Solder-jetted eutectic PbSn bumps for flip-chip

The commonly used deposition technology for solder bumps (evaporation or electroplating) requires thin-film processing. The compatibility of the solder-wettable metallizations does not allow the use of the same production equipment as installed in the wafer-fabrication facility. In this study, a maskless bump process is described. Here, solder droplets are ejected from a capillary and impinge on a wettable bond-pad metallizations of electroless-deposited Ni/P-Au. Droplets impinging on rough surface layer often bounce away. It is shown that this roughness layer is mainly determined by the Zn nucleation on the bond-pad metallizations. Nucleation conditions are optimized to deposit only small particles of the same size. The volume of the droplets depends on the product of pulse amplitude and pulse length. Degradation of the interconnection between the piezoelectric actuator and the glass capillary requires a larger pulse amplitude for stable jetting behavior. In addition, it is found that every first droplet on a new position is larger than all other droplets ejected directly thereafter. The diameter distribution of the latter are within the requirements for the final bump. The quality of the solder-jetted bump is studied by several reliability tests after flip-chip assembly on printed wiring boards (PWB´s). In combination with underfill, the reliability of solder-jetted bumps are comparable with electroplated bumps.