No single inspection method proves every aspect of a hermetic laser weld. Visual examination can find surface features but not all internal defects; metallography reveals one destructive section but not an entire seam; helium leak testing measures gas flow through a leakage path but does not classify every weld imperfection. A defensible inspection plan combines methods according to the assembly's failure modes, drawing requirements and production risk.
Start with function and acceptance criteria
Before selecting equipment, define what the weld must accomplish and how failure could occur. Questions include:
- Is the primary function structural, hermetic, electrical, corrosion-resistant, biocompatible or a combination?
- Which internal components could be damaged by penetration, heat, spatter or contamination?
- What leak rate, test gas, pressure differential and test method are specified?
- Which surface and internal imperfections are limited by the drawing or contract?
- Is the test for process qualification, first-article approval, production screening or periodic audit?
- Can representative coupons reproduce the production joint and atmosphere?
An applicable standard can provide terminology and method requirements, but it does not replace product-specific engineering acceptance. For example, ISO 13919-2 addresses quality levels for certain laser- and electron-beam weld imperfections. ASTM F2391 covers helium tracer-gas leak detection for package and seal integrity. The governing drawing or contract must identify the required edition, method and limits.
What each inspection method can tell you
| Method | Best suited to | Important limitation |
|---|---|---|
| Visual and magnified examination | Surface cracks, undercut, spatter, seam continuity, discoloration | Cannot confirm internal fusion or hermeticity by itself |
| Dimensional or surface profiling | Bead width, mismatch, distortion, undercut depth | Access and surface geometry affect measurement |
| Liquid penetrant testing | Surface-breaking discontinuities on suitable nonporous materials | Requires compatible cleaning and may be restricted for clean or medical products |
| Radiography or computed tomography | Volumetric pores and some internal geometry | Resolution, material thickness and orientation can hide fine planar defects |
| Ultrasonic methods | Internal reflectors in suitable geometries | Thin, small or complex precision parts can be difficult to inspect reliably |
| Metallographic cross-section | Fusion boundary, penetration, porosity and microstructure at the cut plane | Destructive and local; one section does not represent a complete seam |
| Mechanical testing | Strength, ductility or failure mode of coupons | Usually destructive and may not reproduce the product's exact stress state |
| Helium leak testing | Gas flow through a connected leakage path | Does not identify the metallurgical cause or all non-through defects |
The choice is not a contest between methods. They answer different questions and should be layered where consequences justify it.
Visual and dimensional examination
Visual examination is normally the first screen because it is fast and preserves the part. Define lighting, magnification, viewing angle, surface condition and measurement tools. Inspect the entire seam, with extra attention to starts, stops, overlaps, corners and thickness transitions.
Record seam continuity, width, position, undercut, concavity, excess metal, spatter, cracking and oxidation. Use the undercut guide when a groove is present and the discoloration guide when color or black residue is the concern. Appearance can trigger further investigation; it should not be used to infer internal penetration or a numerical leak rate.
Cross-sections and procedure qualification
Metallography is especially useful while developing and qualifying a precision laser welding process. Sections can measure penetration, fusion width, interface condition, heat-affected features, pores and cracks. Samples should represent normal conditions and relevant limits such as minimum and maximum gap, material thickness, starts and stops.
Use a documented preparation and etching method, calibrated measurement system and defined interpretation. See how to measure laser weld penetration for a practical workflow. Pair cross-sections with the production records and monitoring signals collected during the same welds. This creates evidence for later process limits.
Helium leak testing for hermetic assemblies
Helium is useful because sensitive instruments can detect very small quantities, but the test configuration matters. Depending on the product and governing method, approaches may include helium inside the assembly with external detection, a pressurized “bombing” exposure followed by measurement, or a tracer probe around an evacuated item.
A meaningful specification identifies at least the maximum leak rate, test pressure or pressure differential, exposure and dwell conditions where applicable, calibration practice, background control, fixture volume and acceptance calculation. Large internal volume, flexible walls, temperature and trapped helium can affect interpretation.
Leak testing complements weld inspection. A sealed part can pass a leak screen while containing a non-through crack or internal pore population; another part can leak through a very small connected path that is difficult to locate visually. The inspection plan should therefore connect hermetic function with the material and weld failure modes.
Qualification, production and audit layers
Process qualification
Use representative parts or coupons to establish the window. Typical evidence includes visual and dimensional results, multiple cross-sections, mechanical tests where structural performance matters, and leak tests for hermetic products. Challenge relevant limits rather than validating only a center setting.
Production screening
Production controls may include 100-percent visual inspection and leak testing, automated process monitoring, dimensional checks, or a sampling plan—depending on risk and contractual requirements. Maintain traceability to material lot, program revision, machine, fixture and atmosphere cycle.
Periodic verification
Scheduled witness coupons, destructive sections, calibration checks and trend review can detect drift that a pass/fail screen misses. Define what events trigger requalification: major optical service, fixture replacement, software or program change, material or supplier change, joint revision, shielding-system repair, or an unexplained defect trend.
Build the inspection plan step by step
- List each product requirement and credible failure mode.
- Map every failure mode to at least one capable detection or prevention control.
- Verify that the selected method has sufficient sensitivity and geometric access.
- Define written acceptance criteria, sample locations and frequency.
- Qualify the welding and inspection processes on representative material and joints.
- Train operators and preserve calibration, setup and result records.
- Review trends, not just individual pass/fail results.
- Reassess the plan after design, material, equipment or process changes.
JMLASER's hermetic sealing systems include controlled-atmosphere and vacuum configurations for precision welding tasks. System selection should begin with the required atmosphere, part handling, production flow and validation plan so that inspection is designed into the process rather than added after equipment delivery.