Introduction
A PCBA quotation may list both SPI and AOI, yet those two abbreviations do not describe the same quality check. Solder paste inspection (SPI) checks the printed paste before component placement. Automated optical inspection (AOI) checks visible assembly features, commonly after placement or reflow. Their value comes from inspecting different stages of the process.
For a buyer, the important question is not simply whether a supplier has both machines. It is which defects the inspection program targets, what remains outside its coverage, and how rejected boards are reviewed. This guide explains the distinction and shows how to ask for a meaningful inspection plan without treating a machine pass as proof of a fully working product.
Where SPI and AOI Fit in the Assembly Process
A common SMT sequence is solder paste printing, SPI, component placement, reflow, and post-reflow AOI. Some lines also use pre-reflow AOI. That additional stage can identify placement problems before soldering, but it cannot judge a finished solder joint that has not yet formed.
Equipment manufacturer Koh Young describes SPI measurements of paste volume, height, area, and offset, while its AOI systems inspect component presence, placement, and visible solder geometry. See its overview of 3D inspection equipment. The actual checks available depend on the equipment, access to the feature, and inspection program.
In a quotation review, specify the inspection stage rather than accepting an unqualified entry reading only "AOI." For the wider production sequence, see our PCB assembly process guide.
What Solder Paste Inspection Can Tell You
Whether the Printed Deposit Matches the Intended Geometry
Three-dimensional SPI measures paste deposits against programmed criteria. It can flag missing paste, excessive or insufficient volume, displaced deposits, and detected bridging between deposits. These results describe the print at that point in production, before the later placement and heating operations.
Volume is particularly useful because two deposits can occupy similar areas while having different heights. Koh Young explains the role of 3D measurement in its solder paste inspection technology overview. A top-view image alone does not provide the same information as a measured deposit profile.
Whether a Repeatable Print Pattern Deserves Investigation
As a practical review method, compare repeated failures by location and board sequence. If the same aperture is repeatedly flagged, investigate its design and printing conditions. If many deposits shift together, examine alignment and setup. These are starting hypotheses, not diagnoses supplied automatically by a fail code.
Retain the measurement and the disposition decision together. An SPI record is more useful when it explains whether the board was accepted after review, cleaned and reprinted, or otherwise handled under the agreed process.
What an SPI Pass Does Not Establish
An SPI pass cannot certify component identity, placement, the completed solder joint, or product function. It also cannot rescue an incorrect reference aperture: a deposit may match the inspection program even when that program was derived from unsuitable data. Review the approved stencil revision alongside the results. Our solder mask and solder paste layer guide explains why those manufacturing files need separate treatment.
What Automated Optical Inspection Can Tell You
Visible Placement and Assembly Conditions
Depending on visibility and programming, AOI can flag missing or displaced components, orientation discrepancies, and accessible solder-joint anomalies. A polarity check needs a usable identifying feature and the correct reference. A part that looks similar to the approved component is not necessarily the right electrical value.
Why the Inspection Stage Matters
Use pre-reflow and post-reflow results to answer different questions. A placement error observed before reflow belongs in a different investigation from a joint anomaly first observed after heating. Correlating records for the same board can narrow the investigation, but sequence alone does not prove the root cause.
What 3D AOI Does Not Make Visible
Adding height measurement does not make an optical system see through an opaque package. Hidden solder joints still need a suitable inspection strategy. OMRON discusses this limitation and the role of X-ray inspection in its automated X-ray inspection overview. Avoid interpreting the label "3D" as complete coverage of every joint.
When X-Ray and Electrical Testing Add Different Evidence
X-Ray Addresses Hidden Geometry
For joints beneath packages such as BGAs, X-ray can provide information that an external optical view cannot. Its effectiveness still depends on the inspection method, image interpretation, board construction, and defect being investigated. A clear image is not itself a guarantee of electrical continuity or long-term reliability.
Electrical Testing Addresses Electrical Questions
In-circuit testing checks accessible components and connections electrically. Functional testing exercises specified product behavior. Keysight distinguishes these roles in its in-circuit test guidance. Neither should be described as universal coverage: test access, the implemented program, operating conditions, and agreed limits determine what is actually checked.
For example, confirming that a board powers up is not equivalent to checking every input, output, communication interface, or load condition. Write the intended functional tests into the purchase requirements. Otherwise, two suppliers can both offer "functional test" while delivering very different evidence.
Why "100% Inspected" Does Not Mean "Every Defect Detected"
Separate Board Coverage from Defect Coverage
Inspecting every board describes production coverage. It does not establish that every component feature was measurable or that every possible failure mode was detectable. Ask separately which board sides, components, and characteristics are included, which checks are disabled, and what alternatives cover the exceptions.
Separate Machine Calls from Confirmed Defects
Consider a hypothetical lot of 100 boards. AOI flags 12 boards; review confirms assembly defects on 3 and accepts the other 9 as false calls. The machine's initial board pass rate is 88%, while 3% of the lot has confirmed defects found at that stage. Neither number establishes the number of defects that escaped inspection.
This example assumes one final disposition per board and is not Linked Electronics production data. It illustrates why a report must define its denominator and review status. Board counts, defect counts, and inspected-feature counts are different measures and should not be mixed.
Do Not Improve the Report by Weakening Detection
A program that flags fewer boards may be better tuned, or it may have lost useful coverage. When a threshold changes, record the reason and verify the intended checks with suitable reference samples and independent review. A lower false-call count is not enough evidence on its own.
How to Specify a Useful Inspection Plan
The following is a buyer's checklist, not a claim that every order requires every test. Select requirements from the design risks and agree on scope before production:
- Stages: identify SPI, pre-reflow AOI, post-reflow AOI, and any additional inspection explicitly.
- Coverage: state whether all units or a defined sample are inspected, and identify exclusions.
- References: record the board revision, BOM variant, stencil revision, and inspection program revision.
- Acceptance: agree on the applicable acceptance requirements and any customer-specific limits.
- Hidden joints: identify features needing X-ray or another agreed method.
- Electrical checks: specify the test program, access requirements, limits, and functional conditions.
- Disposition: define who reviews machine calls and how repaired boards are reinspected.
- Records: agree on board or lot traceability, report contents, and retention expectations.
Match the Plan to the Actual Assembly Variant
A component intentionally omitted from one variant should not be treated like an accidental omission. Conversely, accepting an omission under the wrong variant can hide a real defect. Confirm that the inspection reference follows the released BOM and population instructions, not merely a previous order with the same board outline.
Ask for Evidence You Can Interpret
For a flagged location, a useful record identifies the board, reference designator or pad, measurement or image, applicable limit, reviewer decision, and any reinspection result. The exact report format can vary. The essential point is that a reviewer can follow a finding to its final disposition. See our PCBA quality report guide for a broader reporting checklist.
Conclusion
SPI and AOI complement one another because they inspect different stages. SPI provides evidence about the printed deposit; AOI provides evidence about visible assembly conditions. X-ray and electrical testing answer additional questions that those optical checks do not settle.
Choose a quality plan by asking what needs to be detected, how it is measured, and what happens after a failure is flagged. Clear coverage, approved references, and traceable disposition make an inspection statement useful to both the manufacturer and the buyer.
Frequently Asked Questions
What is the main difference between SPI and AOI?
SPI inspects printed solder paste before component placement. AOI inspects visible assembly features, commonly after placement or reflow. They evaluate different stages rather than duplicating the same check.
Can AOI replace SPI?
Post-reflow AOI does not provide the original paste-deposit measurements. Whether a process includes both should follow its inspection plan and risks, not an assumption that their evidence is interchangeable.
Can 3D AOI inspect all BGA solder joints?
No. Optical height measurement does not see through an opaque package. Hidden joints need a suitable additional inspection strategy, such as an agreed X-ray inspection method.
Does an AOI pass prove that the board works?
No. Optical acceptance is not a functional test. Electrical performance needs to be checked under the specified test program and operating conditions.
Does 100% inspection guarantee zero defects?
No. Inspecting every unit is different from detecting every possible defect. Coverage limitations, references, settings, and review decisions all affect what the inspection establishes.
What inspection information should a PCBA buyer request?
Ask for the inspection stages, unit coverage, program revision, acceptance criteria, exclusions, and disposition records. Specify any required hidden-joint inspection and electrical testing separately.
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