I will try to answer each question as you ask them. 
 
In a message dated 1/28/2008 12:32:15 P.M. Eastern Standard Time,  
[log in to unmask] writes:

Dear  Netters,
Is there a preferred process for immersion silver finish in order  to 
minimize microvoids?
Immersion silver vendors have a preferred way for fabricators to operate  the 
immersion silver line to minimize/prevent microvoids.  Please  understand 
there are more effects than just the immersion silver bath itself. 


How  do you choose the preferred immersion silver PCB supplier?
Your board supplier is very interactive in the performance of immersion  
silver boards.  The best plating solution, run poorly by a fabricator can  give 
problems and vice versa.  Obtain boards from more than one fabricator  and from 
various bath suppliers.  Probably, the bath vendor has an opinion,  from 
process audits, how each fabricator is obeying the process information in  the data 
sheets.  See what works best with your solder paste and wave  solder.  
Wetting balance, dip-and-look, solder spread are all tests that  indicate good 
performance. 


Which test for bare boards could give the best  information?
IPC J-standard 003 for board solderability should be a guide for your  
operation.  Do you use international IEC standards?


Should I perform a print and reflow test for each batch, followed  by x-ray?
The print/reflow test is a good idea, with controlled conditions. However,  I 
don't think you will see microvoids with X-ray.  You probably need cross  
sectioning to see microvoids.  


Which tests are the suppliers performing?
By suppliers, do you mean board fabricators or plating chemistry  suppliers?  
This is not a one stop shop. The board shop should be  running process 
controls on their microetch for roughness and silver  immersion plating bath.  The 
plating rate and final thickness are  recommended by the bath supplier, and 
should be obeyed in fabrication. Both shop  and supplier may be running assembly 
simulation and then cross sectioning to see  if there are any microvoids. 


Thanks,
Gaby

Dennis Fritz
MacDermid, Inc. 



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