Fabrication and adhesion strength of Cu/Ni–Cr/polyimide films for flexible printed circuits

The adhesion strength of a Cu/Ni–Cr/polyimide flexible copper clad laminate (FCCL), was evaluated according to the thickness of the Ni–Cr (Ni:Cr = 95:5 ratio) seed layer using the 90° peel test. The changes in the morphology, chemical bonding and adhesion properties were characterized by SEM, AFM and XPS. The peel strength of the FCCL increased with increasing thickness of the Ni–Cr seed layer, due to the increase in the ion bombardment caused by the higher power used in the Ni–Cr sputtering process. This increase in the FCCL peel strength was attributed to the lower proportion of C–N bonds and higher proportion of C–O bonds in the polyimide surface. The adhesion strength between the metal and polyimide was mostly attributed to the chemical interaction between the metal layer and the functional groups of the polyimide.