Development of ball bump forming technology using solder paste and new simplified CSP

We have investigated a new bump forming process and developed a machine using solder paste. The ball bumps formed by the machine satisfied the planned EIAJ standards. The process is as follows. First, cavities are formed by superimposing a stencil onto a base plate. Secondly, they are filled with solder paste using a screening operation and a printed circuit board (PCB) is aligned face down on the stencil. Finally, they are heated together and solder paste is reflowed to be transferred onto the pads. The separable transfer jig is suitable for mass-production compared with block-type one, reducing the flux residue cleaning time, and realizing bump forming with a 0.3 mm pitch. The bump forming machine will be applied to the mass-production of chip scale packages (CSP) with a 0.8 mm pitch at the rate of 100,000 packages a month. We have also developed a new CSP with a unique structure as an application of the bump forming process.