How to Determine the PCB Assembly Inspection Method?

As a platform for the transmission of numerous components and circuit signals, the Printed Circuit Board (PCB) is regarded as a critical component of electronic information products, and its quality determines the quality and reliability of the final product. With the development trend toward high density, lead-free and halogen-free environmental requirements, PCBs may suffer from various failure problems such as poor wettability, cracking and delamination without professional and timely inspection.

To ensure the high quality and reliability of assembled PCBs, PCB manufacturers and assemblers must inspect the boards at different stages of the manufacturing and assembly process to eliminate surface defects. In addition, timely and professional inspection enables the detection of defects exposed before electrical testing, and facilitates the data accumulation for Statistical Process Control (SPC). The widespread application of Surface Mount Technology (SMT) has put forward higher requirements for inspection, as SMT solder joints must withstand greater stress than those using Plated Through Hole (PTH) technology. Since the device leads dependent on SMT must bear a larger structural load, the device cannot be firmly soldered to the board without sufficient solder.

To date, in addition to visual inspection, a variety of structural inspection technologies with different costs, performance and defect coverage are available. Automated inspection technologies include optical inspection, laser triangulation, X-ray inspection and X-ray laminography technology. To implement optimal in-process inspection, manufacturers shall understand the advantages and disadvantages of each inspection method, and clarify the optimal application scenarios for each method. In general, PCB assembly inspection technologies are divided into two categories: visual inspection and automated in-process inspection.

Visual inspection can be performed after numerous steps in the PCB assembly process, and the equipment for visual inspection can be selected according to the position of the inspection target. For example, after solder paste printing and component placement, inspectors can detect obvious defects such as contaminated solder paste and missing components with the naked eye. The most common visual inspection is to inspect reflow solder joints by observing the light reflected from ordinary prisms at different angles. Generally, this inspection can test 5 joints within one second.

The effectiveness of visual inspection depends on the capability of the inspectors, as well as the consistency and applicability of the inspection standards. Inspectors must fully understand the technical requirements of each solder joint, as each solder joint may contain up to eight defect criteria, and there may be more than six types of solder joints available on different assembly equipment. Therefore, visual inspection is not applicable for quantitative measurement to implement effective structural process control. In addition, visual inspection is not suitable for the inspection of hidden solder joints, such as those on J-lead devices with high-density packaging, ultra-fine quad flat devices, surface array flip chips or Ball Grid Array (BGA) devices, which are established based on unified and specific rules.

The digitization and analysis system with real-time and automated video capture can significantly improve the margin and repeatability of visual inspection. Therefore, the Structural Process Test System relies on some form of emitted light, such as visible light, laser beams and X-rays. All these systems acquire information by processing images to identify and measure defects related to solder joint quality. Similar to visual inspection, the implementation of SPTS does not require physical contact with the circuit board. However, unlike visual inspection, SPTS has extremely high repeatability and eliminates the subjectivity in defect measurement.

AOI systems rely on multiple light sources, a programmable LED library and several cameras to illuminate the solder joints and capture images. Under reflected light, leads and solder joints act as mirrors, reflecting most of the light, while both PCBs and SMD components reflect very little light. The light reflected from the solder joints cannot provide practical height data, while the pattern and intensity of the reflected light can provide information on the curvature of the solder joints. Professional analysis will then be performed to determine whether the solder joints are complete, whether the solder amount is sufficient, and whether poor wetting occurs.

In addition, AOI systems also inspect solder bridging, missing or displaced components before or after reflow soldering. AOI equipment operates at a speed of 30 to 50 joints per second with a relatively low cost. However, it cannot inspect certain solder joint parameters, such as the soldering height and the solder volume inside the solder joints, nor can it inspect hidden solder joints, such as those on BGA, Pin Grid Array (PGA) and J-lead devices, which are essential for soldering reliability. In summary, AOI testing delivers the best performance when inspecting ICs and gull-wing devices with a pitch greater than 0.5mm.

ALT is a more direct technology for testing the height and shape of solder joints or solder paste deposits. The system operates to measure the height and reflectivity of certain surface components when the image of the laser beam is focused on one or more position-sensitive detectors maintained at a certain angle to the laser beam. During ALT measurement, the surface height is determined by the position of the light reflected by the position-sensitive detector, while the surface reflectivity is determined by the power of the reflected beam.

Due to secondary reflections, the beam may irradiate on the position-sensitive detector at multiple positions, which requires a scheme to distinguish correct measurements. In addition, the reflected beam may be shielded or interfered with by interfering materials as it travels along the position-sensitive detector. To eliminate multiple reflections and prevent shielding, the system shall test the reflected laser beam along an adjusted independent optical path. When performing multiple height measurements on solder joints, the ALT system provides data for the real-time structural process control of solder paste printing, including viscosity, alignment, cleanliness, fluidity, as well as extrusion speed and stress, before optimizing the solder paste deposition volume and position alignment for component assembly.

The X-ray fluoroscopy system emits a beam from a single-point light source, which passes vertically through the circuit board. During this process, solder joints attenuate the ray intensity to a greater extent than other materials. The intensity change of the ray energy is converted into a digital X-ray image with 256 grayscale levels. The grayscale X-ray image of a given solder joint is essentially a density image representing the thickness, distribution and internal integrity of the solder joint.

On single-sided PCBs, the X-ray fluoroscopy system can accurately inspect solder joint defects including cracks, insufficient solder, bridging, misalignment, voids, etc., occurring on J-lead devices, gull-wing devices or passive chips. In addition, it can also inspect missing components and reversed tantalum capacitors.

Compared with the X-ray fluoroscopy system, the X-ray laminography system generates a focal plane of the horizontal cross-sectional area by scanning or rotating synchronously with the X-ray detector. The off-axis images generated on the detector then result in a cross-sectional image with a surface thickness of 0.2-0.4mm through homogenization caused by a single swing or multiple swings. In addition, components on the front and back of the focal plane become defocused in the laminated image, thus separating the solder joints within the focal plane from other materials on the PCB.

Depending on the laser rangefinder, the X-ray laminography system can map the position of the board surface relative to the focal plane and correct board warpage. After that, the circuit board is moved in small vertical increments to pass through the focal plane, after which different sections of the same solder joint can be inspected. It is ideally suited for BGA and PTH solder joint inspection. Double-sided printed circuit boards are moved vertically in larger increments across the focal plane to inspect solder joints on both sides of the board. Different magnification factors or visible area sizes can be set by modifying the scanning radius of the beam and the vertically moving focal plane.

The X-ray laminography system can measure the parameters of all physical solder joints on different focal planes, thus providing comprehensive process defect coverage. Due to the indicative relationship between the X-ray cross-sectional image and the given solder paste volume, the grayscale readings can be converted into actual dimensions in adjusted standard or metric units. After analyzing the measurement results, data is provided for characterization and assembly improvement. For example, variations in the average solder paste thickness or solder paste volume of the solder joints will lead to the identification of the quality level of solder paste printing and the source of defects.

The X-ray laminography system operates at an inspection speed of 30-40 joints per second. It operates with a flexible sampling method, which ensures 100% coverage of critical device inspection, but cannot achieve 100% coverage for devices with an assembly time of less than 45 seconds. The X-ray laminography system is the most expensive among all inspection methods, but it significantly reduces troubleshooting and rework time. However, it cannot achieve 100% coverage for devices with an assembly time of less than 45 seconds. The X-ray laminography system is the most expensive among all inspection methods, but it significantly reduces troubleshooting and rework time. However, it cannot achieve 100% coverage for devices with an assembly time of less than 45 seconds. The X-ray laminography system is the most expensive among all inspection methods, but it significantly reduces troubleshooting and rework time.

Despite the wide variety of inspection methods, there are still many questions about the differences between AOI inspection and X-ray inspection. The figure below demonstrates the determining elements of the inspection method, and clarifies the scenarios where AOI and X-ray inspection deliver the optimal performance.

[Illustration Position: Diagram of inspection method determination elements, including 3 elements to be considered: cost 33%, defect type 33%, inspection speed 33%; and comparison table between AOI and X-ray inspection covering defect types, inspection speed and cost] PCBCART NSPEC DEPEND AOI OR X-RAY 3 ELEMENTS TO BE CONSIDERED

Automotive Electronic Hardware Design

Three elements shall be considered when determining the inspection method: defect type, cost and inspection speed.

For the defect coverage of AOI and X-ray, AOI is usually used for inner layer testing before lamination, with defect items ranging from solder paste volume, component position, missing components and polarity to solder joint defects. X-ray, on the other hand, focuses on micro defects after lamination, and can inspect wiring assemblies, semiconductor packaging, BGA solder defects, voids in solder joints, as well as components in high-mix, low-volume production.

In terms of inspection speed, AOI inspection shows a lower speed than X-ray inspection. However, high speed and high precision lead to higher costs.

The manufacturing of printed circuit board assemblies hardly relies on a single inspection method. After all, visual inspection is absolutely unavoidable during the assembly process. Due to the increasing complexity and the demand for high-volume assembly, automated inspection methods must be adopted.

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