Direct Answer
AOI and SPI are not interchangeable inspection steps. SPI should be used when solder paste volume, area, height and stencil-printing stability may drive defects before placement. AOI should be used after placement and reflow to catch component presence, polarity, offset, tombstoning, solder-joint appearance and visible assembly defects. For a buyer, the practical decision is not "AOI or SPI?" but which defect risk must be controlled, what evidence the supplier can return, and whether the project needs both checkpoints.
This article owns the AOI versus SPI selection topic for PCBA buyers. It is related to SMT process window basics, SMT stencil and reflow process control, AI AOI and X-ray quality traceability and the broader PCBA quality and traceability system, but it focuses on the buyer decision boundary between paste inspection and optical inspection.
What SPI Is Best For
SPI, or solder paste inspection, checks the printed solder paste before components are placed. It is most useful when fine-pitch pads, small passives, BGA/QFN pads, dense layouts, stepped stencils or first-build process windows increase paste risk. A buyer should ask whether the supplier can provide paste-volume trends, stencil-cleaning triggers, first-article records and actions taken when paste height or area moves outside the agreed process window.
SPI is especially relevant before a production release described in PCBA manufacturing services or before closing a DFM engineering review. It helps the team prevent defects before reflow instead of discovering them later as solder bridges, opens, insufficient solder or unstable yield.
What AOI Is Best For
AOI, or automated optical inspection, checks visible assembly conditions after placement or after reflow. It is useful for component presence, orientation, polarity, offset, tombstoning, skew, solder-joint appearance and many visible workmanship problems. AOI does not see every hidden joint and does not prove electrical function, so it should be linked with PCB assembly process controls, X-ray when hidden joints are critical, and electrical or functional tests when the buyer needs performance evidence.
AOI evidence should include inspection stage, defect categories, false-call handling, repair records, first-pass findings and how the supplier closes recurring issues. If the project also has firmware, sensor or functional requirements, AOI should not replace a controlled test plan.
Buyer Decision Matrix
- Choose SPI emphasis when paste deposition, stencil aperture, fine pitch or early process drift is the main risk.
- Choose AOI emphasis when placement, orientation, solder appearance or visible assembly defects are the main risk.
- Use both when the board has fine pitch, dense SMT, high-value assemblies, repeat production needs or a low tolerance for rework.
- Add X-ray or other inspection when hidden BGA/QFN/LGA joints cannot be evaluated by AOI.
- Add ICT, flying probe or FCT when the buyer needs electrical or functional evidence.
- Link inspection requirements to the RFQ path so they are quoted and returned as records, not discussed informally.
Evidence To Request In An RFQ
Ask the supplier for the inspection flow, SPI stage, AOI stage, defect categories, first-article records, repair and re-inspection method, escalation criteria, sample report format and who decides whether a finding is acceptable. The RFQ should also state the board risk: fine pitch, BGA/QFN, high-density connectors, double-sided SMT, cosmetic limits, reliability concern or field-failure cost.
Do not treat equipment names as proof of capability. A supplier can list AOI or SPI and still fail to provide useful evidence. The stronger question is whether the supplier can explain what is inspected, when it is inspected, how exceptions are handled, and what records will be returned with the shipment.
Common Mistakes
- Asking only "Do you have AOI?" without defining inspection stage or report output.
- Assuming SPI is unnecessary because AOI is available.
- Assuming AOI replaces electrical test.
- Comparing suppliers only by equipment list instead of defect prevention records.
- Accepting unsupported yield, capacity or certification claims without company evidence.
- Leaving inspection records out of the commercial quote.
Final RFQ Recommendation
For a new or high-risk PCBA build, send the Gerber or ODB++, BOM, CPL, assembly drawing, board photos if available, risk notes and expected inspection records through the KEEP BEST EMS RFQ workflow. Ask the supplier to return whether SPI, AOI, X-ray, ICT or FCT is recommended, what each step will catch, and what evidence will be included after build.