Principle Of Fiber Self-Alignment Using Wetting Force Of Solder And Interfacial Reactions In Laser Soldering

In optoelectronic packaging, alignment and attachment between laser and optical fiber is the key technology to assure component coupling efficiency and reliability. In this paper, a new method of fiber self-alignment by laser soldering has been proposed which uses the restoring force and restoring torque to accomplish the self-alignment of fiber on the pad. The manufacturing cost of optoelectronic packaging can be reduced by using the new process produced by this method. The feasibility of the process was demonstrated by studying the evolution of intermetallic compounds (IMCs) at interface between AuSn solder and pad (Au/Ti pad and Au/Ni/Cu pad) as well as that between AuSn solder and fiber metallization (Au/Ni) during laser soldering and aging process. The basic principle of fiber self-alignment was studied and demonstrated, the driving force of self-alignment educed in theory, effects of material and structure factors analyzed, and alignment precision and error estimated.Research on fiber attachment by laser soldering process showed that AuSn solder alloy was prone to wetting, spreading and forming good solder joint at low laser power and long heating time in argon atmosphere. The characteristics of interfacial reaction between AuSn solder and Au/Ti pad during laser soldering process showed that with the increase of laser input energy the morphology of preeutecticζ-phase changed from scallop protrusions to large cudgel shape and dendritic shape. During aging process,ζ-phase formed at the interface of AuSn-Au/Ti began to coalesce and grow, while the needle-like (Au,Ni)3Sn2 formed at the interface of AuSn-Au/Ni began to dissolve into the solder and disappear, replacing by a layer of (Au,Ni)Sn at the interface, at the same time, the eutectic microstructure of solder began to spheroidize and coarsen. During aging process, the needle-like (Au,Ni)3Sn2 formed at the interface of AuSn-Au/Ni/Cu as well as AuSn-Au/Ni under the condition of laser soldering began to flatten gradually, and a continuous layer of (Au,Ni)Sn formed at the interface, the thickness of which increased with aging time; The flower-like (Au,Ni)3Sn2 formed near the interface of AuSn-Au/Ni/Cu as well as AuSn-Au/Ni began to dissolve into the ambientδ-phase and disappear during aging, at the same time, the content of Ni inδ-phase increased.Based on the reasonable hypothesis, the mathematical model of three dimensional (3-D) shape of solder joint in fiber attachment soldering was established, the energy, the volume and the boundary condition of solder joint shape was described in reason, 3-D shape of solder joint was calculated by employing finite element method (FEM), and the influence of material and structure factors on 3-D shape of solder joint was analyzed. The results showed that the extent of solder wetting on fiber increased with the increase of solder volume and the decrease of pad size; With the increase of aspect ratio of pad, the bending extent of curve at cross section increased. The 3-D shape of solder joint was measured by laser scanning confocal Microscope (LSCM) and the results were compared with those of calculated, the results showed that the measured results of 3-D shape of solder joint were in accordance with the calculated results, and the method of calculating and measuring of 3-D shape of solder joint was reasonable and credible.The formulation of self-alignment driving force was deduced theoretically by analyzing the force acting on fiber. Base on the formula, the restoring force and normal reaction force could be calculated which mainly depended on the wetting force acting on fiber by solder, and the hydrostatic pressure could be ignored. The theoretical equilibrium positions of pad with different shape were calculated by FEM, and the influences of material and structure factors on self-alignment restoring force and restoring torque were analyzed. The results showed that the self-alignment restoring force changed linearly with horizontal offset and the restoring torque changed linearly with yaw angle. Bigger solder volume within limits, bigger aspect ratio of pad, bigger surface tension of solder and smaller solder-pad contact angle were propitious to the self-alignment for horizontal offset, while smaller solder volume within limits, bigger aspect ratio of pad, bigger surface tension of solder and smaller solder-fiber contact angle were propitious to the self-alignment for yaw angle.Calculation results on stand-off height (SOH) between fiber and pad showed that the SOH existed when the solder volume exceeded the critical volume. SOH increased with the decrease of pad size, the decrease of aspect ratio and the increase of solder-fiber contact angle. SOH was measured by LSCM, and the measured results were in accordance with the calculated results.