How outgassing in materials is tested

How Outgassing in Materials is Tested: Key Parameters and Procedures

What is outgassing?

Outgassing occurs when gas is released from a solid or liquid material due to increasing temperature, decreasing pressure, and certain chemical reactions like decomposition. Ultra-low pressure levels in vacuum environments result in outgassing as gas molecules always tend to move from a high to a low pressure area. On the  other hand, increasing temperature increases the kinetic energy of the molecules within the solid or liquid material, making these molecules more volatile, leading to a potential phase change, and finally releasing gas.

Four Mechanisms of Outgassing in Solids and Liquids

Surface atoms, unlike those within the material, have incomplete bonds, making the surface an active site for the adsorption of vapors and gasses. Needless to say, this makes nearly all materials undergo outgassing. Although some materials, including polymers (plastics, elastomers, thermosets, and adhesives), and porous ceramics and metals, have it worse.

Four primary mechanisms contribute to outgassing– vaporization, desorption, diffusion, and permeation. Outgassing rate is the sum of these four mechanisms.

Vaporization. This includes the phase change from solid or liquid to gas of the material surface itself. Vaporization is negligible for metal surfaces under normal operating conditions (temperature and pressure). 

Desorption. This involves the release of gas molecules adsorb on the material surface.

Diffusion. This refers to the movement of molecules from the material’s inner (bulk) to its surface.

Permeation. Outgassing through permeation occurs when molecules from an external atmosphere move through the material’s surface then across the bulk and finally released to the other surface.

Why does outgassing matter?

Outgassing poses a challenge for electronic devices designed for high-vacuum and/or high-temperature environments. The substances released during outgassing, which commonly are water vapor, volatile organic compounds, and oil (or grease) can contaminate the vacuum environment, influencing the performance of delicate equipment or experiments. Take semiconductor manufacturing, for instance. It involves highly sensitive processes that demand ultra-clean environments with stringent standards. The reason is simple: even tiny amounts of contaminants can significantly affect the desired outcomes.

In the automotive industry, outgassing is also crucial. Modern vehicles are equipped with a wide range of electronic components like sensors, control units, and displays, all containing outgassing-prone polymeric materials, such as semiconductor and optoelectronic molding compounds, along with die attach and thermal interface materials.  The outgassing from these components has the potential to impact their long-term performance and reliability. Within electronic systems, the presence of contaminants resulting from outgassing can lead to issues like corrosion, reduced conductivity, and compromised functionality.

Outgassing also holds vital significance in aerospace applications. Firstly, the typical spacecraft conditions characterized by low pressures and extreme temperatures can trigger outgassing. Secondly, similar to the automotive industry, the particles released during outgassing can disrupt the proper functioning of essential electronic equipment crucial for spacecraft operation. Due to this significance, outgassing tests are conducted on numerous aerospace components to guarantee optimal performance and reliability. The National Aeronautics and Space Administration (NASA) developed a screening tool to assess material outgassing performance, later translated into ASTM E595 by the American Society for Testing and Materials (ASTM).

How is outgassing tested?

The test method in ASTM E595-77/84/90 involves exposing the materials to specific conditions to assess their outgassing behavior. First, the components are held at 25 °C and 50% relative humidity for 24 hours in a controlled environment. Then, they are weighed to measure any initial mass changes. To simulate space conditions, the materials are then exposed to vacuum (7×10–3 Pa or 5×10–5 Torr) and held at 125 °C for another 24 hours. During this phase, a cooling collector plate at 25 °C collects gas condensate for further analysis. Finally, the samples and the collector plate are weighed to determine the material’s outgassing performance. 

Outgassing Testing

Key parameters, like total mass loss (TML) and collected volatile condensable materials (CVCM), play a vital role in evaluation. TML refers to the percentage of a material’s initial mass that is lost during the outgassing test. It quantifies the total weight change of a specimen as it undergoes exposure to specific environmental conditions, typically high temperatures and vacuum. CVCM represents the percentage of volatile materials released during outgassing that condense and are collected on a designated surface, often a cooling collector plate. These are the substances that transition from a gaseous to a condensed state during the testing process. Monitoring CVCM is crucial because it helps assess the potential for contaminants released during outgassing to condense on critical surfaces within a spacecraft or aerospace system. NASA sets target benchmarks for acceptable TML and CVCM at less than 1% and 0.1%, respectively. You can find the NASA outgassing data table here.

CAPLINQ’s low outgassing thermal interface materials, die attach solutions, assembly adhesive inks and films, and underfills match NASA’s standards, ensuring excellent performance and reliability in aerospace applications. Among the different types of thermal interface materials, silicone-free phase change materials, such as PTM 7000 and PTM 7900, stand out for their minimal outgassing, with TML levels as low as 0.030% and 0.021%, respectively, after baking at 160 °C for 48 hours. On the other hand, the following table summarizes the TML and CVCM values of low outgassing die attach materials, underfills, and assembly adhesives available at CAPLINQ.

ProductTML [%]CVCM [%]
Die Attach Pastes
Assembly Adhesive Inks and Films
LOCTITE ABLESTIK CE3103WLV Assembly Adhesive Ink0.140.02
LOCTITE ABLESTIK 5020K Assembly Film Adhesive0.240.02
LOCTITE ABLESTIK 563K Assembly Film Adhesive0.440.04

Understanding and testing for outgassing are pivotal in ensuring the optimal performance and reliability of materials in critical industries like electronics, automotive, and aerospace. The testing standards, such as ASTM E595-77/84/90, provide a rigorous methodology to evaluate a material’s outgassing behavior.  At CAPLINQ, we recognize the significance of material reliability in these industries. Our range of molding compounds, die-attach materials, liquid encapsulants and underfills, and thermal interface materials adhere to stringent standards. Take the next step toward material excellence and reliability– reach out to our team today. 

About Rose Anne Acedera

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