Juergen Schulz-Harder

Recent developments of direct bonded copper (DBC) substrates for power modules

DBC substrates are the standard circuit boards for power modules. Using DBC technology, thick copper foils (0.125 mm-0.7 mm) are cladded to alumina or aluminum nitride. The strong adhesion of the copper to ceramic bond reduces the thermal expansion coefficient (TEC) in the horizontal direction only slightly above the TEC of the ceramic itself. This allows direct silicon attach of large dies without using TEC controlling layers. As DBC technology is using copper foils, integral leads overhanging the ceramic can be realized. A new via technology, combined with integrated leads, allows the design of low weight hermetic packages with improved thermal performance. High end multichip modules with extremely low thermal resistance (<0.03 K/W) can be achieved by the integration of 3D micro channels for liquid cooling underneath the power circuit area. A new type of alumina DBC with a flexural strength of >1000 MPa and excellent temperature cycling resistance has been developed.

New heat exchanger concept for high-power diode laser systems

In order to achieve a thermally stable diode laser system based on high power diode laser bars, micro channel heat sinks are used to face the dissipated power with a density of 106 W/m2. Passively cooled diode lasers are either lower in power or facing higher junction temperatures. As a matter of principle the cooling with micro channel heat sinks requires a sealing between the heat sink itself and the system around. The leakage of this sealing, normally achieved by O-rings, can be reduced but never avoided. Sensible systems and extreme lifetime requirements, like in the telecom applications, already require passively cooled diode lasers with no water in the inner system boundaries. To achieve a minimized temperature shift in the junction, we developed a new copper based heat sink, spreading the dissipated heat in an optimised manner. Based on this, our further research shows that the higher temperature shift in a passive submount compared with active ones can be tolerated for a system, if the heat resistance to the external water heat exchanger is minimized. For applications either with or without the requirement of a thermo electric cooling element (TEC), we developed a technical solution for a heat exchanger, to keep water out of the inner system boundaries. The thermal resistance is low enough to run up to 12 passively cooled diode lasers on an regular ambient temperature and a minimum of junction temperature mismatch.