| The miniaturization and light-weight requirements of electronic devices continue to increase the density of electronic package.Two-dimensional planar packaging has reached its bottleneck and three-dimensional packaging is imperative.The wire bonding method can realize the vertical interconnection of multilayer plane and and horizontal interconnection of the same plane so that it is most expected to become the fundamental solution of three-dimensional package.The traditional method has difficulty in bonding coarse wire,for example,low strength,difficult process,wholly heating,the risk of damaging the device,etc.So it is urgent to develop a new method of coarse wire bonding suitable for three-dimensional package.In this paper,the parallel gap micro-resistance welding method is used to bond the aluminum wire and the gold copper pad.Its process,bonding mechanism and reliability are studied in depth.In this paper,the process parameters of 100μm aluminum wire and 3μm/50μm gold copper pad were optimized,the proper process range was selected and the effects of welding current and welding time on the tensile properties of welding joints were studied.The morphology and section constituent of the welding joints under different process parameters were analyzed,the tensile fracture was characterized,and the failure mode of the welding joint under the tension load was studied.The temperature,dynamic resistance and current density of welding joints were obtained by welding process detection system and finite element method.The phase at the interface of welding joint was characterized by transmission electron microscope,and the mechanism of micro-resistance welding bonding of aluminum wire/gold copper pad was put forward.Through the reliability test of electrothermal aging,the variation tendencies of the interconnection resistance and tension force were studied,the transformations of section structure and constituent of welding joint were analyzed under thermal and electrical load.The crack initiation and propagation under tensile force were analyzed as well.The results showed the process range obtained after optimization: welding current 160A~200A,welding time 10ms~20ms,welding pressure 4N.With the increase of input power,the morphology of welding joints changed from complete to incomplete,and a variety of ternary compounds at the basis of Au-Al and Cu-Al compounds grew from Au-Cu-Al solid solute in the Au coating.With the increase of welding current,the tensile force of the welding joint increased first and then tended to remain,and the minimum tensile force of the welding joint under different process parameters was 27.5g.Under the load of tensile force,low current welding joint failed at the interface,medium and high current welding joint failed at the junction of welding joint and aluminum wire.The temperature of the welding joint increased with the the input energy,the aluminum wire underwent the transition from solid to liquid.The temperature and current density of the welding joint were symmetrical distribution in relation to the center of the welding joint.The dynamic resistance of welding joints was relativly stable during welding.The larger welding current,the smaller the average resistance after the welding was stabilized.Transmission electron microscopy characterization confirmed the formation of a variety of ternary compounds at the basis of Au-Al and Cu-Al compounds.A Cu-rich γ phase formed at the side of Cu interface at the presence of the a crystal with a cubic structure and the twin with a three oblique structures.The lattice constant of the three-oblique structure γ phase was then calculated.The martensite phase transformation in the Cu-side interface easily leads to the formation of microcracks in the Au pad,which affects the performance of the welding joints.The microstructure of the martensite is microtwins.The Cu-rich gamma phase and high temperature stable phase ζ phase formed in the Cu side interface.The γ phase had a three oblique or body-centric tetragonal twinning structure and a cubic structure.The lattice constants of the twin structures were calculated,and different contents of the body-centric tetragonal twin,had slightly different lattice constant.Three bonding mechanisms of micro-resistance welding of aluminum wire/gold copper pad were put forward: the solid bonding mechanism at the low input power,the mixed mechanism of partial liquid partial solid bonding at the medium input power,and the mechanism of liquid bonding with complete melting at high input power.In the electrothermal aging test,the interconnect resistance of the solder joint increased,the tensile performance decreased.The crack initiated and propagated at different interfaces,and finally led to the failure of the solder joint under the load of tension force. |
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