Gregg: Both carbon and gold will work; in fact Sn/Pb either fused or 
HAL will work. We use all three types of finishes, each has it's own 
limitations and requirements when used for switches or contacts.	  
Carbon can be used where contact resistance values >20-30 ohms/sq are 
acceptable and can be printed directly over Sn/Pb. Do not use large 
termination pads because the Sn/Pb can melt and shift during reflow or flow 
solder causing the carbon to crack or flake. It's best to print directly onto 
the laminate and terminate outside the actual keypad area to the cicuit.	
 Gold will give you the lowest contact resistance of the three. A thickness 
of 25-35 microinches of electroplated hard gold over 100-200 microinches of 
electroplated nickel should provide the life and wear cycle characteristics 
you're looking for.							     
 Sn/Pb can also be used however it develops oxides on the surface that can 
affect contact resistance. We use a self cleaning mating contact on the 
membrane keypad to get around the oxide issue. Sn/Pb thickness is 300-500 
microinches that is electroplated and fused. HAL thickness may be harder to 
control based on termination pad geometery and location. 		     
  The most bullet proof choice is the gold although the others could also 
work well depending on your application.				
						Regards			
							Michael Barmuta	
							Staff Engineer	
							Fluke Electronics	
							Everatt Wa.	
							425-356-6076	
			              				      


On Tue, 29 Jul 1997 04:54:36 -0700 Gregg Klawson wrote:

> From: Gregg Klawson <[log in to unmask]>
> Date: Tue, 29 Jul 1997 04:54:36 -0700
> Subject: Carbon ink vs. gold - ink vs. mesh/foil
> To: [log in to unmask]
> Cc: [log in to unmask]
> 
> I'm looking for any reference information/user experience on the
> reliability of carbon-ink versus gold used as a switch contact material in
> tactile membrane switches.  I searched our company's IEEE/IEC database
> (last 10 years) for relevant papers but didn't get any good hits.
> 
> Carbon ink over Pb/Sn over Cu has been suggested as "just as good as gold"
> for the contacts.  In our application, reliability is the over-riding
> concern - we're looking for a contact life of 1 to 2 million actuations and
> useable life of 15-20 years.  
> 
> We know that meshes and foils are proven EMI shields for membrane switch
> assemblies but what about the shielding effectiveness of silver or carbon
> inks?  "Just as good as" or worse?
> 
> Thanks in advance for your help.
> 
> -Gregg
> GTE Government Systems Corp
> [log in to unmask]
> 
> 
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