directly into the aluminum chassis. These are available with closed
bottoms (blind threaded) to avoid liquid ingress. Obviously, these must be
pressed in after anodizing and before painting.
Hope this has been of some help.
Regards,
Seth Goodman
Goodman Associates, LLC
tel 608.833.9933
fax 608.833.9966
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Mike Fenner
> Sent: Friday, April 12, 2002 2:52 AM
> To: [log in to unmask]
> Subject: Re: [TN] connecting aluminum to copper
>
>
> Aluminium to anything will fairly quickly go electrically open circuit,
> although you will retain thermal conductivity. Depending on the conditions
> of use and the contact metal you will next slowly or quickly achieve all
> sorts of corrosion issues. Immunise the aluminium from the outside world
> with something more stable like nickel
>
> Kind Regards
>
> Mike Fenner
>
> Applications Engineer, European Operations
> Indium Corporation
> T: + 44 1908 580 400
> M: + 44 7810 526 317
> F: + 44 1908 580 411
> E: [log in to unmask]
> W: www.indium.com
> Leadfree: www.Pb-Free.com
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Phil Nutting
> Sent: Wednesday, April 10, 2002 9:12 PM
> To: [log in to unmask]
> Subject: [TN] connecting aluminum to copper
>
>
> Good afternoon TechNet,
>
> Would anyone like to share their knowledge about using an
> aluminum standoff
> as a conductor when connecting two circuit boards together. I
> suspect that
> if it were aluminum to copper there would be an issue, but what about
> aluminum to HASL surface of circuit boards?
>
> Thanks in advance for your wisdom,
>
> Phil Nutting
>
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