Inner bump bonding technology for CSP

D/sup 2/BGA (die dimension BGA) is an NEC CSP, consisting of die, flexible printed circuit tape, resin, solder bumps and reinforcement resin. Assembly consists of bonding, lamination, encapsulation, solder ball placement and singulation. The bonding technique uses IBB (inner bump bonding) technology. IBB is similar to ILB (inner lead bonding), using an ultrasonic thermocompression single point bonder for TAB. Instead of the TAB inner lead, inner bumps are bonded to an Al pad. The polyimide film is drilled on the Cu trace by laser ablation. The inner bumps are made of an electrodeposited Cu core and Au plating. Various factors affected the Al-Au intermetallic bond: (1) inner bump shape; (2) inner bump deformation; (3) polyimide base film thickness; and (4) the adhesive properties. The Au layer deformation ratio was one of the most important factors; in the three bonded parts, the Au layer, Cu core and Cu trace, Au layer deformation affected bonding quality most significantly. A lower Au layer deformation ratio resulted in poorer bonding. A low Cu core deformation ratio resulted in a high Au layer deformation ratio and gave a good intermetallic bond between the Au plated Cu bump and the Al pad. Flexible PC tape had a thermoplastic polyimide adhesive layer on the die side, and the bonding area was reinforced by the adhesive layer during the bonding operation. The adhesive properties were also found to affect the intermetallic bond. Lower adhesive strength caused damage to the Al pad or Si chip during bonding. Strong flexible PC tape-die surface adhesion resulted in highly reliable bonding.