Anonymized case study. No customer or product is identified.
The assumption
Many people assume wirebond inspection just means: measure the wire height. Reality is messier.
What actually goes wrong
Wirebonds introduce:
- Reflection issues from curved metal surfaces.
- Material variation between parts.
- Shape variation in the bond loops.
- Focus challenges across the height of the loop.
A 25 um gold wire is effectively a curved mirror. Under any directional light, each wire shows one thin bright glint line whose position depends on the light direction - not on the wire geometry you actually want. Two identical wires under slightly different local lighting look like two different wires. That single physical fact drives most of the difficulty.
The measurement set that actually mattered
"Wire height" unpacked into a list once we asked what failure each measurement prevents:
- Loop height - too high risks touching the lid; too low strains the wire.
- Wire presence and count - the trivial check that catches the most real failures.
- Bond placement - offset bonds reduce adhesion area.
- Wire sag and spacing - adjacent wires that approach each other risk shorts under vibration.
- Bond shape - deformed feet correlate with weak bonds.
Each of these wants a different imaging strategy. Presence and placement are easy 2D tasks. Height and sag genuinely need 3D. Bond-foot shape sits in between and is mostly a lighting problem.
Approach
A combination of 2D appearance checks and 3D height measurement worked better than either alone: 2D to locate and screen, 3D to measure loop height and detect shape defects.
For the 3D part, multi-directional illumination mattered more than the height algorithm: capturing with light from several azimuths and keeping the per-pixel maximum turned the unpredictable glint into a stable, fully outlined wire before any measurement ran.
Lessons learned
The reflective, curved geometry meant that lighting and focus dominated the result far more than the height-measurement algorithm itself.
The general lesson travels well: when someone states an inspection requirement as a single number ("wire height"), the real requirement is usually five different checks, and unpacking them early is cheaper than discovering them one field failure at a time.