01 Why Secondary Degassing Matters
Secondary degassing serves two main purposes.
1. Further removal of non-condensable gases (NCGs)
After a vapor chamber has been charged with working fluid and evacuated (i.e. after primary degassing), a small amount of non-condensable gas usually remains inside — including hydrogen, nitrogen, carbon dioxide, and oxygen.
Research shows that these non-condensable gases reduce a heat pipe's thermal performance and service life (for example, by promoting oxidation). When NCGs are present inside a vapor chamber, vapor flow is obstructed and the return of the liquid working fluid slows down, which increases the temperature difference across the chamber and degrades heat-transfer performance.
Because secondary degassing directly affects the internal vacuum level and residual NCG content, it deserves close attention on the production line.
2. Finishing the fill-tube opening (the "rat tail")
The fill tube is often called a "rat tail" in the industry because of its shape. The fill-tube opening generally takes one of four structural forms: recessed, protruding, both protruding and recessed, and flush (neither protruding nor recessed).
For products that are unusually complex or large, multiple degassing ports are sometimes used, though this is uncommon in mass production. Some patent designs also separate the fill port from the degassing port, in which case the part will have multiple rat tails.
Because primary-degassing equipment is relatively complex and fill-tube designs vary widely, the seal made during primary degassing can be thought of as an initial (rough) seal. Secondary degassing can then use simpler tooling to finish the fill-tube opening into its final form.
02 Methods of Secondary Degassing
The goal of secondary degassing is to further remove non-condensable gases from the cavity. There are three common methods; in practice the first two are used most often.
1. Vacuum degassing
The principle is straightforward — evacuate again, sometimes multiple times. From a process and equipment standpoint, there are two approaches:
- Approach A — long degassing tube, repeated cycles. Each cycle follows the sequence "evacuate (pre-vacuum) → break (open the tube seal) → evacuate (connect to the cavity and continue pumping while heating the base) → seal (pinch off the fill tube)." After each cycle, the part must be removed, a section of the degassing tube trimmed off, and the part re-clamped and restarted.
- Approach B — valve replaces tube breaking and sealing. The sequence is still "evacuate → break → evacuate → seal," but the tube-piercing and pinch-off actions are replaced by a vacuum valve. Its advantage is that secondary degassing can be performed continuously over several cycles without repeated clamping and unclamping.
2. Evaporation-purge degassing
In fact, the third step of Approach A above already incorporates evaporation-purge degassing.
The principle: the product is heated so its temperature rises and the internal moisture evaporates. As the vapor flows upward, it carries the non-condensable gases trapped in the cavity and wick structure vertically up into the degassing tube. The tube is then sealed at its base, and the upper section — containing the NCGs and vapor — is cut off.
This is currently the most widely used method in production, because the equipment is simple, the cost is low, throughput is high, and it lends itself well to automation.
3. Evacuate first, then charge fluid
This method is uncommon because the equipment is extremely complex and the process is difficult to control, so it will not be covered in detail here.
03 Parameters That Affect Degassing Results
Many researchers have studied the secondary degassing process experimentally, and manufacturers routinely tune parameters and run trials for different products. The core objective is to remove as much NCG as possible while limiting working-fluid loss and maintaining throughput.
The main influencing parameters include:
- Heating temperature
- Holding (soak) time
- Heating area and heating location
- Evacuation time (for the vacuum degassing method)
- Length of the degassing tube
- Diameter of the degassing tube (one variable-diameter design deliberately builds a larger gas-reservoir volume in the middle of the tube)
There is no fixed formula for designing these structures and parameters — you can only understand the general trends. The specific values have to be dialed in through repeated adjustment during actual production.
04 Summary
Overall, secondary degassing is an important step. Whether for heat pipes or vapor chambers — and especially for long, slender products or parts with many corners and edges — primary degassing alone struggles to clear out the non-condensable gases, so secondary degassing is needed to bring the residual NCG content down.
It's worth noting that NCGs can never be removed 100%; roughly 5% may remain, for example. In that case, the shortfall can be compensated by modestly increasing the condenser-section area (say, by about 5%).
The overall process involves several sealing operations, so pinch-seal tooling design is equally critical. A pinch seal can close the opening quickly, but to pass reliability testing it should be backed up by more robust sealing methods such as TIG (argon arc) welding or resistance welding. That topic is left for a future article.
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Written by
CoolingThermal Engineering TeamCoolingThermal is an automation equipment manufacturer based in Kunshan, China, specializing in heat pipe and vapor chamber production equipment since 2017. Our engineering team designs, builds, and commissions complete production lines covering forming, degassing, welding, testing, and assembly processes. The technical content on this blog is written by the same team that develops the equipment — based on real production experience, not secondary research.
