Solder reaction-assisted crystallization of electroless Ni-P under bump metallization in low cost flip chip technology

Solder reaction-assisted crystallization of electroless Ni-P under bump metallization in the Si/SiO2/Al/Ni-P/63Sn-37Pb multilayer structure was analyzed using transmission electron microscopy, scanning electron microscopy, energy dispersive x-ray, and electron probe microanalyzer. The electroless Ni-P had an amorphous structure and a composition of Ni85P15 in the as-plated condition. Upon reflow, the electroless Ni-P transformed to Ni3Sn4 and Ni3P. The crystallization of electroless Ni-P to Ni3P was induced by the depletion of Ni from electroless Ni-P to form Ni3Sn4. The interface between electroless Ni-P and Ni3P layer was planar. From the Ni3P thickness-time relationship, the kinetics of crystallization was found to be diffusion controlled. Conservation of P occurs between electroless Ni-P and Ni3P, meaning that little or no P diffuses into the molten solder. Combining the growth rates of Ni3Sn4 and Ni3P, the consumption rate of electroless Ni-P was determined. Based upon microstructural and diffusion results, a grain-boundary diffusion of the Ni or an interstitial diffusion of the P in the Ni3P layer was proposed. (C) 1999 American Institute of Physics. [S0021-8979(99)02912-6].