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 ${\mathrm{Si/SiO}}_{\mathrm{2}}\mathrm{/Al/Ni–P/63Sn–37Pb}$ 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 ${\mathrm{Ni}}_{\mathrm{85}}{\mathrm{P}}_{\mathrm{15}}$ in the as-plated condition. Upon reflow, the electroless Ni–P transformed to ${\mathrm{Ni}}_{\mathrm{3}}{\mathrm{Sn}}_{\mathrm{4}}$ and ${\mathrm{Ni}}_{\mathrm{3}}\mathrm{P}.$ The crystallization of electroless Ni–P to ${\mathrm{Ni}}_{\mathrm{3}}\mathrm{P}$ was induced by the depletion of Ni from electroless Ni–P to form ${\mathrm{Ni}}_{\mathrm{3}}{\mathrm{Sn}}_{\mathrm{4}}.$ The interface between electroless Ni–P and ${\mathrm{Ni}}_{\mathrm{3}}\mathrm{P}$ layer was planar. From the ${\mathrm{Ni}}_{\mathrm{3}}\mathrm{P}$ thickness-time relationship, the kinetics of crystallization was found to be diffusion controlled. Conservation of P occurs between electroless Ni–P and ${\mathrm{Ni}}_{\mathrm{3}}\mathrm{P},$ meaning that little or no P diffuses into the molten solder. Combining the growth rates of ${\mathrm{Ni}}_{\mathrm{3}}{\mathrm{Sn}}_{\mathrm{4}}$ and ${\mathrm{Ni}}_{\mathrm{3}}\mathrm{P},$ 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 ${\mathrm{Ni}}_{\mathrm{3}}\mathrm{P}$ layer was proposed.