**1. Detection Method for Assembled PCB Multilayer Circuit Boards**

To meet the testing requirements of PCB multilayer circuit boards, a variety of testing equipment has been developed. The Automatic Optical Inspection (AOI) system is typically used to inspect the inner layers before delamination. After delamination, the X-ray system ensures alignment accuracy and detects small defects. The scanning laser system offers a way to inspect the pad layer prior to reflow soldering. The integration of these systems, along with the visual inspection technology on the production line and the component integrity checks from automated component placement, helps ensure the reliability of the final assembly and soldering of the panels.

Despite these efforts to minimize defects during the production process, a final inspection of the assembled PCB multilayer circuit board is still necessary. This final inspection is crucial, as it serves as the last step in evaluating both the product and the overall manufacturing process.

The final inspection of the assembled PCB multilayer circuit board can be performed using dynamic methods or automated systems, and these two approaches are often combined. “Manual” inspection refers to operators visually inspecting the board with optical instruments, making accurate judgments about any defects. The automated system, on the other hand, uses computer-aided graphical analysis to identify defects. Many experts consider automated systems to encompass all detection methods, except for manual visual inspection.

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X-ray technology offers a method for assessing solder thickness, distribution, internal voids, cracks, desoldering, and the presence of solder balls (Markstein, 1993). Ultrasound can detect voids, cracks, and unbonded interfaces. Automatic optical inspection (AOI) evaluates external features such as bridging, flux, and component shape. Laser inspection can generate three-dimensional images of external features. Infrared detection compares the thermal signature of the solder joint with a known good joint to identify internal faults.

It is important to note that all defects which are beyond the capabilities of automatic inspection technology in assembled PCB multilayer circuit boards have been identified. Therefore, visual inspection methods should complement automated systems, especially for less frequently used applications. A combination of X-ray inspection and manual optical inspection is the most effective way to detect defects in assembled boards.

Assembled and soldered PCB multilayer circuit boards are prone to the following defects:

1) Missing components;

2) Component failures;

3) Installation errors and misplacement of components;

4) Component failures (again);

5) Poor soldering quality;

6) Solder bridges.

### 2. SMT Process Surface Assembly Inspection

The quality and reliability of surface-mount products primarily depend on the manufacturability and reliability of components, electronic materials, process design, and assembly procedures. To successfully assemble SMT products, both the quality of incoming electronic components and materials must be strictly controlled (incoming inspection), and a manufacturability (DFM) review of the SMT process design must be performed. Throughout the assembly process, continuous quality checks must be conducted before and after each stage, including processes such as printing, component placement, and soldering.

#### 1) Inspection of the Solder Paste Printing Process

Solder paste printing is the starting point of the SMT process and is one of the most complex and variable stages, influenced by many factors. It is also the stage where most defects originate, accounting for 60%-70% of all defects. Setting up an inspection station post-printing to check solder paste quality in real time and catch defects early in the production line helps minimize losses and costs. As a result, more and more SMT lines are integrating automatic optical inspection (AOI) systems specifically for solder paste printing, with some printing machines even incorporating AOI and other inspection systems. Common defects in solder paste printing include missing solder pads, excessive solder, scratches on large pads, solder paste sticking to the edges of small pads, misalignment, bridging, and smearing. Factors contributing to these issues include improper stencil thickness and hole wall treatment, incorrect printer settings, insufficient precision, inappropriate squeegee material and hardness, and poor PCB processing.

#### 2) Inspection Content of the Component Placement Process

The component placement process is critical to the assembly line’s level of automation, accuracy, and productivity. It has a decisive impact on the overall quality of the product. Real-time monitoring of this process is essential to ensuring the product’s quality. A typical pre-furnace inspection (post-placement) flow chart is shown in Figure 6-3. The most basic approach is to use AOI after the high-speed placement machine and before reflow soldering to inspect the quality of the components. This prevents defective solder paste printing and misaligned components from reaching the reflow soldering stage, where they could cause further issues. It also facilitates timely adjustments and maintenance of the placement machine, ensuring that the equipment remains in good condition. The key inspection elements during the placement process include component placement accuracy, handling of small-pitch devices and BGAs, pre-reflow defects such as missing or misaligned components, collapse or misalignment of solder paste, PCB surface contamination, and insufficient contact between component pins and solder paste. Character recognition software can be used to check component values and polarities, helping to identify placement errors.

#### 3) Inspection Content of the Soldering Process

After soldering, a 100% inspection of the product is necessary. The following aspects should be checked: whether the surface of the solder joints is smooth, and if there are any holes or voids; whether the shape of the solder joint is correct (e.g., avoiding a half-moon shape) and if there is an adequate amount of solder; whether there are defects such as tombstoning, bridging, component misplacement, or missing components; whether there are solder balls or other similar issues; checking the polarity of all components; and verifying that there are no short circuits, open circuits, or other soldering defects. Additionally, color changes on the PCB surface should be monitored as a potential indicator of defects.
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