Low temperature bonded Cu/LCP materials for FPCs and their characteristics

A new cladding process has been developed using the surface activated bonding (SAB) method. In this process, the surfaces of materials to be bonded are cleaned, activated by Ar ion sputter etching, and immediately rolled together with low distortion at room temperature in a vacuum chamber . This process is applicable not only to cladding metals but also to laminating polymer film on metal without adhesives. We focused on laminating liquid crystal polymer (LCP) on copper (Cu) foil using the SAB method , which is different from other conventional methods . We also investigated the chemical state at the interface between the Cu foil and LCP film before and after heat treatment (up to 300°C), by comparing with that on raw LCP film. All laminated materials were etched and cropped out, and the LCP surfaces were analyzed with X-ray photoelectron spectrometer (XPS). After the heat treatment, the ratio of C=O increased with increasing heat treatment temperature. It indicated that, under the high temperature of the heat treatment, the oxidation occurs not only at the Cu foil/sputtered Cu but also the LCP surfaces. We reported previously that the peel strength of laminated material made by the SAB method was improved by using Cu sputtering on the LCP film , . But after a heat-resistance test, the peel strength significantly decreased. This decrease resulted from the oxidation around the interface between the Cu foil/sputtered Cu and the LCP film due to gas permeation through the LCP film, and the softening of the Cu foil by its recrystallization due to high temperature. To prevent this loss of peel strength, we used Cu-0.02%Zr alloy foil with a higher recrystallization temperature and sputter Cu-Ni alloy or Ni-Cr alloy on the LCP film instead of Cu. It is also shown that the Cu/LCP materials produced by the SAB method have excellent electrical properties and etching ability.