June 19, 2007

Folks, we have an OEM who designed a laminated bus/power plane that has 
"boss" connections brazed to copper bus bars within the assembly.  
Basically, the construction is similar to a PWA since it uses FR4 and copper 
conductors laminated together under heat and pressure similar to a PWA, 
except the "boss' terminations serve the purpose of a PTH.

The OEM inserts a helicoil insert into a tapped hole in the "boss".  
However, when doing this, there are small areas of the top of the "boss" 
where the matte tin-plating is scraped off (when the helicoil insert is 
compressed, the first several threads go in easily, but toward the end, the 
diameter is not compressed as much and this scrapes off some of the 
plating).  Electrical connections to the "boss' are made via  ring type 
terminal lugs (MIL-T-7928) and a screw that threads into the insert.  
However, the electrical connection is made directly to the top surface of 
the "boss', and does not rely on the threads of the insert, nor does it come 
in contact with the exposed copper surface.

My question is, if we left the copper areas exposed, is there any technical 
concern having exposed copper.  If there is, would conformal coating (Type 
UR) be acceptable to cover the exposed copper areas, using caution to make 
sure none of the coating comes in contact with any electrical mating 
surface.

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