Study On The Dielectric And Thermal Conductive Properties Of Polytetrafluoroethylene Materials

With the rapid development of microelectronic technique,the signals transmission velocity of electronic components grows faster.Meanwhile,the operating frequency and integration density increase rapidly,leading to the rising operating temperature.As the key component of electronic packaging,the substrate not only domains the signals transmission in the circuit,but also play the role on dissipating the heat generated.The substrate,which possesses high thermal conductivity and good dielectric properties,is the key to ensure the stable long-term operation of electronic instruments at high speed.Polytetrafluoroethylene(PTFE)possesses outstanding dielectric properties,which is one of the common substrate materials.While the low thermal conductivity of PTFE limits its application as the high-performance substrates.Although the orientation stretching can increase the degree of crystallinity and regularity of polymers,improving their intrinsic thermal conductivities.While the enhanced thermal conductivity is limited to the specific direction.Introduction of thermally conductive ceramic fillers is a common and most studied approach for improving the thermal conductivities of polymers.Although some progress has been made in the correlational researches in dielectric materials based on polymer composites,there still exist some problems demanding prompt solution.For example,the interfacial polarization inside composites would decrease the frequency stability of dielectric properties.The thermal conductivity enhancement efficiency of fillers is low.The traditional porous materials with low dielectric constant have the shortcomings of high moisture absorption and decreased thermal conductivity.In view of the above problems,in this study,two kinds of ceramic fillers,Al N and h BN,are used as thermally conductive fillers and PTFE is used as polymer matrix.Novel PTFE based dielectric composites are fabricated.On the basis of analysis of the dielectric and thermal conduction mechanism of polymer composites,the effect of polarization mechanism and interface structure on the dielectric properties of composites,the effect of filler packaging structure,filler distribution and orientation,and the contact thermal resistance at the filler-filler interface on the thermal conductivity of composites are investigated.Al N/PTFE composites were fabricated via the solution mixing method.The effect of polarization mechanism on the dielectric properties of composites was investigated.The results showed that: The electrical properties differences between polymer matrix and filler led to the interfacial polarization.The introduction of Al N leads to the local field distortion and the high dielectric constant of Al N can't be fully embodied.The interaction between filler was beneficial to the improvement of dielectric constant.Micro-nano size Al N co-filled there-phase PTFE composites were fabricated and the effect of filler packaging structure on the thermal conductivity of polymer composites were investigated.The results showed that: there was much interspace between fillers in Al N/PTFE composites and the thermal conductive network was imperfect.While in micro-nano Al N/PTFE composites,nano-size Al N particles were filled into the gaps between micro-size Al N particles,resulting in more compact filler stacking structure and more continuous thermal conductive networks.Moreover,the thermal percolation is not obvious compared with the traditional electrical percolation due to the low ratio of filler thermal conductivity to polymer matrix and high interfacial thermal resistance.The surface of h BN was treated by the silane coupling agent and the effect of interface structure on the dielectric properties of polymer composites was investigated.The results showed that: Surface treatment by the silane coupling agent decreased the hydrophily of inorganic fillers and the interface adhesion between the filler and polymer matrix was improved.Not only the electrical properties differences between polymer matrix and filler were reduced and the space charge accumulation at the interface was suppressed,thus decreasing the interfacial polarization intensity in the composite.The orientation of h BN in PTFE matrix was characterized through XRD of composites and the thermal conductivities of composites were analyzed by EMA model.The results showed that: The cold pressing resulted in the high in-plane orientation degree of h BN in PTFE matrix.This was not beneficial to the formation of thermal conductive networks along the thickness direction of composite materials,resulting in low thermal conductivity enhancement efficiency along the thickness direction of h BN/PTFE composites.Ag nanoparticles were deposited on the surface of h BN platelets through a liquidphase chemical reduction method and the effect of the contact thermal resistance at the filler-filler interface on the thermal conductivities of polymer composites was investigated.The results showed that: Due to the nanoscale size(6 to 12 nm)of Ag nanoparticles,they exhibited a very low melting temperature of 120.3 °C.Ag nanoparticles coalesced together linking adjacent h BN platelets during the sintering process of PTFE composites,resulting in that the interfacial force between adjacent h BN platelets turned to be metallurgical bonds rather than mechanically contact.The enhanced filler interaction between fillers effectively decreased the contact thermal resistance,leading to the improvement of the thermal conductivity of composites.Meanwhile,the attachment of Ag nanoparticles to the surface of h BN platelets efficiently restrained the continuous contact between them through PTFE matrix,and the h BN-Ag/PTFE composites still maintained excellent electrical insulation.Three-phase composites were prepared through the introduction of Al N and h BN into PTFE matrix.The filler connectivity was calculated by Hashine-Shtrikman model and the effect of filler morphology and orientation on the filler connectivity was investigated.The results showed that: The spherical morphology of Al N particles and high in-plane orientation degree of h BN platelets resulted in their low filler connectivity.While in three-phase composites,Al N particles effectively disrupted the alignment of h BN platelets along the lateral direction during the compression processing and formed compact packaging structure,leading to the increase of filler connectivity through PTFE matrix.When the volume ratio of Al N to h BN is 1:2,the composites achieved the highest thermal conductivity 1.04 W/m K.Core–shell structured hexagonal boron nitride coated hollow glass microsphere(HGM@h BN)hybrid particles were prepared using an electrostatic-assembly process and the effect of HGM@h BN hybrid particles on the dielectric constant and thermal conductivity of composites was investigated.The results showed that: The dielectric constants of composites decreased due to HGM and the h BN layer on the surface of HGM effectively enhanced composite thermal conductivity.The 30 vol% HGM@h BN/PTFE composite exhibited an ultralow dielectric constant of 1.68,a low moisture absorption of 0.11% and the thermal conductivity was slightly higher than pure PTFE,overcoming the shortcomings(high moisture absorption and decreased thermal conductivity)of traditional porous materials with low dielectric constant.In view of the problems in the polymer dielectric composites,a series of novel PTFE dielectric composites were fabricated and the dielectric and thermal conductive Properties of PTFE materials were systematically investigated.The interfacial polarization was decreased due to the improved interface structure and the frequency stability of dielectric properties was enhanced.Three ways: synergistic filling,reducing the contact thermal resistance at the filler-filler interface and decreasing the in-plane orientation of planar fillers were used to increase the thermal conductivity enhancement efficiency of fillers.Moreover,a kind of novel dielectric materials with ultra-low dielectric constant and low moisture absorption was fabricated.These researches mentioned above are of considerable referential importance to the fabrication of highperformance electronic packaging substrates.