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

摘要

A series of semi-alicyclic polyimide (PI) resins with the number average molecular weights (M-n) in the range of 34901-74790g/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 35-45wt% 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 35wt%, the PI varnishes showed absolute viscosities of 2144mPas for PI-3, 9971mPas for PI-5, and 3156mPas for PI-3, respectively. The values were 6877mPas for PI-3, 42370mPas for PI-5, and 13700mPas for PI-3, respectively when the solid contents of the PI varnishes increased to 40wt%. 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 35wt%. The HBPDA-PI passivation layers exhibited good thermal stabilities with the glass transition temperatures (T-g) in the range of 171.5-296.3 degrees C and 5% weight loss temperatures higher than 470 degrees C. In addition, they showed good optical transparency with the transmittance higher than 72% at 450nm at a thickness of 25 mu m and low yellow indices and haze values.