Adhesion improvement of epoxy resin/copper lead frame j oints by azole compounds

Abstract

The adhesion strength of epoxy resin/copper joints is often very poor, due to the naturally formed copper oxide having a low mechanical strength. To improve the adhesion strength of epoxy resin/copper lead frame joints, copper lead frames were treated with azole compounds as adhesion promoters. The azole compounds used were benzotriazole (BTA), benzotriazole-5-carboxylic acid (CBTA), 8-azaadenine, imidazole, 2-methyl imidazole, urocanic acid, adenine, benzimidazole, and polybenzimidazole (PBI). The dependence of the adhesion strength of epoxy resin/azole-treated copper joints on the structure of the azole compound, the azole treatment time, and the azole treatment temperature was investigated. The surface coverage of azole-treated copper was examined by contact angle measurements, a surface defect test, optical microscopy, and scanning electron microscopy (SEM), and the locus of failure was studied by X-ray photoelectron spectroscopy (XPS). Triazole compounds such as CBTA and 8-azaadenine showed excellent adhesion strength; imidazole-based azole compounds did not improve the adhesion strength. However, the adhesion strength of CBTA- and 8-azaadenine-treated joints decreased with increasing treatment time, since thick porous Cu-azole complexes had a weaker mechanical strength when formed. The polymeric azole compound PBI showed the highest adhesion strength, 785 N/m, because of complete coverage of the copper surface. The thermal stability of azole compounds and epoxy resin/azole-treated copper joints was also investigated. CBTA and 8-azaadenine did not decompose up to 250°C, while PBI was stable up to 500°C in an air atmosphere.