Inclined CT, the best inspection solution for 3-terminal power semiconductor devices

July 10, 2017 | Keith Bryant

An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch developed to combine high efficiency and fast switching.

An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch developed to combine high efficiency and fast switching. It switches electric power in many modern appliances: variable-frequency drives (VFDs), electric cars, trains, variable speed refrigerators, wind turbines, lamp ballasts, air-conditioners, rice cookers and even stereo systems with switching amplifiers.

These devices are highly complex, expensive and place huge demands on their component parts. The majority of the failure mechanisms in IGBT units in the field are traceable back to excessive internal temperatures and/or thermal stress. It is well proven and accepted that voiding at critical interfaces is the major contributing factor to these failures. 

We will explore X-ray technology as a non-destructive and non-damaging method of inspection during and immediately after production. The aim is to improve the process by reducing voiding, which will increase the long term reliability of these often expensive to replace parts. Not only the unit cost but the location and also the expenses incurred until the unit can be replaced. These units are often in remote or critical situations, and failure causes huge cost and warranty issues. 

Home appliances increasingly require inverter-based motor drives, which provide better performance, comfort and efficiency: all “musts” for high-end products. Consumers are also using more advanced home solutions, like induction-based plates for rice cookers. These new applications will contribute to IGBT growth in consumer applications, but put increasing price pressure on the manufacturing cycle. Reducing voiding is critical to the on-going success of this technology. 

The purpose of the IGBT module's layered design is to create an efficient low thermal resistance pathway from the circuitry atop the die to the bottom side of the heat sink where the heat is carried away. Including the die, heat in this typical design will travel through five layers of material. The ceramic substrate and the heat sink are chosen in part for their low resistance to the propagation of thermal energy, as are the Thermal Interface Materials (TIMs).

As can be seen from the figure above there are many dissimilar materials and stressed junctions in these assemblies, add voiding into this, and you have a recipe for many failures. Most of these failures happen in the field. 

Little thermal energy crosses a gap (void). Instead, it is redirected back toward the die. In gap-free regions, conduction and radiation continue across the material inter­ faces until, at the bottom of the heat sink, the heat is carried away from the module. The greater the collective x-y area of the gaps, the more thermal energy is aimed back at the die, and the higher the risk of IGBT failure. The image below demonstrates this very clearly: 

Some companies resorted to modeling to predict failure, but voiding cannot be factored into these equations easily as it is not a constant so this method is unsatisfactory. Traditional X-ray struggles to show this voiding well due to the number of thermal interfaces and the very thick copper heatsinks found on IGBT assemblies, which can be seen in the image below.

In addition to this, a 3D model of the device or part of it can be easily and quickly rebuilt and sliced through to investigate key areas.

Want to learn more about preventing failue in IGBTs? Read the full whitepaper on this topic, a free PDF download.

"Failure Mechanisms in IGBTs Related to Voiding"


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