Sam McCorkle asks about the appropriateness of using an immerion tin 
     coating as a finish on a board to be soldered within a month.  The 
     answer involves two facts of chemistry and one fact of economics:
     
     1. Immersion tin is inherently very thin.  Once the tin has covered 
     the copper so that no more is exposed, the reaction stops.
     
     2. Tin reacts with copper, even at room temperature, to form a product 
     which, if exposed to the air, is difficult to solder to.
     
     3. It doesn't take the rework of very many boards which have lost 
     solderability to swamp any savings from the use of a cheaper coating.
     
     A thin tin coating offers very limited protection to the reaction 
     product simply because so little is there.  It doesn't take any time 
     at all for air to react with the reaction product (called "intermetal- 
     lic compound") once the tin above it is gone.  For a board already on 
     hand, there is one (and only one) thing that can be done to prolong 
     its solderability: reduce the reaction rate by reducing the temper- 
     ature.  Put it in a freezer.  Keeping it in a dry inert room- 
     temperature environment does nothing to reduce the rate at which the 
     tin and copper react; putting it in a freezing environment drastically 
     reduces the rate not only of the copper-tin reaction but of any 
     corrosion reaction that might occur due to the presence of porosity in 
     the tin.
     
     Gordon Davy

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