Study On Bonding Mechanism And Performance Of Flexible Graphite Films Joints Ultrasonically Soldered At Low Temperature

Graphite film exhibits good flexibility,low density,excellent electrical and thermal conductivity,which shows broad application prospects in the fields of flexible electronic devices,heat-dissipation modules of power device and so on.To fulfill the preparation and assembly of graphite film and electronic devices,it is inevitable to connect graphite film with other electronic components.Brazing can achieve reliable connection of graphite films.However,active brazing method which is commonly utilized for carbon materials requires high temperature,high vacuum environment,and long brazing periods,which is not suitable for the connection of electronic devices.In addition,the interlayers of graphite film have exfoliation tendency,and the electrical and thermal conductivity between interlayers of graphite film are both poor,which will bring difficulty in connection.Therefore,in this study,Sn-based coatings were prepared on the surface of graphite film(graphene assembled film(GAF)and graphite film for heat-dissipating(GF))with ultrasonic assistance,subsequently soldered joints of graphite films were realized by remelting metal coating,the connection mechanism between metal coatings and graphite under the action of ultrasonic waves were mainly studied,and the mechanical properties,electrical properties and heat dissipating ability of graphite film soldered joints were analyzed.With ultrasonic-assisted process,pure Sn coatings(solder spots)were successfully prepared on GAF surface,and SAC305 coatings were prepared on GF surface.Connection mechanism between coatings and graphite films were studied.Results showed that under the action of ultrasound,good combination between coatings and graphite films was formed,and there existed no defects such as cavities and cracks.Specially,Sn solder penetrated into GAF interlayers under the action of ultrasound to form the ramified structure.SAC305 did not penetrate into GF interlayers,which may be attributed to the direction change of ultrasonic vibration.In addition,the shock waves and micro-jets induced by ultrasonic waves bombarded the surface of GAF and GF,and both form an amorphous carbon layer at the film/Sn-based solder interface,which was beneficial to the transitions of lattice and coefficient of thermal expansion at the interface,and the enhancement of bonding stability.GAF soldered joints were obtained by remelting Sn solder spots.The mechanical properties and electrical performance of joints were studied.Silver conductive adhesive(SCA)was utilized for comparative experiments during experimental process.Results showed that the average tensile breaking force of GAF with the cross-sectional size of5 mm×25 ?m,and its soldered joints,SCA joints were 12.44 N,9.96 N and 3.99 N respectively.During tensile test,the soldered joints were failed due to the fracture of GAF,and SCA joints were failed owning to the interlayer exfoliation of GAF,indicating that soldered joints can overcome the interlayer exfoliation tendency of GAF.Besides,the electrical performance of GAF soldered joints was significantly improved compared with SCA joints,and soldered joints of GAF can remain excellent electrical conductivity after high temperature,thermal cycling and aging treatments,which can be resulted from the excellent electrical conductivity of solder,together with the ramified structure of GAF and the connection mechanism of amorphous transition layer under the action of ultrasound.GF soldered joints were obtained by remelting SAC305 coatings.The mechanical properties and heat-dissipating performance of joints were studied.Resulted showed that the average tensile breaking force of GF with the cross-sectional size of 10 mm×25?m and its soldered joints were 12.52 N and 12.51 N,the tensile load did not destruct the connection interface between GFs and coatings,and the soldered joints were failed due to fracture of GF.In addition,the solder joints of GF can maintain excellent heat dissipation performance after thermal cycling and thermal aging treatments,due to excellent heat dissipating performance of solder and good interface combination under the action of ultrasound.