Frictional metallurgy induced formation and evolution of solder/Cu column interconnect microstructure and properties

A novel friction micro-welding (FMW) technology was used for copper column connection of column grid array (CGA) packaging. The investigation found that its temperature and strain fields could induce formations of diverse interface layers and new solder microstructures. This discontinuous scalloped Cu 6 Sn 5 or Pb(Ag)-rich adsorption layer could form at the upper interface with insufficient hold-tight force. The thin Cu/Sn diffusion layer formed at middle and lower interfaces with greater hold-tight force. The lower position friction pair could be transferred from the solder/Cu column interface to the inside of solders in the later stage of welding, and the reliable join where solder adhered to the Cu/Sn diffusion layer could form. High-temperature visco-plastic solders were not fully extruded from joints and became a micro-zone named stir flow zone (SFZ). In SFZ and dynamic recrystallization zone (DRZ), the friction could induce annealing softening of solders, weak bonding between dispersed intermetallic compound (IMC) particles and β-Sn matrix and low microstructure compactness, but could provide atomic rapid diffusion channels conducive to solid-phase welding. However, above adverse effects disappeared after aging. The average pull-out loads of the SAC305/Cu column and Sn37Pb/Cu column FMW joints increased respectively from 39.6 N and 31.9 N to 76.5 N and 59.3 N. By atomic diffusion, aging self-healing of FMW joints benefited from regeneration of the interface IMC layer, healing of crystal defects caused by friction deformations, dispersion reinforcement of scattered IMC particles in SFZ after solid-phase wetting and the generation of reticulated segregation phases with high melting points at grain boundaries of DRZ.