The impact of zincation on the electroless nickel UBM for low cost flip chip technology

Electroless nickel on aluminum bondpads followed by solder bumping is regarded as one of the promising solutions of low cost flip chip technology. Zincation is a key process step to form a strong, electrical conductive connection between nickel and aluminum. The morphology of the bondpad changes slightly after the degreasing and oxide removal procedures. Large zinc grains grow during zincation. To get more uniform and smooth zinc films, a second zincation is applied. The newly deposited zinc film leads to better adhesion of the flip chip interconnection. Therefore double zincation is employed instead of single zincation. The change of the surface roughness corresponds well with its morphology. The surface of the bondpad is examined by SEM and AFM. Another key factor of the zincation procedure is the immersion time in the bath. And in this paper the immersion time for the first and second zincations is 45 and 15s respectively. For a properly prepared nickel UBM on an 80μm×80μm Al bondpad followed by a eutectic SnPb solder bump (250μm in diameter), its shear strength is as high as 150 MPa and the electrical resistance of a bump is as low as 0.05Ω.