Ever been to see a doctor for an X-ray and/or a CT scan? This is a common process that allows medical professionals to build a complete picture for diagnosis.
In the electronics world we have similar X-ray techniques for non-destructive failure analysis. A quick 2D X-ray inspection provides a good insight into signs of defective connections and a 3D CT (computer tomography) scan can verify the problem with the added benefit of more detailed data, building the complete picture ready for diagnosis.
BGA (ball grid array) packages are common devices that are inspected non-destructively utilising X-rays. Each solder joint is an individual electrical connection – a good joint will be reasonably circular in shape and the size of the joints should be reasonably consistent throughout the device array. Figure 1 shows several different types of defects which may occur during the BGA assembly reflow process.
2D X-ray inspection is a great tool for quick analysis and one type of analysis that can be easily performed is a ball diameter measurement. In Figure 2, it can be seen that the ball at the bottom right is approximately 70 µm smaller in size compared to its neighbouring joints and is highlighted with a red outline. This is the first sign that there is a potential problem with this connection.
The image in Figure 3 is a 2,5D oblique-angle image of these joints. This image builds our diagnosis further, as we now see another sign that the joint in the centre is not completely fused on one side. This is seen through a different greyscale displayed on the interface of this joint and is highlighted with a red arrow.
Having identified strong signs that there is potentially a defective connection, this can be verified further with a 3D CT scan. CT is a technique which captures several images from different angles and then uses reconstruction algorithms to create a 3D model. The results of this can be seen in Figure 4.
It can clearly be seen that there is a difference in the ball shape highlighted with red arrows compared with surrounding balls. The image to the top right shows the quality of ball wetting to the device interface. It is clearly seen that there is an interfacial difference compared to neighbouring joints (note the top surface of the balls, shaded in red).
Now that both 2D and 3D images have been analysed, there is enough data to build a complete picture and diagnose this BGA device. The solder joint is confirmed as ‘non-wet open’ and the result would be a significant detrimental impact on its integrity and ultimately the integrity of the BGA device and the product it ends up in.
So, whether you are a doctor or are responsible for manufacturing quality, combining 2D and 3D X-ray images makes performing a diagnosis much easier.
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