Direct Answer
Choose AOI for visible placement, polarity, solder bridge, tombstone, and many surface solder defects. Choose X-ray when BGA, QFN, LGA, bottom-terminated parts, or via-in-pad features hide the solder joint from optical inspection.
Engineering Context
AOI and X-ray inspection are not interchangeable labels on a quote form. They change the process risk, the inspection conversation, and the assumptions the assembler has to accept before release.
The first-principles question is not which option is more popular. The question is which option removes the limiting risk in this board: pad condition, storage window, component pitch, inspection access, material behavior, process capability, or drawing control.
Keep the decision bounded to the released design package. If the drawing, BOM, fabrication notes, or assembler requirements do not state the controlling assumption, the quote should treat the choice as unresolved rather than silently substitutable.
Decision Criteria
- engineering fit
- risk trigger
- RFQ handoff
Comparison Table
| Criterion | AOI | X-ray inspection | Decision note |
|---|---|---|---|
| Best fit | defects are visible from the board surface and placement quality is the main concern. | hidden solder joints, bottom-terminated packages, or via-in-pad features control inspection risk. | Choose the option that removes a real build risk. |
| Main limitation | cannot prove hidden solder-joint quality or product function. | does not replace functional test or correct design inputs. | Neither side is universally better. |
| RFQ input | state AOI scope, sides inspected, polarity-sensitive parts, and acceptance expectations. | state hidden-joint packages, X-ray sampling or coverage expectation, and any voiding or bridge review need. | Make the selected assumption explicit. |
Decision Tree
1. If the decision is controlled by engineering fit, compare the smallest component, fastest signal, or most restrictive inspection requirement first. 2. If AOI removes that risk, specify it directly in the drawing or RFQ. 3. If X-ray inspection meets the same electrical, assembly, inspection, and schedule constraints with less complexity, keep the simpler option. 4. If the answer depends on missing stackup, BOM, finish, or inspection data, keep the page as a decision guide and close the RFQ inputs before release.
What Changes The Decision
Use AOI when the board risk is controlled by the conditions listed in the AOI column and the added control is justified by assembly yield, electrical behavior, inspection, reliability, or storage margin.
Use X-ray inspection only when the X-ray inspection column remains true after checking component geometry, process assumptions, drawing notes, and assembler acceptance.
Do not use this comparison to invent numeric tolerances, thicknesses, shelf-life promises, alloy composition, compliance statements, or supplier capability claims. Those belong in the released specification, applicable standard, or supplier capability review.
Recommended Use Cases
- Use AOI when defects are visible from the board surface and placement quality is the main concern.
- Use X-ray inspection when hidden solder joints, bottom-terminated packages, or via-in-pad features control inspection risk.
Limitations
- AOI: cannot prove hidden solder-joint quality or product function.
- X-ray inspection: does not replace functional test or correct design inputs.
RFQ Handoff
Before RFQ, state the selected option, allowed alternates, inspection expectations, and any component packages or performance constraints that make one side unacceptable.
If X-ray inspection is allowed, confirm that the assembler accepts the actual package mix and process condition. If AOI is required, state it directly instead of relying on a generic default.
A quote that does not normalize these assumptions is not a comparable quote; it is a different manufacturing condition.
Source Notes
- Automated inspection process-control boundary - IPC-9716A requirements for automated inspection process control (standard).
Source fact: IPC-9716A is the source boundary for automated inspection process-control context, including SPI, AOI, and AXI for printed board assemblies. Omini interpretation: Use this to frame automated inspection as a scoped process-control activity that needs board side, component coverage, method, and acceptance handoff before production release. Allowed usage: Use on AOI, SPI, AXI, SMT inspection, AOI versus X-ray comparison, and quote-handoff pages when explaining automated inspection scope.
- BGA inspection and assembly boundary - IPC-7095E Design and Assembly Process Guidance for Ball Grid Arrays (standard).
Source fact: IPC-7095E covers BGA and fine-pitch BGA design and assembly implementation topics, including inspection, rework, repair, and troubleshooting scope. Omini interpretation: Use this to justify X-ray review when critical solder joints are hidden below BGA or fine-pitch area-array packages and optical inspection cannot see the joint interface. Allowed usage: Use on X-ray inspection, BGA assembly, AOI versus X-ray comparison, via-in-pad, and hidden-joint inspection pages.
- IPC standards source boundary - IPC official resources (standard).
Source fact: IPC official resources are used to locate standards context and public summaries, while paid standard text remains outside the repository. Omini interpretation: Use this to separate standards-backed engineering framing from copied acceptance criteria or unsupported numeric claims. Allowed usage: Use when a page needs IPC context but does not have a narrower public source registered yet.
- Existing OminiPCB repository content (internal): May seed structure and internal links; engineering claims still need external verification when specifications are involved.
This page stores original engineering guidance only. It does not reproduce paid standard text, datasheet tables, competitor claims, prices, or certification claims.
CTA
Request a PCB quote when the selected material, finish, stackup, inspection scope, quantity, and lead time are ready for engineering review.