Authors: Keith Bryant and Thorsten Rother Company: SMT Solutions and Yxlon International Date Published: 1/25/2016
Pan Pacific Symposium
Abstract: An insulated-gate bipolar transistor (IGBT) is a threeterminal power semiconductor device primarily used as an electronic switch which, as it was developed, came to combine high efficiency and fast switching. It switches electric power in many modern appliances: variablefrequency drives (VFDs), electric cars, trains, variable speed refrigerators, lamp ballasts, air-conditioners and even stereo systems with switching amplifiers. Since it is designed to turn on and off rapidly, amplifiers that use it often synthesize complex waveforms with pulse-width modulation and low-pass filters. In switching applications modern devices feature pulse repetition rates well into the ultrasonic range—frequencies which are at least ten times the highest audio frequency handled by the device when used as an analog audio amplifier. The IGBT combines the simple gate-drive characteristics of MOSFETs with the high-current and low-saturationvoltage capability of bipolar transistors. The IGBT combines an isolated-gate FET for the control input and a bipolar power transistor as a switch in a single device. The IGBT is used in medium- to high-power applications like switched-mode power supplies, traction motor control and induction heating. Large IGBT modules typically consist of many devices in parallel and can have very high current-handling capabilities in the order of hundreds of amperes with blocking voltages of 6000 V. These IGBTs can control loads of hundreds of kilowatts. 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. This paper will investigate these voiding issues and 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 and therefore increase long term reliability of these often expensive to replace parts. Not only the unit cost, but the location and also the costs incurred until the unit can be replaced. These units are often in remote or critical situations and failure causes huge cost and warranty issues.