Pulse plating process for deposition of gold-tin alloy

Abstract

The invention relates to a solution for use in connection with the deposition of a gold-tin alloy on an electroplatable substrate. This solution generally includes water, stannous tin ions, a complexing agent to render the stannous tin ions soluble, complexed gold ions, and an alloy stabilization agent that includes an imine functional group. The alloy stabilization agent is present in an amount sufficient to stabilize the composition of the gold-tin deposit over a usable current density range. The solution has a pH of between about 2 and about 10 and the deposit having a gold content less than about 90% by weight and a tin content greater than about 10% by weight. An advantageous way for providing the desired deposit is by a pulse plating technique.

Description

[0001] This application claims the benefit of provisional application 60 / 645,949 filed Jan. 21, 2005, the entire content of which is expressly incorporated herein by reference thereto.BACKGROUND OF THE INVENTION [0002] Gold-tin alloys are useful in many microelectronic applications including chip bonding and wafer bump plating. The 80-20 wt % (70-30 at %) gold-tin eutectic alloy is particularly desirable as a solder. The alloy may be applied by vacuum deposition or as a solid preform, however, electrodeposition, due to its low cost, is a preferred method of application. [0003] Prior art electroplating baths for the deposition of gold-tin alloy have been found by the current inventor to be incapable of depositing the eutectic alloy over a usable current density range. This was clearly demonstrated in “Film growth characterization of pulse electro deposited Au / Sn tin films” by Djurfors and Ivey (GaAs MANTECH, 2001), where in FIG. 1 they show a step transition from 16 at % Sn to 50 at ...

AI Technical Summary

Benefits of technology

[0021] This method is preferably used for electroplating a gold-tin alloy deposit on composite articles that include electroplatable and non-electroplatable portions in order to provide a gold-tin alloy metal electrodeposit on the electroplatable portions of the articles without deleteriously affecting the non-electroplatable portions of the articles.

Problems solved by technology

Prior art electroplating baths for the deposition of gold-tin alloy have been found by the current inventor to be incapable of depositing the eutectic alloy over a usable current density range.

This has been further confirmed by our work which has shown that prior art electrolytes will not typically yield the desired eutectic alloy.

Moreover, many prior art baths suffer from poor stability making them of little practical interest.

However, this bath gives deposits with less than 1% Sn, and therefore is not useful for depositing a eutectic alloy.

As noted above these baths are not always stable and have been found to be insufficient in providing eutectic gold tin alloys on electroplatable substrates, particularly when small parts for electronic components or composite substrates are to be plated.

While certain alloys of gold-silver and gold-nickel are disclosed, gold tin is not mentioned, possibly due to the difficulties in obtaining a stable bath of such alloys.