Hi TechNet -
     
     Flatness has been the biggest reason for the "unfused" requests for 
     tin or solder finishes per many discussions in the solderability 
     committees. However, there are two very divided, distinct groups: 
     people who have no solderability problems with "unfused" tin/solder 
     finishes and people who have disastrous solderability problems with 
     "unfused" tin/solder finishes. The IPC Steam Aging task group will be 
     publishing its report on various finish reactions with steam aging and 
     solderability testing - the "unfused" solder finishes performed very 
     well. My experience with "unfused" solder finishes has be bad - heavy 
     oxidation of the finish with diffusion down/up the grain boundaries to 
     the copper leaving oxidized intermetallic. This unfused finish was 
     reasonable thick enough that you would not have suspected the 
     diffusion problem. I have published data that states the reason for 
     "fusing" the finishes is to change the solder grain structure to 
     reduce oxidation and prevent diffusion problems. Again there are two 
     camps on the issue. 
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]
      


______________________________ Reply Separator _________________________________
Subject: Re[2]: Unfused Solder and/or Tin
Author:  [log in to unmask] at ccmgw1
Date:    10/17/96 3:15 PM


     Rudy, Dave,
     
     I concur with the flatness issue. Have seen several customers request 
     "no reflow" to achieve flat pads without changing their processes to 
     accommodate for gold or OSP's. I would believe Rudy's comment to be 
     true about requiring a more aggressive flux being needed to activate 
     the surface. I have also had customers request a minimal thickness of 
     plated tin/lead unfused on an MCM-L. They were "pasting" the pads, 
     which were in fact becoming the lead-frame attachments for a QFP, with 
     a higher temp solder (90/10 I think). This is so that the frame would 
     withstand the subsequent solder operations when the MCM-L is mounted 
     on the PWB. 
     
     Regards, 
     
     Tom
     
     
______________________________ Reply Separator _________________________________
Subject: Re: Unfused Solder and/or Tin
Author:  [log in to unmask] at INTERNET_GATEWAY 
Date:    10/17/96 11:18 AM
     
     
Rudy,
     
I believe the primary reason is that it is Flat.  Finish flatness is 
critical to many SMT component placements and if the assembler can solve 
the other issues you mention they might have a winner. I do know a 
company that customer was requesting a very thin unfused tin-lead plate 
for flatness issues.
     
Regards
__________________________________________________
     
David W. Bergman, V.P. of Technical Programs 
IPC
2215 Sanders Road
Northbrook, IL  60062-6135
847-509-9700 x340 Phone
847-509-9798 Fax
email  [log in to unmask]
www  http://www.ipc.org
faxback support 800-646-0089
---------------------------------------------------
     
     
On Thu, 17 Oct 1996 [log in to unmask] wrote:
     
> Recently had occaision to have some problems associated with recovering
> scrapped PCB's that have as final finish UNFUSED plated Tin or Solder (wasn't 
> sure which).  Is there anyone who can enlighten me as to why an
> assembler/user would want such a finish.  The finish oxidizes heavily, and 
> quickly, and I suspect likely needs to be cleaned immediately before
> soldering, or alternately requires a flux so active that it will etch the 
> substrate if not washed off in a few minutes....
> 
> Other than that, it is likely wonderful?? 
> 
> Can anyone help me understand this strange (to me) finish? 
> 
> Rudy Sedlak
> 
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