Low-temperature bonding of LSI chips to polymer substrate using Au cone bump for flexible electronics

PEN (polyethylene naphthalate) is a promising polymer candidate of the substrate material for flexible electronics. But its glass transition temperature is as low as 150°C. We show that interconnection bonding of LSI chip to metallization on PEN film can be realized by using cone-shaped compliant bump. 20 um-pitch area array of cone-shaped Au bump was fabricated on a Si wafer by using a photolithography and electroplating. Counter electrode on the PEN film was composed of Au (top)/Ni/Al (bottom) layered structure, where Ni and Au layers were deposited by using electroless plating on patterned Al. We have investigated two designs of the counter electrode; one is simple metal pad and the other is an electrode in which cross-shaped slit was formed. The bonding between the cone-shaped bump and the simple pad electrode was found to be realized at 150°C. More than 10,000 connections with about 100 mQ/bump were formed. When the cross-shaped slit was employed, room temperature bonding was achieved.