Physical chemistry of gas liquid solder reactions
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I.
INTRODUCTION
S O L D E R S used in joints in electronic devices may have porosity or pinholes. There are several possible mechanisms responsible for this porosity. One is the difference in the solubility of gases, such as nitrogen and hydrogen, between the liquid and solid solder. In general, the solubility in liquid alloys is considerably higher than in solid alloys. Therefore, during solidification the equilibrium pressure of nitrogen and hydrogen in the liquid will increase due to enrichment causing gas evolution and pinholes or porosity. A second possible mechanism is that the hydrogen resulting from the reaction of water vapor with metal may cause a surface pinhole. In order to understand the possible origin of pinholes or porosity, the solubility and solution thermodynamics of nitrogen and hydrogen in solder alloys must be known. There are no data available on the solubility of gases in the lead-tin system in the composition and temperature ranges employed in soldering processes. Therefore, it is one of the purposes of this study to determine the solubility of nitrogen and hydrogen in pure lead and lead-tin alloys. The H2-H20 gas mixture that is in equilibrium with leadtin alloys can be calculated from the activity of tin in the lead-tin solders. The activity of tin in lead-tin alloys has been measured by Goto and St. Pierre I at temperatures between 973 and 1173 K by using a ZrO2"(CaO) electrochemical cell. Their results indicate that the activity of tin shows a large positive deviation from the Raoultian ideal behavior. Similar results also have been reported by Elliott and Chipman 2 at 773 K. In general the temperatures used in soldering processes are lower than those used in previous investigations. Therefore, it was also decided to measure equilibrium H2-H20 gas mixture and compute the activity of tin in the lead-tin system at 600 K using a flowing gas equilibration technique. BAHRI OZTURK, Research Faculty, and R. J. FRUEHAN, Professor, are with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. P. BARRON, formerly Graduate Student with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213, is employed at Lanxide Corporation, Newark, DE 19711. Manuscript submitted October 14, 1986. METALLURGICALTRANSACTIONS B
II.
EXPERIMENTAL
A. Solubility Study A modified constant volume Sieverts' technique was used to measure the solubility of nitrogen and hydrogen in leadtin alloys at 698 K. A schematic diagram of the modified Sieverts' apparatus is shown in Figure 1. An alloy of known composition was contained in a quartz crucible. Then the sample was placed in a furnace and connected to the system. The sample was evacuated by a mechanical pump for about an hour at room temperature. Heating was done under a pure hydrogen atmosphere in order to remove all the oxygen from the system. When the desired temperature was obtained, the system was evacuated by means of a mechanical pump for
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