## 5 Conclusions  
In this paper, a high thermal conductivity and low CTE composite with PI matrix and DN/SCW reinforcement is proposed. A preparation method of the composite film is developed, in which the two reinforcement materials are pre-dispersed respectively. By this preparation method, a good dispersion result can be obtained and the whiskers and particles are randomly distributed in the matrix.  
A series of tests are carried out to evaluate the performance of this composite film. The thermal conductivity and the CTE of the composite film are tested when the mass fraction of the SCWs is fixed at 7 wt% and the mass fraction of the DNs is 0 wt%, 1 wt%, 2.5 wt%, 5 wt%, 7.5 wt% and 10 wt%, respectively. As the concentration of the DNs increases, the thermal conductivity increases and the CTE decreases. When the concentrations of the DNs and SCWs are 7 wt% and 10 wt% respectively, the composite film has the maximum thermal conductivity of 1.63 W/m-K and the minimum CTE of 16.7 ppm/\({}^{\circ}\)C (compared with 0.19 W/m-K and 55.6 ppm/\({}^{\circ}\)C of PI). The dielectric property, insulation property and mechanical strength of the composite film are also tested with the optimized component ratio. The results show that the electrical and mechanical properties of the composite are eligible for spin-coating dielectric, although some properties are degraded to some degree, at the cost of the improvement of thermal conductivity and CTE.  
Interposer test samples are fabricated and the effects of the composite on the improvement of the interposer properties are investigated. Simulations by FEA are performed and the results indicate that the DN/SCW/PI composite used as an RDL dielectric has significantly enhanced the thermal-mechanical properties of the interposer compared with the PI dielectric. The thermal resistances of the interposer decreases from 8.04 \({}^{\circ}\)C/W to 1.15 \({}^{\circ}\)C/W and the temperature difference decreases from 20 \({}^{\circ}\)C to 9.8 \({}^{\circ}\)C. The maximum von Mises stress decreases from 72.8 MPa to 16.9 MPa and the warpage decreases from 1.13 \(\upmu\)m to 0.15 \(\upmu\)m.  
The thermal distributions of the interposers with different RDL dielectrics are also investigated by experiments. The result shows that, compared with the PI dielectric, the composite dielectric can reduce the maximum temperature of the interposer from 64 \({}^{\circ}\)C to 45.1 \({}^{\circ}\)C, the temperature difference from 46.6 \({}^{\circ}\)C and 26.8 \({}^{\circ}\)C.  
The composite can significantly reduce the temperature and enhance the reliability of the interposer chips. As a matter of experience, for every 10 \({}^{\circ}\)C increase in the junction temperature, the lifetime of the chip will be reduced by about half [31]. Thus, the composite can extend the lifetime of the interposer chips as well. In conclusion, the composite developed in this study can enhance the properties of the  
Figure 11: (**a**) The thermal distribution test sample. (**b**) Thermal distribution of the interposer with PI dielectric. (**c**) Thermal distribution of the interposer with the proposed composite dielectric.  
interposer chips and has the potential to expand the application of the interposers, especially in high thermal density integration and high reliability devices.  
Conceptualization, G.D. and J.L.; methodology, J.L. and Y.S.; software, Y.W. and J.L.; validation, X.Z., G.D.; formal analysis, J.L. and Y.W.; investigation, Y.L. and G.W.; resources, G.D.; data curation, J.L.; writing--original draft preparation, J.L.; writing--review and editing, Y.S. and G.W.; visualization, Y.W.; supervision, X.Z.; project administration, G.D.; funding acquisition, G.D.  
This research was funded by China Postdoctoral Science Foundation (2018M630440).  
The authors would like to thank supports from the Shanghai Professional Technical Service Platform for Non-Silicon Micro-Nano Integrated Manufacturing.  
The authors declare no conflict of interest.