Good day,

	We have a design that relies on intimate contact between the bottom 
side of an SMT PWB and an aluminum carrier.  The board gets fastened down with 
screws.  The design requires a good ground between the board and carrier for a 
period of 10 years in an outside environment.  The carrier is irridite coated, 
but the surface is abraded where the board is located.  The original board 
finish was tin/lead.

The question arose as to what effects oxidation will have on the grounding 
function.  Which will oxidize faster, the tin/lead or the aluminum?  Is gold a 
better choice for the board finish?  What alternatives are there for the 
carrier finish?

Also, this same product includes components which have either nickel or gold 
plated bodies.  These components must also maintain intimate contact for 
electrical ground between the component body and a .003" thick copper shim.  
What would be the best finish for the shim?  Keep in mind that the shim has to 
be formed slightly, therefore the finish must be somewhat pliable.

Thanks in advance...

			Jim Marsico
			[log in to unmask]
			(516) 595-5879

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