Many OEM users have demonstrated that exposed copper in the assembly 
does not pose any reliability problems, in fact, this has been going on 
for over 10 years (with OPSs).  Obtaining actual reliability data from 
these OEMs is more difficult. 

PCBs have been manufactured for years with electrolytic tin or gold as 
the surface finish.  These boards contain a significant amount of 
exposed copper along all traces, connectors, etc.  The difference is 
the copper cannot be readliy seen as it is located on the sidewalls and 
not the board surface.

I have several years of experience working with customers converting to 
the OSP surface finish.  There are a number of things that can be done 
to minimze the exposed copper, such as increasing the solder 
paste stencil aperture to a 100% opening (most are about 80%), printing 
paste on test points and vias as you suggested, minimizing through-hole 
annular ring, etc.  I find most customers accept exposed copper as it 
is not detrimental to the final product.  Of course, the product 
application plays a role.

Working with the October Project, I've seen a couple of good articles 
that may now be available through IPC.  One good paper was a study done 
by Batelle Labs with Digital Equipment Corporation -- it basically 
concluded that bare copper was more reliable than solder down to a four 
mil line and space.


Dave Boggs
Merix Corporation
phone (503) 359-9300
EMAIL:  [log in to unmask] 





Kaylor Karl <[log in to unmask]> Wrote:
| 
| 
| 
| We are contemplating using OSP's but in doing so will 
| leave exposed copper 
| on the final assembly.  Specifically, the test points, 
| fiducials and vias. 
|  We have discussed printing solder paste on the test 
| points and fiducials 
| but are not sure how it will affect test and the placement 
| equipments vision 
| system.
| 
| What are the long term affects of leaving exposed copper 
| in these areas?
| Does anyone else leave exposed copper on the final 
| assembly?
| What affect will printing solder paste on test points and 
| fiducials have on 
| placement and test?
| How are assemblers dealing with this issue?
| 
| Any information would be greatly appreciated.
| 
| Karl Kaylor
| Thomson Consumer Electronics
| [log in to unmask]
|