On a single PCBA, there can be anywhere from a few hundred to tens of thousands of solder joints. If any one of them fails, the entire product can "collapse." But solder joints are not like a human pulse. You cannot simply touch one and tell whether it is healthy.
In electronics manufacturing, determining solder joint quality follows a rigorous system, much like the "four diagnostic methods" of Traditional Chinese Medicine: inspection, auscultation, inquiry, and palpation.
Why Does Solder Joint Quality Matter?
Solder joints are the "joints" that connect electronic components to the circuit board. Poor solder joints can cause signal interruptions, intermittent equipment failures, or even short circuits, burns, and safety incidents. In fields such as automotive electronics and medical devices, a single cold joint could mean tens of millions in recall costs and irreversible safety risks.
However, as components continue to miniaturize, with BGA, QFN, and 0201 components becoming increasingly common, traditional visual inspection is no longer sufficient. For example, 0201 components are only about 0.6 mm × 0.3 mm, while BGA solder joints are completely hidden beneath the chip and cannot be seen by the naked eye.
As a result, the electronics manufacturing industry has developed a complete set of "TCM-style" inspection methods to evaluate solder joint quality from different angles.
Inspection: AOI – The "Smart Eye" of PCBA
AOI, or Automated Optical Inspection, is one of the most critical real-time monitoring tools on an SMT production line.
How does AOI work?
AOI equipment uses high-resolution cameras and computer vision technology to automatically inspect components and solder joints on the PCB. It illuminates solder joints with multi-angle, multi-color temperature LED ring lights, then analyzes the reflected light to assess quality.
The typical AOI process includes four steps: image acquisition, data processing, image analysis, and report output.
What can AOI detect?
AOI is highly effective at identifying surface-visible defects, including component misalignment, poor solder joints, solder bridging, insufficient solder paste, excessive solder paste, missing components, and polarity errors.
What is the difference between 2D AOI and 3D AOI?
Traditional 2D AOI mainly identifies appearance defects. In contrast, 3D AOI uses laser phase-shift mapping to obtain real height data of solder joints, making it more effective for detecting lifted leads and coplanarity issues of IC pins.
However, AOI also has limitations. It can only "see" the surface, not the inside of a solder joint. It is like a doctor observing a patient with the naked eye. You can judge the general condition, but you cannot see internal fractures.
Auscultation: X‑Ray – The "X‑ray Vision" into Solder Joints
For components such as BGAs, where solder joints are entirely hidden beneath the chip, X-Ray inspection is one of the most important non-destructive testing methods.
How does X-Ray inspection work?
X-Ray inspection uses X-rays to penetrate the circuit board. Different materials, such as solder alloy, copper, and molding compound, absorb X-rays to different degrees, producing images with different grayscale levels.
In simple terms, X-Ray inspection takes an "X-ray photo" of the solder joint, allowing engineers to examine what cannot be seen from the surface.
What can X-Ray detect?
X-Ray inspection can detect internal defects such as voids, cracks, solder ball shifting, and bridging. For BGA devices in particular, voids are one of the most common issues. Some BGA solder balls may already contain voids, and if the reflow profile is not properly controlled, these voids can expand further during soldering.
Why should voiding rates be controlled?
According to IPC-A-610 standards, the cross-sectional area of voids within a BGA solder joint should typically be less than 25%. For high-reliability products, such as automotive electronics, many factories adopt stricter internal standards of 10% to 15%.
X-Ray inspection can cover a very high percentage of manufacturing defects and has become a core quality assurance tool for high-density electronic assembly.
Inquiry: ICT and FCT – The "Pulse Diagnosis" of Electrical Performance
Inspection and auscultation examine the appearance and internal structure of solder joints, but good-looking solder joints do not always guarantee good electrical performance. This is where ICT and FCT come in. They are similar to "pulse diagnosis" in PCBA quality control.
What does ICT check?
ICT, or In-Circuit Testing, uses test probes to contact test points on the PCBA. It detects open circuits, short circuits, and the soldering condition of individual components. It also verifies whether the parameters of resistors, capacitors, inductors, and other components are within specification.
What does FCT verify?
FCT, or Functional Testing, places the PCBA into its actual working environment, powers it on, and verifies whether its functions meet design requirements. Simply put, FCT makes the PCBA "go to work" and checks whether it performs its job properly.
Palpation: IPC Standards – The "Judgment Criteria" for Solder Joints
With multiple testing methods in place, manufacturers still need a clear standard of judgment. IPC-A-610 is the globally recognized standard for the acceptability of electronic assemblies.
It classifies electronic products into three levels:
- Class 1, General Industrial Products: This class applies to products such as toys and household appliances. Requirements are relatively lenient, and minor cosmetic defects may be allowed.
- Class 2, Dedicated Service Products: This class applies to communication equipment, industrial controls, and similar products. It requires full, well-wetted solder joints with no cold soldering.
- Class 3, High-Performance Electronic Products: This class applies to automotive safety components, medical devices, and other high-reliability applications. It requires near-perfect solder joints with virtually zero defects.
Different classes have specific quantitative requirements for solder joint shape, component shift, voiding rate, solder balls, and board cleanliness.
Why Is Multi-Level Inspection a Combination, Not a Choice?
No single inspection method is without blind spots. AOI cannot see inside the solder joint. X-Ray inspection is more costly and slower than optical inspection. ICT cannot directly inspect internal BGA joints. Therefore, a mature PCBA factory combines AOI, X-Ray, ICT, and FCT to ensure that every critical solder joint and function is properly evaluated.
This is much like the "four diagnostic methods" in Traditional Chinese Medicine: observing complexion, listening to sounds, asking about symptoms, and feeling the pulse. All four methods must be used together for a reliable diagnosis.
The same logic applies to PCBA solder quality. AOI checks the surface, X-Ray looks inside, ICT tests electrical connectivity, and FCT verifies function. Only when these methods are combined can a PCBA be truly qualified for demanding applications.
| Inspection Method | Main Purpose | Best For | Limitation |
|---|---|---|---|
| AOI | Surface visual inspection | Missing parts, polarity errors, bridging, and solder appearance | Cannot inspect hidden BGA solder joints |
| 3D AOI | Surface and height inspection | Lifted leads, coplanarity, and solder volume | Still limited to visible areas |
| X-Ray | Internal solder inspection | BGA, QFN, LGA, hidden joints, and voids | Slower and more costly than AOI |
| ICT | Electrical connectivity testing | Open circuits, short circuits, wrong values, and solder continuity | Requires test access and fixture design |
| FCT | Functional validation | Real operating behavior and system-level performance | May not locate the root defect by itself |
Conclusion
XWONDER has been deeply involved in PCBA manufacturing for over 12 years. We are equipped with 8 SMT production lines, fully automated in-line AOI, and X-Ray non-destructive inspection equipment.
We provide more than PCBA manufacturing. Our one-stop service covers circuit design, PCB layout, software development, production, and quality management. Whether for consumer electronics or automotive-grade products under IATF 16949 quality requirements, XWONDER can tailor the inspection level and acceptance criteria to match your reliability needs.
From design to mass production, from "TCM diagnosis" to quality delivery, XWONDER safeguards every PCBA we make.
