SPI vs AOI: Choosing the Right Inspection Investment
Compare SPI and AOI by process stage, defect coverage, line capacity and operating workload. Build an evidence-based inspection equipment shortlist.
4 min read
SPI and AOI are not interchangeable purchases. Solder paste inspection checks the printed deposits before placement. Automated optical inspection checks visible assembly features after placement or reflow, depending on where the station is installed.
For an equipment buyer, the useful question is not which technology is universally better. It is which inspection coverage your process needs, where defects should be caught and whether the proposed configuration can inspect your actual boards within the line takt.
Match the inspection station to the risk
SPI after printing — Can we verify the required paste-deposit characteristics before adding components?
AOI after placement — Can we identify the placement problems relevant to this assembly before soldering?
AOI after reflow — Can we detect the visible assembly and soldering defects required by our quality plan?
Koh Young’s AOI explanation describes this distinction between paste inspection and assembled-board inspection. Its discussion is useful for understanding process stages; performance claims for a particular vendor’s platform should not be treated as guarantees for other equipment.
Start with your defect history and customer requirements. If you do not have production history yet, identify likely risks from the design and planned process, then validate the inspection strategy during trials.
Build a coverage matrix before requesting quotations
List each defect or measurement requirement and the stage at which it should be checked. Include missing parts, polarity, offset, visible bridges and paste-related concerns where relevant.
For every requirement, ask the supplier to identify the inspection method, visibility limitations, required options and evidence from a representative board. Do not settle for a broad “detects solder defects” statement.
Optical systems inspect what their imaging and measurement arrangement can access. They should not be assumed to verify every hidden solder joint beneath a package. Where hidden-joint integrity or electrical function matters, define the additional inspection or test method with the quality team.
Understand what 2D and 3D mean for your purchase
Two-dimensional AOI evaluates image features such as shape, brightness and color. Three-dimensional systems add height-related measurement. That distinction can matter for the defects you need to detect, but it does not by itself decide the purchase.
Ask how the candidate handles your components, board finishes, shadows and height variation. Evaluate the actual coverage, cycle time, escape risk and review workload. Avoid assuming that a technology label guarantees lower false calls on every assembly.
For SPI, identify which deposit measurements are required and how the system produces them. Request evidence of repeatability appropriate to the process tolerances, not only sample images of an obviously defective board.
Check the mechanical envelope first
Confirm panel dimensions, thickness, weight, edge clearance and maximum component height above and below the board. Include the fixture if one travels with the assembly.
If the process needs long-board inspection or both sides of a panel, state that explicitly. Do not assume a product-family option is included in the quoted machine. Check whether the intended process requires a second pass and include that time in capacity calculations.
The FAROAD M400 page identifies the model as a 2D double-sided inline system. It should not be described as a 3D AOI machine. Ask for an application-specific coverage review and the exact configuration proposed for your assembly rather than inferring capabilities from camera resolution alone.
Measure capacity with the intended inspection program
Camera resolution and time per field of view are not complete board-cycle specifications. Board size, coverage, movement, image acquisition and analysis settings can all affect throughput.
Ask for a timed run using the required inspection coverage. Record automatic inspection time separately from the time operators spend reviewing calls. A system can keep up mechanically while creating an unmanageable review queue.
For example, a hypothetical line producing 200 boards per hour with an average of three review calls per board creates 600 calls each hour. At five seconds per review, that is 50 minutes of review labor before any rework or interruptions. These illustrative assumptions show why review workload belongs in the business case.
Test good boards as well as known defects
A useful trial includes known-good production variation and representative known defects. Define how results are independently checked so the trial does not merely confirm the machine’s own decisions.
Record false calls, missed defects, programming effort, tuning time and board-cycle time. Identify which results depend on operator adjustment and whether the same program remains usable across normal lot variation.
Agree how inspection thresholds will be governed after installation. Excessively permissive settings can hide defects; excessive sensitivity can overwhelm review. The acceptance criteria should reflect your quality plan and real process variation.
Specify data and integration requirements precisely
Decide whether you need board-level identity, stored images, measurement trends, repair records or links to factory software. Confirm the export format, software licenses and retention requirements.
If closed-loop correction is requested, identify the exact upstream equipment and permitted control functions. A general network connection or board-transfer handshake does not establish automatic process feedback. Require a model-specific compatibility confirmation and a functional demonstration within the agreed scope.
Compare lifecycle workload, not just equipment price
Include programming, training, calibration or verification routines, software, storage, maintenance and review staffing. Ask who owns the inspection programs and what happens when a new product is introduced.
If budget requires a phased investment, document the coverage available at each phase and the risks that remain. Do not describe a partial inspection plan as complete simply because one station has been installed.
Request the right inspection proposal
Prepare a sample board, design data, defect requirements, target takt, board envelope and traceability needs. Include examples of difficult features and define the expected trial evidence.
Review FAROAD SPI systems and AOI systems, then request a configuration review. Ask for a coverage matrix and timed application trial. The best inspection investment is the one that addresses your process risks with evidence—not the longest list of generic defect names.