Hi TechNet
     
     You may be able to use SERA (sequential electrochemical reduction 
     analysis) testing to determine what the oxidation is and/or use the 
     SERA technology as a process control tool. SERA analysis has been used 
     by a number of people for similar oxide identification and process 
     control. Contact Peter Bratin at ECI Technology 201-773-8686 for 
     additional help. The results of the IPC/EIA SERA Gauge R&R study will 
     also be presented at the IPC Expo in a couple of weeks which may be of 
     interest too. Good Luck.
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Re[2]: FAB: Discoloration in solder plating
Author:  [log in to unmask] at ccmgw1
Date:    2/20/97 3:00 AM


Thanks for details explaination on Sn+2 AND Sn+4 Oxidation.
This is reffering to Plating solution discoloration. We  manage control this 
using Solderon RD by Lea Ronal or Hydroquinone in Technic system. 
     
I was reffering to discoloration on the "work/lead frame".
     
The leadframe is copper. Thus we plating solder (tin + lead) over copper. 
My problem is;
When the plating machine stop immediately the partially plated leadframe will 
change to gray or dark color. When the production resume this gray/dark color 
remain on the leadframe. Some time this portion will fail solderability test 
(fail to mount to PCB). 
Somebody told me that this is due to Methane Sulfonic Acid remain on the 
leadframe and oxidise the leadframe.
     
I tried to rinse the leadframe everytime the production stops. Slightly better 
but not good enough. 
     
What is actually the grey/dark color on top of solder layer?
     
Any other suggestion to solve this problem?
     
_______________________________________________________________________________ 
Subject: Re: Discoloration in solder plating
From:    [log in to unmask]@INTERNET at #EMAIL 
Date:    2/17/97  12:01 PM
     
Misri Mohd replied to me personally that they are using the Methane Sulfonic 
Acid based electrolyte.
     
The Methane Sulfonic Acid based system has certain properties that must be 
attended to when plating Tin or its alloys that may be causing some of the 
problems alluded to in the previous message.   Tin has two oxidation states 
besides the metal, Stannous ( 2+) and Stannic (4+).   The Stannous Tin, which 
is the normal, and preferred, state in this bath,  readily oxidizes to 
Stannic Tin in the presence of either atmospheric Oxygen, or if the anode 
area is too small for the current being pushed through the bath.
     
When Stannic Tin forms, it is insoluble, and forms a milky looking 
precipitate in the bath.   The Methane Sulfonic Acid bath is clear, (not to 
say uncolored) when there are is no Stannic Tin in the bath.   This milky 
precipitate normally stays suspended in the bath, and can plate out, 
(co-deposit) and become part of the plating.  The amount of the Stannic Tin 
plated out will change with the current density, with greater current density 
tending to plate more Stannic Tin.   This could be the cause of some of the 
color change of the plating.
     
The Stannic Tin is difficult to filter out, but there are ways to get it out 
of the bath, and my guess is that there are people reading this that can give 
this gentleman some directions at finding the appropriate technology.
  Alternately the plating bath supplier may be able to assist in this matter.
     
Hope this helps
     
Rudy Sedlak
RD Chemical Company
     
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