Theoretical Optimization Design Of Composite Dielectric Substrate With Low Dielectric Constant And Low Coefficient Of Thermal Expansion

With the rapid development of the Internet of Things（IoT）and fifth generation（5G）mobile communication technology,high-frequency and high-speed printed circuit board（PCB）are highly desirable for widespread applications ranging from satellite communication and radar detection to automotive electronics and communication stations.Dielectric substrate is an important basic component of PCB.It is composed of dielectric substrate layer and copper foil deposited on one or both sides.It plays a key role in structural support and connection of various load electronic components.In order to ensure high-speed transmission and high-frequency signal quality（>5 GHz）,the composite substrate must have ideal dielectric properties（>10 GHz）and high-frequency signal stability.The current bottleneck limiting the further development of substrate material performance is the contradiction between reconciling electrical and thermal performance requirements,and the different requirements for ceramic fillings with low thermal expansion coefficient and low dielectric constant and dielectric loss are the origin of the contradiction between electrical and thermal performance.Furthermore,the rational design of polymer-based composites to meet the demand has been challenging research due to the difficulty in maximizing the structural modulation of composite properties by inorganic fillers due to the lack of understanding of the paradoxical role of inorganic fillers for dielectric-thermal-mechanical property modulation.This paper takes polytetrafluoroethylene PTFE-based composite dielectric substrate model as an example.The effects of fillers on dielectric constant（ε_r）and thermal expansion coefficient（CTE）of composite dielectric substrate were systematically studied by high-throughput finite element calculation.A series of influencing factors are considered in the model,including the inherent properties,volume fraction,shape,distribution and orientation of fillers.The structure property mapping image of dielectric composites is established,which provides theoretical guidance for regulating the balance of thermal/electrical properties of composite dielectric substrate materials.The main research contents are as follows:（1）Firstly,the structural model of PTFE/particle filler composite dielectric substrate material is established.Based on the simulation of coupled physical field,the types of fillers are selected through high-throughput calculation,and the low dielectric constant is determined（ε_r=3.9）and Si O₂with low coefficient of thermal expansion（CTE=0.5 ppm/K）were used as fillers for subsequent discussion.The effects of Si O₂fillers on the thermal stability and dielectric properties of PTFE based composite dielectric substrate materials were discussed respectively.（2）Through the thermal displacement distribution and stress distribution images of the simulation model,the correlation between the size,shape and aggregation degree of filler and the CTE of composite dielectric substrate is studied.The results of thermal/electrical coupling model show that the thermal stability of the composites is not sensitive to the change of filler size when Si O₂particles are from 20μm changes to 6μm,the CTE of the composite dielectric substrate basically does not change.With the increase of filler aggregation,the stress can be transferred along the particles gradually,and the thermal stability of the substrate material is enhanced.The effect of filler shape and orientation on CTE anisotropy of composite dielectric substrate material was confirmed by fiber filler comparison.When the filler content was maintained at 10 vol%,it increased with the fiber orientationθfrom 90°to 0°,the CTE of composite dielectric substrate decreased significantly from 98.8 ppm/K to37.7 ppm/K.（3）Through the electric field distribution image of the simulation model,the relationship between the size,shape and aggregation degree of the filler and the composite dielectric substrate is studiedε_r.The dielectric response model results show that the local electric field magnitude inside the composite receives a combination of particle size and location The degree of aggregation of the filler has almost no effect on the dielectric constant of the composite dielectric substrate material,due to the fact that the effect of the interfacial phase is not considered in the model.By comparing the electric field distribution of a single filler with different shapes,it is confirmed that the shape and orientation of the filler determine the size of the internal depolarization field.At a filler volume fraction of 10 vol%,the dielectric constant decreases from 2.17 to 2.12 as the fiber orientationθchanges from 0°to 90°.Due to the small difference between the dielectric constants of Si O₂and PTFE,the variation range of electric field caused by the above factors is small.（4）Through the above discussion,two different structural models are constructed.The hollow Si O₂particle structure system CTE shows a weak correlation with the air volume fraction,because the introduction of air hardly affects the deformation of Si O₂particles;Due to the introduction of air,the internal polarization of the material decreases and itsε_rshows a strong correlation with air volume fraction.The Si O₂skeleton structure image shows that the stress is transmitted through the connected skeleton,so as to better play the supporting role.Therefore,the Si O₂skeleton can reduce the CTE of the composite dielectric substrate to 60 ppm/K and the dielectric constant to less than 2.3 at the volume fraction of 14 vol%.