Study On Preparation And Properties Of Low Dielectric Constant Polyimide Films

Polyimide(PI)is one of the high-performance engineering plastics,and widely used in microelectronics industry due to its excellent mechanical properties,and thermal stability,as well as electrical insulation properties.Recently,low-dielectric-constant PI is required to meet the rapid development of ultra-large-scale integration and high frequency printed circuit.However,the traditional PI is not available.Therefore,how to develop a new type PI with low dielectric constant has become a hot topic in this field.In order to obtain the multilayer PI with low dielectric constant and high comprehensive performance,in this thesis,graphene oxide/polyimide(GO/PI)composite film,fluorographene/polyimide(FG/PI)composite film,porous PI film and multilayer PI composite film were prepared by using3,3?,4,4?-benzophenonetetracarboxylic dianhydride and 4,4?-diaminodiphenyl ether as monomers.PI films were systematically characterized by scanning electron microscope,X-ray diffractometer,tensile tester,impedance analyzer,thermo-gravimetric analyzer,etc.The dielectric properties and mechanical properties of the films were analyzed in depth using the Clausius-Mosott equation,series capacitance theory and parallel structure stiffness theory.The research promotes the development of low dielectric constant PI materials.Detailed research contents are as follows:Three kinds of PI films(BTDA-PI,PMDA-PI,and BPDA-PI)were prepared by a two-step method,and their mechanical properties,dielectric properties and thermal stabilities were compared,respectively.It found that BTDA-PI film had the most comprehensive performance,which showed the great difference from PMDA-PI,and BPDA-PI film.The dielectric constant of BTDA-PI film is 3.56,tensile strength is 114 MPa,and the characteristic breakdown strength is 478.90 kV/mm.Therefore,BTDA-PI type PI was selected as the basis for subsequent research.On the basis of the synthesis condition of BTDA-PI film,the GO/BTDA-PI composite films were prepared by an improved in-situ polymerization.The results show that the addition of GO can improve their mechanical properties,but it cannot affect the dielectric constants.Therefore,we chose FG instead of GO,the high-quality FG was prepared by liquid phase ultrasonic stripping method,and the excellent dispersive FG/PI composite films were obtained by solution blending.The results show that a small amount of FG can reduce the dielectric constant and improve the mechanical and hydrophobic properties of the PI film.When the content of FG was 0.5 wt%,the dielectric constant of the PI film can be reduced to 2.49,the tensile strength increased to160 MPa.In addition,porous PI films were prepared by solution phase inversion method.The influence mechanism of solvent composition and the temperature of the coagulationbath on the structure and dielectric properties of the films was investigated.The porous PI film obtained from N,N-dimethylacetamide(DMAC)showed the finger-like macro-void structure,high porosity and relatively low dielectric constant.On the other hand,the porous PI film obtained from 1,4-butyrolactone(GBL)showed the spongy-like micro-voids structure,low porosity and relatively high dielectric constant.As the porous PI film derived from GBL-DMAC co-solvent,finger-like structure combined with the spongy-like structure was obtained,and the dielectric constant of PI film decreased with the increasing content of DMAC.Moreover,the effect of coagulation bath temperature on the structure and dielectric properties of different solvent system films was also studied.In the single solvent(GBL or DMAC)system,the new small pores or grooves were formed when increasing the temperature of the coagulation bath,leading to reduce the dielectric constant.On the contrary,in the GBL-DMAC mixed solvent system,the formation of the finger-like macroporous structure was inhibited by increasing the temperature of the coagulation bath.Therefore,the decline of the porosity leads to an increase of the dielectric constant.FG/PI nonporous composite film was served as a dense exterior surface layer.Porous film was used as the core layer to reduce the dielectric constant.The double and three layers of low dielectric PI composite film were prepared by a layered coating method.The influence mechanism of composition and structure of the film layer on the dielectric properties,mechanical properties,and thermal stability was analyzed.Compared with the single-layer porous film,the mechanical properties,breakdown performance and thermal stability of the double-layer and three-layer films have been improved.The double layer PI-1 and FG/PI-1 films with good dielectric and mechanical properties were obtained.The dielectric constants of them were 2.01 and 1.98respectively;the breakdown strength was 180.75 kV/mm and 198.14 kV/mm respectively,the tensile strength was 42.69 MPa and 58.62 MPa respectively.The three-layer PI-2 and FG/PI-2 film with density surface,low dielectric constant,good breakdown performance,mechanical properties,and thermal stability were obtained.The dielectric constants of them were 1.95 and 1.92 respectively,the characteristics breakdown strengths were 184.20 kV/mm and 170.49 kV/mm respectively,the tensile strengths were 45.67 MPa and 65.76 MPa respectively,and the tensile moduli were 1.93 GPa and 2.96 GPa respectively.