Why Helium Leak Tests Fail: Five Common Causes in High-Purity Gas Piping
A leak test fails. The system visually looks fine. The project is now behind schedule, and someone has to explain why.
In high-purity gas piping for semiconductor manufacturing, the acceptance standard is SEMI F1-96. Most leak test failures trace back to a small set of interferences that the standard describes directly. Knowing them helps you avoid a false reading, interpret a real one, and decide what to do next.
A brief refresher on what a helium leak test measures
A helium leak test uses helium as a tracer gas to detect leaks in a sealed system. SEMI F1-96 defines three leak directions. Inboard leakage moves from outside to inside when external pressure is greater than internal pressure. Outboard leakage moves from inside to outside when internal pressure is greater. Internal leakage occurs across a closed flow barrier such as a valve seat.
Cause 1: Helium permeation through polymeric seals
Helium has high permeability through polymeric materials. When you spray or pressurise with helium, the seals absorb it. Two effects follow.
The detector shows a delayed reading and a continually rising apparent leak rate, even when the mechanical seal itself is sound. Later sensitive tests can also be inhibited by desorption of helium from the seals.
SEMI F1-96 addresses this in two ways. First, it specifies a test sequence: inboard, then internal, then outboard. Second, when polymeric seals are present in the piping, the internal and outboard leak rate limits in the standard are multiplied by 100. If you ignore the sequence, you create the interference yourself.
Cause 2: Dilution from inadequate purging
For low pressure tests at 1 MPa (147 psig) or less, the test space must be purged or evacuated before tracer gas is introduced. If air is left in the space, it dilutes the helium and the detector reads a lower leak rate than is actually present.
This applies to enclosures (when you bag a component) and to the interior of the test object. Dilution can hide real leaks completely.
Cause 3: Restricted conductance to the detector
The leak detector needs adequate conductance to the test object. Anything between the two that restricts flow, such as a regulator or check valve, reduces sensitivity.
SEMI F1-96 recommends testing the test object before installing downstream components when conductance is a concern. Long tubing runs or small-diameter tubing at the inlet may also need extra purging so the tracer gas physically reaches the test object before measurement starts.
Cause 4: Air flow disturbing the detector probe
Air movement at the probe tip sweeps leaking gas away before the detector can capture it. This is why area airflow needs to be reduced as much as possible during testing.
When testing has to happen in an area with significant airflow, SEMI F1-96 references ASTM E 499 Method B: place a protective film around the probe tip when examining each joint. Without that, you can probe directly over a real leak and still miss it.
Cause 5: Testing outside the correct temperature range
SEMI F1-96 specifies a leak test temperature between 18°C (64°F) and 26°C (78°F). Outside that band, leak performance can shift. The standard notes the effect exists but does not quantify it. If your ambient was outside the window, the result is not directly comparable to the leak rate limits in the standard.
What to do if your test fails
Three checks before you escalate.
Confirm the tracer gas pressure did not exceed the manufacturer’s rating during the test. Check that joints, including welds, were uninsulated and exposed for examination.
If repairs or additions were made after the original leak test, the affected portions need to be retested under SEMI F1-96. A clean retest record matters at handover.
If the test still fails after these checks, engaging a qualified helium leak testing provider with SEMI F1-96 experience is the next step.
Need helium leak testing for your gas piping system?
Mechlink is one of Singapore’s recognised providers of helium leak testing for high-purity gas piping systems. We support Singapore and Taiwan, with <3 hour response subject to availability. Contact us to discuss your project.
