Hi Steve -
     
     No, you are not oversimplifying the problem. It's just that getting 
     the "rework" portion of the recipe right is not very simple. A fair 
     share of the industry looks at solderability problems as having two 
     main source causes: oxidation and intermetallic growth. Chris Hunt at 
     NPL has some really good info on intermetallic growth and growth 
     prediction equations - check out the 96 SMI proceedings for his paper. 
     
     The area I see lots of misconceptions is how to restore an oxidized 
     solder finish. If you had a thick solder finish that is just plain 
     oxidized without any copper/tin intermetallic complications then a 
     controlled process reflow with the right flux chemistry will make 
     things solderable again. If you have a thin solder finish that has 
     converted (via diffusion) itself into a copper/tin intermetallic you 
     need to use a very aggressive flux to attack the intermetallic oxide 
     and then build up/deposit a reasonable thickness of solder (say 60-120 
     microinches). If you just re-process the thin solder scenario without 
     increasing the solder thickness the copper/tin intermetallic just 
     reforms and oxidizes again -now you are back at square one. If I would 
     quickly (within a few days) process these thin solder scenario boards 
     in my assembly line with the right flux chemistry I would have 
     adequate yields - solderability goes down hill if the boards sit 
     around.
     
     That's why you will hear some camps say a reflow will restore an 
     oxidized board (the JIT type of processes) or hear some camps say a 
     reflow won't restore a board (the long storage time processes). There 
     are some new restoration/fluxless solder technologies that are in the 
     work too (e.g. ROSA, PADS etc.) that may help everyone in the future 
     if they prove viable.
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]
     


______________________________ Reply Separator _________________________________
Subject: Re: "Tinning house"
Author:  [log in to unmask] at ccmgw1
Date:    1/23/97 11:35 PM


Dave,
I have been following your discussion on solderability (root causes of 
excessive oxidation) and am lead to believe that if porous/thin tin leads are 
"reworked" properly by these tinning houses we could avoid the many inventory 
problems (storage time, slow moving parts, bagging with desicant, etc.) and 
still maintain or improve solder connection reliability.
The eventual/ultimate solution being to get the OEM's with poor tinning 
corrected or off the approved supplier list.
Am I over simplifying with this conclusion? 
Steve Ross
     
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