Hi Bill:
     
     Solderability is such a fun subject! And it will hopefully keep us 
     metallurgical engineers employed for a while longer! Here are my 
     opinions on your questions:
     
     1. Does anyone know of a good test coupon or test method which can be 
     used to evaluate pwb surface solderability?
     
     I would suggest using the IPC JSTD 003 specification. The coupons in 
     the specification have some history and industry use. Their designs 
     are not set in stone - you can modify them to reflect your particular 
     design or fabrication/assembly specifics.
     
     
     
     2. What role does surface finish play in solderability and will a 
     rough plated Ni finish have more solderability problems than a 
     smoother finish?
     
     
     I believe that the surface finish is responsible for 80% of the 
     solderability problems the industry runs into today (boy, this 
     statement will get the TechNet fired up!). Solderability of a surface 
     finish is impacted by two sources: diffusion and oxidation. Diffusion 
     will be defined by the specific finish (copper, silver, gold, etc.), 
     the interaction of the finishes (copper on silver, solder on copper, 
     gold on nickel, etc.) and the thickness of the finishes. Many 
     solderability problems disappear provided that a sufficient thickness 
     is applied. Oxidation is a characteristic of the specific finish and 
     it's interaction with the environment. This interaction is why so many 
     people are investigating steam aging, dry aging, and other 
     "durability" conditioning steps looking for a recipe that will provide 
     some predictive capability. Surface roughness would be one variable 
     that for many finishes would impact the oxidation characteristics thus 
     impacting solderability. Look at the SERA testing that is occurring in 
     the industry - a method of looking at a surface's oxidation 
     characteristics that many people are relating to process control. 
     There have been several papers published in the IPC on the subject. 
     Depending on what type of nickel you are using (low P or high P) the 
     roughness may not matter because of grain structure considerations and 
     the fact that nickel oxide is a self limiting, dense structure.
     
     
     The paper in Plating & Surface Finishing "Factors Influencing 
     Solderability of Electroless Ni-P Deposits", July 1992 mentioned on 
     the TechNet last week is informative. Also try "Contact Materials for 
     Electronic Connectors: A Survey of Current Practices and Technology 
     Trends in the USA" in Plating & Surface Finishing, June 1991 - it has 
     a great reference section which contains some nickel info. There will 
     be a great deal of information on diffusion/oxidation/solderability at 
     this year's SMI conference which may be of interest to you. Good luck.
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]
     


______________________________ Reply Separator _________________________________
Subject: Re: Solderability of Electoless Ni
Author:  [log in to unmask] at ccmgw1
Date:    7/26/96 4:57 PM


We have been starting to have solderability problems with some electroless 
nickel plated pwbs.  Nickel has always been a challenge to solder... but we 
have been successful until recently by simply doing a pumice scrub prior to 
solder stenciling, and by minimizing the time through assembly and into reflow.
     
There have been a large number of Technet communications on Electroless Ni 
and Immersion Au over the last few weeks.  We are tying to learn from those 
who are having similar problems.
     
A few topics which were not fully addressed and would be of great value are:
     
1. Does anyone know of a good test coupon or test method which can be used 
to evaluate pwb surface solderability?
     
2. What role does surface finish play in solderability and will a rough 
plated Ni finish have more solderability problems than a smoother finish?
     
     
Any help would be appreciated.
     
Bill Dieffenbacher,  Lockheed Martin Control Systems 
[log in to unmask]  Phone 607-770-2961  Fax 2056 
_______________________________________________________________
     
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