Nanoscale physics

Copper plates bonded by solder pastes: effects of copper crystallinity and powder coatings on joining kinetics
Yu.O. Lyashenko, O.Yu. Liashenko, S.Derevianko, V. Morozovych
Physics Department, Cherkasy National University, Shevchenko, 81, Cherkasy, Ukraine.
E-mail: urico@ukr.net

Soldering technology in microelectronics has faced a lot of difficulties during last two decades due to the ban of the well-developed lead-based solders and the significant miniaturization of interconnects. Recently the transient liquid phase (TLP) bonding became a method of choice for bonding the components of high-power electronics. In this method solid Cu substrates with liquid filler between them are till thermodynamically equilibrium solid intermetallic layer is formed between Cu substrates. In the frame of our investigation we suggest to overcome the difficulties of long thermal treatments by sintering of powder of pre-alloyed Cu-rich intermetallic phases coated with thin nanometric tin layer between the Cu substrates. In this approach the intermediate layer of the joint assembly will reach thermodynamically equilibrium state much faster (in time less than 1h) comparing to the conventional TLP processes (duration of process normally more than 5 h).
Due to the brittle nature of the Cu-Sn intermetallics the ball milling will be an appropriate method for production of fine powder. Prior to the sintering the pastes, in form of a mixture of Cu-Sn intermetallic particles and organic liquids, were prepared. After thermal annealing an organic liquid evaporates, so the scanning electron microscopy can be used for studying the densification of filler layer and joining between powder particles and substates. In some cases the mechanical compaction of powder was performed prior to sintering experiments. 
The electroplating of Cu layers in reversed pulsed and stochastic regimes was performed in order to study the effect of the Cu plates surface layer structure (composed of nanometric or micrometric grains) on the joining kinetics between plates and intermediate layer. The x-ray diffraction methods were used for determining the microstructure of the surface layers of copper depending on the type of the applied electroplating regime. In some cases we found broadening of the diffraction peaks, which indicates the grain refinement. We also found preferential (220) orientation of Cu crystallites. An effect of such texturing and grain refinement on the surface and bulk diffusion kinetics and, consequently, on the joining kinetics of Cu substrates and intermediate layer was elucidated and modeled.