Crystallization of electroless Ni-P under bump metallization induced by solder reaction

Solder reaction-assisted crystallization of electroless Ni-P under bump metallization in the Si/SiO/sub 2//Al/Ni-P/63Sn-37Pb multilayer structure was analyzed using transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray, and electron probe microanalysis. The electroless Ni-P had an amorphous structure and a composition of Ni/sub 85/P/sub 15/ in the as-plated condition. Upon reflow, the electroless Ni-P transformed to Ni/sub 3/Sn/sub 4/ and Ni/sub 3/P. The crystallization of electroless Ni-P to Ni/sub 3/P was induced by Ni depletion from electroless Ni-P to form Ni/sub 3/Sn/sub 4/. The crystallization kinetics were found to be diffusion-controlled.