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www.us- tech.com


May, 2020


Automated X-Ray Inspection for Power Electronics


By Astrid Sassen, Head of Marketing, Viscom AG M


anufacturing power electron- ics is efficient and highly automated. However, until


now, there have been restrictions during final inspection. X-ray inspec- tion of concealed solder joints and enclosed power electronics has not been conducive to fully automatic inline inspection. But, improvements in inspection object handling, as well as in X-ray technology are making this possible. The advances in power electron-


ics have been immense. Switching semiconductor elements, such as bipolar power transistors, power MOSFETs, thyristors, IGBTs, triacs, and power diodes, ensure efficient conversion of electrical energy. Power density is continuously increasing on ever smaller surfaces. The elimination of mechanical switching procedures has made semi- conductors extremely robust and suitable for applications in the renewable energy and electromobili- ty sectors. For example, they are used in DC-DC converters, inverters and frequency converters, as well as for control and loading electronics in battery modules. These developments are creat-


ing new challenges in production and quality control. “Systems with semi- conductor transistors that switch outputs of 100 kW and more are con- fronted with tough requirements for electrical and thermal connections,” says Axel Eschenburg, head of prod- uct development — new products division, Viscom. “The quality of these connections is crucial for heat dissipation, and thus the whole switchable output. This poses new challenges for the final inspection.” Eschenburg cites the inspection


of flat-soldered components with con- cealed solder joints, which can no longer be optically inspected. In addi- tion, the installation of components and assemblies in cooling elements and in housings often results in large, heavy inspection objects. Optical inspections and electrical


function tests are no longer good enough.


Voids in Power Electronics Bubble-shaped gas inclusions in


solder joints, known as voids, pose a


results of electrical function tests may even be error-free, though the solder joints are not stable over the long term and product reliability is at risk from voids. Only 3D X-ray inspection can


the process. This enables AXI with shorter exposure times and faster analyses, even for high-quality 3D reconstructions. The inspection of power elec-


tronics is done in a closed-loop X-ray system, which places additional requirements on the handling of heavy, solid and large assemblies in particular. The PCB is often inte- grated in a complex product, such as a motor control device. Connectors for external electrical connections are built into these components. Poorly executed or incomplete solder joints rapidly lead to malfunctions during operation. This requires X-ray inspections for larger devices as well, as other inspection methods are not suitable.


Simplified System Integration “We have now developed our X-


Viscom plans to add AI support to its verification stations in the future.


major challenge for components in power electronics. Particularly in power electronics, such gas inclu- sions act as a problematic isolator, both electrically and thermally. Too many voids disrupt heat dissipation and can reduce solder joint stability, which can lead to impairments in performance and damage to the com- ponents. This can affect the robust- ness and reliability of the assemblies over the entire service life of the product. Wired components, such as con-


nectors, larger switches, electrolytic capacitors, power coils, and power semiconductors, are frequently attached using through-hole technol- ogy. Here, voids or insufficient filling levels reduce the mechanical and electrical reliability of the compo- nents. AOI cannot detect voids. The


reveal the position and distribution of voids. Eschenburg emphasizes that voids in the marginal area of a solder joint may not cause any prob- lems, but voids in the critical points where the most heat is generated can pose a significant risk. X-ray inspection has been a reli-


able method of examining solder joints for many years. According to Eschenburg, the technology has sig- nificantly improved in two areas. “In the meantime, high-resolution and low-noise X-ray detectors are also available in standard devices, which Viscom uses as flat panel detectors,” he says. The second area of progress, Eschenburg mentions, is improved computer analysis, which can now be integrated as standard technology. These computers use the GPU


and the CPU to execute calculations in parallel, considerably accelerating


ray systems to the point that we can carry out inline X-ray inspection even for large, heavy objects auto- matically and within normal cycle times,” says Eschenburg. Viscom has designed a loading system for trans- port systems that use workpiece car- riers for its new X8068 SL X-ray inspection system. The loading sys- tem automatically transports objects


Distribution of voids in the flat-soldered areas of a QFN component.


weighing up to 33 lb (15 kg) with the workpiece carriers. “We were looking for a standardized solution for non-


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