Semi-Aromatic Polyimides with Low Dielectric Constant and Low CTE

Semi-aromatic polyimides (PIs) possessing simultaneously a low dielectric constant ( K) and low linear coefficient of thermal expansion ( CTE) were molecularly-designed. Two PI systems were focused on, i.e. the s-BPDA/CHDA polyimide derived from 3,3′,4,4′-biphenyltetracarboxylic dianhydride with trans-1,4-cyclohexanediamine and CBDA/TFMB derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride and bis(2,2′-trifluoromethyl)benzidine. In the s-BPDA/CHDA system, poly(amic) acid (PAA) polymerization using the common procedure was prevented by strong salt formation at the initial stage. However, heating the salt-forming reaction mixture at a selected temperature for a short period allowed the system to polymerize and resulted in a high-molecular-weight PAA (reduced viscosity η red > 2 dl g −1 ). On the other hand, in the CBDA/TFMB system, a high-molecular-weight PAA ( η red > 3 dl g −1 ) was readily obtained by choosing the solid content without any salt formation. The chosen PIs almost achieved the current goal; CTE < 20 ppm K −1 , K < 3.0, and T g > 300°C concurrently with adequate film flexibility. In particular, CBDA/TFMB accomplished a very low K (∼2.6). Although these PIs have a much lower thermo-oxidative stability than wholly aromatic PIs, the results suggested that there is no serious problem for the short-term thermal stability required for soldering processes.