Reliability evalution of low-cost bumping by laser jetting Sn-Ag-Cu solder on electroless Ni/Au UBM

The technology of electroless Ni/Au UBM and laser jetting solder are attractive in flip chip package for its low-cost and flexibility. In this study, the solder ball was bumped on electroless Ni/Au UBM through laser jetting technology. Scanning Electron Microscope (SEM) was used to investigate the pad surface morphology after electroless Ni/Au deposition. The deposition itself was isotropic, and a smooth nickel surface was obtained. The die shear test was conducted to investigate the correlation between the thickness of electroless Ni and shear force. With the increase of electroless Ni thickness, the shear force was much improved. With the thickness of electroless Ni being 5.4um and 7.7um respectively, the failure occurred at the interface of sold joint and substrate. In such cases, the interface stress at electroless Ni layer and pad was comparatively small. Hence the interconnect kept highly reliable. The die shear force achieved 2.8Kg with electroless Ni thickness of 5.4um, more than the requirement of 2.5Kg by MIL-STD-883H.