Highly transparent preimidized semi-alicyclic polyimide varnishes with low curing temperatures and desirable processing viscosities at high solid contents: preparation and applications for LED chip passivation

A series of semi-alicyclic polyimide (PI) resins with the number average molecular weights (M_n) in the range of 34901–74790 g/mol were prepared from a hydrogenated 3,3′,4,4′-biphenyltetracarboxylic dianhydride (HBPDA) and various aromatic diamines by a high-temperature polycondensation procedure. For comparison, the analogous PI reference resins were prepared from hydrogenated 1,2,4,5-pyromellitic dianhydride (HPMDA) and the same aromatic diamines. The prepared PI resins were all soluble in polar aprotic solvents, such as N-methyl-pyrrolidinone (NMP) and N,N-dimethylacetamide (DMAc). Various PI varnishes with the solid contents as high as 35–45 wt% were successfully prepared by dissolving the PI resins in NMP. Some of the PI varnishes, including PI-3 derived from HBPDA and 1,3-bis(3-aminophenoxy)benzene (133APB), PI-5 from HBPDA and 2,2′-bis(trifluoromethyl)benzidine (TFMB), and PI-3′ from HPMDA and 133APB exhibited desirable processing viscosities at high solid contents. For example, when the solid content was 35 wt%, the PI varnishes showed absolute viscosities of 2144 mPa s for PI-3, 9971 mPa s for PI-5, and 3156 mPa s for PI-3′, respectively. The values were 6877 mPa s for PI-3, 42370 mPa s for PI-5, and 13700 mPa s for PI-3′, respectively when the solid contents of the PI varnishes increased to 40 wt%. This unique feature made the current PI varnishes good candidates as thick-film passivation layers for chip protection. Bare light emitting diodes chips were successfully passivated via once coating by using PI-5 (HBPDA–TFMB) varnish with a solid content of 35 wt%. The HBPDA–PI passivation layers exhibited good thermal stabilities with the glass transition temperatures (T_g) in the range of 171.5–296.3 °C and 5% weight loss temperatures higher than 470 °C. In addition, they showed good optical transparency with the transmittance higher than 72% at 450 nm at a thickness of 25 µm and low yellow indices and haze values.