AOI Programming Strategy for Mixed Technology Boards

An AOI programme is a measurement routine, not a photograph album. It decides what the machine looks for, how much variation it tolerates and what it reports, and those decisions determine whether the station finds real defects or trains operators to ignore alarms. Programming a mixed technology board well takes library discipline, package specific lighting and a plan for tuning thresholds as production settles.

What AOI Programming Has to Achieve

Thresholds should be derived from the acceptance criteria rather than from the machine defaults. When the standard requires a defined wetting angle on a fillet, the measurement window is set from that requirement, and the correspondence between the two should be written down so an auditor can follow it. Where the programme and the inspector disagree, the cause is usually a translation problem rather than a real difference of opinion.

The programme has to detect the defects that the process is actually capable of producing, including missing parts, reversed parts, wrong parts, insufficient or excess solder and displacement beyond a defined limit. It also has to pass good boards, because a station that rejects acceptable product is quickly bypassed.

Those two goals pull against each other, and the balance is set by the threshold on every measurement. A programme tuned only for detection produces false calls, while one tuned only for yield produces escapes, and the correct point between them depends on the cost of each failure.

Detection capability should be stated in the programme documentation, because a new product engineer needs to know which defects the station is expected to catch rather than assuming full coverage.

Library Based Programming Versus Learning

Library based programming builds each component from a stored definition: body shape, lead count, pad geometry and expected fillet. The library is maintained once and reused across products, which makes programmes consistent and fast to build.

Learning based programming derives the reference from a known good board, which is quicker for a new design and less rigorous. It captures whatever variation happened to exist on the sample, so a defect present on that board becomes part of the accepted standard, and the error is invisible until the customer finds it.

Lighting and Camera Settings per Package

Lighting determines whether the measurement is possible at all. A shiny lead needs diffuse light to reveal its shape, while a solder fillet is best judged with angled illumination that produces a characteristic contrast pattern. The same settings rarely work for both.

Package size and height also change the required settings, because a tall component throws a shadow that falls across its own pads. Programmes therefore group components into families with shared lighting and exposure, and odd form parts are usually given their own recipe.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/24-layer-pcb-1536×640-1.webp" alt="AOI screen showing solder joint inspection results” />

Camera resolution has to resolve the smallest feature that will be measured, so a fine pitch device and a large connector cannot always share the same field of view without a change of lens or magnification.

Thresholds for Solder Joint Inspection

Solder joint thresholds are expressed as ranges for the visible fillet, the wetting angle and the amount of exposed pad. They should mirror the acceptance criteria used by the inspectors, so that the machine and the people are applying the same standard.

Thresholds are normally tightened after a defect escapes and loosened after a spike in false calls, and both adjustments should be recorded with the reason. Without that record, the programme drifts over a few months until nobody can explain why a particular limit is where it is.

Reducing False Calls Without Losing Detection

False calls usually come from a handful of components rather than from the whole board. Analysing which references generate the calls, and reviewing the images of those calls, identifies the specific lighting or threshold that is wrong rather than the whole programme being too tight.

Masking a component is a last resort, because it removes detection rather than fixing the measurement. Where masking is used, the reference should be listed in the programme notes so that the loss of coverage is visible to everyone who relies on the station.

Covering Mixed Technology and Odd Form Parts

Mixed technology boards combine reflow joints, through hole joints and hand soldered connections, and each type looks different to the camera. Through hole joints on the solder side are often inspected from a different angle or with a different programme than the reflow side.

Cycle time shapes programming choices as much as detection does. Every additional measurement and every extra camera position adds time, so the expensive checks are concentrated at the positions where defects cost the most. That allocation should be deliberate and should be reviewed whenever the product mix or the line speed changes.

Odd form parts, connectors and cables that sit above the board plane are the hardest to cover. They are usually inspected for presence and orientation rather than for solder quality, with the joint quality handled by visual inspection or by defect trending on the process data.

Reviewing and Auditing the Programme

Auditing means checking that the programme actually detects what it claims to detect. A simple method is to seed a board with known defects, or to use a library of images from past failures, and confirm that each one is reported.

The audit should be repeated after any significant programme change and at a defined interval, because a threshold adjusted for one product can weaken detection for another if the two share a component definition.

Debugging Escapes and False Failures

When a defect reaches the customer, the first question is whether the machine could have seen it. Reviewing the stored image of that reference from the same lot answers the question directly, and the answer determines whether the fix belongs in the programme or in the process upstream.

False failures deserve the same analysis. A call that an operator clears without investigating is a measurement problem, and the images usually show which setting is responsible. Treating both directions as data rather than as noise keeps the station credible.

Change Control and Programme Revision

Programmes should be version controlled with the product revision, and a change to the placement data should trigger a review of the AOI programme rather than an automatic update. A new component position changes the lighting, the field of view and sometimes the package definition.

gopcb keeps programme revisions aligned with the assembly revision so that a board built to a known revision has a known inspection standard. That alignment is what makes an escape traceable, and it keeps the quality record meaningful when the same design is built again months later.

FAQ

Is learning based programming faster? Yes, but it accepts whatever was present on the sample board. Library based programming takes longer to set up and produces a more consistent standard across products.

How do I reduce false calls? Identify the references that generate them and review the images. Most false calls come from a few components with unsuitable lighting or a threshold copied from a different package.

How is an AOI programme audited? By seeding known defects or replaying images of past failures and confirming that each one is reported, then repeating the audit after any significant change.