To enhance the adhesion of copper-based epoxy/metal interfaces, copper is oxidized, in 95℃, $NaClO_2$(37.5g/l), NaOH(50g/l), $Na_3PO_4 \cdot 12H_2O$(100g/l) solution to form black oxide composed of cuprous oxide and cupric oxide, or 70℃, $NaClO_2$(160g/l), NaOH(10g/l) solution to form brown oxide composed of cupric oxide and then laminated with prepreg which is glass cloth impregnated with a resin.
In the case of black oxide coating, the peel strength decreases by the 30g/mm as oxidation time increases till 1min, then becomes constant. This behavior of peel strength is similar to that of thickness of cuprous oxide. The failure locus is cuprous oxide. Therefore, weak adhesion of this system is due to the existence of cuprous oxide. For brown oxide coating, the peel strength increases by the 76g/mm as oxidation time increases till 1min, then becomes constant. The oxide is in the form of long thin needle growing approximately perpendicular to the copper surface. Failure occurs transverse between cupric oxide and epoxy. Strong adhesion of this system is mainly due to mechanical interlocking between cupric oxide and epoxy.
Reliability test reveals that the peel strength of black oxide coated copper/epoxy system is constant with aging time at 120℃. But, in the case of brown oxide coated copper/epoxy system, the peel strength decreases by the 82g/mm with aging time at 120℃, which is considered owing to the formation of cuprous oxide.
