This paper ultimately presents the method to reduce the thermal stress which is built at interlayers of leadframe, adhesive and chip during LOC die attach process by lowering the CTE(coefficient of thermal expansion) of die attach adhesive or by changing the adhesive thickness. To reduce the CTE of adhesive, alumina powder is mixed with polyimide solution which is popularly used to make LOC die attach adhesive. The properties of alumina, polyimide solution and additive are briefly reviewed and the proper method to make alumina-polyimide composite is empirically selected. Four different volume fractions of alumina are mixed with polyimide solution, and the CTE and specific heat of each alumina-polyimide composite are determined by TMA and DSC. To estimate the thermal stress at interlayer of LOC die attached leadframe, each composite is coated on the wafers and the biaxial thermal stress of coated composite film is determined by a Laser scanning method. These empirical results are compared with 2-dimensional FEM analysis. Finally, 3-dimensional FEM analysis of LOC die attached leadframe model is performed with a specific LOC leadframe model by changing the CTE of adhesive and by changing adhesive thickness.
Through comparing and synthesizing the empirical results and FEM analysis, this paper presents the relationship between the CTE or thickness of adhesive and the thermal stress or deformation of LOC die attached leadframe. And this paper presents the possibility that alumina-polyimide composite which has lower CTE than the CTE of pure polyimide can lower the thermal stress and thermal deformation at interlayer of LOC die attached leadframe.
