Kinetic interactions of copper, lead and tin on solder coated PC boards studied using X-ray diffraction

X-ray diffraction has been used to monitor the growth and decay of the constituent phases existing in solder (60% Sn-40% Pb) coated Cu printed circuit boards. Samples were aged from 145°C to 175°C for times sufficient to allow the Cu₃Sn intermetallic phase X-ray peak count rate to exceed that for the Cu₆Sn₅ phase. Activation energies have been determined for the growth of (1) Cu₆Sn₅ and Cu₃Sn and (2) the dissolution of crystalline from the solder matrix. The data indicate that the activation energy for Cu₆Sn₅ growth is Sn concentration dependent and can be related to the Sn solubility in Pb. The activation energy for Sn dissolution is not comparable to Sn lattice diffusion. A compressional d-space change in the Pb lattice induced by dissolved Sn is observed. The Pb(111) orientation is observed to grow preferentially during aging at all measured temperatures