Hi TechNet -
     
     I can't provide any info on how much resistance change occurs as 
     copper oxide grows but the question concerning the use of tin as a 
     contact surface is pretty straight forward. Tin oxide is not very 
     coherent - that is it doesn't bind tightly to the metallic surface it 
     grows from. Nickel oxide is an example of a coherent oxide. Most folks 
     using tin contacts follow two rules: a) they use high insertion forces 
     so that they make good contact with the tin and/or solder surface 
     under the oxide; b) they minimize the number of insertions that need 
     to be made - i.e. one or two at the most. There have been a number of 
     documented cases were tin oxide "spalls" off a contact surface leading 
     to fretting corrosion and/or wear problems. The old IPC spec 
     IPC-GH-850 "Handbook of Interconnection Contact Finishes" is a good 
     (but dated) reference to read. Good luck.
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]
     


______________________________ Reply Separator _________________________________
Subject: RE: DES/FAB:switchpad contacts
Author:  [log in to unmask] at ccmgw1
Date:    1/27/97 11:41 AM


     
     
Bill Parlin wrote:
     
     
Hello Technetters,
     
I am writing as a fabricator, but my question is about design...more 
specifically, the functionality of keypad contacts.
     
In jobs we produce, the "contact coating" of the circuit geometry is usually 
specified as carbon ink or electrolytic gold where the elastomeric button 
makes contact. However, there is a new job in the design stage right now 
that
utilizes a stainless steel pill/button to make contact, and (for various 
reasons) I was wondering why bare copper would not work. The field 
environment
is "indoor" but there is expected to be high temperture and humidity swings, 
and the contact resistance must remain very, very low I am told (less than 
what is achievable with a carbon coating), and remain so "forever".
     
I know solderability degrades as the copper surface ages and oxidizes, but 
does it also become less conductive? Is there any other reason bare copper 
should not be considered for this application?
     
Thanks in advance for your help.
     
Bill Parlin
     
     
I received a  similar question from our design department:
     
     
Jan -
     
Right now I am working on a current profiler that's battery powered.  One 
side of the battery stacks will go against a PWB contact.  I thought this 
contact had to be gold to make it reliable, but my supervisor thinks it can 
be a
typical solder coating or tin plating.  Any thoughts?  Do any of our 
TECH-NET
downloads help with this question?
     
thanks
Sark
     
Thanks in advance for any inputs to these questions.
     
Jan Satterfield
     
*************************************************************************** 
* TechNet mail list is provided as a service by IPC using SmartList v3.05 * 
*************************************************************************** 
* To unsubscribe from this list at any time, send a message to:           * 
* [log in to unmask] with <subject: unsubscribe> and no text.        * 
*************************************************************************** 
* If you are having a problem with the IPC TechNet forum please contact   * 
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      * 
***************************************************************************
     
*************************************************************************** 
* TechNet mail list is provided as a service by IPC using SmartList v3.05 * 
*************************************************************************** 
* To unsubscribe from this list at any time, send a message to:           * 
* [log in to unmask] with <subject: unsubscribe> and no text.        * 
*************************************************************************** 
* If you are having a problem with the IPC TechNet forum please contact   * 
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      * 
***************************************************************************
     

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************