Improving mold compound adhesion to Ni/Pd/Au pre-plated lead frames

Abstract

A unique approach for investigating new epoxy mold compounds with improved adhesion to Ni/Pd/Au leadframes is described. This approach consists of three parts: 1) the calculation of polar gammap and dispersive gammad surface free energy components of various model epoxy mold compounds and Ni/Pd/Au-plated leadframes, respectively, derived from dynamic contact angle measurements at room temperature using water and diiodomethane, 2) interpretation of the results in terms of the geometric mean theory of Owens, Wendt, Rabel and Kaelble, which includes the construction of the wetting envelope for the Ni/Pd/Au leadframe under various environmental conditions, and 3) a measurement technique for the true contact angle thetas and hence the thermodynamic work of adhesion Wa according to the Young-Dupre equation compared to actual adhesion. The calculated thermodynamic work of adhesion was compared to actual adhesion values obtained using a modified die shear adhesion test. Reasonable correlation was obtained between the two tests, allowing the contact angle measurement to be adapted as a rapid screening tool for identifying promising leadframe and EMC material candidates. Actual epoxy molding compounds are formulated based on information from the wetting angle experiments and die-shear adhesion tests of unfilled model compounds. Fully-formulated epoxy molding compounds are characterized using traditional tab-pull adhesion tests (Ju et al., 2004) and thermal cycle reliability tests on actual leaded packages. Further results from the surface energies, the resulting adhesion strengths, and the interfacial shear stresses between the EMC and the leadframe are discussed