Salt spray testing is a crucial testing procedure to verify environmental reliability of PCB&PCBA. This article will provide a comprehensive and detailed explanation.
I. Test Purpose and Significance
Salt spray testing is primarily used to evaluate the corrosion resistance of PCB&PCBA and their surface treatment processes in a salty, humid atmospheric environment. It's core purpose is not only to simulate a real marine environment, but rather to provide an accelerated, controlled corrosion environment for:
1. Evaluation of Protective Performance: Testing the protective effect of solder mask layers and surface treatments (such as HASL, immersion gold/ENIG, OSP, etc.) on the copper substrate.
2. Discovery of Potential Defects: Expose defects such as micropores, scratches, and contamination on the solder mask layer that may exist during the manufacturing process.
3. Comparing Process Advantages and Disadvantages: Providing comparative data on the corrosion resistance of different surface treatment and materials or inks from different suppliers.
4. Predicting Long-Term Reliability: Providing reliability references for electronic products which will be used in harsh environments such as coastal areas and chemical plants.
II. Testing Standards
Common applied standards from the following major systems:
- IEC (International Electrotechnical Commission): IEC 60068-2-11 and IEC 60068-2-52. IEC 60068-2-52 is more commonly used, as it includes various severity levels (from Ka to Kd).
- ISO (International Organization for Standardization): ISO 9227, one of the most mainstream salt spray testing standards globally.
- ASTM (American Society for Testing and Materials): ASTM B117, a long-established and traditional testing method.
- Chinese National Standards (GB): GB/T 2423.17 and GB/T 2423.18, essentially equivalent to IEC standards.
In the electronics industry, IEC 60068-2-52 and ISO 9227 are the most commonly applied.
III. Test Methods and Procedures
1. Solution Preparation:
A 5% 卤 1% NaCl (sodium chloride) solution is typically used, with a pH value in the neutral range (6.5-7.2). Sometimes, an "acetic acid accelerated salt spray test" is used to further accelerate corrosion.
2. Test Conditions:
- Temperature of salt spray chamber: Generally constant at 35掳C 卤 2掳C.
- Relative humidity: Approximately 100%.
- Settling rate: The amount of salt spray settling on the collector should be controlled at 1.0 ~ 2.0 ml/80cm虏路h.
3. Test Preparation:
- The test sample should be a PCB with completed surface treatment, preferably with representative graphics (such as lines and pads).
- The sample must be clean, free of fingerprints and oil.
- The placement angle is crucial; typically, the sample's surface should be at an angle of 15掳 ~ 30掳 to the vertical to ensure that the salt spray can settle freely on the surface and does not accumulate and drip.
4. Testing Cycle Times:
- Depending on requirements and standard levels of products, common cycles are 24h, 48h, 96h, 168h (7 days), or even longer.
- Neutral Salt Spray Test (NSS): The most basic test.
- Acetic Acid Accelerated Salt Spray Test (AASS): Acetic acid is added to the solution, lowering the pH to ~3.2, resulting in faster corrosion; used for more stringent evaluations.
- Copper Accelerated Acetic Acid Salt Spray Test (CASS): Based on AASS, copper chloride is added, resulting in the strongest corrosiveness and the highest acceleration rate.
5. Recovery: After testing, the sample should be gently cleaned under room temperature water to remove surface salts, and then recover within standard atmospheric conditions (e.g., 23掳C, 50%RH) for 1-2 hours before evaluation.
IV. Result Evaluation and Failure Criteria
After testing, it's not simply a matter of checking for "rust," but rather a detailed inspection:
1. Visual Inspection:
- Substrate: Check for blistering, delamination, and discoloration.
- Solder Mask: Check for blistering, peeling, cracking, and discoloration.
- Conductors and Pads:
1) Copper Corrosion: Green, blue, or black copper rust appears on the surface.
2) Surface Treatment Corrosion: For example, blackening of the tin-plated surface, "red rust" (nickel layer corrosion) under the immersion gold layer, and damage to the OSP film exposing copper oxide.
3) Electrochemical Migration: Under voltage bias, dendrites may form, leading to short circuits.
2. Testing of Electrical Performance: Measure whether the insulation resistance decreases or whether short circuits/leakage exist.
3. Testing of Mechanical Performance: Check whether the solderability of the pads decreases after testing.
4. Common Criteria: For example, after a 96-hour neutral salt spray test, the solder mask should show no blistering or peeling, and the copper corrosion width at the conductor edges should not exceed a certain range (e.g., 100碌m).
V. PCB Design and Manufacturing Requirements
To improve pass rates for salt spray test, the following should be considered during the design and manufacturing stages:
1. Surface Treatment Selection:
- Immersion Gold (ENIG): Generally performs well, but be careful to prevent "black pads".
- Immersion Tin, Immersion Silver: Offer good corrosion resistance, but be mindful of storage and oxidation.
- HASL: Thicker layer, but uneven surface may trap corrosion.
- OSP: Thin protective layer, generally weaker salt spray resistance.
2. Solder Mask: Choose inks with high adhesion, high density, and good chemical resistance. Coverage is crucial; ensure all copper surfaces are well covered, especially the edges of conductors.
3. Design Specifications:
- Avoid placing critical, fine traces along board edges.
- Consider adding a protective coating, such as conformal coating. This is one of the most effective ways to significantly improve salt spray resistance.
- Proper spacing design to prevent corrosion bridging.
VI. Important Considerations
- Limitations of Acceleration and Correlation: Salt spray testing is an accelerated corrosion test, and its results do not have a direct, linear relationship with the lifespan of the circuit board in real outdoor environments. It is mainly used for qualitative comparison and defect screening.
- Consistency Control: The calibration of the salt spray chamber, solution concentration, pH value, and placement angle must be strictly controlled; otherwise, the repeatability of the results will be poor.
- Comprehensive Evaluation: Salt spray testing is usually not used as a standalone reliability criterion. It needs to be combined with other environmental tests such as damp heat testing and temperature cycling testing to comprehensively evaluate the product's reliability.
Summary
Salt spray testing for PCB&PCBA is a fundamental and critical reliability test to measure&verify the resistance to environmental corrosion. It mainly verifies the manufacturing quality of the PCB, especially the integrity of surface treatment and solder mask processes. By selecting test standards appropriately, strictly controlling the testing process, and combining it with correct failure analysis, the long-term reliability of PCBs in harsh environments can be effectively improved.
Benlida has been manufacturing PCBs and assembling PCBAs for 14 years and now has a fully equipped quality laboratory. If your PCB&PCBA or electronic products need salt spray testing, please contact Benlida!
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