Modern electronic – and even electrical – subassemblies are complex devices that can fail for many reasons, either during manufacturing or in the field. Some failure modes, such as delamination, voiding or solder bridging, are mechanical, yet they cannot be easily seen as they occur internally. Breaking the subassembly open if sealed is unattractive, especially if the action destroys the evidence of the fault.
Accordingly, a non-destructive way of viewing the subassembly’s internal construction and components is needed; one that supplies images of sufficient resolution to clearly show where the fault lies.
In fact, two complementary approaches are available: X-ray inspection and Confocal Scanning Acoustic Microscopy or CSAM. Broadly, we can say that CSAM reveals air gaps, voids, and delamination in materials that X-Ray cannot see so, for example, delamination in a circuit board would be invisible to X-Ray but easily seen in CSAM. Conversely, voids in a BGA ball would be difficult to visualise with CSAM but easy with X-Ray.
Below, we look at each approach in more detail.
CSAM
CSAM techniques detect material-weakening flaws such as surface cracks, voids, delaminations and internal porosity better than any other inspection method. CSAM systems non-destructively inspect materials layer by layer, delivering accurate and comprehensive results for failure analysis, strength, durability and reliability testing, and other insights.
SAM uses ultrasound waves to detect changes in acoustic impedances. Pulses of different frequencies are used to penetrate various materials to examine sample interiors for voids or delamination. At interfaces between materials having different acoustic impedances, an acoustic reflection (an echo) occurs. The intensity and polarity of this echo is recorded and presented as a colour map of the sample.
One application that highlights the benefits of CSAM inspection involves SiC and GaN semiconductor assemblies used to switch hundreds of kilowatts of power for high-voltage electric vehicle batteries. These assemblies comprise devices mounted on substrates and interconnected through copper conductors that must be very heavy gauge to manage the required power levels. The associated die bonding process can produce voids; these can compromise thermal properties and result in early failures.
While X-ray technology may normally be used, it would be challenged in this case; the X-rays cannot penetrate to the required depth within the copper conductors. By contrast, a CSAM system with its acoustic imaging can penetrate sufficiently within the copper conductors – air gaps and voids reveal themselves clearly.
It also runs advanced operating software, with intuitive operator interface menus which help maximize results, while saving operator time.
Nordson Dage GEN7 CSAM inspection system
X-ray inspection
Whereas acoustic imaging works by collecting reflected sound waves, X-ray images are created by shadow imaging instead of reflection. All material features are shown at once. Rounded objects that would scatter acoustic waves can be imaged in detail. X-ray imaging can reveal wire breakages, for example in BGA or other IC packages, transistors, and diodes, as well as displacement or destruction of small internal parts, that would be invisible to CSAM.
In practice, this means that X-ray inspection systems allow quick and easy discovery of soldering defects such as open circuits, solder bridges and shorts – including BGA and other area array package shorts and open circuit connections – as well as insufficient or excess solder, solder voids, and solder quality. It can also identify component defects including bond wire attachment quality, lifted leads, missing, misaligned, misplaced, or faulty components, or incorrect component values[i].
There are different types of X-ray inspection systems, from the ubiquitous 2D types to 3D tomosynthesis and computed tomography (CT) machines.
In a 2D system, X-rays produced by an X-ray tube pass through the sample under investigation and into an X-ray detector. This converts the X-rays into visible images that the operator inspects. Any object or material of higher density than its surroundings absorbs more X-rays and casts a darker shadow on the detector. This is very effective for imaging electronics non-destructively since solder locations, device terminations and copper tracks cast different, darker shadows than a laminated PCB circuit board.
The greater the density difference between materials, the more clearly the contrast can be seen on the X-ray image. For example, voids or air bubbles within BGA solder balls are much less dense than the surrounding solder and can easily be seen in tin/lead or lead-free solders.
Computed tomography (CT) is a popular technique for creating 3D models from multiple 2D X-ray images taken at different angles around the object. The term µCT is often used when looking at electronic samples since it can give spatial information on features approaching micrometre sizes. In a typical system a sample is loaded into a motorised mount which rotates through 360 degrees. X-ray images are taken at regular intervals and powerful reconstruction software uses a technique called back projection to create a 3D model of the sample under inspection. However, for larger samples, the region of interest must first be removed from the board into a smaller format so that it can be rotated close to the tube to create a µm resolution model. In many cases this technique cannot be used non-destructively.
Tomosynthesis is a variation of µCT where the sample remains in one position on a sample tray and the detector orbits through 360 degrees around its top. The benefit is that the sample can be positioned very close to the X-ray tube for high magnification images and does not need to be cut in any way since it is no longer rotated.
The trade off with this geometry is that only relatively shallow models can be created since the tomosynthesis arrangement yields less height information, however since most PCB samples are relatively thin this is not usually a practical limitation.
The Nordson Dage QUADRA 7 is an advanced X-ray inspection machine which provides all the X-ray modes; it has built in tomosynthesis with a quick change to CT. It supports high resolution imaging with a 4k image of 100nm resolution.
The QUADRA 7’s industry leading Aspire FP detector captures 6.7 MP images at 30 fps. These images are shown on two 4K UHD monitors, as conventional HD monitors lack the resolution needed to show them.
High resolution, high magnification CT image slices can be captured quickly and easily from anywhere on a board or subassembly, using Nordson Dage’s X-Plane technology. Hard to see defects such as interfacial voids can be spotted quickly.
Accessories include a heated stage which recreates reflow oven conditions, so solder processes can be watched in real time.
Nordson Dage QUADRA 7 X-ray system and failure analysis tool
Conclusion
From the above, we can see that X-ray and CSAM are somewhat overlapping yet also complementary technologies. While purchasing a system of each type may appear as a comprehensive and attractive solution, it could be hard to justify for many applications.
One alternative is to use an inspection and test house like Cupio. This will give you access to either technology when you need it, without having to invest in capital equipment. It also gives you access to their expertise, as they can advise on which technology should be used, and how it should be set up. They also work closely with the system manufacturers, so they always have the latest technology in terms of updates and new equipment.
[i] https://www.electronics-notes.com/articles/test-methods/automatic-automated-test-ate/axi-x-ray-inspection.php
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