The interaction between Cu6Sn5 particles in the bulk of a solder and a Ni substrate was examined during solid-state aging using Cu/Sn/Ni and Cu/Sn/Cu/Sn/Ni diffusion couples with initially thin Cu layers. The results clearly demonstrated that the (Cu,Ni)6Sn5 particles dispersed in the bulk solder decomposed in order for a ternary (Cu1-xNix)6Sn5 layer to grow at the solder/Ni interface during solid-state aging. The interaction between the (Cu,Ni)6Sn5 particles and the (Cu1-xNix)6Sn5 layer occurs owing to the driving force for the (Cu,Ni)6Sn5 compound to become saturated with Ni. A (Ni,Cu)3Sn4 layer forms at the (Cu1-xNix)6Sn5/Ni interface only after the Ni composition of the (Cu,Ni)6Sn5 phase in the bulk solder approaches that of the (Cu1-xNix)6Sn5 layer. Once the (Ni,Cu)3Sn4 layer has formed, it grows at an exceptionally rapid rate by consuming the (Cu1-xNix)6Sn5 and Sn layers, which can be problematic in solder joint reliability.
Inkjet-printed Ag metallization is a promising method of forming front-side contacts on Si solar cells due to its non-contact printing nature and fine grid resolution. However, conventional Ag inks are unable to punch through the SiN(x) anti-reflection coating (ARC) layer on emitter Si surfaces. In this study, a novel formulation of Ag ink is examined for the formation of fire-through contacts on a SiN(x)-coated Si substrate using the single-step printing of Ag ink, followed by rapid thermal annealing at 800 degrees C. In order to formulate Ag inks with fire-through contact formation capabilities, a liquid etching agent was first formulated by dissolving metal nitrates in an organic solvent and then mixing the resulting solution with a commercial Ag nanoparticle ink at various volume ratios. During the firing process, the dissolved metal nitrates decomposed into metal oxides and acted in a similar manner to the glass frit contained in Ag pastes for screen-printed Ag metallization. The newly formulated ink with a 1 wt% loading ratio of metal oxides to Ag formed finely distributed Ag crystallites on the Si substrate after firing at 800 degrees C for 1 min.
To simulate the growth of Ni 3 Sn 4 phase layers in Sn-based solder joints with Ni substrates during solid-state aging, Sn/(Cu 1Àx Ni x ) 6 Sn 5 /Ni and Sn/Ni diffusion couples were aged isothermally at 180°C and 200°C, and the growth kinetics of the (Ni,Cu) 3 Sn 4 and Ni 3 Sn 4 layers in the respective couples were monitored during the isothermal aging. Once the (Ni,Cu) 3 Sn 4 layer was formed at the (Cu,Ni) 6 Sn 5 /Ni interface, it grew unexpectedly fast with concurrent growth of voids formed in the Sn layer during prolonged aging at both temperatures. The results obtained from the various types of diffusion couples revealed that the voids formed in the Sn layer were Kirkendall voids, due to the (Ni,Cu) 3 Sn 4 layer growing predominantly at the (Ni,Cu) 3 Sn 4 /Ni interface by fast diffusion of Sn across the (Ni,Cu) 3 Sn 4 layer. It is proposed that the accelerated growth of the (Ni,Cu) 3 Sn 4 and Ni 3 Sn 4 layers after the formation of voids in the Sn layer is due to the relaxation of vacancy oversaturation and the enhanced annihilation rate of incoming vacancies in the presence of the voids in the Sn layer.
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