Production of Gold Powders

The reliability of gold conductors makes them highly desirable for a variety of applications in electronics and communication systems. In hybrid microcircuitry it is the bonding behavior of gold as well as its electrical conductivity that makes it desirable. The reliability of gold is largely due to its superior resistance to oxidation and tarnish. Using gold initially may cost more, but much more would be sacrificed in terms of lost operating time and repair expenses if the electrical properties of the nongold substitute were to deteriorate due to the formation of oxide or sulfide films (Ref 6, 12).

Pure gold conductors have high conductivity, excellent wire bondability, excellent migration resistance, and are generally compatible with other components of thick-film materials. This makes gold particularly suitable for resistor terminations, wire- and die-attach pads, and highly reliable electrical interconnections (Ref 13). Applications for these devices include medical equipment, computers, satellite telecommunications, automotive control devices, and defense-related electronic components.

Pure gold conductors are not readily solderable using conventional solders because of poor solder leach resistance. The gold conductors can be made to be solderable through the addition of platinum. This imparts solderability at the expense of conductivity. Where high reliability and solderability is required and cost is not an issue, platinum-gold conductors are the industry standard (Ref 13, 14).

Many processes for making gold powder use an aqueous solution of chloroauric acid (HAuCl4) as the starting salt solution. This gold chloride solution is made by dissolving gold grain or shot in aqua-regia (a 3-to-1 mixture of hydrochloric acid and nitric acid) followed by heating to decompose and remove the nitric acid.

A variety of reducing agents can be used to reduce gold to gold powder including other metal powders (zinc, magnesium, lead, aluminum, and iron), sulfur compounds (sulfur dioxide, sodium sulfite, potassium sulfite, and ferrous sulfate), hydrazines (hydrazine hydrate, hydrazine hydrochloride, and hydrazine sulfate), aldehydes (formaldehyde), oxalates (oxalic acid and potassium oxalate), hydrogen peroxide, hydroquinone, hydroxylamine, and hypophosphorous acid (Ref 15, 16).

The morphology of the gold particles is determined by the conditions of the precipitation process. Using reducing agents without the addition of particle morphology modifiers produces gold particles with nodular or irregular-shaped particles. Heating the solution increases the rate of reduction. The presence of free hydrochloric acid can retard or even prevent the reduction. Solutions that are basic can increase the rate of reduction (Ref 17). Sodium hydroxide, sodium carbonate (or bicarbonate), and amines (including ammonia) can be used to make the solutions basic and regulate the pH during the reaction.

Reproducibility and morphology are the keys to the applications for gold powder and are the driving forces for choosing one process over another. Gold powder can be precipitated in a wide range of particle size distributions and shapes. Different reducing systems produce different types of gold powders. Modifiers can be added to control the nucleation stage to closely control particle size, and colloidal materials can be added to inhibit particle-to-particle aggregation (Ref 18).

The shape characteristics of gold powder are almost entirely spherical where the reductants are sulfur dioxide, sulfites, and related compounds. Potassium sulfite or sodium sulfite can be directly added to aqueous gold chloride solution to produce spherical gold particles with controlled size and density (Ref 19):

2HAuC14 + 3Na2S03 + 3H20 >2Au(S) + 3H2S04 + 6NaCl + 2HCl

This reaction is very dependent on pH, the ratio of gold chloride to sulfite, the mixing, and the temperature. If high temperatures are used, a mixture of spheres and flat plates can be made (Ref 16).

Gold powder containing a mixture of gold flakes and gold spheres can be made by reducing an aqueous gold chloride solution with a mixture of hydroquinone and oxalic acid in the presence of a protective colloid such as gum arabic, methylcellulose, sodium alginate, gelatin, and so forth (Ref 20). Fatty acids, fatty acid alcohols, and fatty acid amines can also be used as coatings for the gold particles to control the particle size and act as lubricating agents when using the gold powders (Ref 21).

An example of spherical gold powder is shown in Fig. 7, and a gold powder containing a mixture of spheres and flakes is shown in Fig. 8.

Fig. 7 Spherical gold powder
Fig. 8 Gold powder containing flakes and spheres

Gold powder can also be made mechanically. Sheets of gold are reduced in thickness by rolling and beating to produce a foil between 0.001 and 0.002 cm thick that is then beaten to produce a gold 0.2 to 0.5 /'m thick. These leaves are broken down further during a dispersion process to enable them to be used as fine gold powder. Larger gold powders and gold flake can be made through standard ball milling of gold powder (Ref 18).

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