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How To Use X-ray For PCB Inspection?

In this blog post, you’ll read:X-ray test is used to detect the sample that cannot be detected by the appearance of the position. X-ray penetrates the material of different densities due to the change in its light intensity. The resulting contrast effect can form an image to show the internal structure of the object to be measured.

Table of Contents

The role of printed circuit board (PCB) is to provide electrical connections for electronic components. The advantages of PCB include reduction of assembly and wiring errors, automation improvement, and production of labor rates. Depending on the number of circuit board layers, it can be divided into single-sided, double-sided, four-layer, six-layer, and other multilayer circuit boards.

 

Seven inspection methods in PCB manufacturing

 

To ensure the production quality of PCB boards, there are seven inspection methods during the production process. Each inspection method targets different PCB board defects. Let us go through these methods one by one.

 

1. Manual visual inspection

This is the oldest visual traditional method to determine the failure of the circuit board using magnifying glass or calibrated microscope. Here, you can predict when the correction is required. The advantage of this method is the low cost. The disadvantages include high long-term costs, human subjective errors, difficulty in data collection, and discontinuous defect detection.

 

2. Online test

This electrical method detects manufacturing defects and test analog, digital, and mixed-signal components using needle bed tester and flying probe tester. The major advantage of this method includes low test cost of each board, fast and thorough short and open test, programming firmware, strong digital and functional test capabilities, high defect coverage, and easy programming. The disadvantages include the need to assess fixtures, programming and debugging time, the cost of making fixtures, and the difficulty of using them.

 

3. PCB board function test

This method tests the functional modules of the circuit board at the middle stage and the end of the production line using special test equipment to confirm the quality of the circuit board. There are two types of functional testing namely latest solid model and stacked testing. This method does not provide in-depth data such as foot-level and component-level diagnostics for process improvement and requires specialized equipment with a well-designed test process. Since it is difficult to write functional test programs, it’s not suitable for most circuit board production lines.

 

4. Automatic optical inspection

This method confirms manufacturing defects using a combination of image analysis, computer, and automatic control technologies. This test is done before the electrical testing to improve the pass rate of electrical processing or functional testing. Hence, the cost of correcting defects is much lower than the cost of the final test.

 

5. Laser detection system

 This latest method uses a laser beam to scan the printed board, collects all measurement data, and compares the actual measurement value with a preset pass limit value. Since this method works on light boards, it is also considered for assembly board testing at speeds sufficient for mass production lines. Its major advantages include fast output, no fixtures and visual unobstructed access. The disadvantages include high initial costs and maintenance issues.

 

6. Size detection

This method measures hole size, length and width, position and other dimensions accurately using two-dimensional image measurement since PCB is small, thin, and soft.

 

7. Automatic X-ray inspection

This method is used to detect defects such as ultra-fine pitch and ultra-high-density circuit boards and bridging, chip loss, and misalignment during the assembly process using high-definition X-rays (PCB X-ray machine). Using tomography technology, it can also detect IC chip internal defects. This is the only method to evaluate the soldering quality of ball grid arrays and shaded solder balls. Its major advantages include no fixture costs.

Hence, it’s mandatory to ensure the quality of PCB by inspecting them during the production process. Right inspection at the right time can maximize efficiency.

 

How to inspect PCB with X-Ray?

 

Principle of X-ray inspection

X-Ray is a stream of particles caused by the transition of electrons between two energy levels with energies between 0.1 and 1,000 kV. These are electromagnetic waves with a short wavelength of about 0.01 ~ 100 angstroms between ultraviolet and gamma rays. Since the German physicist W.K. Roentgen discovered the X-rays in 1895, it is also called as the Roentgen ray. The principle of X-rays involves the use of high-energy electrons and metal target collision. During the collision process due to the sudden deceleration of electrons, the loss of kinetic energy will be released in the form of an X-ray.

This X-ray inspection devices are composed of three elements:

· X-ray tube to generate X-ray

· A sample operation platform to position the sample so that it can be inspected from varied angles

· Detector to capture the X-ray through the sample and convert it into an image.

X-ray test is used to detect the sample that cannot be detected by the appearance of the position. X-ray penetrates the material of different densities due to the change in its light intensity. The resulting contrast effect can form an image to show the internal structure of the object to be measured. Hence, this X-ray test can detect the problem areas within the object to be tested without destroying its properties. It is good at detecting hidden defects such as opens, shorts, misalignments, missing electrical components, and so on.

 

Types of PCB defects inspected by X-ray

1. Inspect the defects in the IC package, such as layer peel, bursts, voids, and integrity testing of wire.

2. Detect the defects in the printed circuit board process, such as poor alignment or bridging, and open circuits.

3. Detection and measurement of voids in SMT solder joints.

4. Tests the possibility of open circuits, short circuits, defects, or improper connections in connection circuits of varied types.

5. Integrity testing of solder balls in solder array packages and cladding chip packages.

6. Detect cracks in higher density plastic material.

7. Measures chip size, marking and wire arc, and tin component area ratio.             

 

Methods of PCB X-ray inspection methods

 

X-ray detection equipment is used during the entire process of PCB assembly, at the beginning, middle and end of a batch start-up. This equipment has strong resolution, high magnification, and has brand-new and excellent detection effects. It can detect PCB components, lead-free components (especially BGA) production and assembly of internal quality problems. This equipment can also assess SMT, Flip-Chip, IC semiconductor, CSP connector, cable, pv module, battery, ceramic products and other electronic products.

There are two methods of X-ray inspection of PCBs:

· On-line inspection

· Off-line inspection

 

On-line

On-line X-ray system for PCB can interface with the PCB assembly line, and automatically detect the defects. It reduces manual inspection of the fine pitch components that cannot be fully inspected using other methods at the end. Hence, it is possible to reduce repair and rework costs. It can process large quantity of complex PCBs. But on-line X-ray inspection systems seems to be the slowest part in the assembly line, making fabrication line capacity low, and more time-consuming.

 

Off-line

Off-line X-ray system for PCB is used for debugging purpose. They can be operated offline to achieve sampling inspection and panel inspection. Offline tools are quick to operate and less cost-effective. It can inspect PCBs at any stage in the assembly.

 

Classification of X-ray Inspection Devices

 

X-ray Inspection Devices are mainly classified into two types:

· 2D (two dimensional) X-ray system for PCB

· 3D (three dimensional) X-ray system for PCB

There are varied factors such as quantity of the product, inspection quality, and the required time to complete the inspection step. Hence, we need to be careful in selecting the best system for this operation.

 

2D system

This X-ray system can display the two-dimensional images from both sides of the PCB at the same time by creating picture of its components. It can be operated online and offline. This method is similar to the traditional X-ray system, which was used to check bone fractures in the past.

There are two main types of X-ray tubes used in 2-D X-ray inspection systems

· Closed (or sealed) tubes

· Open (or demountable) tubes

 

3D system

This X-ray system can display the three-dimensional images of the PCB by building a series of 2D cross-sections. It can only be operated offline due to the high cost and the complications in its algorithms. This method is similar to the medical CT scanning. Hence, this system is used only in the research analysis.

3D system can also function using laminography method. It combines the image of certain cross-sections and eliminate the image from other cross sections, thereby rebuilding the image of a specific cross section. It can be operated both online and offline.

 

How to inspect PCB with X-ray?

 

Figure 1: Basic layout of PCB X-ray inspection system

 

Procedure

In this procedure, the X-ray source is placed below the lead-lined chamber, and a detector is positioned above (as seen in the figure 1). Here, digital image intensifier is used as a detector. This detector converts the X-rays it receives into visible images on a computer monitor.  A PCB is placed on a manipulator with the help of edge conveyer belts which lies between the source and the detector. As the X-rays pass through the PCB sample, the varied densities within the component reduce the rays by varied amounts, thereby resulting in darker and lighter areas on the final image. This X-ray system can completely inspect the various components on the PCB by changing the position of PCB. It is possible to magnify the specific area of interest by moving the board closer to the source. The greater the density difference, the more clearly the contrast can be seen on the X-ray image. For example, voids, or air bubbles, within BGA solder balls are comparatively less dense than the surrounding solder.

There are several specifications taken into consideration while inspecting PCB with X-ray system:

· The X-ray system is placed within a vacuum and monitored continuously by a penning gauge.

· The X-ray inspection system should allow only less than 30 seconds for the inspection of PCB.

· It has a microfocus electron gun intercepting a 15µm air-cooled tungsten foil target behind 0.5mm thick aluminum window.

· The X-rays are focused with a fan-angle of 14°. The cathode voltage can vary from 40 to 160 kV, and the maximum current is 500µA, emitting 20 watts target power at 40 kV.

· This system has 6mm lead shielded ply tunnels up to 1m long, on either side of the X-ray chamber to store the internal conveyer belts. The PCBs will be placed through a small gap, approximately 30mmhigh.

· Using a X-ray beam angle of 14°, the dimensions of the X-ray chamber, the absorption of lead, simulations of the scattered and secondary radiation within the chamber can be calculated. Thus, it is possible to determine the size of the entrance gap to ensure that X-ray safety regulations are met.

· The detection media contains 6-inch aluminium window with an intensifier coupled via a large aperture lens to a digital camera. This camera produces images of 758 x 580 pixels and 12-bit depth. It has an adjustable frame rate of 25 fps and less and is fully computer controlled.

· The nominal source to focus distance is 600mm. It allows magnifications up to 1200 times depending on the sample position.

 

Mechanical manipulation

Manipulator is used for turning and tilting the PCB in X, Y and Z planes of motion. It can be controlled using a joystick to inspect manually. It moves according to a pre-programmed script to conduct the overall inspection. To detect defects with higher magnification, a PCB sample should be in a distance of not more than 20 mm from the source. Tilting the PCBs can precisely reveal more defects.

However, compromise should be made between tilt angle and magnification. Tilting the PCB will increase the distance between the PCB and source, to prevent its collision with either the source or detector.

Figure 2a: Sample manipulation

Hence, the manipulation of sample in the x, y and z directions (see figure 2a) within the X-ray inspection system allow movement to different locations (x and y-plane movements) and change the height of the sample relative to the X-ray tube position (z-direction). This sample movement in the z-direction can also change the magnification. For example, moving the sample closer to the X-ray tube can increase the magnification (see figure 2b).

Figure 2b: Geometric magnification in an X-ray inspection system

This X-ray system for PCBs can provide a high level of grey scale sensitivity in the image capture device. Additionally, it provides oblique angle views of the sample, along with the top-down view. Without oblique angle views on one side of the X-ray board, the analysis of the X-ray system will be severely compromised. Ideally, oblique angle views should be available in the system without any loss of geometric magnification. As devices continue to shrink, lack of magnification will compromise the analytical efficacy.

But in the recent X-ray inspection systems, it is possible to provide oblique views without losing the available magnification. It’s enough to move the system image capture device, in some way, into an oblique angle relative to the plane of the X-ray tube and sample (see figure 3).

Figure 3: Tilting the sample to achieve an oblique view compared to moving the detector and the effect on the geometric magnification

Unlike traditional X-ray, the sample need not be moved away from the X-ray tube to prevent collisions. Hence, there will be no loss in available geometric magnification during oblique view. This X-ray system has can visually separate very small density differences in the samples in a precise manner. Hence, it is possible to detect more subtle variations/faults easily.

 

Let us discuss how to detect defects automatically using the PCB X-ray inspection:

 

Voids are the presence of air bubbles or other nonmetallic material trapped within the solder joint. It is the most common defect that takes place during the PCB production by either a fault in achieving the profile peak temperature and/or the X-ray board pending insufficient time above the liquidus temperature of the solder-paste alloy. The limit of the presence of voids is always questioned. However, the quantity of voiding depends on the sample, detection sensitivity, and specification of the X-ray inspection system being used. Hence, the presence of voids is the sensitive indicator of the overall production quality.

Double-sided reflow products, pads that have micro-vias, lead-free solder pastes, and surface finishes are considered as samples to detect voids. The term’ micro-void’ is often used to describe the voids within BGA [Ball Grid Arrays] solder balls with a diameter of around 1-10 microns (Figure 4).

Figure 4: Voids in BGA [Ball Grid Arrays] solder balls highlighted by color filtering of the X-ray image

There are two types of voids:

· Process or bulk’ voids

· Interfacial voids

 

Process or bulk’ voids

They are relatively larger in size. It can be positioned in the middle of the ball or associated with one of the solder ball interfaces (pad side or device side). Its size can be reduced by slightly increasing the time above liquidus during reflow for a few seconds. This step allows the full escape of the volatiles during the reflow process.

 

Interfacial voids

They are smaller in size than the process void. These voids can be associated with the solder ball interfaces, either to the pad or to the device.

The position of these voids can be detected by observing the solder ball at different oblique angle views within the X-ray system and check how the voids move relative to the rest of the ball and the pad and device interfaces.

Till the discovery of newer digital X-ray systems and detectors, it was difficult to see the interfacial voids. Only the larger process voids can be viewed.

 

The need for oblique angle views for BGA inspection

 

The interfacial areas can help in predicting the joint quality. Inspecting the solder bumps from directly above the sample, its mass can mask the subtle variations in the interfacial areas between the solder ball and the device, on one side. and the pad on the other side.

By viewing the joint at an oblique angle in the newer digital X-ray systems, at several different angles, it is possible to eliminate the interference from the solder ball. This helps in clearly visualizing the solder ball shape and the associated interfacial variations (including voids) if any.

Imagine a BGA solder ball joint to be like a soccer ball sandwiched between two flat pieces of wood (see figure 5). According to the figures 6 and 7 below, a top down view will limit the information compared with the oblique angle view. The larger the oblique angle, the better the separation between the bulk of the ball and the interface.

Figure 5: Schematic of BGA solder ball interfaces seen in X-ray inspection

Figure 6: How BGA solder balls appear under -ay inspection with a top down view and an oblique angled view

Figure 7: Oblique view X-ray image showing reflowed and non-reflowed open joints

Digital X-ray inspection, with the help of its greater grey scale sensitivity, it is possible to capture more information within the BGA solder balls. Its enhanced sensitivity highlights the differences between interfacial voiding and bulk voiding within the solder joint. This system can also distinguish the actual joint interfaces from the bulk of the solder ball and highlights the presence of non-reflowed, or open joints (see figure 8).

Figure 8: Oblique view X-ray image of BGA solder balls 

In the above figure 8, we can notice a missing joint interface (highlighted by the red arrow). The difference between good and bad can be clearly seen with the oblique view and the high grey scale sensitivity of the detector

 

Conclusion

 

Therefore, X-ray inspection service helps in detecting the defects of printed circuit board (PCB) automatically and precisely in detail. This inspection technique can be followed after reflow. X-ray inspection should always be conducted after rework of any area array devices or land grid ay components. By doing so, it will quickly and non-destructively confirm the rework quality and help in establishing the correct temperature or time profiles for device rework. Its precise results prove that this system can also improve the process control during the BGA component population of the PCB efficiently. Additionally, modifications can also be made to enable a conveyor add in. The automated control of its manipulator helps in allowing the in-line detection of defects in BGAs.