Gold is known for its glitter and glamour, but it also has significant value from an electroplating perspective. Why should gold be plated instead of using pure solid gold components or base materials? While plating with gold can be relatively expensive when compared to using less costly materials, this lustrous metal provides a number of important benefits in a wide variety of metal finishing applications. As a rule of thumb, if the additional upfront expense is manageable for your company, gold is usually the best plating option.
Gold Plating Specifications
In some instances, it may be necessary to perform gold plating to meet military or engineering specifications.
MIL-DTL-45204D
The MIL-DTL-45204D gold plating spec meets the requirements for the U.S. Department of Defense and its various support agencies and supersedes the previous MIL-G-45204C standard. This spec classifies gold type based on the purity of the material: Type I features a minimum gold content of 99.7 percent, Type II includes 99.0 percent and Type III consists of 99.9 percent. There are also four hardness grades based on the Knoop scale: Grade A (90 max.), Grade B (91-129), Grade C (139-200) and Grade D (201 and over).
Possible combinations for purity vs. hardness are the following:
- Type I: A, B or C
- Type II: B, C or D
- Type III: A only
There are also gold plating thickness standards under this military spec. There are eight classes corresponding to the acceptable minimum thickness:
- Class 00-0.00002
- Class 0-0.00003
- Class 1-0.00005
- Class 2-0.00010
- Class 3-0.00020
- Class 4-0.00030
- Class 5-0.00050
- Class 6-0.00150
Also, there are specific requirements regarding the underplating for the gold coating. Depending on the application and environment, an undercoating of nickel, copper or a Cu/Ni may be used to fulfill the contract requirements. Avoid using a silver or silver/copper underplate unless specifically required. Regardless of the composition of the underplating, it is essential to apply a soft gold strike atop the undercoating before adding the final coating of gold to promote adhesion and reduce the risk of contamination.
ASTM B488 Gold Plating Specification
The ASTM B488 gold plating standard applies to engineering applications. Gold coatings that comply with this spec typically are used to increase the substrate’s resistance to corrosion and to prevent tarnishing. The classes and types under this specification mirror those of the MIL-DTL-45204D gold plating standard. However, there are some differences in the thickness requirements. The seven thickness classes under ASTM B488 gold plating include:
- 0.25-0.25 um
- 0.50-0.50 um
- 0.75-0.75 um
- 1.0-1.0 um
- 1.25-1.25 un
- 2.5-2.5 um
- 5-5.0 um
Nickel is the preferred choice for underplating, except for applications requiring a coating thickness of 5.0 um or greater when working with substrates made of copper or a copper alloy. Others reasons that nickel makes the best undercoating material include serving as a brightener for the leveling layer, acting as a corrosion inhibitor in surface pores, preventing tarnish creep on the gold topcoat and acting as a load-bearing underlayer for the contacting surfaces.
The issue of this standard establishes an ASTM Type designation that is new involving purity, which is established for electrodeposited gold:
- Mass percent gold, minimum, excluding potassium, carbon and nitrogen 99.70: New ASTM Type I, MIL-DTL-45204 Type I, Old ASTM Type 2
- Mass percent gold, minimum, excluding potassium, carbon and nitrogen 99.00: New ASTM Type II, MIL-DTL-45204 Type II, Old ASTM Type 3
- Mass percent gold, minimum, excluding potassium, carbon and nitrogen 99.90: New ASTM Type III, MIL-DTL-45204 Type III, Old ASTM Type 1
The hardness values are specified by ASTM Code:
- Knoop Hardness Range 90 HK25 maximum: ASTM Code A
- Knoop Hardness Range 91–129 HK25: ASTM Code B
- Knoop Hardness Range 130–200 HK25: ASTM Code C
- Knoop Hardness Range >200 HK25: ASTM Code D
When it comes to the relationship between purity and hardness for good commercial practice, the following combinations are fully representative:
- New ASTM I – Old ASTM Type 2 – Code A, B and C
- New ASTM II – Old ASTM Type 3 – Code B, C and D
- New ASTM III – Old ASTM 1 – Code A only
Additional Requirements
Examples of other requirements for complying with the MIL-DTL-45204D and the MIL-DTL-45204D gold plating spec include ensuring the base metal is free of surface defects and implementing appropriate pretreatment steps before plating. Avoid electroplating steel parts and components that have a tensile strength of 220,000 psi unless stipulated in the contract. Acceptable post-plating procedures include removing all plating salts from the substrate and inspecting the part for blisters that will render the piece defective.
SPC Is a Gold Electroplating Leader
Over our more than eight decades of service, Sharretts Plating Company has developed a finely honed gold plating process. While we can perform gold plating for a variety of applications, our primary focus is in the electronics industry. In addition to gold electroplating, we also offer a highly effective immersion plating process that does not require the use of electricity. Request Quote
