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 *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * *************************************************************************** * If you are having a problem with the IPC TechNet forum please contact * * Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] * ***************************************************************************