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.

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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