KP Chan requested info on the causes of solder blow-out in soldered 
     PTH assemblies other than insufficient copper thickness.  Several 
     people blamed water in the board and suggesed baking, but this doesn't 
     explain why one set of boards is more susceptible to the problem than 
     another.
     
     Guenter Grossmann's comments help to identify the underlying cause: 
     bad drilling, leaving a rough surface to be electrolessly plated.  But 
     there's one more part to the puzzle: plating of the rough hole surface 
     leaves copper plating salts trapped between the laminate and the 
     metal.  Those salts are hydrated.  That means that water molecules - 
     lots of them - actually make up a part of the crystal structure of the 
     salt.  Thus the source of the vapor that blows the solder out of the 
     hole is not just water molecules that are dissolved in the resin, but 
     water that is attached, rather strongly, to the salt.  Some of these 
     molecules can be driven off by the kind of baking that has been 
     recommended, but not all, no matter how long you wait, so it is a 
     matter not of luck but physical chemistry.
     
     You can demonstrate the phenomenon easily with a crystal of copper 
     sulfate.  (There are five water molecules for every copper atom in 
     this crystal.)  Using a long pair of tweezers or forceps, immerse the 
     crystal in molten solder.  You will observe bubbles that will con- 
     tinue for as long as you are willing to stand there. During soldering, 
     the heat drives off the water molecules in the crystals of plating 
     salts in just the same way, with sufficient pressure to break through 
     the plating, or, if the copper is thick enough, push it away from the 
     hole wall.  (Incidentally, this experiment will not contaminate the 
     solder, as copper sulfate does not react with and is not soluble in 
     solder - it just floats - and to the extent that water reacts with the 
     solder, the product is just dross.)
     
     The _Handbook of Chemistry and Physics_ shows that four of the five 
     can be driven off at 110 C, but the last one requires 150 C - just a 
     bit hotter than you'd like to use for baking, and other hydrated 
     copper salts may well be present in the board as well, with even more 
     tightly bound water.  Even with solder at 260 C, the evolution is 
     steady but not explosive.  This explains why baking doesn't solve this 
     problem (which is not new): the conditions required to remove the 
     water from the crystals before soldering are just too rigorous to be 
     practical.
     
     Should the connections be reworked?  In an earlier posting to TechNet, 
     (12/23)I stated that very little solder is actually needed to make a 
     through-hole connection reliable.  The connections you are getting, 
     even the worst-looking ones, will almost certainly have enough solder 
     remaining to be reliable.  To be consistent, a person who disagreed 
     would have to tell you to scrap the assembly - rework doesn't make 
     sense.  Here's why.
     
     Were you to X-ray or cross section the board, you would find at least 
     as many ordinary-looking connections that had bubbles frozen beneath 
     the surface as there are holes where the solder has been blown out.  
     The difference is only a matter of timing: where was the bubble when 
     the solder froze?  Trying to resolder the holes from which solder had 
     blown out, even if you were successful, would not address the hidden 
     but comparable condition for the remaining holes.
     
     As for prevention, the answer is much simpler: buy boards from a 
     fabricator who knows how to (and does) drill smooth holes.
     
     Gordon Davy

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