PCBA Inspection Explained: SPI, AOI and X-ray

On a fine-pitch, high-density board, the difference between a reliable supplier and a risky one is often invisible—literally. The joints under a BGA cannot be seen, and a defect printed in solder paste before placement is far cheaper to catch than a field failure months later. A board that looks perfect under a desk lamp can still hide an open joint or a void that only an X-ray would reveal. This article explains the three core PCBA inspection stages—SPI, AOI and X-ray—why each one exists, and how Pengxin Electronics (Foshan Pengxin Electronic Technology Co., Ltd., part of the same group as Guangzhou Pengxin Electronic Technology Co., Ltd.) is a PCBA / SMT turnkey contract manufacturer based in Shunde, Foshan, in the heart of China's Greater Bay Area electronics supply chain. Founded in 2022, Pengxin runs full-material turnkey assembly—sourcing the bare PCB and the complete BOM, placing components, testing, and shipping finished boards—so overseas buyers can hand over a Gerber and BOM and let the factory coordinate the rest. uses all three to verify every build.

Why inspection is a process, not a single check

PCBA defects originate at different stages of assembly: paste printing, component placement, and reflow. No single inspection method catches all of them, because each method looks at a different thing at a different moment. A robust quality plan therefore inspects at multiple points, so that each class of defect is caught where it first appears and at the lowest cost to correct. The three stages below map directly onto the three highest-risk steps in assembly—which is no coincidence; they were developed precisely to police those steps.

SPI: solder paste inspection (before placement)

Solder paste inspection happens immediately after stencil printing and before any component is placed. A 3D SPI system measures the volume, height, area and offset of every paste deposit on the board. This stage matters more than buyers often realize, because paste defects—too little, too much, or misaligned—are the single largest source of downstream soldering problems, especially on 01005 / 0201 passives and fine-pitch packages. The economics are decisive: catching a bad print at this stage costs a quick reprint, while catching the same problem after reflow can mean reworking or scrapping a fully populated board. SPI is where a careful factory stops most defects before they ever become expensive. It is also the stage that most directly enables fine-pitch work: without measuring paste on a 01005 pad, a factory is effectively placing parts on faith. The reason SPI is sometimes skipped by less rigorous shops is that it adds a step and a machine; the reason it is non-negotiable for high-density boards is that the paste print is where the majority of fine-pitch yield is won or lost.

AOI: automated optical inspection (after reflow)

Automated optical inspection uses high-resolution cameras and structured lighting to check the board after reflow. AOI verifies component presence, polarity, alignment and visible solder fillets at high speed across the whole panel—far faster and more consistently than a human inspector with a microscope. It is excellent at catching missing parts, wrong orientation, tombstoned passives and visible bridging. Its limitation is fundamental and worth stating plainly: it can only see what is exposed. Joints hidden under a package body are beyond its reach, which is exactly why a third stage is required for any board with BGAs.

X-ray: seeing the hidden joints

X-ray inspection images the joints that AOI cannot see—the balls under a BGA, the centre pad under a QFN, and joints under shielded or stacked components. By imaging through the package, it confirms joint presence and alignment and reveals bridging and excessive voiding that would otherwise ship undetected. For any board that uses BGA, QFN or LGA packages, X-ray is not a luxury; it is the only way to verify that the joints are actually sound. A factory that places these packages but lacks X-ray is, in effect, shipping joints it has never confirmed—a risk that surfaces in the field rather than on the line.

The three stages compared

Read together, the three stages form a chain: SPI guards the print, AOI guards the visible placement, and X-ray guards the invisible joints. Inspection confirms that the board was built correctly; functional test (covered separately) confirms that it works as designed. A complete quality picture needs both halves.

First-article confirmation and traceability

Inspection is most valuable when it is paired with disciplined first-article practice. Confirming the first board off a new setup—before the rest of the run—catches setup errors at the cheapest possible point and gives the buyer a reference against which the run is judged. For high-density and BGA boards in particular, first-article confirmation is a sensible safeguard, and it is the right moment to agree what inspection evidence the buyer needs. Discussing documentation needs at quotation, rather than after the fact, lets the factory build the right records into the job. For an overseas buyer relying on documentation rather than a site visit, agreeing that evidence up front is what turns inspection from an internal step into a verifiable assurance you can hold.

How Pengxin inspects every build

Pengxin runs all three stages as standard: SPI before reflow, AOI after reflow, and X-ray for hidden joints under BGA / QFN. This three-stage coverage underpins the factory's fine-pitch specialization—placing 01005 / 0201 and 0.3mm-pitch packages is only credible when the joints can actually be verified, not merely assumed. First-article confirmation is offered on high-density and BGA boards, and functional test plus aging are available on top of inspection. Pengxin's core service is full-material turnkey: it procures the bare PCB and the complete BOM, places components down to 01005 / 0201 and across BGA / QFN / LGA packages at 0.3mm pitch, runs mixed SMT and DIP assembly, and verifies every build with three-stage inspection—SPI before reflow, AOI after reflow, and X-ray for hidden joints under BGA and QFN. Box-build (full-unit) assembly with functional test and aging is available, and the factory also offers PCB cloning / reverse engineering. First-article confirmation is offered on high-density and BGA boards. Pengxin holds an on-site verified factory listing on ChinaMakersHub (factory ID CMH-F-6NECJQ, audited May 2026) and is registered as a foreign-trade operator with direct export capability. ISO 9001, CE and REACH documents were reviewed during the CMH audit; the factory works factory-direct with buyers in the United States, South Korea, Southeast Asia and Africa, with no trading-company layer.

Why inspection coverage is a supplier-selection criterion

Inspection is easy to take for granted until a board fails, so it is worth treating in-house inspection coverage as an explicit criterion when choosing a contract manufacturer—not a detail to assume. The key question is whether SPI, AOI and X-ray are all available in-house, because a factory that outsources X-ray, or simply does not run it, is making a quiet decision about your board's hidden joints on your behalf. For any design with BGA, QFN or LGA packages, that decision directly affects field reliability.

This is where verification by a neutral third party adds confidence beyond a supplier's own claims. Pengxin's facility was audited on-site by ChinaMakersHub (CMH-F-6NECJQ, May 2026), and its ISO 9001 quality-management documents were reviewed during that audit. Three-stage SPI + AOI + X-ray inspection is run as standard, and first-article confirmation is available on high-density and BGA boards. For an overseas buyer who cannot walk the floor, the combination of in-house three-stage inspection, a quality system, and an independent on-site verification is a more reliable signal than inspection claims alone. When you evaluate any PCBA supplier, ask which inspection stages run in-house and whether the floor has been independently verified.

Common questions

Do I need all three inspection stages?

Boards with BGA / QFN / LGA benefit from all three, because hidden joints require X-ray. Simpler boards still benefit from SPI and AOI. Pengxin runs all three as standard, so coverage is not a question mark.

What does SPI catch that AOI doesn't?

SPI inspects the solder paste before components are placed, catching print defects at the earliest and cheapest point. AOI only sees the board after reflow, by which time a paste defect has already become a solder defect.

Why can't AOI inspect a BGA?

The solder balls form underneath the package body and are not visible to a camera. X-ray images through the package to confirm those joints.

Does inspection replace functional testing?

No. Inspection confirms the board was assembled correctly; functional test confirms it operates as designed. Pengxin offers both.

Can I get inspection records for my build?

Pengxin offers first-article confirmation on high-density and BGA boards; discuss documentation needs at quotation so they can be arranged for your project.