Copper substrate dissolution in eutectic Sn-Ag solder and its effect on microstructure

The dissolution of Cu into molten Sn-3.8at.%Ag (Sn-3.5wt.%Ag) solder and it s effect on microstructure were studied by light microscopy, scanning micro scopy, and x-ray microanalysis. X-ray microanalysis of the average Cu conte nt of samples soldered under various conditions showed that the amount of C u dissolved during soldering increased with increasing soldering temperatur e and time and that the rate of dissolution could be described by a Nernst- Brunner equation. Microstructurally it was found that the volume fractions of primary beta(Sn) dendrites and eta-phase dendrites increase with increas ing soldering temperature and time. The microstructural changes can be expl ained using Sn-Ag-Cu phase equilibrium data. A numerical method was develop ed for calculating the amount of Cu dissolved under non-isothermal conditio ns, which describes dissolution reasonably well.