SPI vs Aoi: when to Run Each in PCBA

Definition, scope, and who this guide is for

Understanding the distinct roles of Solder Paste Inspection (SPI) and Automated Optical Inspection (AOI) is critical for optimizing yield in modern electronics manufacturing. While both technologies aim to detect defects, they operate at different stages of the SMT line and target different root causes. This guide clarifies the decision-making process for spi vs aoi: when to run each in pcba, helping procurement leads and quality engineers balance cost against reliability risks.

SPI focuses on the very first step of the SMT process—solder printing—measuring the volume, height, and area of paste deposits before components are placed. In contrast, AOI is typically deployed after reflow soldering to verify component placement, polarity, and the quality of the formed solder joints. For high-reliability sectors like automotive or medical, using both is standard; however, for cost-sensitive consumer goods, understanding where to allocate inspection resources is vital.

This playbook is designed for engineers and buyers sourcing from manufacturers like APTPCB (APTPCB PCB Factory). It moves beyond basic definitions to provide actionable specifications, risk mitigation strategies, and a supplier qualification checklist. By the end of this guide, you will have a clear validation plan to ensure your PCBA provider uses the right inspection strategy for your specific design complexity.

When to use spi vs aoi: when to run each in pcba (and when a standard approach is better)

Having defined the fundamental scope of inspection technologies, we must now determine the specific scenarios that dictate their deployment in a production environment.

The decision of spi vs aoi: when to run each in pcba often depends on component density and the cost of rework. SPI is indispensable when using fine-pitch components (0201, 01005, or µBGA) because 70% of SMT defects originate at the printing stage; catching them here allows the board to be wiped and reprinted for pennies. If you skip SPI, a printing defect becomes a soldering defect, requiring expensive rework or scrapping the board entirely.

AOI is universally recommended for almost all production runs, regardless of complexity. It serves as the final gatekeeper before functional testing. However, relying only on AOI is a reactive strategy—it catches defects after they are permanent. A standard approach for simple boards (0603 components and larger) might rely solely on AOI to save setup costs. However, for any design requiring high reliability or containing lead-less packages (QFNs/LGAs), the "vs" becomes a "plus"—running SPI to prevent defects and AOI to catch placement errors is the only safe path.

spi vs aoi: when to run each in pcba specifications (materials, stackup, tolerances)

Once you have determined the inspection strategy, you must define the PCB design and material specifications that enable these machines to function accurately.

Surface Finish Flatness: Specify ENIG or ENEPIG over HASL if using SPI for fine-pitch parts; uneven HASL topography can cause false height failures in SPI.
Fiducial Markers: Require a minimum of 3 global fiducials and local fiducials for fine-pitch ICs to ensure machine alignment for both SPI and AOI.
Solder Mask Dams: Define a minimum 3-4 mil solder mask dam between pads to prevent bridging, which AOI must be programmed to detect.
Silkscreen Clearance: Ensure silkscreen is at least 6-8 mils away from solder pads; ink on pads confuses AOI algorithms regarding solder joint quality.
Component Height Data: Provide accurate component height data in the BOM/pick-and-place file to enable 3D AOI to detect lifted leads or tombstoning.
Stencil Thickness: Specify stencil foil thickness (typically 0.10mm to 0.15mm) relative to the smallest component pitch to ensure SPI volume targets are achievable.
Pad Design: Follow IPC-7351 standards for pad geometry; non-standard pads lead to variable solder fillets that trigger false calls in AOI.
Panelization Frames: Include break-away rails (5-10mm) with fiducials to allow the PCB to travel through inline SPI and AOI conveyors securely.
Material Color: Avoid white or yellow solder masks if possible, as they reflect light differently and can reduce contrast for older 2D AOI cameras.
Test Points: Designate test points that do not interfere with component bodies to avoid shadowing effects during optical inspection.
Warping Tolerance: Specify a maximum bow and twist of <0.75% (or <0.5% for BGA) to ensure the board stays in the focal depth of inspection cameras.

spi vs aoi: when to run each in pcba manufacturing risks (root causes and prevention)

With specifications locked in, the next step is to anticipate potential failure modes and understand how spi vs aoi: when to run each in pcba addresses them.

Insufficient Solder Paste:
- Root Cause: Blocked stencil apertures or low squeegee pressure.
- Detection: SPI detects low volume immediately.
- Prevention: Automated stencil cleaning cycles and real-time SPI feedback loops.
Solder Bridging (Shorts):
- Root Cause: Excessive paste release or slump during pre-heat.
- Detection: SPI detects area violation; AOI detects physical bridge post-reflow.
- Prevention: Proper aspect ratio in stencil design and correct reflow profile.
Tombstoning:
- Root Cause: Uneven wetting forces or placement offset.
- Detection: AOI identifies the lifted component.
- Prevention: Balanced thermal pad design and high-accuracy pick-and-place.
Missing Components:
- Root Cause: Feeder failure or nozzle vacuum loss.
- Detection: AOI flags the empty pad.
- Prevention: Feeder maintenance and vacuum sensing on placement heads.
Polarity Errors:
- Root Cause: Incorrect reel loading or tray orientation.
- Detection: AOI checks component markings/chamfers.
- Prevention: Barcode scanning verification during feeder setup (IQC).
Coplanarity Issues (Lifted Leads):
- Root Cause: Bent leads on the component or warped PCB.
- Detection: 3D AOI measures Z-height of leads.
- Prevention: Strict component handling and moisture sensitivity control (MSL).
Solder Balls:
- Root Cause: Paste oxidation or rapid temperature ramp.
- Detection: AOI (if programmed for debris) or visual inspection.
- Prevention: Fresh solder paste and optimized reflow profile.
Voiding:
- Root Cause: Outgassing from flux or PCB finish.
- Detection: X-Ray (neither SPI nor AOI sees inside the joint, but SPI ensures enough flux is present).
- Prevention: Correct reflow soak time.
Skewed Placement:
- Root Cause: Board movement or placement speed too high.
- Detection: AOI measures X/Y deviation.
- Prevention: Secure board support pins and conveyor speed calibration.

spi vs aoi: when to run each in pcba validation and acceptance (tests and pass criteria)

Identifying risks is only effective if you have a rigorous validation plan to verify that the inspection equipment is calibrated and catching defects as intended.

SPI Volume Threshold Validation:
- Objective: Ensure SPI rejects insufficient paste.
- Method: Run a test board with deliberately blocked stencil apertures.
- Criteria: SPI must flag 100% of pads with <70% volume.
SPI Height Accuracy Test:
- Objective: Verify Z-axis measurement.
- Method: Use a calibration block or golden board with known paste heights.
- Criteria: Measurement must be within ±10% of the known standard.
AOI False Call Rate Tuning:
- Objective: Minimize false alarms without missing real defects.
- Method: Run 50 known-good boards through AOI.
- Criteria: False call rate should be <500 ppm (parts per million) to avoid operator fatigue.
AOI Escape Study:
- Objective: Verify detection of actual defects.
- Method: Introduce "rabbit" boards with known defects (missing part, wrong polarity) into the line.
- Criteria: AOI must catch 100% of the introduced defects.
Golden Sample Comparison:
- Objective: Establish a baseline for visual acceptance.
- Method: Create a signed-off Golden Board that represents the perfect standard.
- Criteria: All production boards are compared against this digital twin in the AOI library.
SPC Data Review:
- Objective: Monitor process stability.
- Method: Review CpK data from the SPI machine for critical components.
- Criteria: CpK > 1.33 indicates a stable printing process.
Algorithm Validation:
- Objective: Ensure OCR (Optical Character Recognition) works.
- Method: Verify AOI reads text on ICs from different vendors.
- Criteria: Successful identification of alternate part numbers if approved.
Shadowing Verification:
- Objective: Ensure tall components don't hide small ones.
- Method: Inspect layout for tall capacitors next to small resistors.
- Criteria: 3D AOI cameras must have multi-angle projection to see "behind" obstacles.

spi vs aoi: when to run each in pcba supplier qualification checklist (RFQ, audit, traceability)

To ensure your manufacturing partner can execute the validation plan, use this checklist during the RFQ and audit phases. This includes elements of an incoming quality control (iqc) checklist for pcba to ensure inputs are correct before inspection even begins.

RFQ Inputs & Requirements

Does the RFQ explicitly request "100% 3D SPI and 100% Inline AOI" for the project?
Are IPC Class 2 or Class 3 inspection criteria specified?
Is the requirement for X-ray inspection on BGAs/QFNs included alongside SPI/AOI?
Have you requested a DFM report that specifically highlights "Shadowing" or "Inspection Access" issues?
Is the maximum allowable false call rate defined in the quality agreement?
Are specific data reporting requirements (e.g., SPI volume histograms) listed?
Is the surface finish (ENIG/OSP) specified to support inspection accuracy?
Are panelization drawings provided with fiducial locations clearly marked?

Capability Proof

Does the supplier use 3D SPI (volumetric) rather than 2D SPI (area only)?
Can the AOI equipment handle the smallest component size in your BOM (e.g., 01005)?
Does the supplier have offline programming stations to minimize downtime?
Is the AOI networked to the repair station to guide operators to the exact defect location?
Can the supplier demonstrate a closed-loop system (SPI feedback to Printer)?
Do they have specific algorithms for inspecting connector pins and through-hole parts?

Quality System & Traceability

Is there a robust incoming quality control (iqc) checklist for pcba components before they hit the line?
Are SPI and AOI images archived for a minimum period (e.g., 1 year) for traceability?
Does the MES (Manufacturing Execution System) link inspection results to the board serial number?
Is there a procedure for "False Call" verification (double-check by certified inspector)?
Are maintenance logs for inspection cameras and lighting sources up to date?
Is there a "Stop Line" trigger if consecutive defects are detected by SPI/AOI?

Change Control & Delivery

Is there a process for updating AOI programs if component vendors (and visual appearance) change?
Are revision controls in place to ensure the old AOI program isn't used on a new PCB revision?
Does the Certificate of Compliance (CoC) certify that all boards passed SPI and AOI?
Is there a procedure for handling "marginal" passes (process drift warnings)?
Can the supplier provide a yield report separating SPI fallout from AOI fallout?
Are repair/rework actions logged and re-inspected by AOI?

How to choose spi vs aoi: when to run each in pcba (trade-offs and decision rules)

After reviewing capabilities and risks, the final decision often involves trade-offs. Here is a logic framework to guide your choice.

If you prioritize Zero Defects on BGAs: Choose SPI + AOI + X-Ray. SPI is the only way to guarantee the correct paste volume under a BGA before the part hides the pads.
If you prioritize Cost on Simple Prototypes: Choose AOI Only. For large pitch (>0805) components on a 5-piece run, the setup time for SPI may exceed the value of the inspection. Visual inspection plus AOI is usually sufficient.
If you prioritize Throughput (Speed): Choose 3D SPI + High-Speed 2D/3D AOI. Catching print defects immediately with SPI prevents wasting time placing components on bad boards, keeping the line moving efficiently.
If you prioritize Rework Reduction: Choose SPI. Since most defects are print-related, stopping them here means you can simply wash the board. Catching them at AOI requires soldering irons and heat, which stresses the PCB.
If you prioritize Component Verification: Choose AOI. SPI cannot tell you if the wrong resistor value was placed or if a part is missing entirely; only AOI (or electrical test) can confirm this.
If you prioritize Data/Process Control: Choose Both. The combination provides a full data picture—SPI tells you about your process stability (printing), and AOI tells you about your placement accuracy.

spi vs aoi: when to run each in pcba FAQ (cost, lead time, Design for Manufacturability (DFM) files, materials, testing)

Q: How does adding SPI impact the cost of spi vs aoi: when to run each in pcba? Adding SPI typically adds a small NRE (Non-Recurring Engineering) charge for programming but saves significant money in mass production by reducing scrap. For APTPCB, 3D SPI is often standard for complex boards to ensure quality.

Q: Does running both SPI and AOI increase production lead time? Generally, no. Modern SPI and AOI machines operate "inline" at line speed. The inspection happens while the next board is being printed or mounted, so it does not create a bottleneck unless the defect rate is extremely high.

Q: What DFM files are required to program spi vs aoi: when to run each in pcba equipment? You must provide Gerber files (specifically Paste Layer for SPI and Silkscreen/Copper for AOI), Centroid/Pick-and-Place (XY) data, and a BOM with manufacturer part numbers to identify component package types.

Q: Can AOI replace electrical testing (ICT/FCT)? No. AOI checks for physical defects (orientation, solder joints), but it cannot verify if a chip is electrically functional or if a capacitor has the correct capacitance. AOI and E-Test are complementary.

Q: How do PCB materials affect spi vs aoi: when to run each in pcba accuracy? Highly reflective materials (like white solder mask) or very flexible materials (like thin Flex PCBs) can challenge optical cameras. Flex PCBs often require rigid carriers to ensure the flatness required for accurate 3D inspection.

Q: What are the acceptance criteria for spi vs aoi: when to run each in pcba? Acceptance is usually based on IPC-A-610 (Acceptability of Electronic Assemblies). Class 2 is standard for consumer/industrial, while Class 3 is for high-reliability (medical/aerospace) and requires tighter inspection thresholds.

Q: Does spi vs aoi: when to run each in pcba cover through-hole components? SPI is strictly for SMT paste. AOI can inspect through-hole solder joints (fillet quality) and part presence, but manual visual inspection is sometimes still used for complex THT assemblies.

Q: Why is an incoming quality control (iqc) checklist for pcba important for inspection success? If the raw PCB has oxidized pads or the components are the wrong size (inputs), SPI and AOI will generate massive false calls or fail to detect issues. IQC ensures the inputs match the program parameters.

SPI Inspection: Deep dive into how Solder Paste Inspection works and the specific defects it prevents at the printing stage.
AOI Inspection: Detailed overview of Automated Optical Inspection capabilities for detecting placement and soldering errors.
Incoming Quality Control: Learn how raw materials are verified before assembly, a crucial precursor to successful automated inspection.
DFM Guidelines: Design tips to ensure your PCB layout is optimized for automated inspection equipment.
SMT & THT Assembly: Understanding the full assembly process helps contextualize where SPI and AOI fit into the production line.
Quality System: Explore the broader quality framework that governs inspection standards and certifications.

Request a quote for spi vs aoi: when to run each in pcba (Design for Manufacturability (DFM) review + pricing)

Ready to move from planning to production? Request a Quote from APTPCB to get a comprehensive DFM review that includes an analysis of your inspection requirements.

To get the most accurate quote and inspection plan, please provide:

Gerber Files: Including paste, mask, and silkscreen layers.
BOM: With full manufacturer part numbers.
XY Data: Centroid file for component placement.
Volume: Prototype vs. Mass Production quantities (influences the inspection strategy).
Special Requirements: IPC Class 2 vs. Class 3, or specific testing needs.

Conclusion (next steps)

Deciding on spi vs aoi: when to run each in pcba is not about choosing one over the other, but about building a defense-in-depth strategy for your electronics. SPI secures the foundation by ensuring the solder print is perfect, while AOI acts as the final verifier of assembly integrity. By defining clear specifications, understanding the risks, and validating your supplier's capabilities, you can significantly reduce rework costs and ensure reliable delivery.