Mechanical Stress Reducing Heat Sinks for High-Power Diode Lasers

摘要

During the last years high power diode lasers have become increasingly established for direct material processing. The advantages are the high efficiency (more than 50%) and long lifetime of more than 10.000h. An important factor believed to be responsible for the aging of diode lasers is the thermo-mechanical stress. High stress levels arise from the packaging process. The mismatch between the thermal expansion coefficient of the heat sink (typically copper 16.5x10-6 K-1) and the laserbar (GaAs 6.7x10-6 K-1) cause high mechanical stress. The change in length during the cooling process of a 10mm wide laserbar is more than 10μm. If a hard solder is used, the stress is much higher, because hard solder typically has a higher melting point and stress can not be reduced by relaxation. Typically material with lower thermal expansion coefficient have a lower thermal conductivity than copper. This increases the thermal load of the laserbar, which decreases the life-time in this sense. The expansion-matching and the lower thermal conductivity of this material are working against each other. In order to find a good compromise, different active cooled expansion matched heat sinks are simulated. Very promising heat sinks have been fabricated and characterized. Also the solder selection has influence on the long term stability. A very soft solder is more critical in terms of long term stability. A higher diffusion takes place, so that the properties of the solder change during the lifetime of the diode-laser. Hard solder, especially AuSn, are well tested solders with a very high long term stability, (no changes of the intermetallic structure even at higher temperature) The disadvantage of the hard solder is the incapability to reduce the mechanical stress through relaxation. Different solders are being used and investigated.