Preparation Of Polyimide-based Composite Films And Their Dielectric Properties Under High Frequency Conditions

The development of microelectronics technology has greatly improved the integration of integrated circuits,but this will be accompanied by problems such as resistive-capacitive delays,high power consumption and inter-line crosstalk noise.Especially at high frequencies,the polarization loss of the dielectric films increases significantly,which will lead to thermal runaway inside the integrated circuit and a decrease in signal transmission speed.As the base layer of flexible printed circuit board（FPC）,flexible insulating films play an insulating role for high-density circuits and electronic components on FPC.Their mechanical properties,heat resistance and dielectric properties are crucial to ensure high-speed and reliable transmission of 5G communication signals.Aromatic polyimide（PI）is widely used in flexible circuit boards due to its excellent dielectric,mechanical and thermal properties.However,the dielectric constant of PI films is relatively high in the high-frequency range,which cannot meet the requirements of high-frequency communication.In order to develop PI-based composite films with low dielectric constant,low dielectric loss and high temperature resistance under high-frequency operating conditions,this paper synthesized fluorine-containing polyimide（FPI）as a substrate with a prepared porous hyper-crosslinked polymer（HCP）and commercially available intrinsically low-dielectric polytetrafluoroethylene（PTFE）as modified fillers,respectively.The all-organic flexible insulating composite films with low dielectric constant and low dielectric loss were prepared in order to meet the demand of high frequency communication such as 5G.The specific research work is as follows:（1）To obtain intrinsically low dielectric PI films,dianhydride monomer containing trifluoromethyl and diamine monomer containing ether bond were selected as raw materials in this thesis to obtain precursor polyamic acid solution,and then imidized through stepped high temperature to finally obtain flexible FPI films with excellent film-forming properties.The introduction of fluorine-containing groups can also reduce the overall electron polarization rate by using flexible or large steric hindrance effect groups in the monomer to reduce the molecular chain stacking density and increase the void fraction,ultimately obtaining intrinsically low dielectric FPI films.The results show that the dielectric constant of the synthesized FPI films is below 2.8 at 8.2-10.5 GHz.Its 5%thermal decomposition temperature(T_5%)is 508°C,glass transition temperature（T_g）is305.9°C,and the mean value of coefficient of thermal expansion（CTE）is 54.6 ppm/K.The synthesized FPI film shows good dielectric properties and thermal stability.The film exhibites a tensile strength of 100 MPa,a tensile modulus of 2.1 GPa,and an elongation at break of 7%.These results prove that it has good dielectric properties,thermal stability and mechanical properties.（2）HCP with high porosity was prepared by mechanical ball milling and dispersed in polyamic acid,the precursor of FPI.All-organic flexible insulating composite films were prepared by the two-step method.The filling amount of HCP was regulated to study its effect on the dielectric,thermal and mechanical properties of the composite films.The experimental results show that the pore size of the prepared HCP’s micropores is concentrated around 1.3 nm and the specific surface area is as high as 1957 m²/g.When the content of HCP in the composite film is 5-20 wt%,the T_5%of the composite films are above 425℃,and the range of T_g is 295-306℃.When the content of HCP in the composite film is 15 wt%,the CTE of the composite film reaches the lowest value of46.67 ppm/K,which is 14.5%lower than that of the pure film,showing good heat resistance performance and thermal stability.When the content of HCP in the composite film is 10 wt%,the composite film reaches an ultra-low dielectric constant of 1.7 at 8.2-9.6 GHz,and the dielectric loss is less than 0.01 in the 9.5-10.0 GHz band,with a tensile strength of 66.5 MPa,tensile modulus of 2.2 GPa,and elongation at break of 4.6%.From the above analysis,it can be seen that the introduction of HCP can effectively reduce the dielectric constant and dielectric loss of FPI-based composite films at high frequencies,while maintaining the heat resistance and mechanical properties of the films well.The method can be easily extended to other organic component-filled all-organic PI systems.（3）The water-soluble polyamic acid salt（PAAS）was produced by adding organic bases to polyamic acid,the precursor of FPI.A concentrated dispersion of low dielectric commercialized polytetrafluoroethylene（PTFE）was selected and compounded with the dried PAAS to form an aqueous solution system.The effects of PTFE on the dielectric,thermal and mechanical properties of the composite films were studied.The experimental results show that the dielectric constant of the composite films decreases with increasing PTFE content and reaches a minimum value when the PTFE filling is 50 wt%（D_k=1.5@8.5 GHz）.The dielectric loss of 10 wt%PTFE/FPI in the range of 9.25-10.25GHz is less than 0.03.The T_g range of the composite films is 289-297°C for PTFE content of 10-50 wt%,and T_5%are all greater than 508°C,indicating that the composite films have excellent thermal stability.With the increase of PTFE filling,the CTE of 40 wt%PTFE/FPI was as low as 59.67 ppm/K,which is 28.4%lower than the pure FPI in this chapter.When the PTFE filling is 20 wt%,the tensile strength of the composite film is 70MPa,the tensile modulus is 1.59 GPa,and the elongation at break is 7.7%,indicating that it has good mechanical properties.The all-organic PI composite films,with ultra-low dielectric constant at high frequencies and good overall performance,have the potential for industrial applications.The pure FPI film and the all-organic FPI composite insulating films prepared in this thesis achieve ultra-low dielectric constant at high frequencies and low dielectric loss at specific frequency points,while combining good film formation,heat resistance and mechanical properties.The method of preparing these low dielectric FPI composite films can be easily extended to other composite systems of porous polymer components,low dielectric aqueous phase dispersed polymer components and PI matrix.It provides an experimental reference for the design and preparation of low dielectric polymer films suitable for FPC for high frequency communication.