Fluxless flip chip assembly on rigid and flexible polymer substrates using the Au-Sn metallurgy

The application of the Au-Sn system for flip chip assembly offers the advantage of a fluxless soldering process. This can be attributed to the excellent wetting properties of Au-Sn and Sn alloys on Au-surfaces. Depending on the chosen Au-Sn or Au-Sn/Pb alloy composition, soldering in a wide range of temperatures between 200 to 380°C is possible. For flip chip interconnections, two approaches are presented. One consists in the application of electroplated eutectic Au/Sn 80/20 solder bumps on substrates with Au or Ni/Au as final metallizations. The second approach uses gold bumps on substrates with Sn or eutectic SnPb solder as final metallization. In this case, the Au-Sn alloy is formed during the flip chip soldering process. This solution permits soldering of the flexible and low cost Au- stud bumps produced using thermosonic wire bond equipment. The bond parameters, the material requirements and limitations of the polymer substrates consisting of rigid and flexible printed wiring boards are discussed. A metallurgical study of the formed Au-Sn alloys and intermetallic compounds and their influence on the reliability of the FC-assembly is presented