In recent years, the commercial application of 3D CT X-Ray has become increasingly noteworthy. As the technology matures, more companies are investing in its development, making the equipment more affordable, though still not quite at a consumer-friendly price point. Additionally, more laboratories equipped with 3D CT X-Ray are emerging, indicating that more electronics manufacturers are willing to invest in non-destructive testing. Workingbear has encountered at least two labs in Taiwan with 3D CT X-Ray equipment.
What is 3D CT X-Ray?
3D CT X-Ray is equipment that can produce 3D images through X-Ray imaging. The basic principle involves scanning at different depths with X-Rays and then using computer calculations to synthesize the 3D image. This might sound familiar because it’s similar to the CT (Computerized Tomography) scans used in hospitals for full-body imaging.
The Limitations of Current 2D X-Ray
As many of you know, the capabilities of the commonly used 2D X-Ray in electronics assembly manufacturers are quite limited. It can mostly check for wire bond continuity in ICs, inspect for obvious open or short circuits in PCB traces, and detect solder joint shorts under components like BGA, QFN, or LGA. It can also identify bubbles and voids in solder joints to some extent.
However, critical issues like HIP/HoP (Head-in-pillow/Head-on-Pillow), NWO (Non-Wet-Open), and BGA cracks are difficult to detect with traditional 2D X-Ray. Although some experienced engineers can use 2D X-Ray to judge BGA solder quality, the capability is limited.
There’s also a technology referred to as 2.5D X-Ray, where the test object can be tilted without opening the X-Ray machine cover. This allows for limited-angle observations of BGA solder balls, potentially detecting soldering issues. However, this method heavily relies on personal experience and has a high rate of misjudgment.
The Basics and Benefits of 3D CT X-Ray
3D CT X-Ray works by rotating the sample 360° at a 45°/60° tilt angle to create a 3D image. Scanning a single 3D X-Ray image typically requires 10-15 minutes for preparation, another 15-20 minutes for scanning, and an additional 5-10 minutes to assemble the 3D image. Therefore, producing a basic 3D X-Ray image takes at least 30 minutes.
Since 3D images are generated by combining layers of 2D images using software, the internal structure of the object can be sliced and displayed at various depths. This precision allows even small defects to be clearly identified, achieving the goal of defect detection.
Compared to previous 2D X-Ray, the current 3D CT X-Ray offers more detailed scans and presents 3D images, making it easier to detect obvious defects like BGA HIP/HoP and NWO. However, identifying micro cracks depends on the accuracy of the crack location.
This is because each 3D X-Ray scan takes about 30 minutes to produce a 3D image. To detect micro cracks, the resolution needs to be increased, which means limiting the scan area to about 1-4 BGA solder balls (depending on the size of the crack). This process is time-consuming and labor-intensive. For outsourced labs, the cost of a single scan ranges from US500 to US1,000 (consult the lab for specific fees).
Major Applications and Limitations of 3D CT X-Ray
To understand how 3D CT X-Ray can be used for failure analysis in electronic products, it is essential to grasp the characteristics of X-Ray imaging, which are significantly influenced by the following properties of the materials being inspected:
- Atomic number in the periodic table
- Density
- Thickness
Generally, materials with a higher atomic number have larger atomic structures, making them more challenging for X-Rays to penetrate, resulting in darker images. The same principle applies to density and thickness; the greater the obstruction, the harder it is for X-Rays to pass through.
In IC packaging, if there are voids or bubbles, the significant density difference between the encapsulating material and the voids makes it easy to distinguish their locations using X-Ray. Voids appear grayish-white, while the encapsulating material appears dark. Gold wires (atomic number: 79) and copper wires (atomic number: 29) in IC packaging can be distinctly differentiated from the silicon die (atomic number: 14). However, for COB (Chip-On-Board) with aluminum wires, distinguishing them from the silicon die is difficult because aluminum (atomic number: 13) is too close to silicon (atomic number: 14) in atomic number, making differentiation challenging.
Main Applications of 3D CT X-Ray:
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Defect Inspection in IC Packaging: For example, checking the integrity of gold wire bonding, cracks in encapsulation material, and bubbles in silver paste or black glue.
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Defects in PCB and Substrate Manufacturing: Such as misalignment or bridging of traces, open circuits, quality of plated through holes, and analysis of multilayer board trace layouts.
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Detection of Open Circuits, Short Circuits, or Abnormal Connections: In various electronic products.
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Inspection of Solder Ball Integrity in BGA and Flip Chip Packages: Including deformation, cracks, non-wetted opens (NWO), head-in-pillow (HIP), head-on-pillow (HoP), solder voids, and short circuits.
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Inspection of High-Density Plastics or Metal Voids: To detect cracks or voids.
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Testing of Various Passive and Active Components:
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Material Structure Analysis: Such as alloy composition and fiberglass weave angles.
Other Applications of 3D CT X-Ray:
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Reverse Engineering: Beyond inspecting hidden areas, 3D CT X-Ray can be used for reverse engineering. For example, it can uncover layers of ultrasonic welded enclosures or other techniques designed to prevent disassembly.
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Finished Product Inspection: Used for precision mechanical parts, where high dimensional accuracy is required. A full-size inspection can be performed and the 3D image can be compared to the original 3D specifications to determine acceptance or the need for rework.
Precautions When Using 3D CT X-Ray:
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Sample Size Limitation: Most 3D CT X-Ray machines limit the size of samples. Generally, mobile phone boards can fit without issue, but it’s best to confirm the size, as larger panels might not fit.
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Focus Distance: The clarity of the X-Ray image depends on how close the area is to the focus center. Areas closer to the focus center will be clearer, while those further away may appear blurred.
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Scanning Time: A basic scan takes about 30 minutes, but higher precision scans can take 3-4 hours. The time depends on the imaging frequency, so it’s important to discuss this beforehand.
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Power and Capability: The capacity of a 3D CT X-Ray machine is usually indicated by its kV/W rating. Higher numbers mean stronger energy, which can penetrate thicker and denser materials. Noise reduction capability is also crucial.
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Image Resolution: Standard 3D CT X-Ray image resolution can reach 0.5-0.7 micrometers (µm) and nanometer (nm) levels. However, as previously mentioned, achieving the best resolution requires limiting the scan area to increase precision.
Below is an example of using 3D CT X-Ray to scan a BGA package, showing the side view of the solder balls.
Below is another example of using 3D CT X-Ray to scan a BGA package, showing the PCB surface solder ball quality.
Below is an example of a cracked solder ball (Crack) imaged with CT scanning.
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