Using Computer Tomography for 3D Board Level X-Ray Inspection

A number of talented researchers have contributed to the development of the computerized tomography technique (CMT) as we currently know it. Non-destructive 2D X-ray inspection is an essential operation for electronics manufacturers for process development and control.

Figure 1 shows a 2D X-ray image of a two-sided PCBA where components overlap each other.

2D x-ray image

Figure 1. 2D x-ray image of a double-sided PCBA where the components from either side overlap each other in the view.

Certain clutter can be removed using oblique angle x-ray views of the sample. This is possible when the sample is tilted with respect to the x-ray tube to detector axis within the x-ray inspection system. This method does reduce the achievable magnification. Hence an alternate approach is offered by system manufacturers wherein the detector is tilted relative to the sample (Figure 2)

The sample remains close to the X-ray tube and retains the available magnification, which becomes highly important as the feature sizes within electronics continue shrinking.

x-ray system manipulator movements

Figure 2. Schematic of x-ray system manipulator movements enabling oblique angle views without compromising the available magnification.

Micro Computer Tomography

Micro computer tomography (µCT) analysis for electronic applications has been used for several years but has been limited for failure analysis and specialist applications. The µCT model represents a sample with a 3D density array that can be sliced and diced virtually in the computer to offer the needed analysis.

The constant increase in the computational processing speed along with the number-crunching power and ready availability of off-the-shelf Graphics Processor Units (GPUs) implies that realistic µCT reconstruction and analysis can be achieved in a few minutes or seconds.

Hence there are three CT techniques applicable to electronics issues:

  • Full µCT
  • In-line partial µCT
  • Off-line partial µCT

Full µCT

Full µCTis the method wherein a series of 2D x-ray images is taken around the sample and precise maintenance of the position and image geometry is maintained relative to the sample.

In electronics applications, the axis of the X-ray tube to the detector remains fixed and the sample rotation is done relative to this plane. (Figure 3). A dataset of 2D X-ray images is obtained from all angles around the sample. The time to obtain the original 2D x-ray images for full µCT is not trivial.

µCT system configuration

Figure 3. Schematic representation of a µCT system configuration where the axis of x-ray tube to detector remains fixed and the sample rotates perpendicular to this axis. The limitation on making a large sample get close to the x-ray tube and therefore limit magnification is also shown.

The most commonly used µCT algorithm to make the final µCT is known as the Feldkamp Cone Beam algorithm, which makes use of a filtered rear projection approach to create the 3D model.

There is a difference between medical and electronic µCT as electronics is needed to take high magnification 2D x-ray images so as to view the ever shrinking features. The more the magnification, the more should be the proximity of the sample to the tube. This is the reason for the sample rotation method being used in full µCT for electronics since maintaining sample proximity relative to the tube when the detector and tube rotate in the other method would be tricky to achieve.

CT model voxel

Figure 4. CT model voxel array showing how the sample is defined within the final ^CT model.

The final resolution in full µCT is determined by the original sample size, the field of view for CT scan and the ability of the image detector and the X-ray tube.

In-line Partial CT & Off-Line Partial µCT

While full µCT offers several benefits for failure analysis, it requires the board to be cut up and destroyed, making it a technique that will be used as a last resort for printed circuit board assemblers. Hence the possibility of separating different board layers and de-cluttering the 2D view for analysis is highly desired especially if there is no need to cut up the sample. This can be done using the partial µCT technique.

For both in-line and off-line methods, the method for undertaking PCT is the same. Several 2D images are taken surrounding the sample with the detector inclined with respect to the axis of the tube and the sample (Figure 5).

PCT system

Figure 5. Schematic showing an off-line PCT system configuration where the detector moves at an angle around the area of inspection to obtain the images from which the PCT reconstruction can be made.

2D x-ray image

Figure 6. 2D x-ray image (figure 1) and 2 off-line PCT slices showing the separation of the components on different sides of the board.

Below: slices at two different layers in the board created by Off- Line PCT

Off-line PCT generates excellent views into the plane of the board without any cutting required and is available in any place within the inspection area of the x-ray system. Off-line PCT also offers useful information in the other planes but now like in full µCT would offer since the dataset for the CT reconstruction has restricted information in the original 2D images compared to the data all around the sample gathered in full µCT.

In Table 1 shows a comparison of the relative merits of all three CT techniques

Table 1. Comparison of relative capabilities of CT techniques used for electronics inspection.

2D Planar View image Quality Full CT Off-Line PCT In-Line PCT
Z (into board) Excellent Very Good Good
X, Y (left to right and front to back through sample) Excellent Good to Acceptable Very Poor or Not Available
Other Planes Excellent Good to Acceptable Very Poor or Not Available or Shown
Limited sample size Yes No No
Cut sample Yes (unless very small) No No
3D Rendering of data? Yes Yes No

Example

The difference between full µCT and off-line PCT can be done using the example of a Head on Pillow (HOP) joint under a BGA.

Assuming that one can completely rotate the BGA in the full µCT system, then as images are taken all around the BGA then some of those 2D views will include images from the side of the device showing the gap in the separation of the head and the pillow of the joint. In full µCT the separation between the head and pillow is seen but there is a different bond shape in PCT probably similar to an 'hourglass' type shape.

Real Life Examples for PCT

A Package on Package (POP) is a device which uses vertical integration with the advantages of saving space and enhancing the electrical inter-connection characteristics. Vertical integration, unfortunately makes the conventional 2D x- ray inspection technique more difficult as features overlap each other automatically in the 2D view.

Head on Pillow Joints

Head on Pillow / Head in Pillow (HOP / HIP) defects occur commonly and are difficult to identify using AXI. 2D X-ray / oblique angle view systems are normally the choice for diagnosing HOP. A 2D-angled x-ray image is shown in Figure 7 showing a suspected HOP joint.

2D angled x-ray image of suspected HOP joint

Figure 7. 2D angled x-ray image of suspected HOP joint (red arrow).

Connectors

Determining how well a complex connector is soldered or engaged within the assembled product has always been a challenge. The full µCT option is also destructive and as the connector is mounted on a large board in a large assembly, the CT scan can only be done if the connector is removed from the board. Figure 8 shows how challenging it is to find out whether the connector actually engages using 2D X-ray even with oblique angle viewing. A complete smart phone assembly is shown.

Angled 2D x-ray view of a connector within a smart phone assembly

Figure 8. Angled 2D x-ray view of a connector within a smart phone assembly. The connector is in the area marked by the red box. Correct engagement cannot be verified even using multiple viewing angles and higher magnification.

In Figure 9, virtual cross sections of the same connector in Figure 8 are shown, produced by PCT, while keeping the whole smart phone assembly intact.

PCT virtual cross sections of a connector within a smart phone assembly

PCT virtual cross sections of a connector within a smart phone assembly

Figure 9. PCT virtual cross sections of a connector within a smart phone assembly. a) section in the horizontal plane, b) section in the vertical plane. Satisfactory engagement is verified in the areas shown by the yellow arrows.

Solder Bumps

While PCT was developed especially as a PCB inspection method, it is also a 3D non-destructive method for examining micro bumps on a wafer or package level. Wafer bumps become smaller and smaller with 20 to 30µm diameter bumps

Area Voids

Figure 10 shows a PCT model of a QFN device where 10a shows a section of the complete QFN device.

PCT 3D model of a QFN device

PCT 3D model of a QFN device

Figure 10. PCT 3D model of a QFN device. Extremely large void is shown going through the whole solder joint - suspected in overview (a) and confirmed in section view (b).

Precise locating of voiding positions is a known strength of the CT technique and nicely demonstrated here with this PCT case.

Conclusion

In this article, the different CT techniques available for electronics inspection including the offline partial µCT technique also called board level CT. This new technique helps in the creation of high-quality 3D CT models within electronic assemblies or large PCBs.

The PCT technique has been shown with several examples and real life defects. Performing non-destructive defect diagnosis in a fast and convincing manner through virtual cross sections of samples makes PCT a powerful alternative to mechanical cross sectioning.

About Nordson DAGE

Dage was founded in 1961 and is a market leader in its chosen markets of Semiconductor and PCBA Manufacture. It has an award winning portfolio of Bondtester and X-ray Inspection Systems for destructive and non-destructive mechanical testing and inspection of electronic components.

Dage was acquired by the Nordson Corporation in 2006.

Nordson DAGE has a strong portfolio of award winning products for destructive and non-destructive mechanical testing and inspection of electronic components. It has an excellent, wholly owned distribution and support network of seven offices covering Europe, Japan, China, Singapore, and the USA, and maintains representative offices in other territories.

With its self-contained R&D facilities, Nordson DAGE has developed world-leading products for testing wire bonds on semiconductor packages such as BGA, Chip Scale Packages (CSP) and other electronic components. More recently Nordson DAGE has been heavily involved in the testing of the newest technology 300 mm wafer bump shear.

Nordson DAGE has developed an excellent suite of award-winning X-ray inspection equipment targeted at both the Semiconductor and PCBA markets.

Control of the patented core technology of X-ray tube manufacture ensures that the high-resolution X- ray will detect, identify and measure even the smallest of features. Nordson DAGE's high precision, state-of-the-art inspection equipment when joined with their sophisticated software offerings, ensure that the equipment is simple to use.

Our core strategy:

  • To support our customers wherever they do business,
  • To continually develop new products that provide its customers with enhanced competitive advantages,
  • To develop products which bring new advantages to its customer base
  • To control the core technologies that underpin its products.

Nordson DAGE

This information has been sourced, reviewed and adapted from materials provided by Nordson DAGE.

For more information on this source, please visit Nordson DAGE.

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