Strategies for Reducing Outgassing Issues in Printed Circuit Boards

Reducing Outgassing in PCBs for Space and Vacuum Applications

Outgassing in printed circuit boards (PCBs) refers to the release of gases trapped or absorbed within the PCB materials when exposed to heat, vacuum, or during device operation. This phenomenon can cause significant issues, particularly in high-vacuum environments such as aerospace, vacuum electronics, or scientific instrumentation.

Effects of Outgassing in PCBs

Outgassing can have several detrimental effects on PCBs and the systems they are a part of. Some of these include:

Contamination of Vacuum Environments: Released gases can deposit on sensitive surfaces, optics, or electronic components, degrading performance or causing failures.
Material Degradation: Repeated outgassing can deteriorate the structural and electrical integrity of PCB materials.
Electrical Performance Loss: Voids or bubbles from escaping gases may reduce dielectric properties or induce delamination.
Reliability Issues: Outgassing can contribute to device failure by causing shorts or open circuits due to contamination or material cracking.

Mechanisms of Outgassing

Outgassing in PCBs can occur through several mechanisms:

Thermal Desorption: Elevated temperatures during manufacturing (soldering, baking) or operation cause volatile components or moisture trapped in polymers to evaporate.
Decomposition: Thermal or radiation-induced breakdown of organic materials releases gases such as water vapor, CO2, or other organic volatiles.
Diffusion: Gases dissolved or absorbed inside PCB polymers slowly migrate to surfaces over time, especially under vacuum.
Porosity or Surface Roughness: Material defects like pores or rough surfaces can trap gases which are later released; however, studies indicate that porosity and surface roughness in some 3D-printed plastics are less critical for outgassing.

Prevention and Minimization Strategies

To mitigate outgassing in PCBs, several strategies can be employed:

Material Selection: Use PCB laminates and polymers with low volatility and high thermal stability. For example, halogen-free, antimony-free, and red phosphorous-free compositions reduce outgassing risk, as seen with materials like Nanya NPG151.
High-Temperature Baking (Pre-Bake): Heating PCBs before application in vacuum drives off volatile compounds early, reducing later outgassing under UHV.
Optimized Manufacturing Processes: Controlling solder reflow profiles and cleaning procedures to balance sufficient heating to remove moisture and volatiles without damaging materials is critical.
Surface Treatments: Applying conformal coatings or using materials with low porosity and low moisture absorption helps reduce gas release.
Material Testing and Selection in Vacuum: Assessing candidate materials via residual gas analysis under ultra-high vacuum (UHV) to identify low-outgassing options, including advanced 3D-printed plastics like COC and PEEK, which have shown promising behavior.

Examples of Low-Outgassing Materials

Some examples of materials with low outgassing rates include Rogers' RT/duroid series and Nelco's meteorwave series. Materials specifically developed for vacuum environments, such as Rogers Corporation's RT/duroid series, should be used to reduce offgassing. Flexible circuit materials like Kapton offer excellent outgassing properties.

Testing and Standards

NASA has designed stringent procedures to test circuit board laminates for outgassing in space applications. ASTM E595-07 is a test procedure to assess PCB materials, evaluating TML (total mass loss) and CVCM (collected volatile condensable material). IPC-1601 describes the standard for circuit board baking to eliminate outgassing.

In high-vacuum applications, employing PCB materials with high glass transition temperature (>170°C) and excellent chemical and heat resistance, such as Nanya NPG151, combined with thorough pre-baking and controlled processing, significantly reduces outgassing and preserves vacuum integrity and PCB reliability. To avoid outgassing during soldering, hole plating thickness should be at least 25μm.

To prevent PCB outgassing in space applications, designers should verify that the circuit board and its components can resist extreme temperature swings, employ heating devices, and direct vapor away from sensitive surfaces. To avoid offgassing, designers need to ensure that the materials have low outgassing rates and can withstand fluctuating pressure and temperature. The conductance method is used to measure PCB outgassing, involving a chamber of known outgassing rate, a pump, and specific conductance.

In the ASTM E595-07 test, the sample is placed in a vacuum environment for 24 hours at 125°C and weighed to determine its volatile condensable content and total mass loss. A board material with collected volatile of less than 0.1% and total mass loss of less than 1% when placed in a vacuum for 24 hours at a temperature of 125°C is considered as the substrate with low outgassing rate. Moisture removal from the PCB is vital, and boards should be baked to achieve this.

In summary, understanding the mechanisms of outgassing and employing appropriate strategies for material selection, manufacturing, and testing can help minimize outgassing in PCBs, ensuring optimal performance and reliability in high-vacuum applications.

References:

Low Outgassing 3D Printed Plastics for Space Applications
Nanya NPG151: A Low-Outgassing PCB Material
Rogers Corporation: Low-Outgassing PCB Materials
Understanding and Minimizing Outgassing in PCBs
IPC-1601: Standard for Circuit Board Baking to Eliminate Outgassing

Technology plays a crucial role in addressing outgassing issues in PCBs, as it enables the development of low-outgassing materials such as Nanya NPG151 and materials from Rogers Corporation's RT/duroid series.
Advances in technology, including high-precision manufacturing processes, conformal coatings, and optimization techniques, help minimize outgassing in PCBs, ensuring optimal performance and reliability in space and vacuum applications.