Sub-Pixel Circle Fitting for Wafer Via-Hole Inspection

Background

Via-hole roundness is a critical quality metric in PCB and semiconductor wafer fabrication. A deviation in hole circularity directly correlates with electrical impedance variance and long-term reliability risk. Traditional pixel-level inspection tools report ±1–2 px error — insufficient for holes smaller than 200 µm at 4× magnification.

This project integrates VisionLab's FitCircle module into an existing MES-connected inspection station, replacing the legacy threshold-based AOI system.

Algorithm Pipeline

VisionLab's circle fitting follows a multi-stage robust pipeline:

Annular ROI Masking → Edge Detection (Devernay subpixel) → Sector Segmentation (24×15°) → RANSAC 3-point Fit → Least-Squares Refinement

Annular ROI

Only pixels within the ring [inner_radius, outer_radius] around the estimated hole center are processed. This eliminates background clutter and reduces computation by 60–80%.

// Approximately 8 lines to set up VisionLab plugin and call the algorithm
PluginHostLauncher* launcher =
    PluginHostLauncher_Create("visionlab.exe");
PluginHostLauncher_Launch(launcher, 5000);
PluginHostHandle* host = PluginHostLauncher_GetHost(launcher);

PluginHostImage img = { frame.data, frame.cols, frame.rows, 1 };
PluginHostResult result;
PluginHost_SendAndWait(host, &img, paramsJson, &result, 3000);

RANSAC + Least-Squares

Three randomly sampled sector centroids uniquely determine a circle. The RANSAC loop runs 1000 iterations, each time counting inliers within inlier_dist pixels of the candidate circle. After convergence, all inliers are fed into a Levenberg–Marquardt least-squares refinement:

Devernay Subpixel Edge

Unlike Canny (which snaps to integer pixel coordinates), Devernay's algorithm localises edges at sub-pixel positions by fitting a 1D quadratic to the gradient profile perpendicular to the edge direction:

This is the primary contributor to the < 0.1 px centre accuracy.

Results

Integration

The VisionLab Plugin SDK exposes a pure-C API, so the inspection station's existing C++ controller required no framework changes. The plugin process communicates via Windows shared memory (IPC), keeping latency under 2 ms per frame transfer.