Thermal and mechanical optimisation of diode laser bar packaging

Aachen / Publikationsserver der RWTH Aachen University (2007) [Dissertation / PhD Thesis]

Page(s): III, 124 S. : Ill., graph. Darst.

Abstract

Increasing the reliability of high power diode laser bars as a viable photon source in pumping and direct applications is necessary as increased reliability reduces costs and makes diode lasers more attractive for use. High stress levels and high thermal loads are the limiting factors for the lifetime of diode lasers. Large amounts of stress can be caused by the packaging process due to the expansion mismatch of the semiconductor and heat sink materials. Ductile solder is used to compensate for the mismatch. By using an expansion matched submount, a brittle solder such as gold-tin can be used. At the same time, due to the lower thermal conductivity of this expansion matched material, the thermal load increases. Here, the effects of expansion matching work against the reduction of the thermal load. For higher reliability of diode lasers, the packaging induced stress and thermal load need to be reduced.The thermal and mechanical influence of the packaging process on diode laser bars is described. One method to reduce mechanical strain is to use expansion matched heat sinks. To demonstrate this, expansion matched packages using two different types of actively cooled expansion matched heat sinks were developed. The integrated design has the advantage of having low thermal resistance. As the simulated as well as the experimental results show, the expansion mismatch of the expansion matched heat sink is nearly 1 ppm/K, instead of the 10 ppm/K mismatch seen in a conventional copper package. The thermal resistance is 0.55 K/W compared to 0.45 KW seen in the reference copper heat sink. The expansion matched heat sink increases the thermal load when compared with copper heat sinks. Stress measurements show that the stress is also reduced by a factor of 2.5. These results lead to an increase in the lifetime of diode lasers mounted on these expansion matched heat sinks compared to the reference bars packaged on copper heat sinks. The FEM-simulations have been confirmed as accurate, so that this simulation tool can be used to design new types of expansion matched heat sinks with advanced materials such as diamond composites. Another method utilised to reduce mechanical stress, which is also used in standard packages with copper heat sinks, is to optimise the solder layer. The soft and ductile indium solder has the task of reducing the packaging induced stress during plastic deformation. The possibility of reducing stress by adapting the solder metallurgy was investigated in detail. Diffusion takes place during the soldering process which allows all of the gold available in the contact plating of the heat sink and the laser bar to diffuse into the indium solder. There, the brittle AuIn2 intermetallic compound is formed. It is shown that the possibility of plastic deformation decreases with an increase of the intermetallic compound fraction. If the indium layer is thick enough, the intermetallic compound has nearly no influence on the relaxation behaviour of the solder joint. The thermal influence of a thicker solder layer is similar to that of a thin solder layer. Simulations and experiments show that the temperature increases caused by increasing the thickness from 2 µm to 10 µm is less than 1 K. The increased solder thickness is a way to reduce significantly the packaging induced stress for packages on copper heat sinks and has nearly no effect on the thermal behaviour. It was also shown, that a thicker solder layer and expansion matched heat sink can lower packaging induced stress.

Authors

Authors

Scholz, Christian

Advisors

Poprawe, Reinhart

Identifier

  • URN: urn:nbn:de:hbz:82-opus-19765
  • REPORT NUMBER: RWTH-CONV-114831