Mechanical Properties ofSolder Bumps formed on FPCB for Continous Flip Chip Bonding Process

Roll-to-Roll(R2R) processes have been offered high productivity for fabricating flexible PCB and flip chip bonding. Generally, solder bumps were formed on the IC chip pad by separate process after front end process. If solder bumps are formed on the FPCB during the R2R process, the productivity for flexible devices fabrication would be much higher than conventional bump formation.

In this study, low temperature solder bumps were formed on the FPCB by electroplating and microstructure and mechanical properties of the solder joints were observed to determine the feasibility of the novel process. To prevent warpage problem during high temperature bonding process, low melting temperature Sn-58Bi was used as solder materials.

To evaluate joint properties, two type modules were manufactured. For Type A module, solder bumps were formed on flexible substrate and for Type B, solder bumps were formed on chip. The Sn-58Bi eutectic solder bumps were fabricated by electroplating using commercial plating solution.

Diameter of the solder bumps were 35um and the pitch was 75um. Si chips were bonded on the flexible substrate under the condition of temperature of max. 180? and force of 40 N. Bonding interfaces were observed by SEM in order to observe IMC for Type A and B modules. The IMC thicknesses of Type A and B were 2 um and 3 um, respectively.

Shear strengths of each module measured 10 times were 35gf/bump and 30gf/bump, respectively. In case of A type, the fracture plane consist of brittle mode of 64% and ductile mode of 36%.