D. Drake is looking for info on how much the solidus temperature of a 
     high-melting solder will be reduced in forming a connection where the 
     connection members are finished with a lower-melting metal or alloy 
     such as tin or eutectic tin-lead.  He/she gives as examples a compo- 
     nent lead with 200 to 500 micrometers (8 to 20 mils) of tin or 3 to 4 
     mils (75 to 100 micrometers) of solder.
     
     I suspect that the problem is one of perception, since real finish 
     thicknesses are much less than the examples.  Component manufacturers 
     work hard to get 200 microinches (5 micrometers) of solder on the 
     major flats of rectangular leads and 60 microinches (1.5 micrometers) 
     of solder on round leads by dipping.  A plated lead finish thickness 
     is typically a little thicker, but nowhere near a mil.  A printed 
     board, whether through-hole or surface-mount, also has a finish 
     thickness at least an order of magnitude less than the examples.
     
     Hence, for any ordinary type of connection, the fraction of the solder 
     in the final connection that is contributed by the finish rather than 
     by addition (solder paste, preform, wave, or other) is small enough to 
     make the reduction in the solidus temperature insignificant.  This 
     could be confirmed without too much difficulty by attaching a fine 
     thermocouple to a connection and watching for a break in the rate of 
     rise or fall as it is heated or allowed to cool, or just watching for 
     the first sign of melting and noting the temperature.
     
     Gordon Davy

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