Hello Tracy - you characterized the concerns with electroplate tin 
     finish very well. Just a couple of comments:
     
        - Tin whiskers can be prevented by "poisoning" the tin bath with a 
     very small percentage of another element. There have been numerous 
     papers in the industry on the subject. Bismuth, antimony, lead, etc. 
     can all be used to prevent the whiskering. I suggest you talk with 
     your plating supplier and check on what they use to poison their bath.
     
        - I believe that you will have fewer problems with oxidation of 
     electroplate tin than HASL, that has been my experience with pwbs we 
     process. We have done quite a bit of SERA solderability testing and 
     the oxides (SnO and SnO2) that develop can be removed with the fluxes 
     we are using. There was a paper published within the last year by Urmi 
     Ray of AT&T on immersion tin that may interest you too (IEEE 
     Transactions on Components, Packaging, and Manufacturing Technology - 
     part A, Vol. 18, No.1, March 1995).
     
     
     David Hillman
     Rockwell Collins
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Tin electroplate finish for PCBs
Author:  [log in to unmask] at ccmgw1
Date:    4/17/96 8:42 PM


     
I want to evaluate the possibility of converting to a 100% tin electroplate 
finish on PCBs that requires a tin or tin-lead for contact pads.  Primarily 
72 pin SIMM memory modules which are destined for use in tin or tin-lead 
(usually high tin content 90/10 - 95/5 tin-lead alloy).  
     
Overall thickness is an issue with SIMM products (.050" + .004" -.003").  The 
HAL process results in a adequate finish most of the time, however this process 
can not be tightly controlled and is a root cause for thickness rejects and 
contact rel problems.
     
I've a couple of goals:
     
1)  Improve quality and performance of our product by improving contact
    reliability.
     
2)  Give our PCB suppliers a way to improve yields for overall thickness. 
     
There are other pluses (for us and the PCB suppliers) associated with a 
conversion to tin that I won't bring up.  I'm primarily concerned with 
overcoming any potential quality or reliability problems.  Here are some 
of my concerns:
     
1)  Tin whisker growth.  Dependent on stress induced during electroplating.  
    Low/no stress plating is critical.  Whisker growth begins at ~50 'C.  
    Spacing between contact pads on a 72 pin SIMM is .010" so whisker growth 
    has to be eliminated.  Whiskering is not a problem with lead present. 
     
    Test conditions:  High temperature storage, 150 'C unbiased.  Length ?  
                      1080 hrs 125 'C high temp operating life, biased. 
                      85/85 APB T&H 
     
2)  Tin undercut must be minimized to prevent tin slivers.
     
3)  Solderability, tin-oxides form readily at room temperature.  This problem
    exists with HAL, but to a lesser same extent.  
     
4)  Copper-tin intermetallic growth, tin should be a minimum of 200 microinches
    thick.  Again, this problem exists with HAL.
     
    Test conditions: Same as in #1.
     
************************************************************
     
My question is, has anyone experience with a tin electroplate finish in a 
similar application?  Any potential problems I've overlooked?
     
All comments are appreciated.  Thanks in advance.
     
Tracy Tennant   ([log in to unmask])
     
ph (208) 368-5963