Many composite materials that are used in industrial processes, including epoxy-resin laminates used in PCB production, will give off gaseous byproducts during processing. PCB outgassing can also occur after production, when the board is fully assembled and deployed in its intended operating environment. Offgassing is unavoidable and will always happen at different stages of the production process, as well as during the lifetime of the board in its operating environment.
The question that should come up when thinking about outgassing is this: is outgassing from a PCB assembly a “bad” thing? The best answer to this is “it depends,” particularly with regards to application and the intended environment. This article will look at the causes and effects of PCB outgassing, and some application areas where it is undesirable.
Why PCB Outgassing Occurs
The main outgassing mechanisms are: desorption, vaporization, and diffusion/permeation. Outgassing can occur from a bare PCB after fabrication, as well as from a completed assembly that includes components, cabling/wiring, and even the material that was used to build the enclosure. Outgassing from electronics assemblies is most commonly associated with high vacuum systems, where a system brought under vacuum begins emitting compounds from exposed surfaces.
Outgassing occurs in situations where the PCB encounters its first exposure to elevated temperature after the PCB has been laminated and cured during fabrication. This is typically during assembly, so the PCB can begin outgassing during the soldering process. For most assemblies, this is not a problem and will not interfere with soldering. It could be problematic in high density PCBs where component pads are small and dense.
Environmental testing chamber used for outgassing tests.
A PCB could also begin outgassing when placed in its intended operating environment. For example, the outgassing rate will be larger when the environmental pressure around the PCB is lower, allowing greater desorption flux from the board surface. Even at atmospheric pressure, there will be some outgassing, but the rate may be very slow such that the resulting outgassing is not problematic. As a designer, it is your job to determine when to take some steps during manufacturing to prevent outgassing.
Is PCB Outgassing Bad?
PCB outgassing is sometimes a bad thing. Outgassing will always occur to some extent except in certain operating conditions with certain material sets. But in general, there will be some outgassing that will occur in the majority of electronics assemblies.
There are a few instances where measures should be taken to reduce or eliminate outgassing:
- When an assembly will operate in high vacuum environments
- When outgassed materials can be a contaminant
- When the PCB will be thermally cycled above 100-125 °C
- When the PCB will run continuously about the boiling point of water
When outgassing occurs, the material that exits from the PCB can deposit on other surfaces. It can then act as a contaminant in some other portion of the system. Two instances where this would be very undesirable include deposition of contaminants on optical elements and on chemical sensors. In the case of high vacuum environments, such as in space, rough vacuum will be reached quickly when a device is initially put into orbit, which will begin the progression of outgassing.
PCB Outgassing Standards
There are some standards that dictate outgassing requirements for PCB laminate materials.
This standard describes a baking process to remove contaminants that would appear during outgassing. The process involves baking the bare PCB at 100 to 125°C.
This testing standard is used to directly measure the amount of volatiles that outgas from a PCB laminate material.
This NASA standard provides outgassing limits for materials used in aerospace PCBs. A testing procedure is also described in the standards document.
In addition to these three standards, there are other standards limiting outgassing from plastic materials and wiring materials used in electronics assemblies. The PCB solder mask can also outgas during assembly.
Given these standards, some simple steps can be taken to prevent or greatly reduce outgassing from bare PCBs. These include:
- Pre-bake: prior to soldering, bake the PCBs in the 100-125 °C range within a clean oven to force moisture desorption.
- Hole wall thickness: Insufficient hole wall plating thickness can allow diffusion through small pores, so make sure plated holes have at least 1 mil wall thickness.
- Operating temperature: Keep the board’s temperature below the water desorption range (100 °C) to ensure residual water remains trapped in the board.
- Low outgas materials: There are some PCB materials that have been evaluated against outgas standards and can be shown to produce low outgassing.
Just like many other aspects of high-reliability design, the most important place to start designing a PCB is in material selection and planning for manufacturing. Starting here will ensure the final product has a long lifetime and complies with relevant outgassing standards.
When you need to specify your material requirements and outgassing prevention measures for your PCB, you can include your fabrication in your PCB layout with the CAD tools in OrCAD from Cadence. OrCAD is the industry’s best PCB design and analysis software with utilities covering schematic capture, PCB layout and routing, and manufacturing. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.