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

Abstract

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.

Formation and growth of intermetallics around metallic particles in eutectic Sn-Ag solder

“…12a, these IMC layers are a very thin chunky shape, as has been documented in published litera ture. 17 The morphology of this IMC layer is similar to that found around the Ni particle when Cu is not available in the system to form the petals, as can be seen in the insert SEM image in Fig. 12a.…”

The microstructure of eutectic Au-Sn solder bumps on Cu/electroless Ni/Au

“…The coarse-grained layer beneath this, which was not observed in the Pb-Sn system on Ni-P, [6,7] has an average grain size of about 60nm and a P content of 5~8 at%. The grain size gradually decreases across this layer towards the Cu side with an associated increase in the P content.…”

“…[22,23] The presence of Au may contribute to the formation of this interlayer, since no amorphous layer was observed at the interface between eutectic Pb-Sn and electroless Ni. [6,7] Since an amorphous thin layer is metastable, it is expected that the layer found here will transform into stable crystalline phases, (Ni,Au) 3 Sn 2 and Ni 3 P, during aging.…”

“…[4][5][6][7] However, there has been some concern about the reliability of these joints, given the weak interface between the eutectic Pb-Sn solder and the electroless Ni/Au metallization. [8][9][10][11] This concern has motivated recent investigations of interfacial reactions [6,7] and failure mechanisms [12,13] in joints that contain electroless Ni layers.…”

Intermetallic Compound Formation and Growth Kinetics in Flip Chip Joints Using Sn–3.0Ag–0.5Cu Solder and Ni–P under Bump Metallurgy

“…The FC technology is generally considered as the ultimate first level connection because the highest density can be achieved and the path length is shortest so that optimal electrical characteristics are achieved. [3][4][5] However, as the flip chip technology is beginning to evolve from the researching area to commercial production, cost becomes an important issue for its successful implementation. In the early 1990s, a mask-less chemical wafer bumping technology, which combines electroless Ni plating and immersion Au process with stencil solder printing, was developed.…”