Class 6 375 pm 1500 pin

Type I purity cannot have hardness D, and Type II purity cannot have hardness A. Type III purity can only be hardness A.

Strike Plating. Gold is a noble metal and deposits at a very low applied potential. These characteristics can cause nonadherence of the gold deposit if the substrate is either passive or not perfectly clean. Poor adhesion can be prevented by using a gold strike bath. A strike is generally a solution with very low metal concentration that is operated at high voltage and high current density for a very short period of time. For rack plating, the strike plating time is less than 1 min at a current density of 1 to 3 A/dm2 (9 to 28 A/ft2). A gold strike generally is not needed when plating from an acid gold solution unless the gold concentration is greater than 8 g/L or the substrate is passive.

Noncyanide Gold Plating Solutions. Sulfite gold industrial baths are used for their unique physical properties in addition to the desirable property of being noncyanide. As discussed above, sulfite golds have exceptional microthrowing power, which makes them the only gold formulations that build brightness. Furthermore, they have the best infrared reflectivity of any gold plating solution. The following table shows the composition and operating parameters of sulfite gold industrial baths:

Component or parameter

Value

Gold as sodium gold sulfite, g/L (oz/gal)

4-16 (0.5-2)

Sodium sulfite and sulfate, g/L (oz/gal)

90 (12)

pH

8.5-10.0

Temperature, °C (°F)

50-60 (122-140)

Brightener

As required

Current density, A/dm2 (A/ft2)

0.1-0.4 (1-4)

Current efficiency, %

100

Electroplating Calculations. Factors to use with gold electroplating calculations are:

• The price of gold, as given in newspapers and on the radio, is expressed in dollars per troy ounce (1 troy ounce = 31.1 g).

• A deposit of gold that is 1 pm thick = 19.58 g/m2 (1.82 g/ft2).

• At 100% cathode current efficiency, 7.35 g of gold can be electrodeposited in 1 ampere-hour, or 0.123 g in 1 ampere-minute.

• At 100% cathode current efficiency, 160.5 ampere-minutes are required for a gold deposit that is 1 pm thick and covers 1 m2.

Time, temperature, and amperage can be accurately measured and controlled in gold electroplating. The largest errors that can affect gold calculations are the inaccuracies in the current density and the current efficiency. Current density is determined by calculating the area measurement, which is not always an easy task. Outside surface areas may be correctly calculated, but inside surfaces and holes, such as solder cups, must be calculated and then their effective plating area must be estimated.

Current efficiency is determined by current density, metal concentration, electrolyte concentration, and impurity content. The impurities that change the current efficiency are the metallic impurities, the organic impurities from masking materials and resists, and airborne dust. Current efficiency can be measured with a weighed coupon plated in the laboratory using a sample of the solution.

In practice, a good way to measure the efficiency of a solution is to estimate the required amperage and time based on theory, increase the amount by, say, 10%, and then plate a load under these conditions. The thickness of the gold on the plated work can be measured by microsection, x-ray diffraction, beta-ray backscatter, or other means. The thickness actually measured should be used to correct the estimated efficiency and to modify the plating conditions. It is best to measure the thickness periodically, because the cathode current efficiency of a gold bath will change not only with the variability of all the chemical constituents but also with the age of the bath. Periodic monitoring of the thickness ensures consistent quality control.

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