Research And Development Of Solder Alloys And The Soldering Process For Ultrasonic-iron-assisted Active Soldering ZnO Devices

As a wide-band-gap semiconductor material,ZnO has been widely used in electrical and electronic engineering,photovoltaic modules,optoelectronic devices etc.For example,ZnO varistor is used as overvoltage protection devices in circuits,and depositing a layer of ZnO thin film on the solar glass panel can improve the light transmittance and photoelectric conversion efficiency of a thin-film silicon solar cell.Both the above ZnO devices need to be soldered with leading wires to achieve their electrical functions.However,as a metal oxide,ZnO is a hard-to-solder material.Surface metallization is usually used as a metallic electrode layer to improve solderability and electrical performance.In order to reduce the manufacturing cost of ZnO devices,using cheap metals as electrode or directly soldering ZnO without a metallic layer is significantly important in research highly valuable in applications.The purpose of this thesis study is to explore a low-cost,easy-to-implement ultrasonic-iron-assisted soldering process for ZnO devices.Firstly,Sn-Ag-Ti based active solders were used to investigate the soldering mechanism of Al alloy and evaluate the feasibility of replacing Ag electrode with an arc-sprayed Al electrode.Subsequently,bare ZnO without a metallic electrode layer was subjected to ultrasonic-assisted direct soldering using Sn-Ag-Ti based active solders.Ultrasonic parameters were optimized and soldering mechanism was studied.Finally,in order to reduce the high cost of Ti-containing solders,Zn,Al and Sb were selected as active elements to evaluate the solderability and further reduce the material cost.The experimental results show that when Sn-Ag-Ti based active solders were used to solder Al alloy with ultrasonic-assistance,the oxide layer on aluminum surface can be removed under the effect of ultrasound,and then Al grains were dissolved by the solder.The interconnection mainly depends on the mechanical interlock formed by the contact area between the solder and the uneven surface or inner holes of Al substates.The amount of Ag-Al phase formed near the interface decreases with decreasing Ag content in the solder,but the effect of the Ag content on the bonding strength of the solder joint is not obvious.The chemical adhesion of element Ti with Al₂O₃ layer promotes wetting and spreading of the solder on Al substrate.When using Sn-Ag-Ti based active solder to solder ZnO substrates,the solder joints with high strength can be obtained when the ultrasonic time is appropriate（e.g.,5 s）,while too short or too long ultrasound time is not conducive to the improvement of joining strength of the joints.Moreover,shear strength of the solder joints increases gradually with increasing Ti content in the solder.Chemical reaction between Ti and ZnO takes place and metallurgical bonding is formed to ensure the strength of the joints.The peeling force values of the leading wires soldered on both types of ZnO substrates meet the industry standard.Finally,when Zn,Al and Sb were selected as active elements during soldering,Zn element obviously promotes wetting and spreading of the sloder on the surface of the ZnO substrate.However,high Zn content will lead to aggravation of the the corrosion of ZnO surface,decreasing surface roughness and then reducing the strength of solder joints.The strength of solder joints using the solder containing Al and Sb as active elements is higher than that using Zn as active element.Nevertheless,due to the inability of forming an effective metallurgical bonding,the strength of solder joints using Zn,Al and Sb-containing solders is still lower than that of solder joints using the solder containing active element Ti,which is mainly attribute to mechanical interlock between the solder and ZnO substrate and chemical adhesion of active elements.