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Outgassing is a defect that occurs when trapped gas/vapor is released from the printed circuit board when exposed to heat or vacuum, creating blowholes and voids. Offgassing affects a wide range of electronic applications that are used in high-vacuum conditions such as satellites, space-based equipment, and medical devices. Low outgassing board materials are the right choice for these kinds of applications.
Mechanisms contributing to outgassing
Outgassing is caused by four different mechanisms.
Vapourisation — Release of molecules from the material’s surface.
Desorption — Release of molecules trapped at an internal fixture.
Diffusion — Flow of molecules from a material’s interior structure to its outer surface.
Permeation — Flow of molecules from the external environment to the vacuum surface through the material.
When does outgassing occur in a PCB?
Outgassing occurs during the soldering process and can also occur when the board is exposed to high temperature and vacuum conditions. It is also caused by poor material choices and manufacturing errors.
- HDI PCB material selector by Sierra Circuits
- Outgassing during the soldering process
- When a circuit board is soldered, any moisture present near the hole is heated and converted to vapor. This gas causes gaps in the solder mask when it escapes. Outgassing can occur due to the following:
Presence of moisture in the circuit board: Moisture removal from the PCB is vital. To achieve this, boards should be baked. Baking is the process of eliminating volatile molecules from circuit board materials by heating them to a high temperature (100 to 125°C). IPC-1601 describes the standard for circuit board baking to eliminate outgassing. This procedure should be carried out in a clean oven to protect the board from any contamination that may occur during the process. It’s also a good option to place the PCBs in such a way that air can readily move around them throughout the baking process. To know how contamination leads to board failure, read why we perform PCB ionic contamination analysis?
Thin through-hole plating (less than 20µm): Air can come through the plated hole wall if the coating is thin or if there are voids in the copper plating. To avoid this, hole plating thickness should be at least 25µm.
Exposure to an ultra-high vacuum environment
Circuit boards that are implemented in space and military applications operate under ultra-high vacuum (UHV) environments. Dielectric materials will lose their mass when exposed to the UHV environment. The materials start evaporating and the adsorbed material on the circuit board surface will start desorbing (releasing gas). This results in outgassing. The expelled vapor can condense on optical devices such as cameras and can affect their image capturing quality.
Use of inappropriate material
Board materials and component packages should be selected based on their mechanical and thermal abilities as they impact the lifecycle of the PCB. To avoid offgassing, designers need to ensure that the materials have low outgassing rates and can withstand fluctuating pressure and temperature. PCB materials such as Rogers’ RT/duroid series and Nelco’s meteorwave series have low offgassing rates. These materials are thermally stable and impose outstanding resistance to this defect.
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