What is the difference between low outgassing and standard conformal coatings

Conformal coatings protect circuit boards from moisture, dust, vibration, and contamination, but not every coating behaves the same way in every environment. When electronics are headed into high-vacuum settings, such as satellites, imaging payloads, or sealed instrument housings, the difference between a low outgassing and a standard conformal coating becomes far more than a specification detail. This article explains what the two material groups are, how outgassing is measured, and how to pick the right option for your board.

What a standard conformal coating does

A standard conformal coating is a thin protective polymer film applied over an assembled PCBA. It typically adds 25 to 75 microns of protection and is designed to shield solder joints, traces, and components from moisture, humidity, dust, mild chemical attack, and mechanical stress. For most commercial and industrial boards, this level of protection is the everyday workhorse: it keeps a controller running in a damp workshop or an automotive ECU surviving thermal cycling.

The most common standard coating chemistries are:

• Acrylic — solvent-based, air-curing, easy to apply and rework. A solid choice for inner boards that are well shielded, but it can yellow and turn brittle over time.
• Silicone — flexible and stable over a wide temperature range, good for vibration and thermal-shock environments, though its moisture-vapour permeation is relatively high.
• Polyurethane — durable and abrasion-resistant, often used where tougher film properties are required.
• Epoxy — hard and very protective, but difficult to rework after curing.
• Parylene — applied by chemical vapour deposition rather than spraying, giving pinhole-free, truly conformal coverage.

These are the coatings most people picture when they think of circuit board conformal coating. They apply well to high-volume lines, are well understood, and are inexpensive. The catch is that many standard formulations contain solvents or resin fractions that release a measurable amount of volatile material, especially under vacuum and at temperature.

What outgassing actually means

Outgassing is the release of trapped or dissolved gases and volatile compounds from a material when it is exposed to reduced pressure, elevated temperature, or both. Every material does this to some degree. Under normal atmospheric conditions the effect is negligible, but in a vacuum the small molecules that would ordinarily stay trapped are free to migrate out of the coating.

This matters because the released material does not simply disappear. In a sealed or space environment, the outgassed molecules can re-condense onto nearby surfaces and form a thin film. On an optical lens, a camera sensor, or the contact pads of a relay, even a sub-micron layer of contamination can degrade performance. On some missions the contamination grows slowly over years of operation, turning into a reliability risk that is impossible to fix once the hardware is sealed or launched.

A low outgassing conformal coating is formulated and processed specifically to minimise this release, so that vapour condenses on critical neighbours as little as possible.

How outgassing is measured: ASTM E595

Whether a coating counts as low outgassing is not a marketing claim; it is measured against a recognised standard. The most common screening method is ASTM E595, the standard test method for total mass loss and collected volatile condensable materials from outgassing in a vacuum environment.

The procedure is simple in concept. A sample is weighed, placed in a vacuum chamber, and typically held at 125°C for 24 hours under vacuum in the low 10⁻⁵ to 10⁻⁶ Torr range. The results are expressed as two figures:

• TML — Total Mass Loss: how much of the sample's mass was lost to the vacuum.
• CVCM — Collected Volatile Condensable Materials: how much of the released material condensed onto a nearby collector plate, which is the fraction most likely to contaminate neighbouring optics or contacts.

Historically, the values used to screen space materials were a TML of 1.00% or less and a CVCM of 0.10% or less. These figures come from long-standing NASA screening practice rather than from the ASTM standard itself, which leaves the acceptance level to the user. In practice many programmes set even tighter internal limits, such as a TML below 0.50%, for high-value optics or sensors. A coating that comfortably beats these numbers, for example a qualified Parylene or polyurethane formulation with a TML around 0.4% and a CVCM below 0.03%, is generally regarded as low outgassing.

Verification can go further. Materials that appear in NASA's MAPTIS database have published outgassing results, and programmes concerned about corrosion can also check ionic cleanliness according to MIL-STD-883 method 5011, since ionic contamination encourages dendrite growth under bias. Outgassing is only one part of the reliability picture, but it is the part that vacuum environments force you to take seriously.

The key differences at a glance

Both material groups protect the same board, but they differ in chemistry, performance, and cost. The table below summarises the practical differences.

Notice that the line between the two is not always a fixed chemistry. Some standard chemistries can be made in low outgassing grades, and a coating labelled low outgassing still needs certificate-backed E595 data to be trusted. The material name alone is not enough; the actual TML and CVCM numbers are what an engineer should cite.

Why low outgassing matters for sensitive systems

The practical consequences of ignoring outgassing are easy to underestimate, and they tend to show up only after the hardware is sealed or in flight. Optically, a thin condensate film can fog a lens or reduce the sensitivity of an imaging sensor. Electronically, volatiles that settle on relay contacts can form an insulating layer that grows over time and causes intermittent open circuits. In navigation and control, small mass changes from outgassing can even disturb precise calculations over long missions.

This is why satellite imaging cameras, telescope payloads, medical implants, and sealed instrument housings routinely specify low outgassing materials — and why the same rule applies to the conformal coating on their circuit boards. For these systems, the correct coating is rarely a matter of preference; it is a documented selection backed by E595 data.

How to choose the right coating

Choose the coating group according to the environment the final product will live in:

• Standard, well-shielded boards — commercial, industrial, and most automotive applications benefit from the low cost and easy rework of acrylic, silicone, or polyurethane standard coatings.
• Vacuum or sealed systems — optics, space electronics, and compression-sealed instruments should use a low outgassing coating and require the supplier to provide TML/CVCM test certificates.
• Extreme thermal or mechanical duty — silicone handles wide temperature swings, while harder films like polyurethane or epoxy resist abrasion; check compatibility with rework and with the board's other materials before finalising.

Whatever you ｓｅｌｅｃｔ, the value of the coating depends as much on process control as on the material. A coating that is applied unevenly, masked incorrectly, or cured with the wrong profile will not protect the board as intended. That is why it is worth working with a manufacturing partner that runs an automated line, controls masking, and validates results, rather than treating coating as a last-minute add-on.

Partners who understand the difference

Farway Electronic is a Shenzhen-based electronics manufacturing services provider that handles the full chain from PCB fabrication and component sourcing through SMT assembly, DIP welding, testing, and finished-product assembly. For boards that need surface protection, its automated conformal coating line applies the film using selective masking, double-sided spraying, and controlled baking, supporting boards up to 550 mm × 470 mm with average spray times of 0.5 to 3 minutes per board.

Because Farway also designs and performs PCBA testing under IPC-oriented controls, the engineering team can discuss whether your application truly needs a low outgassing grade or whether a standard coating is sufficient — and, if it does need low outgassing performance, help you verify the material data before committing to production. For electronics bound for harsh, high-reliability, or vacuum-adjacent applications, that up-front conversation is where the real protection begins.