What is Outgassing on a PCB?

Understanding PCB Outgassing and Its Effects

Printed Circuit Boards (PCBs) are essential components in modern electronics, forming the backbone of countless devices we use daily. However, during the manufacturing process and throughout their lifecycle, PCBs can experience a phenomenon called outgassing. In this article, we will delve into the concept of PCB outgassing, its causes, effects, and mitigation strategies to ensure the longevity and reliability of electronic devices.

What is PCB Outgassing?

PCB outgassing is the release of trapped gases, vapors, or volatile organic compounds (VOCs) from the materials used in the construction of a printed circuit board. These substances can be released due to various factors, such as high temperatures, vacuum conditions, or chemical reactions within the PCB.

The outgassing process can occur during the manufacturing stage, especially when the PCB is subjected to high temperatures during soldering or curing. Additionally, outgassing can continue throughout the PCB’s operational life, particularly when exposed to elevated temperatures or other environmental stressors.

Causes of PCB Outgassing

Several factors contribute to the occurrence of outgassing in PCBs:

1. Material Composition: The materials used in PCB fabrication, such as substrates, laminates, adhesives, and solder masks, can contain volatile compounds that are prone to outgassing. These materials may release gases when exposed to high temperatures or vacuum conditions.

2. Manufacturing Processes: During PCB Assembly, various processes involve high temperatures, such as soldering, reflow, and curing. These elevated temperatures can trigger the release of trapped gases within the PCB materials.

3. Environmental Factors: Exposure to moisture, humidity, and extreme temperatures during the PCB’s operational life can accelerate the outgassing process. Additionally, exposure to certain chemicals or solvents can cause the degradation of PCB materials, leading to increased outgassing.

Effects of PCB Outgassing

PCB outgassing can have several detrimental effects on the performance and reliability of electronic devices:

1. Contamination: Outgassed substances can contaminate sensitive components, such as optical sensors, lenses, or mirrors, leading to reduced performance or complete failure. The deposited residue can also interfere with electrical connections, causing signal integrity issues.

2. Corrosion: Some outgassed compounds can be corrosive, causing damage to metal surfaces and connections within the PCB. This corrosion can lead to decreased conductivity, increased resistance, and potential short circuits.

3. Insulation Degradation: Outgassing can degrade the insulating properties of PCB materials, such as the substrate or solder mask. This degradation can result in reduced dielectric strength, increased leakage currents, and potential electrical failures.

4. Delamination: Outgassing can cause the separation of PCB layers, known as delamination. This can occur when trapped gases expand and create voids or bubbles within the PCB structure, weakening the adhesion between layers and compromising the board’s structural integrity.

5. Reliability Issues: The cumulative effects of outgassing can lead to reduced reliability and shortened lifespan of electronic devices. Contamination, corrosion, insulation degradation, and delamination can all contribute to premature failure of PCBs and the devices they support.

Mitigating PCB Outgassing

To minimize the impact of PCB outgassing and ensure the reliability of electronic devices, several strategies can be employed:

1. Material Selection: Choose low-outgassing materials for PCB fabrication, such as specialized substrates, laminates, and solder masks that have been formulated to reduce outgassing. Consult with PCB manufacturers and material suppliers to identify suitable low-outgassing options.

2. Manufacturing Process Optimization: Optimize the manufacturing processes to minimize the exposure of PCBs to high temperatures and prolonged heating cycles. Implement controlled heating and cooling rates, and ensure proper ventilation to allow outgassed substances to dissipate efficiently.

3. Cleaning and Baking: After the manufacturing process, thoroughly clean the PCBs to remove any residual contaminants or outgassed substances. Additionally, consider baking the PCBs in a controlled environment to accelerate the outgassing process and remove trapped gases before final assembly.

4. Conformal Coating: Apply conformal coatings to the PCBs to create a protective barrier against environmental factors and prevent the ingress of moisture or contaminants. Conformal coatings can also help contain outgassed substances and prevent them from depositing on sensitive components.

5. Vacuum Baking: For critical applications or high-reliability devices, consider vacuum baking the PCBs before final assembly. This process involves exposing the PCBs to a vacuum environment at elevated temperatures to accelerate the outgassing process and remove trapped gases and volatiles.

6. Design Considerations: Incorporate design features that promote outgassing mitigation, such as proper component spacing, adequate ventilation, and the use of gas-permeable materials. Consider the thermal management of the PCB to minimize exposure to high temperatures during operation.

7. Testing and Monitoring: Implement testing and monitoring procedures to assess the outgassing behavior of PCBs and detect any potential issues early in the manufacturing process. Conduct outgassing tests, such as ASTM E595 or NASA SP-R-0022A, to evaluate the outgassing properties of materials and assemblies.

Outgassing Standards and Specifications

Several industry standards and specifications provide guidelines and requirements for managing PCB outgassing:

1. ASTM E595: This standard test method, developed by ASTM International, measures the outgassing properties of materials in a vacuum environment. It evaluates the total mass loss (TML) and collected volatile condensable materials (CVCM) of a sample.

2. NASA SP-R-0022A: This specification, established by NASA, sets requirements for materials used in spaceflight applications. It defines acceptable levels of outgassing and specifies testing methods to ensure compliance.

3. IPC-4101: The IPC-4101 standard, developed by the Association Connecting Electronics Industries (IPC), provides specifications for base materials used in printed boards. It includes requirements for outgassing properties and defines test methods for evaluation.

4. ESA PSS-01-702: The European Space Agency (ESA) has established this specification for materials and processes used in spacecraft applications. It defines outgassing requirements and testing procedures to ensure the suitability of materials for space environments.

Adhering to these standards and specifications helps ensure that PCBs and their constituent materials meet the necessary outgassing requirements for their intended applications.

Frequently Asked Questions (FAQ)

1. What is the difference between outgassing and offgassing?
   Outgassing and offgassing are often used interchangeably, but there is a slight distinction. Outgassing refers to the release of trapped gases, vapors, or volatile compounds from materials due to exposure to heat, vacuum, or other environmental factors. Offgassing, on the other hand, typically refers to the release of gases or vapors from materials at normal atmospheric pressure and room temperature.

2. Can outgassing be completely eliminated in PCBs?
   While it is challenging to completely eliminate outgassing in PCBs, it can be significantly reduced through proper material selection, manufacturing process optimization, and mitigation strategies. Using low-outgassing materials, implementing controlled heating and cooling cycles, and employing techniques like vacuum baking and conformal coating can help minimize the impact of outgassing.

3. How does outgassing affect the performance of electronic devices?
   Outgassing can have several detrimental effects on electronic device performance. Outgassed substances can contaminate sensitive components, causing reduced performance or failure. Corrosion caused by outgassed compounds can degrade electrical connections and increase resistance. Insulation degradation and delamination can compromise the PCB’s structural integrity and lead to electrical failures. Collectively, these effects can reduce the reliability and lifespan of electronic devices.

4. What are some common low-outgassing materials used in PCB fabrication?
   Several materials are known for their low-outgassing properties and are commonly used in PCB fabrication. These include:

5. Polyimide substrates, such as Kapton or Upilex
6. Low-outgassing FR-4 laminates
7. Ceramic substrates, such as alumina or beryllia
8. Low-outgassing solder masks and conformal coatings
9. Specialized adhesives and encapsulants formulated for low outgassing

It is essential to consult with PCB manufacturers and material suppliers to select the most suitable low-outgassing materials for specific applications.

1. How can outgassing be measured and tested in PCBs?
   Outgassing in PCBs can be measured and tested using standardized methods, such as ASTM E595 or NASA SP-R-0022A. These tests typically involve exposing the PCB or material samples to a vacuum environment at elevated temperatures and measuring the total mass loss (TML) and collected volatile condensable materials (CVCM). The results are then compared against specified acceptable limits to determine compliance with outgassing requirements.

Conclusion

PCB outgassing is a critical consideration in the design, manufacture, and operation of electronic devices. The release of trapped gases, vapors, or volatile organic compounds from PCB materials can lead to contamination, corrosion, insulation degradation, and delamination, compromising the performance and reliability of electronic systems.

By understanding the causes and effects of PCB outgassing, manufacturers can implement mitigation strategies, such as careful material selection, process optimization, cleaning, baking, conformal coating, and vacuum baking. Adhering to industry standards and specifications, such as ASTM E595, NASA SP-R-0022A, IPC-4101, and ESA PSS-01-702, ensures that PCBs meet the necessary outgassing requirements for their intended applications.

As technology advances and the demand for reliable electronic devices grows, addressing PCB outgassing becomes increasingly crucial. By proactively managing outgassing through informed design choices, rigorous manufacturing practices, and comprehensive testing, manufacturers can enhance the longevity and performance of their products, ultimately benefiting end-users and the electronics industry as a whole.