Understanding Helium Leak Testing for Critical Components and Assemblies

A satellite in orbit. An EV fuel cell. A nuclear submarine. These three types of critical assemblies span the heavens, the earth, and the seas, yet they all have one thing in common: the need to reliably join components together while preventing gases or fluids from leaking in or out. 

These and other mission-critical applications often rely on brazed or diffusion-bonded joints to achieve hermetic sealing, but how can manufacturers be certain that an assembly can keep liquids or gasses in or out? An assembly may pass a visual inspection, but it can still hide microscopic leak paths that can compromise performance or jeopardize operational integrity. 

That is where helium leak testing can provide a highly reliable method for verifying the hermetic integrity of a joint. A joint can look good, and still leak. Defects are often internal, obscured, and difficult to access. Helium leak detection offers an extremely precise way to detect even the smallest leaks in an assembly. 

At VPE, our team regularly produces vacuum brazed and diffusion bonded joints for a wide range of mission-critical assemblies and performs helium leak testing to the specific tolerances our clients require. This allows us to manufacture and verify a joint’s performance before it is put into rigorous duty. 

What is Helium Leak Testing?

In many critical applications with vacuum brazed or diffusion bonded joints, a hermetic, leak-tight seal isn’t just optional, it’s essential. When manufacturers need to be absolutely sure that an assembly is hermetic to their specification, they use helium leak testing as a non-destructive method as a key quality gate. 

Conceptually, a helium leak testing procedure is similar to running water through a garden hose to find a leak. Only instead of water, helium is used to trace microscopic leak paths. 

Why helium? Helium’s small atomic size allows it to penetrate extremely small leak paths, while its inert properties mean that it does not readily interact with the materials being tested.

When a manufacturer uses a helium leak test, the goal isn’t just to determine if a leak is present. The helium leak detector uses a mass spectrometer to detect how much helium actually passes through a leak path. That provides more useful information than whether a leak is present. 

In practice, there is virtually no limit to the geometry of parts we can test, making our helium leak testing services highly versatile. We can build special fixturing that allows us to adapt to universal fittings capable of connecting virtually any type of part to the helium testing unit and mass spectrometer. 

While there is no such thing as an “absolute hermetic seal”, helium leak detection provides quantitative confirmation that a component or joint meets the client’s specified leak-rate requirements.  

How Does a Helium Leak Test Work?

While the garden hose example works conceptually, the actual helium leak testing procedure is far more complex. 

At VPE, after an assembly has been processed using vacuum brazing or diffusion bonding, we follow a helium leak testing procedure designed to ensure the component meets each client’s acceptance criteria.

Preparing the Component

Assemblies that have been vacuum brazed leave the furnace in an extremely clean state, ready for high-vacuum (HV) or ultra-high-vacuum (UHV) applications.  

Because VPE has helium leak detection systems onsite, assemblies can move directly into the testing process without the additional lead time required of sending them to an outside testing provider. 

Creating a Vacuum

When the parts are ready for testing, we connect one end of the assembly’s process flow path to the helium leak detector. The opposite end is sealed, creating a high-vacuum condition inside the assembly. 

Introducing Helium and Detecting Leaks

Once the vacuum is stabilized, a stream of helium gas is sprayed along all brazed or bonded joints. If a leak path exists in the component, the helium will penetrate the joint and enter the assembly. A mass spectrometer inside the system detects any helium that penetrates the joints. The helium leak detection system measures the leak rate, which allows VPE to confirm whether the joint meets the client’s specific acceptance criteria. 

Dry vs Wet Helium Leak Testing

VPE can perform both wet and dry helium leak testing. 

Most helium leak detection tests we run at VPE are dry, meaning they are performed without oil or other lubricants in the detector’s pump system. This is preferable when contamination must be minimized, such as in high-vacuum, ultra-high-vacuum, and semiconductor applications. 

Some clients do not have strict contamination requirements and choose wet helium leak testing. This type of testing uses a vacuum system with oil-lubricated components and is generally more cost effective than dry testing. When the client does not have a preference, wet testing may be appropriate. 

Leak Rate Requirements

A leak rate measures how much helium passes through a joint or assembly. Depending on their risk tolerance, manufacturers will specify an acceptable leak rate based on the application’s requirements.

The industry standard for expressing a leak rate is atm-cm³/sec (air). VPE’s helium leak testing services can detect leak rates down to approximately 1 × 10⁻¹⁰. 

Most leak rates vary by the application and are determined by the client and the gases or fluids to be used in the assembly. 

For liquids, a leak rate of 10⁻⁵ to 10⁻⁷ atm-cm³/sec is generally considered acceptable. In applications with vacuum, cryogenic, or toxic working fluids, a leak rate as low as 1 × 10⁻¹⁰ atm-cm³/sec may be required to ensure operational integrity. 

Why Do Components Fail Helium Leak Testing?

Both vacuum brazing and diffusion bonding are capable of producing extremely durable, leak-tight bonds, but just like any precision manufacturing process, there are times when these joints can fail helium leak checking.

Incomplete Brazing or Bonding

Most failed helium leak tests happen due to an improperly formed joint. This can happen when the components were not properly cleaned prior to the joining process, leaving oil, oxides, or other residues behind. Those contaminants can prevent proper wetting of the braze alloy. 

Improper Joint Design

Sometimes, the design of the joint itself can create problems. This happens often in older joint designs that are locked in and cannot be changed. In other cases, the customer may not have produced a joint of this type before, and their joint design prevents full wetting or diffusion between the joint and parent materials.

Cracking in Successful Joints

Sometimes a joint can be properly brazed or bonded, yet still develop leak paths due to significant differences in the coefficient of thermal expansion (CTE) between the materials being brazed. Different materials expand at different rates, which can cause cracking within the braze joint. In other cases, the braze alloy and parent materials may have incompatible chemistry that creates brittle intermetallic zones that can create cracks in the joint.   

A major advantage of helium leak testing is that the process doesn’t simply tell us if something has failed or not. The results can provide valuable feedback for troubleshooting and improving joint design. 

The Advantages of Integrated Helium Leak Testing

For manufacturers that require vacuum brazed or diffusion bonded components for use in critical assemblies, access to helium leak testing services is essential. However, not every contract manufacturer who provides these advanced joining techniques has access to helium leak detection methods in-house.

As a leading provider of advanced joining techniques, VPE can both produce and test critical assemblies under one roof. Parts can move directly from the furnace to leak testing without the additional lead time of sending components to a separate leak testing provider. 

Whether an aerospace or medical client requires 100% inspection of every finished component or an industrial client only needs AQL sampling, VPE can pair traditional visual quality inspection with advanced helium leak detection systems to meet your testing requirements with minimal additional lead time. 

In addition, we also offer extensive engineering support behind our leak testing capabilities. We can recommend the best method to leak test your components, using our helium leak testing machines calibrated on a strict preventative maintenance schedule to produce consistent results. Our integrated quality-control processes and documentation also support our customers’ QMS and ISO compliance requirements. 

But the benefit of helium leak testing isn’t just in convenience, it can produce real material results. 

Consider a recent client case study. VPE was able to help a client producing radiation detectors for the nuclear industry reduce rejection rates using helium leak testing. 

The customer was using a legacy joint design that was not ideal for modern vacuum brazing practices but was unable to change it. The joint design was not ideal and produced pinholes and internal voids that were difficult to inspect visually, leading to a high rejection rate. Without the possibility of rework, the client was losing a significant amount of product to waste.

VPE worked with the client to integrate visual inspection with helium leak testing. If the assembly passed visual inspection and helium leak testing, it could be accepted. Together, VPE and the client were able to reduce the rejection rate from approximately 70% to 10%, without changing the manufacturing process.

When you choose VPE, our helium leak testing services aren’t simply an afterthought. They can be directly integrated into the development, manufacturing, and ongoing quality control of critical brazed and diffusion bonded assemblies. 

Need to verify the integrity of a brazed or diffusion-bonded assembly? Contact VPE today to learn more about our helium leak testing procedures and services.