Figure 1. Close-up of ultrasonic copper terminal welds on IGBT substrate. (Image: Emerson). Close-up of ultrasonic copper terminal welds on IGBT substrate. (Image: Emerson).
Found in the power modules of electric vehicle (EV) power supplies, insulated gate bipolar transistors (IGBTs) are integral to fully electric and hybrid vehicle operation. These three-terminal power semiconductors provide the high-speed, high-voltage switching required in the inverter system.
The inverter system converts direct current (dc) power from the battery into alternating current (ac) power to drive an electric motor. The inverter relies on IGBTs to make rapid adjustments in the frequency of the ac power that controls the speed at which the vehicle motors rotate and the EV travels.
IGBTs also provide high-frequency motor-speed control in various industrial and consumer products that rely on variable-speed motors and drives.
The continuing challenge for IGBT manufacturers is increasing power density within inverter modules while improving power efficiency by reducing conduction and switching losses. Efficient designs that limit inverter power losses provide highly reliable vehicle speed control and increased battery capacity — resulting in greater vehicle range.
To switch high voltages at high frequencies, IGBT power modules rely on solid design and construction, several materials, and precision assembly processes. One of these processes is ultrasonic metal welding. This clean, reliable joining method uses high-frequency, low-amplitude vibration plus compression to gently and precisely weld metal parts at low temperatures.
Ultrasonic metal welding is preferable to resistance welding because it completes welds without melting the metals or risking heat-related damage to nearby structures. It’s also favored over soldering because it eliminates the need for soldering fluxes or oils, resulting in a more environmentally friendly process.
The relatively gentle, low-temperature bonds provided by ultrasonic metal welding are important given the structure of IGBT modules, which typically employ an outer cover of plastic with a relatively brittle ceramic material as a base material.
Figure 2. A uniquely engineered “long-reach” ultrasonic welding sonotrode (horn) reaches into the IGBT module housing to complete concentric welds of long, round pins, which are linked to the inverter’s electronic controls. (Image: Emerson)
Ultrasonic metal welding uses specialized tooling, in the form of an acoustic sonotrode or “horn” to accomplish two types of welds critical to IGBT module performance.
The first critical weld bonds current-carrying copper terminals to the inverter circuitry housed in the IGBT substrate (Figure 1), where the dc-to-ac power conversion occurs.
Figure 3. The Branson GMX-Micro ultrasonic welder, with a traditional stack. Branson also offers the GMX-Micro D ultrasonic welder with a direct-press stack. (Image: Emerson)
The second type of weld, which bonds long, narrow pins to the substrate, is far more specialized and challenging to complete. These pins protrude upward from the module substrate and out through the housing, linking the inverter circuitry to logic and communications from the digital inverter control, which is mounted above. A round weld is required where the pin base touches the module substrate.
However, because the module substrate is housed in a plastic frame and there are fragile structures nearby, a unique “long reach” welding tool is needed. It consists of a needle-like, “long reach” horn that reaches down into the narrow cavity of a power module to accomplish the weld (Figure 2).
The horn uses a special acoustic design that drives the ultrasonic vibration concentrically, evenly distributing weld energy around the base of the pin to ensure an even, circular bond with the substrate.
As the IGBTs that power EVs continue to advance, assembly technologies such as ultrasonic metal welding must evolve to meet new design, size, and power requirements.
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