On Thu, 11 Apr 1996 11:29:01 -0700 Jim Marsico wrote:

> From: Jim Marsico <marsico%Organization=Industrial & Mfg'g 
[log in to unmask]>
> Date: Thu, 11 Apr 1996 11:29:01 -0700
> Subject: DES: MATERIAL FINISHES
> To: [log in to unmask]
> 
> 
> 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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> 
									
	Jim: 1.The "irridte" coating you refer to is a trade name for 
chromate conversion coating.Class III is typically called out for electrical 
connections.Since it is abraded away at the contact area this may be a mute 
point.Is it abraded from the chasis possibly because you are using a heavier 
coating(Class I)and it is acting as an insulator?			
									
									
	    2.The aluminum will oxidize faster than the Sn/Pb,it forms an 
oxide layer almost immediately when exposed to air.The Sn/Pb however also 
form an oxide layer in a short time.					
									
									
	    3.A gold pcb will not oxidize,however due to it's highly cathodic 
(noble)state and aluminums highly anodic position in the galvanic series the 
possibility of corrosion from a galvanic cell is very probable.This is 
especially true in an outside environment that is subject to moisture and 
humidity conditions.							
									
									
	    4.You may want to condider a zincate/copper/tin finish on the 
aluminum carrier and stay with the Sn/Pb on the board,these are very close in 
the electromotive series.Another option could be to conformal coat the 
aluminum/pcb assembly.							
	    								
	     The real issue here is what do you require for"a good ground 
between the board and carrier".In many cases the connection of the 
board/screw/carrier as it is may be sufficient.How well sealed the enclosure 
is,where it's located,type of environment etc. are all going to have an 
effect on the reliability.						
									
									
									
	    5.I would tin plate the shim.				
									
									
									
					Regards				
					Michael Barmuta			
					Staff Engineer			
					Fluke Corp.			
					Everett Wa.			
					(206)356-6076