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ISO 27874:2008 — Electrodeposited Gold and Gold Alloy Coatings
Specifications and Test Methods for Electrical, Electronic, and Engineering Applications
Overview of Gold Coating Standards
ISO 27874:2008 specifies requirements and test methods for electrodeposited gold and gold alloy coatings for electrical, electronic, and engineering purposes. Gold coatings are essential for applications requiring low contact resistance, excellent corrosion resistance, high wear durability, and superior solderability. This standard classifies gold deposits into four types based on purity and hardness: pure gold (99.7% min), hard gold (with cobalt or nickel codeposits), gold alloys, and flash gold.
Hard gold deposits (99.0-99.7% Au with 0.1-0.5% Co or Ni) are preferred for connector contacts due to their 2-3x higher wear resistance compared to pure gold, while maintaining contact resistance below 10 mOhm.
The standard applies to coatings on metallic substrates including copper, copper alloys, nickel, and nickel-plated surfaces. It defines six service condition classes (SC1 through SC6) from benign indoor environments to severe industrial or marine exposure, with minimum thickness ranging from 0.2um to 5.0um.
Classification System and Technical Requirements
| Type | Gold Purity | Hardness (HV 0.05) | Typical Applications | Thickness Range |
|---|---|---|---|---|
| Type A: Pure gold | >= 99.7% | 40-80 HV | Semiconductor bonding, PCB ENIG | 0.1-1.0 um |
| Type B: Hard gold | 99.0-99.7% | 130-200 HV | Connector contacts, edge connectors | 0.5-5.0 um |
| Type C: Gold alloy | 95.0-99.0% | 100-250 HV | Sliding contacts, brush rings | 1.0-10.0 um |
| Type D: Flash gold | >= 99.0% | 40-80 HV | Solderability preservation | 0.05-0.2 um |
Porosity testing uses nitric acid vapor or sulfur dioxide exposure, with porosity ratings from P0 to P4. Adhesion is verified by bend test, thermal shock (250C for 30 min), or tape test. Wear resistance uses the Taber abraser method.
Nickel underplating (1.0-5.0 um) between substrate and gold coating is critical for preventing copper migration. ENIG processes must maintain phosphorus content in the nickel layer between 7-10% for optimal corrosion protection.
Engineering Design and Application Considerations
Contact Resistance and Connector Reliability
The low contact resistance of gold coatings (typically less than 5 mOhm) derives from its nobility and absence of surface oxides. However, porosity in thin gold deposits can expose underlying nickel or copper, leading to corrosion product formation. The standard’s porosity classification guides engineers in selecting appropriate thickness for the service environment.
Wear Mechanisms and Lubrication Strategy
For connector applications with repeated mating cycles, hard gold with cobalt codeposit provides optimal wear performance. The standard recommends applying a lubricant coating for connectors exceeding 100 mating cycles, reducing wear rate by 50-80%.
For critical aerospace and medical applications, specifying Type B hard gold at 2.5 um minimum with nickel underplate of 2.0-4.0 um provides optimal balance of corrosion resistance, wear durability, and cost.
Frequently Asked Questions
Q: What is the difference between ENIG and electroplated gold per ISO 27874?
ENIG produces a thinner (0.05-0.15 um) gold layer through displacement reaction for PCB finishing. Electroplated gold per ISO 27874 produces thicker, denser deposits for connector contacts and high-wear applications.
ENIG produces a thinner (0.05-0.15 um) gold layer through displacement reaction for PCB finishing. Electroplated gold per ISO 27874 produces thicker, denser deposits for connector contacts and high-wear applications.
Q: How does cobalt content affect hard gold properties?
Cobalt (0.1-0.5%) refines grain structure, increasing hardness from 40-80 HV to 130-200 HV. Excessive cobalt (>0.5%) increases internal stress and reduces ductility.
Cobalt (0.1-0.5%) refines grain structure, increasing hardness from 40-80 HV to 130-200 HV. Excessive cobalt (>0.5%) increases internal stress and reduces ductility.
Q: What is the shelf life of gold-plated components?
When properly packaged, gold-plated components have a shelf life exceeding 5 years. The standard requires coatings remain solderable and corrosion-free within the specified storage period.
When properly packaged, gold-plated components have a shelf life exceeding 5 years. The standard requires coatings remain solderable and corrosion-free within the specified storage period.
