Influences of pad shape and solder microstructure on shear force of low cost flip chip bumps

The bumping process plays a critical role in flip chip technology. A low cost bumping process has been developed using electroless nickel and immersion gold followed by stencil printing. The process flow is described in this paper. The Al pad size is about 100 μm in diameter with a pitch of 400 μm. Different electroless plating solutions were evaluated and different solder pastes were used to evaluate the stencil printing process. Different pad shapes were also tested for shear strength. Ni studs with no bump material were fabricated to evaluate the electroless process. The shear force test result shows a strength value of 230 MPa for Ni studs. The solder bump after reflow has a diameter of 160 μm and a height of 120 μm. There is some difference in the shear force test results for different pad shapes. SEM and EDAX results of the fracture surface indicate that the fracture was cohesive or inside the solder. Cross sections showed some intermetallic layers at the interface. A Ni-Sn intermetallic layer and a phosphorus rich layer formed during reflow, which have compositions of Ni₃Sn₄ and Ni₃P respectively. The low cost flip chip samples were subjected to multiple reflows and shear force tests were performed. Fracture surfaces were analysed and failure modes were differentiated