Wetting reactions of lead-free eutectic solders in the metallisation layer under thick Cu bumps

The wetting reaction of a lead-free eutectic solder to a metallised layer under a thick Cu bump involves a number of aspects, and the following is a detailed analysis of the process: 

We present a comparative analysis of the reactions of four different eutectic solders (SnPb, SnAg, SnAgCu and SnCu) on thick Cu (15 μm) UBM layers prepared by electroplating.

SEM photographs of the interconnect interface of four eutectic solders on Cu UBM layer after 2 reflows (c)

(a)SnPb;(b)SnAg;(e)SnAgCu;(d)SnCu

Optical microscope photographs at the interface between the four solders and the Cu interconnect after solid state aging at 170°C for 1500h

(a)SnPb;(b)SnAg;(c)SnAgCu;(d)SnCu

Formation of scalloped Cu6Sn5:

Scalloped Cu6Sn5 was observed in all specimens, which is a typical intermetallic compound (IMC) formed by the reaction of Cu with Sn-based solder.

The scalloped Cu6Sn5 was larger at the interface of the Pb-free solder with Cu than at the interface of the SnPb solder, which may be related to the composition of the Pb-free solder and the reflow conditions.

Formation of Ag3Sn intermetallic compounds:

Very large flakes or shoots of Ag3Sn intermetallic compounds (IMCs) were observed in SnAg and SnAgCu solders. The formation of these IMCs is related to the Ag content in the solder, which reacts with Sn to form Ag3Sn.

Effect of solid-state aging on IMC morphology:

The solid-state aging process changes the scalloped Cu6Sn5 morphology to a layered morphology, with the continuous diffusion of Cu atoms and the apparent formation of a layer of Cu3Sn. This suggests that IMCs undergo morphological and compositional changes during aging.

SnPb solder generally grows in the solder matrix grain during the aging process, and a Pb-rich layer is formed immediately adjacent to the CuSn layer. This may be related to the phase transition and elemental diffusion of SnPb solder, and Pb does not undergo IMC reaction with Cu.

The grain growth of Pb-free solder is not obvious during aging, which may be related to the composition and stability of Pb-free solder.

Cu consumption:

The amount of Cu consumed during solid-state aging has the same order of magnitude as the amount of Cu consumed in the wetting reaction, although the time difference amounts to four orders of magnitude. This indicates that the rate of IMC formation is much higher in the wetting reaction than in the solid-state aging process, related to the concentration of metal atoms.

This may be due to the fact that the wetting reaction is carried out at high temperatures and the solder is in direct contact with the Cu substrate, which facilitates the rapid formation of IMC, whereas the solid state aging process is carried out at lower temperatures with slower rates of elemental diffusion and reaction.

Conclusion:

There is a significant difference in the reaction between SnPb solder and Pb-free solder on the Cu UBM layer, especially in the formation and morphological changes of IMC.

The lead-free solder showed better stability during solid-state aging and insignificant grain growth.

The rate of IMC formation in the wetting reaction is much higher than that in the solid-state aging process, which may be related to the temperature, the rate of elemental diffusion, and the solder composition.

This information is important for the selection of suitable solder and optimisation of soldering process parameters to ensure the reliability and long-term stability of electronic products.

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