Flipchip assembly is the direct electrical connection of the device die on the substrates or circuit boards using conductive stud bumps. Stud bumps help in the reduction of the number of bond wires and weight, and improve performance. Ueno et al. [1] fabricated copper column bumps with a diameter of 75 μm, a height of 50 μm and a pitch of 200 μm. Ni/Au bumps were also fabricated using electroless plating method with a pitch of 50 μm [2]. Tomita et al. [3] successfully fabricated tip capped copper column bumps with a height of 5 μm that were spaced 100 μm apart. Lead free solder bumps have been fabricated using pure-tin (Sn), tin-bismuth (80wt%Sn:20wt%Bi), tin-copper (99.3wt%Sn:0.7wt%Cu), tin-silver (96.5wt%Sn:3.5wt%Ag), and tin-silver-copper (93.8wt%Sn:3.5wt%Ag:0.7wt%Cu) alloys with a diameter of 200 μm and a pitch of 400 μm [4]. Low cost gold bumps have also been fabricated on polymer substrates with dimensions of 40 μm in diameter, 41 μm in height and a pitch of 100μm [5]. More recently Chu et al. [6] have demonstrated the use of cylindrical indium solder bumps with 80 μm diameter for optical interconnect and microwave applications.

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