Metallic coatings - Determination of porosity in gold coatings on metal substrates - Nitric acid vapour test (ISO 14647:2000); German version EN ISO 14647:2016
1Key Takeaways
This international standard specifies the test apparatus and method for determining the porosity of gold plating by means of nitric acid vapor, particularly for electroplating and coatings used for electrical contacts. The procedure is designed to show whether the porosity is below or above a value that the user consid…
2Expert Interpretation
This article details the ISO 14647:2000 standard for porosity testing of gold plating on metal coatings using nitric acid vapor, covering key technical points such as test principles, equipment requirements, operating procedures, and result determination. This article provides professional guidance for quality testing of gold plating in areas such as electronic connectors and electrical contacts.
Standard Overview and Technical Background
ISO 14647:2000, "Metallic Coatings — Determination of Porosity of Gold Deposits on Metallic Substrates — Nitric Acid Vapor Test," is an authoritative standard developed by the International Organization for Standardization specifically for testing the porosity of gold coatings. This standard was adopted by the European Committee for Standardization as EN ISO 14647:2016 in 2016 and published in Germany as a DIN standard. Gold coatings are widely used in key electronic components such as electrical connectors and plug-ins due to their excellent corrosion resistance and electrical conductivity. However, their protective effectiveness directly depends on the integrity and porosity of the coating.
Test Principle and Mechanism Analysis
The nitric acid vapor test method is based on the principle of chemical corrosion. By exposing the sample to a specific concentration of nitric acid vapor, the chemical reaction between the nitric acid vapor and the base metal is exploited to reveal porosity defects in the gold coating. When there are pores in the gold plating layer, nitric acid vapor passes through the pores and reacts with the underlying metal (such as copper, nickel, etc.) to generate visible corrosion products. These products form obvious spots on the surface of the coating, thereby achieving qualitative and quantitative detection of the pores.
Scope of application and restrictions
| Applicable objects | Restrictions | Remarks |
|---|---|---|
| Inlays or coatings with a gold content ≥75% | Gold plating with a thickness of <0.6μm | The test is too harsh |
| Electroplating with a gold content of >95% | Palladium and its alloy plating | Not applicable |
| Copper, nickel and their alloy substrates | Partial palladium-gold plating | Test invalid |
Test Equipment and Reagent Requirements
Test Container: A glass desiccator with a capacity of 9-12 liters, with the ratio of the air volume to the nitric acid liquid surface area in the container ≤25:1
Sample Holder: Made of glass, polytetrafluoroethylene (PTFE) or other inert materials to ensure that steam circulation is not hindered
Nitric Acid Reagent: Analytical grade concentrated nitric acid (69±2%), relative density (20℃) 1.39-1.42
Detection Equipment: A 10x magnification stereo microscope with a movable light source
Key Points of Test Environment Control
| Environmental Parameters | Control Range | Importance |
|---|---|---|
| Temperature | 23±3℃ | Influence of reaction rate |
| Relative humidity | 40%-55% | Key control parameters |
| Exposure time | 60±5min (standard) | Adjust according to thickness |
Detailed operation process
Sample pretreatment: Use oil-free compressed air to remove surface dust, gently clean with a solvent that does not contain chlorofluorocarbons, chlorinated hydrocarbons or other known ozone-depleting compounds, and finally use hot methanol, ethanol or isopropanol to accelerate the drying process.
Test Procedure: 1. Pour 500ml of fresh nitric acid into the bottom of a clean, dry test container. 2. Immediately close the container lid and wait 30±5 minutes. 3. Place the sample on an appropriate support, ensuring the critical test surface is exposed. 4. Reclose the container and start timing. 5. Control the relative humidity between 40% and 55%.
Post-Processing: After the test, dry the sample in an oven at 125±5°C for 30-60 minutes, then transfer it to a desiccator filled with desiccant and cool it to room temperature.
Result Interpretation and Counting Rules
Observe and count using a 10x magnification stereomicroscope under oblique light (incident angle <15°). Corrosion products, as indicators of porosity, must meet the following counting criteria:
- At least three-quarters of the corrosion products must be located within the test area
- Corrosion product diameter > 0.05 mm (resolvable size)
- Corrosion spots at the edge and outside the test area must be excluded
Technical Points and Precautions
Safety Protection: Testing must be conducted in a fume hood. Operators must wear full-face protective goggles, and an accessible eyewash device must be in place. Avoid unnecessary sample handling and only use tweezers, lens tissue, or clean soft gloves.
Humidity Control: Relative humidity is critical to a successful test. Humidity > 60% will cause microscopic water film to adhere to the metal surface, affecting the accuracy of the test results. Humidity < 40% will result in incomplete results.
Time Control: Adjust the exposure time according to the thickness of the gold plating (see Appendix A): - 0.6-2.0μm: 60±5 minutes - 2.0-2.5μm: 75±5 minutes
Application Cases and Implementation Recommendations
Electrical Connector Industry: For gold-plated pin connectors, it is recommended that the porosity be controlled at <100 per square centimeter, and the critical contact areas should be completely free of pores. During implementation, internal acceptance/rejection criteria should be established and incorporated into the component specifications.
Precision Electronic Components: For RF connectors and precision connectors with high reliability requirements, it is recommended to adopt more stringent porosity standards and conduct correlation verification based on the actual use environment.
Standard Evolution and Technology Comparison
ISO 14647:2000 replaced the earlier porosity test method. Its technical advantages are: 1. High test sensitivity, capable of detecting almost all pores that may participate in substrate corrosion reactions 2. Applicable to contact parts with complex geometries 3. Fast, simple and low-cost operation
Compared with other porosity test methods (such as electrochemical method, gel method, etc.), the nitric acid vapor method has obvious advantages in reproducibility and operability, but it is a destructive test and the tested parts cannot be put into use.
Implementation Recommendations and Best Practices
1. Establish Internal Standards: Develop specific porosity acceptance criteria based on product application scenarios, including parameters such as pore quantity, location, and size
2. Environmental Control: Invest in precise temperature and humidity control equipment to ensure that test conditions meet standard requirements
3. Personnel Training: Provide professional training to operators, especially in microscopic observation and counting techniques
4. Equipment Calibration: Regularly calibrate microscopes and temperature and humidity measurement equipment to ensure the accuracy of test results
5. Complete Records: Test reports should include all information required by the standard to facilitate traceability and analysis
The ISO 14647 standard provides a scientific and reliable test method for porosity testing of gold plating. By strictly adhering to the standard requirements and combining practical application experience, the quality and reliability of gold-plated products can be effectively evaluated and improved, providing a guarantee for the long-term stable operation of electronic equipment.