Pre-Commissioning Testing Checklist for High-Purity Gas Distribution Systems

A new high-purity gas distribution system is mechanically complete. Now it has to be tested before it can be signed over to operations. Pre-commissioning is where projects most often lose time, usually because something gets caught late and needs rework.

This is a five-stage checklist for high-purity gas distribution systems used in semiconductor manufacturing applications. Each stage is tied to the relevant SEMI standard. The project specification always governs, but the items below should be verified on any system before it goes live.

Stage 1: Visual weld inspection

This is where most rework gets identified, so it goes first. The standard for visual inspection of GTA welds in fluid distribution systems is SEMI F81-1103. The acceptance criteria are specific.

Welds must show complete penetration around the entire internal surface. No visible surface cracks, porosity, or inclusions under magnification. No ID concavity at any point. ID and OD convexity not to exceed 10% of nominal wall thickness T. Bead meander not to exceed 35% of T. Minimum ID bead width 1.0 times T, maximum 2.5 times T. The OD weld face should be free of oxidation, with light straw colour permitted. On HP and UHP systems, no visible discoloration on the tube ID or weld ID under bright fluorescent light without magnification.

Inspector qualifications referenced in the standard are ASNT SNT-TC-1A or AWS QC-1. Mechlink inspectors hold AWS CWI and ASNT Level II.

Stage 2: Welding procedure documentation review

Visual inspection only tells you about the weld in front of you. The documentation tells you whether every weld in the system was made the same way.

SEMI F78-0304 requires that all welds be based on a Welding Procedure Specification (WPS), documented with associated Procedure Qualification Records (PQR), and produced only by certified welders and welding operators. Certification includes producing three acceptable welds in a row at the smallest and largest diameters of each alloy. Certification expires after six months of inactivity.

The standard also requires a daily log of all system welds and coupons, with as-built drawings maintained in the welding area. Coupons should be logged with date and operator identification, and retained for at least one year. Confirm these records exist before moving to leak testing.

Stage 3: Helium leak testing

The largest stage in the checklist. SEMI F1-96 sets the acceptance leak rate limits in atm cm³/sec. Verify each of the following before signing off.

• Test sequence performed in the order inboard, internal, outboard, to avoid permeation interference from earlier tests.
• Tracer gas matches the test type: 100% helium for enclosure or spray tests; helium plus nitrogen at design pressure for pressurisation tests.
• Helium purity less than 100 ppb total contaminants, filtered to less than 0.1 µm at point of use.
• Leak rate limits documented per SEMI F1-96, with measured values recorded alongside the limits.
• Personnel qualified per the standard.
• Joints uninsulated and exposed for examination during the test

If any portion was repaired after testing, the affected portions need to be retested.

Stage 4: Trace gas analysis

Once the system passes leak testing, trace gas analysis verifies that the gas delivered downstream meets purity requirements. Three analyses are standard.

Moisture (dew point) analysis verifies water vapour content. Mechlink uses TigerOptics HALO+, HALO3, HALO, and Meeco analysers depending on the application.

Oxygen analysis verifies residual oxygen in inert gas systems. Mechlink uses Servomex analysers.

Total Hydrocarbon Analysis (THA) is a sample-based laboratory service. Clients collect samples and Mechlink performs the analysis off-site. Typical analytes include CO, CO₂, CH₄, H₂, and oil mist.

Each result should be documented with the instrument used, calibration status (NIST-traceable), and the sampling point on the system.

Stage 5: Particle Testing Services

Particle testing verifies the gas meets the applicable particle count specification at the test point. For compressed air, the relevant standard is ISO 8573. Each test point should have a recorded particle count by size class, sample volume, and reference to instrument calibration.

Common items that get missed

A short list of items most often absent from project documentation at handover:

• Calibration certificates not attached to test reports.
• Test method not explicitly named (only “leak test” or “purity test”).
• Units used inconsistently (std cm³/sec or atm cc/sec instead of atm cm³/sec).
• No retest record after a repair.
• Gaps in the daily weld log.
• Coupons not retained for the required one-year period.

Each of these traces back to a requirement in one of the SEMI standards above. If any of them is missing, ask for it before sign-off.

How Mechlink supports pre-commissioning

Mechlink is one of Singapore’s recognised providers of helium leak testing, particle testing services, trace gas analytical testing, total hydrocarbon analysis, pressure testing, boroscope inspection, and welding certification. Coverage spans Singapore, Taiwan and India. Response within 3 hours, subject to availability.

Need pre-commissioning support for a new gas system?

If your project is approaching commissioning, contact Mechlink to scope a pre-commissioning testing programme. We can support all five stages above.