Material and process development of UV curable and epoxy-anhydride isolation material for TO220 FullPAK high voltage application

In TO220 FullPAK high voltage application, matrix leadframe design increases leadframe density and provides consistent isolation thickness. Tie bars removal exposes 0.8 mm length × 0.4 mm width metal holes, filling with minimum 15 kV/mm dielectric strength insulation material is required to prevent arching. Three unique filling materials, which include an UV cure material, one part and two parts components liquid encapsulants respectively have been formulated for isolation test, processability, material analysis, and flammability test. Processability study shows maximum filler cut of 120 um is suitable for 0.34mm inner diameter needle filling and 4.7% viscosity increment reduces 15% filling height. Material analysis shows UV cure material has constant viscosity; 150mW/cm2 UV intensity is required to cure constant 0.3mm filling height. Stringent control in UV intensity, constant filling height and cure time are needed to achieve full UV curing. Additionally, inhomogeneous of two parts filling materials which require mixing encounters CTE mismatch and higher moisture absorption, causing filling material separation and isolation test failure. As a result one component epoxy and anhydride encapsulatant is shortlisted for dielectric strength analysis. Result shows maximum dieletric strength degradation after 96h Pressure Cooker Test is 0.4%, and this passes isolation test. TO220 FullPAK with filling material passes package flammbility test UL1694-SC0, which is equivalent to UL94-V0.