Non-conductive adhesives (NCA), widely used in display packaging and fine pitch flip chip packaging technology, have been recommended as one of the most suitable interconnection materials for flip-chip chip size packages (CSPs) due to the advantages such as easier processing, good electrical performance, lower cost, and low temperature processing. Flip chip assembly using modified NCA materials with material property optimization such as CTEs and modulus by loading optimized content of nonconductive fillers for the good electrical, mechanical and reliability characteristics, can enable wide application of NCA materials for fine pitch first level interconnection in the flip chip CSP applications. In this paper, we have developed film type NCA materials for flip chip assembly on organic substrates. NCAs are generally mixture of epoxy polymer resin without any fillers, and have high CTE values un-like conventional underfill materials used to enhance thermal cycling reliability of solder flip chip assembly on organic boards. In order to reduce thermal and mechanical stress and strain induced by CTE mismatch between a chip and organic substrate, the CTE of NCAs was optimized by filler content. The flip chip CSP assembly using modified NCA showed high reliability in various environmental tests, such as thermal cycling test (-55°C/+160°C, 1000 cycle), high temperature humidity test (85°C/85%RH, 1000 h) and high temperature storage test (125°C, dry condition). The material properties of NCA such as the curing profile, the thermal expansion, the storage modulus and adhesion were also investigated as a function of filler content.