Qualitative evaluation of flip chip solder bumps produced by stencil printing of solder paste on various electroless nickel/gold metallizations

Stencil printing of solder paste in conjunction with electroless Ni/Au UBM on Al metallization offers the most economical method for wafer bumping either for Flip chip applications or for WLCSP packages. This paper deals in detail with the bumping process of 6? wafers at 300µm and 200µm pitch and investigates the bump interface integrity for different Au flash thicknesses on 5µm Ni. The bump height produced is 108±5µm with Sn63Pb37 and Sn4Ag0.5Cu compositions. Bumped chips have undergone high temperature storage at 150oC and up to 10 multiple reflow passes. Interesting results show that the shear strength of bumps with 5µmNi/26nmAu/Sn4Ag0.5Cu interface is about 97.1±8.3 MPa; very close to the standard 5µmNi/80nmAu UBM used so far in electroless processes. Furthermore, Sn4Ag0.5Cu bumps with only 5µm Ni UBM (without flash Au) has shown significant shear strength of about 80.4 ± 8.9 MPa which corresponds to about 17% reduction in strength. The prevailing shear fracture mode for all bumped chips is solder bulk fracture. The results imply that a possible reduction in Au flash thickness or a complete absence of Au can be potentially applied without any compromise in the mechanical integrity and robustness of the bumps.