Conductive substrate inspection

*This is an anonymized case study. No customer or product is identified.*

The problem

Conductive substrates made of ceramic and bonded metal layers carry defects that are subtle at normal exposure: the defect and the surrounding material reflect light almost identically.

A reflective metal substrate with a low-contrast feature — A bonded metal substrate. The defect and the background reflect light almost identically until the lighting is tuned to separate them.

Constraints

Defects micron-scale and low-contrast.
High throughput required for production.
Surface finish varied between batches.

Approaches tried

Single coaxial image - revealed reflectivity differences but missed texture-type defects.
Overexposure - deliberately saturating the image separated the defect brightness from the background.
Multiple lighting angles - brought out topography that flat lighting hid.

Solution

A multi-image strategy: a normal exposure as baseline, an overexposed frame to separate near-identical materials, and angled lighting for topography. The algorithm then fused these into a single reliable decision.

Lessons learned

The contrast was created by the lighting design, not the algorithm.
Batch-to-batch surface variation, not the defect itself, was the hardest part to keep stable.