How to Prepare Your PCB for Outgassing in Ultra High Vacuum Systems

 

Introduction

In ultra-high vacuum (UHV) systems, outgassing is a critical phenomenon that can significantly impact the system's performance and reliability. Outgassing refers to the process of releasing trapped gases from materials when exposed to vacuum conditions. Printed circuit boards (PCBs) are an essential component of many UHV systems, and their outgassing behavior can have a detrimental effect on the vacuum level and system operation. Proper preparation of PCBs is crucial to minimize outgassing and ensure the successful operation of UHV systems.

Understanding Outgassing

Before delving into the preparation methods, it is essential to understand the concept of outgassing and its implications in UHV systems.

What is Outgassing?

Outgassing is the process by which materials release trapped gases, such as water vapor, air, or other volatile compounds, when exposed to vacuum conditions. These gases can be present in the bulk material, adsorbed on the surface, or trapped in microscopic pores or crevices.

Impact of Outgassing in UHV Systems

In UHV systems, outgassing can lead to several detrimental effects:

1. Vacuum Degradation: Released gases can increase the overall pressure inside the vacuum chamber, making it difficult to maintain the desired ultra-high vacuum level.
2. Contamination: Outgassed species can deposit on sensitive components, such as optics, detectors, or vacuum gauges, affecting their performance and accuracy.
3. Interference with Processes: In certain applications, such as particle accelerators or surface analysis techniques, outgassed gases can interfere with the intended processes or measurements.

Preparing PCBs for Outgassing in UHV Systems

To minimize outgassing and ensure the proper functioning of UHV systems, PCBs must undergo specific preparation steps. These steps aim to remove trapped gases, contaminants, and volatile compounds from the PCB materials.

Material Selection

The first step in preparing PCBs for UHV systems is to carefully select the appropriate materials. Some materials are more prone to outgassing than others, and their selection can significantly impact the overall outgassing behavior of the PCB.

Low Outgassing Materials

When designing PCBs for UHV applications, it is recommended to use materials with low outgassing rates. These materials typically have a low moisture absorption rate, low volatile content, and high thermal stability. Examples of low outgassing materials include:

• Ceramic-filled epoxy resins
• Polyimide laminates
• Polytetrafluoroethylene (PTFE) dielectrics
• Ceramic substrate materials

Avoiding Outgassing-Prone Materials

Certain materials should be avoided or minimized in PCB design for UHV systems due to their high outgassing rates. These materials include:

• Standard FR-4 epoxy laminates
• Acrylic conformal coatings
• Polyurethane conformal coatings
• Certain types of solder masks and surface finishes

Baking and Vacuum Dehydration

One of the most effective methods for reducing outgassing in PCBs is baking or vacuum dehydration. This process involves subjecting the PCBs to elevated temperatures and vacuum conditions for an extended period, typically ranging from several hours to several days.

Baking Process

The baking process involves heating the PCBs in a controlled environment, such as a vacuum oven or a dedicated baking chamber. The temperature and duration of the baking process depend on the specific materials used in the PCB and the desired outgassing reduction level.

Typical baking temperatures range from 100°C to 200°C, with higher temperatures generally being more effective in removing trapped gases and volatile compounds. However, care must be taken not to exceed the maximum temperature ratings of the PCB components or materials.

Vacuum Dehydration

Vacuum dehydration is often performed in conjunction with baking. In this process, the PCBs are placed in a vacuum chamber, and the pressure is gradually reduced to a level that facilitates the removal of trapped gases and moisture.

The vacuum dehydration process can be carried out at room temperature or at elevated temperatures, depending on the specific requirements and materials involved. Combining baking and vacuum dehydration can significantly enhance the outgassing reduction effectiveness.

Surface Cleaning and Preparation

In addition to baking and vacuum dehydration, proper surface cleaning and preparation of PCBs are crucial for minimizing outgassing in UHV systems.

Solvent Cleaning

Solvent cleaning can be used to remove surface contaminants, such as oils, greases, and residues, which can contribute to outgassing. Common solvents used for this purpose include isopropyl alcohol, acetone, or specialized cleaning solutions recommended by the PCB manufacturer.

Plasma Cleaning

Plasma cleaning is an effective method for removing organic contaminants and improving the surface cleanliness of PCBs. In this process, the PCBs are exposed to a low-pressure plasma environment, which generates reactive species that break down and remove organic compounds from the surface.

Surface Preparation

After cleaning, the PCB surfaces may require additional preparation steps, such as mechanical or chemical etching, to improve adhesion and reduce outgassing. These processes can remove surface layers and expose fresh, low-outgassing materials.

Encapsulation and Sealing

In some cases, encapsulating or sealing the PCBs can be an effective strategy for minimizing outgassing in UHV systems. This approach involves coating or covering the PCB with a low outgassing material, effectively trapping any remaining gases or volatile compounds within the encapsulation.

Common encapsulation materials include ceramic coatings, parylene coatings, or specialized low outgassing epoxies or resins. However, it is essential to ensure that the encapsulation material itself has a low outgassing rate and is compatible with the UHV system requirements.

Testing and Validation

After preparing the PCBs for UHV applications, it is crucial to perform testing and validation to ensure that the outgassing levels meet the system requirements.

Outgassing Rate Measurements

Outgassing rate measurements involve placing the prepared PCBs in a controlled vacuum environment and monitoring the pressure rise over time. This measurement provides quantitative data on the outgassing rate of the PCB materials and can help determine their suitability for UHV applications.

Several techniques are available for outgassing rate measurements, including:

• Throughput method
• Accumulation method
• Pressure rise method

Throughput Method

In the throughput method, the PCB is placed in a vacuum chamber with a known pumping speed. The outgassing rate is calculated based on the steady-state pressure and the pumping speed.

Accumulation Method

The accumulation method involves isolating the PCB in a sealed volume and monitoring the pressure rise over time. The outgassing rate is calculated from the pressure increase and the known volume.

Pressure Rise Method

The pressure rise method is similar to the accumulation method but involves a continuous pumping system. The outgassing rate is determined by measuring the pressure rise rate and the pumping speed.

Acceptance Criteria

Based on the intended application and system requirements, specific acceptance criteria for outgassing rates should be established. These criteria may be defined by industry standards, customer specifications, or the design constraints of the UHV system.

FAQs (Frequently Asked Questions)

1. Can outgassing be completely eliminated in PCBs? While it is impossible to completely eliminate outgassing in PCBs, the outgassing rates can be significantly reduced through proper material selection, baking, vacuum dehydration, and surface preparation techniques. The goal is to minimize outgassing to acceptable levels that meet the system requirements.
2. How long does the baking process typically take? The duration of the baking process can vary depending on the specific materials used in the PCB, the desired outgassing reduction level, and the baking temperature. Typical baking times range from several hours to several days, with longer baking times generally providing better outgassing reduction.
3. Can the baking process damage PCB components or materials? Yes, the baking process can potentially damage certain components or materials if the temperature exceeds their maximum ratings. It is crucial to carefully consider the temperature limitations of all components and materials used in the PCB before subjecting them to the baking process.
4. Is it necessary to perform outgassing rate measurements for every PCB? While outgassing rate measurements are recommended for validating the effectiveness of the preparation process, it may not be necessary to test every individual PCB. Instead, representative samples can be tested, and the results can be applied to similar PCBs prepared using the same methods and materials.
5. Can outgassing rates change over time in UHV systems? Yes, outgassing rates can change over time in UHV systems. Initially, the outgassing rate may be higher due to the release of surface contaminants and trapped gases. As the system operates and the materials are exposed to vacuum conditions for an extended period, the outgassing rate may decrease as the materials become increasingly degassed.