X-ray Inspection of PCBs, 2D/3D, Tomography or Laminography: A Closer Look

X-ray inspection has become an essential technology in the production and quality control of electronic boards (PCBs). This non-destructive method allows for precise visualization of the interiors of components and solder joints without causing any damage. But what are the differences between 2D/3D inspections, computed tomography (CT), and laminography with 3D reconstruction in the context of microelectronics and PCB manufacturing? How do they work and what systems are used? 

What is X-ray inspection for PCBs? 

X-ray inspection in the field of electronic boards enables non-destructive examination of the internal features and underlying conditions of the objects tested. Leveraging the penetrating nature and differential absorption characteristics of X-rays, this technique provides a precise and detailed view of internal components and connections, often obscured in complex PCB assemblies. Specifically, in the context of PCB inspection, X-ray imaging is used to highlight hidden solder joints, component fixations, and structural details in multi-layer circuit boards, which are not visible through external visual inspection. X-rays, with their short wavelength and high frequency, have the ability to penetrate solid objects. 

Thus, when interacting with an electronic board, for example, variations in material thickness and density result in differential absorption. This generates an X-ray projection image that encodes the details of the internal structure, thereby revealing potential defects. This ability to see beyond superficial layers opens the door to in-depth analysis of electronic components, ensuring the reliability and performance of PCBs in their final application. 

Common defects sought include short circuits, solder discontinuities, component misalignments, and porosities within solders, which might otherwise remain undetected and could potentially cause failures in the finished product. 

 

Advanced packaging leads to smaller solder bump sizes in a three-dimensional arrangement

Advanced packaging leads to smaller solder bump sizes in a three-dimensional arrangement 

2D X-ray Inspection 

2D X-ray inspection is an imaging technique that uses X-rays to produce two-dimensional images. X-rays are emitted from a radiation source, passing through the object to reach a detector on the other side. The X-rays that pass through the object are absorbed to varying degrees depending on the density and thickness of the material, thus creating a contrast image on the detector. This image is then processed to reveal details. 

 2D X-ray inspection is a rapid imaging method, making it a popular choice for PCB inspection. It can detect defects such as short circuits, solder discontinuities, and misalignments of components. However, it does not provide information about the depth or the precise location of defects, which can make it difficult to identify certain types of defects. Additionally, 2D inspection cannot provide a complete visualization of the complex internal structures of PCBs. 

 2D X-ray inspection is often used for inspecting solder joints and components on PCBs. It can also be used to inspect the inner layers of multi-layer PCBs. However, for more complex PCBs with sophisticated internal structures, 3D inspection may be necessary for a complete and accurate visualization. In some cases, multiple 2D scans taken from different viewing angles can be combined to offer a limited 3D visualization, but this cannot achieve the precision and clarity of a true 3D inspection. 

3D Inspection by Computed Tomography with 3D Reconstruction (CT) 

3D inspection by computed tomography (CT) is a non-destructive imaging technique that uses X-rays to create three-dimensional images of an object. Unlike 2D inspection, tomography provides spatial information about the inspected object, allowing for the precise localization of defects and anomalies. 

The operating principle of tomography is based on acquiring a large number of 2D images of the object to be inspected, taken from different angles. These images are then processed by a tomographic reconstruction algorithm to create a 3D image of the object. This 3D image can be viewed from different angles, sliced for more detailed analysis, or used to measure the spatial properties of the object. 

3D inspection by tomography offers several advantages over 2D inspection. First, it provides precise spatial information about the inspected object, allowing for highly accurate localization of defects and anomalies. Additionally, tomography can detect defects that might be hidden in a 2D image, such as voids or inclusions. Finally, tomography can be used to measure the spatial properties of the object, such as the size and shape of defects.

 

CT scan of BGA Package on Package (PoP) showing a Head in Pillow (HiP) failure

CT scan of BGA Package on Package (PoP) showing a Head in Pillow (HiP) failure 

 

However, tomography may lack resolution compared to 2D inspection. Indeed, the resolution of the 3D image depends on the size of the voxels (3D pixels), which can be larger than the pixels in a 2D image. 

3D inspection by tomography is particularly useful in the manufacture of electronic boards for defect detection and failure analysis. It can be used to inspect solders, vias, internal layers, and components of PCBs. It can also be used to measure the spatial properties of PCBs, such as layer thickness and component height. 

However, it may not be necessary for all types of PCB inspection. In some cases, high-resolution 2D inspection may be sufficient to detect defects. 

Laminography Inspection with 3D Reconstruction (CL) 

Laminography is an ideal inspection method for flat objects such as integrated circuits, electronic boards (PCBs), phones, tablets, laptops, and even papyrus manuscripts. This technology combines the advantages of 2D and 3D inspection by adding depth of field information to high-resolution 2D images, thus enabling the detection and localization of defects. 

Unlike tomography (CT), laminography does not require a 360-degree recording to generate spatial information. Instead, it scans objects from a limited angular range, which allows the X-ray tube to be brought closer to the inspection object for higher resolution. 

 

Computed laminography with X-ray tube and detector rotation

Computed laminography with X-ray tube and detector rotation 

 

Laminography is supported by several Comet Yxlon systems, including the Cougar EVO, Cheetah EVO, FF20 CT, FF35 CT, and FF85 CT. This technology is particularly well-suited for inspecting solders on ball grid arrays (BGAs). It ensures that the contact areas are sufficiently large to conduct electricity or heat according to specifications and to determine the existence, size, or distribution of voids. 

 

The laminography scan shows clearly the pads and traces of the BGA's

The laminography scan shows clearly the pads and traces of the BGA’s 

 

During the inspection of double-sided PCBs, systems such as the Comet Yxlon Cheetah EVO and Cougar EVO use laminography to create layered images of the contact area, free from the overlays of components on the other side of the PCB that obstruct the view as in 2D radiographic images. For the final evaluation of the solders, software support is provided by the VoidInspect CL inspection workflow. 

 

Blown wire visualized with computed laminography

Blown wire visualized with computed laminography 

 

Laminography is also used for inspecting connections between different layers in integrated circuits (ICs) and wafers. Since IC packages contain multiple layers, a 2D radioscopic image is generally insufficient for analysis, as it does not provide spatial information and the different internal structures tend to overlap. Systems such as the Comet Yxlon Cheetah EVO and Cougar EVO use laminography to produce high-quality images of the interconnection plane. 

In terms of inspecting flat electronic devices such as tablets, mobile phones, and laptops, laminography is particularly well-suited as an inspection method. During quality control and failure analysis, Comet Yxlon laminography systems reveal whether all parts are assembled and correctly positioned, or if mechanical defects such as connection breaks exist. 

 

3D visualization of a laminography scan of a multi-layer board, using the function "Inverted Clipping Box"

3D visualization of a laminography scan of a multi-layer board, using the function “Inverted Clipping Box 

 

In the rapidly growing sector of electric mobility, battery configurations for electric vehicles are evolving quickly. To ensure safety and quality in mass production, lithium-ion batteries must be inspected in large quantities. Inspection during production requires high throughput in addition to high resolution relative to the size of the battery. 

Unlike traditional 2D testing, laminography allows for more reliable and precise inspection of internal battery structures, revealing, for example, the geometry of stacked electrodes with distances and angles of walls, overlaps of anodes, or the inclusion of foreign objects or voids. 

In summary

Comet Yxlon offers a variety of X-ray and CT inspection systems suited to specific inspection tasks for semiconductor products. 

2D radioscopic inspection is ideal for in-process inspections, with rapid image generation for the inspection of simple wire bonds, vias, and bumps. It is available in the Cheetah and Cougar systems, providing the best image in record time with sub-micron resolution. 

3D tomography (CT) is used for 3D measurements to determine spatial properties, locations, and dimensions of defects. It is primarily used for detailed failure analysis. CT is available in the FF20 CT and FF35 CT systems with an intuitive user interface thanks to Geminy, and as an option for the Cheetah and Cougar systems. 

Laminography (CL) is used to determine the spatial properties of components, as well as the location and dimensions of defects. It is primarily used for random inspection of multi-layer packages and embedded devices in advanced packaging applications. CL is available in the Cheetah and Cougar systems with a vertical beam and the highest resolution, and in the FF20 CT and FF35 CT systems with a horizontal beam. 

 

For more information on our X-ray systems, please contact us

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