High-frequency 5G substrate: Low dielectric biphenyl polyimide with low CTE and high thermal stability

Polyimide (PI) faces new challenges in meeting the requirements of low coefficient of thermal expansion (CTE), low dielectric content/loss, and high thermal stability to be utilized effectively as a substrate for flexible electronic materials. At present research, the CTE of PI films can be reduced to around 5 ppm/K, with Tg > 420 °C. However, it is difficult to balance low CTE and high Tg at the same time, and reducing dielectric performance is even more difficult. This study focused on synthesizing four types of PI films (using BPDA as the dianhydride) by employing biphenyl structure diamines (PDA, BZD, DPT, DMP) with an increasing number of benzene rings. The improvement in CTE, dielectric properties, and thermal stability was observed with an increase in the number of benzene rings and a decrease in the number of imine rings in the PI horizontal structure. The Tetraphenylene PI-DMP exhibited values of ε and tanθ as 3.31 and 4.73 ‰ respectively under 10 GHz. Additionally, the biphenyl PI films demonstrated commendable thermal stability (Tg = 442°C), outstanding mechanical properties (elastic modulus > 10 GPa), and a low CTE (2 ppm/K within the temperature range of 50–300 °C, and close to the 3.6 ppm/K CTE of monocrystalline silicon). The low CTE, high thermal stability, and low dielectric properties that 5G flexible substrate materials need to be qualified are simultaneously reflected in this study. MS theoretical calculations were used to analyze the results. The excellent consistency with experimental results can promote the feasibility of MS theory in PI dielectric properties at high-frequency (10 Ghz). This innovative approach is anticipated to provide foldable chip fields with intrinsic biphenyl PI materials possessing low dielectric, low CTE, and high thermal stability.