Industrial Gold Plating

The printed circuit industry of the late 1950s led to the rediscovery of the stability of potassium gold cyanide on the acid side (below a pH of 7). This was first hinted at in a Ruolz French patent of addition of 1840-45. The stability was described in the English edition of Cyanogen Compounds by H.E. Williams in the 1890s. Finally, the Lukens patent of 1938 made use of low-pH gold cyanide plating to ensure good adhesion on stainless steel. Lukens referred to this bath, made up with sodium gold cyanide, sodium cyanide, and hydrochloric acid as acid gold plating.

The alkaline gold plating solutions in use in the early 1950s caused lifting of printed circuit resists, especially the wax-based resists introduced in an attempt to speed board preparation. The pH of the gold solutions was progressively lowered to minimize this effect. In one case, an accident resulted in too low a drop in the pH. It was not noticed at first because the bath continued to plate and there was no lifting of the resist. However, a drop in cathode current efficiency and a decrease in the thickness of the gold deposit alerted the operator. On investigation it was found that the pH had fallen to 4.0.

Separately, it was discovered by Duva that at a pH of 3.5 to 5, it was possible to add small amounts of cobalt, nickel, iron, and other metals to harden the gold deposit and cause it to plate bright. The purity of the deposit was still over 98% gold, but the hardness could be as high as 230 HK. Later, it was also noticed that the crystal structure of the surface could be plated to yield a (111) crystal plane, which greatly increased the wear resistance of the contact surface. Depending on the added metal or metals, the chemical form of the addition, and the pH of the electrolyte, deposits of various hardnesses and other characteristics could be made (Table 4).

Table 4 Acid gold industrial plating baths

Component or parameter

Bright, hard acid

Weak acid

Regular baths

Gold as potassium gold cyanide g/L (oz/gal)

4-16 (0.5-2)

4-8 (0.5-1)

Potassium citrate, citric acid, g/L (oz/gal)

180 (24)

Mono- and dipotassium phosphate, g/L (oz/gal)

180 (24)

Brightener

(a)

pH

3.5-5.0

5.5-7.0

Temperature, °C (°F)

20-50 (68-122)

65-74 (150-165)

Current density, A/dm2 (A/ft2)

1-10 (9-90)

0.1-0.5 (1-5)

Current efficiency, %

30-40

85-100

High-speed baths

Gold as potassium gold cyanide, g/L (oz/gal)

4-24 (0.5-3)

8-32 (1-4)

Citrates, g/L (oz/gal)

90 (12)

Phosphates/citrates, g/L (oz/gal)

90 (12)

Brighteners

(a)

(a)

Temperature, °C (°F)

49-60 (120-140)

71-82 (160-180)

Current density1™, A/dm2 (A/ft2)

10-200 (93-1860)

5-50 (46-460)

Current efficiency, %

40-50

(b) Values given are typical; they depend on agitation and the individual machine.

At the same time that the above developments took place, the semiconductor industry developed a need for high-purity golds at increased thicknesses. This led to a series of formulations by Ehrheart that plated gold from mild acid solutions. Raising the pH resulted in better covering power and higher current efficiency. At first the hardness and brightness of the acid golds was lost, but it was found that by modifying the neutral electrolytes, these properties could be partially restored (Table 4). So many different solutions were developed that a standard was needed. The most recent MIL-G-45204C (1984) and ASTM B 488-86, the military specification defines the purity, hardness, and thickness of the deposit. Purity is described as:

0 0

Post a comment