One focus of this workshop was the contacting of the top side of the chip (top die connection). So far, the usual connection with aluminum bonding wires has reached its limits for future applications that need higher power densities. Robert Woehl from Danfoss Silicon Power discussed the use of copper bonding wires instead of aluminum. However, the existing metallizations on the top of the chip are not suitable for bonding with copper. To go one step further, Danfoss is not only working on bonding with copper wires but also with copper strips. The speaker looked at the reliability of bonding with copper strips. This increased by over 16x compared to the standard technology with aluminum wires.
To allow ultrasonic bonding with copper wires or ribbons, Andreas Hinrich from Heraeus Electronics presented the Die Top System (DTS), which is based on the Bond Buffer technology developed by Danfoss. A copper foil is sintered onto the chip, onto which in turn copper wires or ribbons can be bonded in order to contact the chip electrically. This sintered copper foil offers several advantages. Firstly, DTS protects the die from the high mechanical stress occurring during ultrasonic bonding with thick copper wires or ribbons. Secondly, it can spread the heat so that the maximum temperature on the power semiconductor is significantly reduced. The current density is also distributed more uniformly.
The lifetime increases significantly with the use of DTS and thick copper bonding wires. With indirect cooling without a base plate in the test layout showcased here, the IGBT's surface temperature drops by 10 K under a load of 136 A compared with soldered and aluminum wire-bonded assemblies. This also means that the service life increases by a factor of 67 (Fig. 3; ). When cooling directly with a system soldered on a base plate, it turns out that all test setups with DTS increase the lifetime according to the CIPS2018 model by a factor of 6 to 10, and that the setups without base plate can be expected to have a longer lifetime than those with a base plate. It is important to note that the setups with base plate had to (and could) tolerate almost 50 percent more current at 197 A to achieve the same temperature swing in the test.
Usually, wire bonding is done using ultrasound. As an alternative, Dr. Hans-Georg von Ribbeck from F&K Delvotec presented bonding using a laser beam. Thereby, not only punctual bonding connections can be realized, but by means of an oscillating laser also planar connections with aluminum or copper ribbons for high currents can be realized. Using this technique, not only stiff contacts can be bonded, but also flexible ones, since laser bonding does not require high contact pressure or special stiffness of the product. In addition, the secondary connections - those from the substrate to the module terminals - can also be realized with this technology. If only little space is available for bonding connections, the other connections can also be stacked on top of each other by this technique (Fig. 4; ).
Peter Beckedahl from Semikron compared two double-sided sintered module technologies – SKiN and DPD (Direct Pressed Die). In both processes, the chip is contacted on the top side with a flexible circuit board sintered onto it. The difference: in DPD, the chip is additionally pressed onto the substrate with an elastic bump made of silicone, and the substrate is not sintered onto a base plate. Instead, a thin layer of heat-conducting paste (approx. 5 µm) is sufficient, so that no voids are left under the chips. Since the substrate and base plate are not rigidly attached to each other, they do not bend due to different coefficients of expansion (bimetal effect), and thus larger substrates are possible. In a direct comparison of SKiN and DPD technology, the thermal resistance from the junction to the environment differs by only four percentage points.
At the end of the first day, Vincent Bley from Laplace Laboratory concluded with the topic "The Interconnection for Power Module 3.0" and looked at future connection technologies. He presented three research topics: Direct copper-to-copper bonding, copper nanoposts and conductive foams. The latter can be used to electrically contact semiconductors embedded in a printed circuit board.