AOI Inspection Calibration: 6 Checks for Cameras and Lighting
An AOI inspection machine decides pass or fail from an image, so everything that shapes that image has to be under control. Camera focus, pixel scale, light intensity and colour settings all change with age, and each one moves the decision point without any alarm. Calibration is what keeps the same board judged the same way two years apart.
Why AOI Inspection Results Depend on Calibration
AOI inspection compares what the camera sees against a model built during programming. If the camera drifts, the image no longer matches the model, and the machine starts calling good joints defective or, worse, accepts joints that have changed. Neither outcome shows up as a machine fault.
The four variables that matter are magnification, lighting geometry, light intensity and colour response. Magnification and scale set the size of the features the algorithm measures; lighting and colour set the contrast that separates a good fillet from a void or a lifted lead. Calibrating all four keeps the measurement meaningful.
Camera Focus, Distortion and Pixel Scale
Focus is checked with a resolution target that carries known line widths, and the result is expressed as the smallest line pair the camera can still separate. A camera that has lost focus cannot resolve the fillet edge, and the algorithm compensates by widening its tolerance, which reduces detection. On a machine with several cameras, check each one, because a scale error on a single camera affects only the products that use that view.
Pixel scale converts image pixels into millimetres, and it must be verified for each camera and each field of view. Mount a target with a known pitch, measure it in the image and compare the two figures. A scale error of a few percent is enough to move a solder fillet measurement outside its acceptance band.
Light Source Aging and Intensity Measurement
LED light sources lose output over their life, and the loss is not the same across the different angles used for a single image. A ring light that has faded on one side produces a shadow that the algorithm reads as a lifted lead, so intensity has to be measured rather than assumed from the setting.
Measure each light channel with a meter at a fixed distance, or use the machine’s own image statistics on a uniform target, and record the value with the date. Replace a light source when its output has fallen outside the range the program was built with, and re check the whole set after any replacement. Record the meter distance with the reading, since intensity falls with distance and a figure without its geometry cannot be compared with the next check.
White Balance, Colour and Contrast Settings
Colour settings affect how the machine separates flux residue, bare copper, solder mask and solder, because those materials differ mainly in hue and saturation. A white balance that has shifted makes flux look like solder and produces false calls that are hard to explain.
Verify colour with a reference chart and confirm that the values sit inside the range recorded at programming. Where an operator has adjusted exposure or gain to clear a specific defect, note the change, because that adjustment affects every measurement made afterwards, not only the one that prompted it. Where an operator has changed exposure to clear one defect, log it, because that setting applies to every board inspected after the change.
Golden Boards and Reference Samples
A golden board is a known good assembly used to confirm that the AOI inspection machine still passes a part it should pass. It has to be stored so that it does not change: no oxidation on the pads, no dust, no handling damage, and a defined orientation and support in the machine.
Keep a second panel with deliberate defects as well, covering the defect types the program claims to detect. Running both boards at each calibration gives a pass result and a detection result, and the pair is far more informative than either board on its own. The accept and reject criteria for the joints themselves are published in IPC-A-610, and the program should be traceable to the class the product is built to.
Program Tuning Versus Machine Calibration
When false calls rise, the first question is whether the machine has moved or the process has. Adjusting the inspection program thresholds to clear the calls hides a calibration fault and quietly reduces detection for every board that follows.
Calibrate first, then tune. If the machine passes its reference boards at the recorded settings and the calls persist, the cause is in the process, and the next step is to compare the affected joints against the accept and reject criteria in solder joint acceptance criteria rather than adjusting the machine.
False Call Rate as a Calibration Indicator
The false call rate is the most useful trend an AOI inspection area has, provided it is recorded per machine and per product rather than in a single monthly figure. A slow rise on one product points to a lighting or scale problem, while a step change points to a program or a settings change. That is why the rate should be recorded per product and per machine rather than as one shop figure.
Read the rate together with the escape rate. A machine that has been tuned until it never calls a false defect usually has thresholds so wide that it also stops detecting real ones, and the escapes appear later at functional test or in the field, where they cost far more.
Intervals, Records and Change Control
Set the calibration interval from the machine’s stability and from the criticality of the product, and shorten it after any lamp replacement, camera service or software update. An update that changes the image processing is a calibration change even though no hardware was touched.
Keep the records in one file per machine: the resolution check, the scale value, the light intensity figures, the colour values, the reference board results and the settings at the time of the check. A camera or light change also affects X-ray judgement on the same product, so read the records with the X-ray and AOI comparison data for the assemblies that go through both.
Camera Calibration Records and Sign Off
Every camera calibration value belongs in one file per machine: the resolution figure, the pixel scale for each field of view, the light intensity of each channel, the colour values and the settings in force at the time. Store it with the AOI inspection program version, because a program change and a calibration change are two different events.
An AOI inspection result is only defensible when those values are traceable to a date. Where a customer asks how a joint was judged, the record that answers the question is the calibration file plus the reference board result from the same week, not the image on its own.
FAQ
How often should an AOI machine be calibrated? Quarterly is a common interval for a stable line, with a check after any lamp replacement, camera service or software update. Where the false call rate has risen, check the calibration before adjusting the program.
What is a golden board used for in AOI? It confirms that the machine still passes an assembly it should pass. Stored so that it does not oxidise or collect dust, and run with a defect panel as well, it verifies both the pass decision and the detection capability of the program.
Why does light intensity matter more than camera resolution? Because a joint is judged by the contrast between the fillet, the pad and the background. A light source that has faded on one side changes that contrast and moves the decision point, even when the camera still resolves the finest feature on the board.
