On the thickness of Cu₆Sn₅ compound at the anode of Cu/liquid Sn/Cu joints undergoing electromigration

In the electromigration tests performed with the Cu/liquid Sn/Cu samples of effective Sn lengths \(450\, \upmu \hbox {m}\) and \(1.243\,\hbox {mm}\) under current densities of \(5.6 \times 10^2\) and \(3.0 \times 10^3\,\hbox {A}/\hbox {cm}^2\) respectively at \(250\,^{\circ }\hbox {C}\), the thickness increments of anode \(\hbox {Cu}_6\hbox {Sn}_5\) intermetallic compounds were in-situ observed using synchrotron radiation imaging technique. The size of the compound attained for the smaller sample at reflow time of 1 h was \(110\, \upmu \hbox {m}\) whereas the larger specimen yielded a thickness value \(152\,\upmu \hbox {m}\) by the end of 45 min. Though the magnitude of effective charge number for electromigration is reduced at higher current density values, it is revealed that the net effect of bigger current intensity is always the built up of thicker intermetallic compound. Additionally, the raise in medium temperature caused by greater joule heating in samples corresponding to bigger current density, can be associated with the increment of transport of Cu from cathode to anode. Owing to the slower diffusion phenomena in larger specimens, the electromigration enhanced growth of the compound is more pronounced at the later stage of the experiment. The numerical model for advection–diffusion of Cu species was implemented using finite element method.