Over the last 10 years, 2D X-ray inspection technology has advanced significantly in capability. This technology has become an essential tool within the test regime of the electronics industry. This is because it provides a way for assessing hidden solder joints and makes it possible to view the interior of microelectronics packages in a non-destructive manner.
Mechanical cross-sectioning is another technique which is extensively used for development and failure analysis purposes. However, this technique has a major disadvantage and involves cutting the expensive printed circuit board (PCB).
2D X-ray Inspection Systems
Present generation of 2D X-ray inspection systems include powerful and sharp X-ray sources with submicron feature recognition down to 0.1µm or 100nm. In addition to maintaining submicron resolution these systems provide high X-ray power/flux of up to 10W at the X-ray target. Moreover, significant developments have been made in X-ray detectors, with advanced image intensifier and flat panel types. These advancements contribute to high resolution and bandwidth X-ray images, with improved automation and speed capabilities. The end result is more enhanced defect detection capabilities. Figure 1 displays a high resolution 2D X-ray image, showing a blown Au wire taken on an advanced X-ray instrument.
Figure 1. Extremely high resolution 2D X-ray image showing blown Au bond wire.
However, in spite of these exceptional performance achievements, 2D X-ray has not become a standard in electronics design, development and production. This is because the 2D X-ray image becomes difficult for multi layered assemblies and devices, as X-rays penetrate through the entire object. Therefore, multi-layered devices make it difficult for operators when trying to study the images. A 2D X-ray image of a stacked die device is shown in Figure 2, where it is very difficult to study the multi-level bond wires structure using the 2D X-ray image alone.
Figure 2. 2D X-ray image of a stacked device. Due to image complexity it is difficult to check for shorted bond wires.
To that end, the 3D X-ray computer tomography (|CT) is being used in the microelectronics industry to deal with projects like this one. This has been facilitated by the remarkable developments in the 2D X-ray technology together with the continuous developments in the computer technology.
Figure 3. 3D µCT e-sections of stacked device. The bond wires are easily examined by changing the e-section plane orientation.
The 3D µCT method can create virtual cross sections or e-sections in a non-destructive fashion, at any plane of the device. A couple of µCT e-sections of a similar stacked device are shown in Figure 3. The 3D µCT model is created at a number of stages. The first one is to obtain a series of high-resolution 2D X-ray images at various angles around the sample and track the geometrical positioning at the highest possible precision level. Figure 4 shows a standard µCT set-up utilized in the electronics industry.
Figure 4. Typical µCT set-up used in the electronics industry.
First, the sample is suspended between the detector and X-ray source and 2D images are obtained at various angles by rotating the sample. Then, the set of 2D X-ray images is processed by means of mathematical algorithms during a step called CT reconstruction. The end result is the µCT model, which represents the sample inside a three-dimensional density array. This array can be virtually diced and sliced within a dedicated computer viewer to provide the preferred e-section analysis. This reconstruction step was achieved in a matter of minutes.
Large Board CT
In order to address the above limitation of the µCT, a technique called Large Board CT, Partial CT (PCT) , or limited angle CT has been developed. Figure 5 shows the fundamental principle of this technique, which involves keeping the sample flat and close to the X-ray source, so that the inspection system can create high resolution 2D X-ray images. The detector is shifted around the object at an oblique angle.
Figure 5. Basic principle of Large Board CT also called limited angle CT or PCT.
Large Board CT Evaluation of Interfacial BGA Voiding and Comparison with 2D Results
Calculation of interfacial voiding percentage is a quality assurance process that is followed within the testing regime of PCBs and microelectronics. Figure 6 depicts a 2D X-ray image that shows the voiding inside a corner section of a BGA device. The large void seen in the middle joint (orange arrow) is certainly a problem as it is way above the 25% criteria set by IPC-A-610. The smaller voids (red arrow) could also be a problem for a reflow process, in spite of the fact that they might pass the IPC-A-610 criteria. These voids are mainly concentrated at the joint interface, making the joint prone to interface failures in the field.
Figure 6. Oblique 2D X-ray image showing volume and interfacial voiding.
The Large Board CT enables the operator to select the location of the slice (e-section) where the voiding calculation is carried out. This way, precise data for the interfacial voiding percentage can be obtained which is otherwise not available using just 2D X-ray analysis.
Comparison was then made between the total voiding percentage as per 2D imaging data versus interfacial voiding percentage as measured using Large Board CT data. The aim was to ascertain the correlation level and find out whether the calculation of the total voiding percentage as per IPC-A-610 gives a sufficient representation of the interfacial voiding.
Figure 7. 2D voiding calculation of a BGA device as per IPC-A-610.
Figure 8. Voiding calculation performed using Large Bard CT e-section.
Figure 7 shows a standard void calculation obtained at a 2D top-down view as per IPC-A-610 of a corner area of a BGA device, and Figure 8 depicts a voiding calculation of the same area, but carried out at an e-section located at the BGA joints interface. It can be seen from the images that the voiding calculations using the two techniques give entirely different results.
CAD-driven Automation for Irregular BGA Devices and Bumps
For manufacturers of PCB and microelectronics, it is important to achieve high levels of automation during the X-ray inspection. The X-ray inspection system can easily handle standard bumped devices when setting up an automation inspection routine. The software detects the diameters, pitch and locations of the BGA bumps and compares these to a database comprising standard patterns and sizes. On the other hand, Irregular patterns of different shapes are becoming popular with the microelectronics manufacturers (Figure 9).
Figure 9. Wafer piece with an irregular bump pattern.
The automatic algorithms within the X-ray inspection system anticipate a standard, regular pattern and could have a problem when tackling with the irregular patterns. In this case, the regular pattern algorithm was able to detect the balls, but there is still some mismatch within the pitch as the software is trying to accommodate this irregular pattern to a regular one available in the database.
Given that these irregular patterns are not standardized, the best approach is to utilize CAD data to "teach" the inspection locations within the inspection practice. Two approaches are available here. The first approach is to utilize the CAD data offered by the electronics manufacturer, while the second one is to create our own CAD data within the software that is unique for the specific irregular pattern device. The latter approach is flexible and results in reliable CAD files, which reflect only the required data for the automatic X-ray inspection.
This article discusses the challenges encountered in the advanced X-ray inspection owing to miniaturization and the need of automation. The Large Board CT technique was used to analyze the interfacial voiding of BGA devices. This technique helps in creating virtual cross sections in a non-destructive fashion, at any plane of the devices. it was found that the total voiding calculation utilizing 2D X-ray images as per IPC-A-610 shows a poor correlation with the voiding calculations at the joint interfaces measured using e-sections. This shows that the 2D X-ray total voiding calculation fails to give a suitable representation of the joint interface condition. Hence, Large Board CT provides the best way to assess the interfacial area in a non-destructive manner. This article also describes the simple CAD generation methodology, which facilitates and streamlines the automation of X-ray inspection of irregular bumped devices.
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.
This information has been sourced, reviewed and adapted from materials provided by Nordson DAGE.
For more information on this source, please visit Nordson DAGE.