Polyimides with low dielectric constants and dissipation factors at high frequency derived from novel aromatic diamines with bistrifluoromethyl pendant groups†

Fan-out wafer-level packaging (FOWLP) urgently demands low dielectric constant and dissipation factor interlayer dielectric materials to mitigate high transmission loss at high frequencies. Polyimides (PIs) are widely used as interlayer dielectric materials in FOWLP due to their excellent comprehensive properties. However, addressing the challenge of decreasing the dielectric constant and dissipation factor of PIs at high frequencies to meet the application requirements remains an active pursuit in both industry and academia. In this study, we designed and synthesized a novel diamine monomer featuring bis(trifluoromethyl) pendant groups, denoted as 4,4′-((3′,5′-bis(trifluoromethyl)-[1,1′-biphenyl]-3,5-diyl)bis(oxy))dianiline (HFBODA). The binary polymerization of this diamine monomer with common dianhydrides led to promising outcomes. Remarkably, among the prepared PIs, 6FDA-HFBODA exhibited excellent properties (T_d,5% = 521 °C, T_g = 240 °C, Dk = 2.63 and D_f = 3.72 × 10⁻³) at 10 GHz. Additionally, BPADA-HFBODA demonstrated an ultra-low D_f value of 2.30 × 10⁻³ at 10 GHz. The relationship between the charge density of imide in PIs and the dissipation factor of PIs was investigated for the first time. By introducing strong electron-withdrawing groups to the side group of PI, the effect of the imide ring on the orientation polarization was greatly declined; thus, the dissipation factor of PI at high frequency was significantly decreased. Besides, the relationship between the structures and other essential properties of PIs in this study was systematically explored. This work provides a novel diamine and demonstrates the role of trifluoromethyl located in the side group in lowering the dissipation factors of PIs at high frequencies. The introduction of a bis(trifluoromethyl) pendant group led to a reduction in polarizability and an increase in free volume within the PIs. Moreover, the electron-withdrawing effect of the trifluoromethyl group substantially minimized the probability of internal friction among dipoles, resulting in reduced dielectric constants and dissipation factors. These findings provide crucial insights and guidance for the future design and research of low dielectric constant and dissipation factor PIs, particularly for high-frequency applications in fan-out wafer-level packaging.