Tin/Nickel has been around for along time. You're absolutely right.
     The conductor overhang can be such as to produce bad slivers that can
     easily create shorts. A good way to overcome that issue is to tightly
     control the plated thickness to an extreme minimal. (Generally this
     must be negotiated with the customer up front due to customary Nickel
     requirements.) If the deposit thickness is <250 u inches, (50-100 
     preferred, 100-200 with fairly good results) you can generally cleanly
     etch the circuitry with the overhang at a minimal or even removed
     during the etching process. If a mild scrub is required, at that 
     thickness you can eliminate the overhang issue(s). Tin/Nickel,
     however, has really only focused on niche markets such as high temp
     boards (BIB)and a few others. It really never was widely accepted
     due to similar issues that you listed. The tin surface is more
     user friendly.
     
        Surface Tin plating is something that many large volume shops
     are doing that has been successful. (No solderstrip required. HASL
     readily takes to the solderable surfaces with tin over copper.)
     I'm waiting to see what their comments are.


______________________________ Reply Separator _________________________________
Subject: Re: Solderability of Electrolytic Tin Plating
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    9/27/95 9:34 PM


In a previous life I worked for a place that made thousands of tin boards. 
The customer (Texas Instruments) specified nickel as a migration barrier 
between the copper and tin.  There is no reason you can't use bright acid 
tin as a surface finish.  The one caveat we encountered was that using 
tin nickel as an etch resist tended to give very sharp edges to the lines 
where the etchant undercuts the nickel.  Operators can cut their hands 
and if you overetch and get slivers they are like little razors.
     
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On Wed, 27 Sep 1995 [log in to unmask] wrote:
     
> Does anyone have any experience soldering surface mount devices to
> Electrolytic Tin plating as opposed to Tin/Lead reflow or HAL?  We have a 
> customer that is interested in this but wants us to ensure that they'll be 
> able to solder the boards.  They are using a BTU convection oven for
> reflowing the boards after assembly.  Thanks in advance for any 
> correspondence.
> 
>