PCB X-Ray Inspection – Meaning, Uses, Techniques, and Application

PCB X-ray inspection is one of the final stages of PCB manufacturing. It captures every aspect of the final board to ensure the board meets quality assurance guidelines.

What is PCB X-Ray Inspection, and Why is it Important?

Printed circuit board (PCB) X-ray inspection is a detailed, minimally invasive, and non-destructive procedure to point out defects in PCBs. The inspection maps out all the boards, one at a time, to reveal any design and manufacturing defects.

As PCBs form an important part of all daily technology applications, manufacturers must subject the boards to high testing standards. The inspection is designed to ensure every PCB passes quality control parameters.

X-ray inspection is a more modern approach to defect detection in PCBs. In the past, manufacturers have used methods such as manual visual inspection, automated optical inspection (AOI), ultrasonic testing, and thermal imaging.

While these methods have been sufficient, modern PCBs are smaller, more detailed, and used in highly sensitive applications.

PCB X-ray inspection has an edge over these methods for the following reasons.

• Unlike other methods, it reveals hidden vulnerabilities in individual PCBs.
• Guarantees the quality of the final PCB product.
• It doesn’t subject the PCBs to further destruction.
• It can detect defects beyond the surface level.
• Provides confidence to the final users in sensitive applications such as medical equipment, communication, safety and defense, and automation.
• Manufacturers can use it on simple and complex boards.

How does X-Ray Inspection Work?

Manufacturers use high-frequency electromagnetic waves. They pass the PCBs through X-ray machines emitting X-rays of about 10nm. The machine shows grayscale images cast as shadows of the various PCB components.

Since the boards have different materials, each material has a different image according to its unique density, thickness, and atomic number.

For context, heavy elements on the PCB appear darker since they take in more X-rays.

The X-ray’s image processing software reveals any flaws that you might miss using traditional inspection methods.

A successful PCB X-ray inspection comes down to three vital components.

• X-Ray Tubes: They generate photons within a 0.1-1,000 kV range that pass through the board to generate an image on the opposite end of the machine.
• The Operating Platform: It moves the inspection object (PCB) at different angles to provide a full coverage image. The images help reveal errors anywhere on and within the board.
• A Detector: This collects the photons and converts them to visible light. A sensor then converts the light into digital data and renders a readable image of the PCB.

The inspected PCB X-ray image passes through an authenticity judgement test and compared to the original PCB image.

What are the Different Types of PCB X-Ray Inspection Systems?

2D X-Ray Systems

2D PCB X-ray inspections are typically faster. You can see 2-dimensional images of both the board’s sides. However, 2D inspection only offers one angle for viewing.

One of the benefits of 2D inspection is that you can perform them online or offline. Despite the advantages, manufacturers have to use images with less depth and less quality. Consequently, they might miss deeper-lying defects, especially in thicker boards.

3D X-Ray Systems (CT and Laminography)

Like 2D inspections, 3D systems can be online or offline. Offline 3D X-ray PCB inspection works like a CT-scan. CT scanning involves processing complex algorithms to produce a 3D outcome.

Basically, the manufacturer gets a series of highly-detailed 2D cross-sections of the PCB. They can see defects beyond the surface since it shows all the board’s layers.

Also, 3D inspection uses online/offline laminography to render images. Although it’s slower than 2D systems, 3D systems produce higher quality images better for detailed defect detection.

Online/Offline Systems

The manufacturer chooses whether to use online or offline systems depending on the complexity of the PCBs and inspection data.

Online systems are preferable for huge amounts of data processing. However, they are likely to extend the throughput time and increase inspection costs.

What are the Benefits of X-Ray Inspection for PCBs?

Whether you use 2D or 3D systems, X-ray inspection for PCBs has substantial benefits.

1. Defect Detection

   You are more likely to uncover defects within the inner layers that would otherwise be hidden when using traditional means.

   Defects such as under-surface cracks, solder bridges and voids, and pin-hole fills commonly come up during inspections.

2. Minimal Destruction

   X-ray inspection doesn’t subject PCBs to disassembly. Therefore, a manufacturer can test final PCB products while maintaining their structural integrity.

3. Higher Precision

   Correct PCB X-ray inspection is almost a guarantee for high-quality imaging. The high-resolution images, especially from 3D inspection systems, provide accurate results compared to earlier testing methodologies.

4. No-Fault PCB Delivery

   Since X-ray imaging has such high resolution, it is less likely that a client will end up with faulty PCBs.

5. Less Post-Manufacturing Costs

   Online inspection systems may be costly. However, inspection saves you the overall costs you would incur from the repair of broken PCBs with missed errors.

6. More Reliability

   You can deliver fully functional defect-free PCBs that can endure different applications and environments. More reliable PCBs ensure end users’ safety.

7. Full Analysis

   Online and offline 2D and 3D inspection systems provide different angles and multisections of the PCBs for more detailed analysis.

8. Data-Informed Reporting

   Online X-ray PCB inspection generates plenty of data. This data can be processed to provide guided decisions to improve design and manufacturing standards.

What Defects can X-Ray inspection Detect in PCBs?

PCB X-ray systems are preferable over other optical and electrical inspection for detecting hidden defects. Manufacturers can spot these common defects using X-ray inspection:

What are the Applications of X-Ray Inspection in PCB Assembly?

X-ray inspection in PCB assembly is necessary where other methods prove insufficient for exposing defects.

1. Soldering Defects Inspection

   It points out solder voids, an error that AOI would miss. Also, it’s more efficient in pointing out other soldering defects, such as:

   • Solder bridges
   • Solder shorts
   • Excess or inadequate solder
   • Low-quality solder
   • Open circuits

2. Contamination Detection

   Metallic contaminants from plating elements might get lodged in small spaces in the PCB. High resolution X-ray imaging can help spot contaminants and debris.

3. BGA and CSP Inspection

   Where AOI would miss most BGA and CSP defects, X-ray points out BGA shorts and open circuits. It ensures ball grid arrays and chip-scale packages are well-placed, connected without any defects.

4. Plated Through-Hole (PTH) Inspection

   Given the size of through-holes and vias, X-ray inspection is efficient in revealing PTH plating, solder fills and blind via defects otherwise invisible upon visual inspection.

5. Component Placement Verification

   Components such as decoupling capacities prevent erratic changes in power equipment.

   It is easy to identify misaligned or missing components using X-ray inspection.

6. Production Control

   PCB X-ray inspection helps the manufacturer meet quality control standards before delivery. Techniques such as CT inspection are vital in ensuring safety, analysing failures in production, and maximizing production efficiency.

What are the Limitations and Challenges of X-Ray Inspection for PCBs?

• Larger components might block some defects, especially when the X-ray systems don’t offer full visualization.
• Smaller electronics typically have miniaturized PCBs. The small size might be too fine for full-resolution imaging.
• 2D X-ray systems lack the ability to produce full coverage during inspection. Also, online 2D inspection uses heavy data and increases overall manufacturing and inspection costs.
• High costs might limit small-scale operations.
• You need qualified and competent inspectors to run the equipment and identify and analyze defects.
• You may need extra special equipment if the PCB includes microwave, fiberoptic, or RF components.
• X-ray PCB inspection requires more safety procedures and equipment that may increase overhead operating and production costs.

What is the History Of X-Ray Technology?

Even before the official discovery, scientists had encountered X-rays in different forms. As far back as 1785, William Morgan shared his findings on passing electric currents through a glass tube to produce a glow with The Royal Society of London.

Physicist Philip Lenard coined the term Lenard Rays while experimenting with cathode rays and Crookes tubes. In 1894, Nicola Tesla was investigating radiant energy that he suspected had damaged film paper in his laboratory.

It wasn’t until November 1895 that Wilhelm Conrad Rontgen studied the Crookes tubes and submitted his first findings. He named the mysterious rays “X-rays”. His findings were used by other researchers in medical and biological tests, yielding similar results.

C.H.F. Müller developed the first commercial X-ray tube weeks after the 1895 discovery. He also got a patent for a water-cooled X-ray tube in 1899.

One of the earliest widely used inventions using X-rays was Thomas Edison’s fluoroscope in the early 1900s. In 1913, Henry Moseley ran crystallography experiments and came up with the Moseley Laws that link X-ray frequency to a material’s atomic number, the basis of modern PCB X-ray inspection.

Modern X-ray technologies such as Talbot and X-ray interferometry have contributed to the development of clearer contrast imaging and specialized equipment such as High-Res X-ray detectors.

What Equipment And Software is Used For X-Ray Inspection Of PCBs?

What Safety Considerations Are Involved In X-Ray Inspection?

All the X-ray equipment must be up to code according to regulations. Authorities such as the FDA, U.S. Environmental Protection Agency, and OSHA provide guidance on the production and maintenance of X-ray equipment to protect against radiation exposure.

X-ray systems must be checked regularly to ensure they have adequate shielding, minimum primary beam scatter, and functional operational lights for warnings.

Operators are trained to stay at least 6 feet away from the machine’s primary beam during normal operation. They receive training on protective personal equipment too.

Manufacturers and inspectors must keep updated logbooks of maintenance and modifications. Inspectors must follow a strict no-hands policy around the machines when in use. They must also ensure the machines are off when not in use.

Summary

While you can produce good PCBs, it is easy to miss some manufacturing and design defects if you use traditional inspection methods that don’t go below the surface. Therefore, you must apply PCB X-ray inspection to ensure the final batch is defect-free.

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