Investigation of Cu stud bumping for single chip flip-chip assembly

Single chip bumping methods [1] are especially of great interest for fast prototyping and small volume production. Useable flip-chip interconnection techniques are thermocompression and thermosonic bonding with Au stud bumps as well as adhesive bonding [2, 3] and reflow soldering. Thermocompression bonding [4] requires temperatures of at least 250 degreesC to achieve a sufficient welding which limits this bonding technique to rigid substrates. The thermosonic bonding of larger chips with higher I/O count is still an issue [51 and adhesive bonding is limited to lower operation temperatures. For most applications a SMT compatible bonding process is the preferred way. Several stud bumping processes on Al metallized single chips are tested or in use. The most common ones are Au, Pd [6, 7] and Pt [8] stud bumping. Au is a softer material compared to the others, but the disadvantage is the formation of Au-Al as well as - if a Sn based solder is in use - Au-Sn intermetallic phases. The disadvantage of applying Pd or Pt bumping is the damage of the Al metallization or (in the worst case) the chip itself due to the hardness of the bump which directly decreases the range of the useable bumping parameter window. In addition, using Pd a rapid growth of Pd-Sn phases was observed. Thus, the preferable single chip bumping method of the above mentioned for flip-chip reflow soldering is the Pt stud bumping. Meanwhile, Cu has become an alternative bumping material for wire bonding, but also for stud bumping, on Al metallization. The advantages compared to Air are the reduction of costs for the wire and the slower growth of CuAl intermetallic phases. Disadvantages are the hardness of the Cu and its fast oxidation which requires the forming of the ball in a reducing atmosphere. The experimental results for single chip Cu bumping and Cu-PbSn bonding are reported in this paper and compared to previouse investigations with Pd and Pt as under bump metallizaton (UBM).