Technical Paper: PIM International, Vol.3 No. 2 June 2009, pages 54-58, 1840 words

Author: H. Schmidt [1], A. C. Rota [1], Ph. Imgrund [1] and M. Leers [2]

[1] Fraunhofer Institute for Manufacturing and Advanced Materials, Bremen, Germany
[2] Fraunhofer Institute for Laser Technology, Aachen, Germany

Abstract

There is an increasing interest in using tiny parts made of special metallic materials for new applications. In this new approach, active cooled heat sinks with a microstructured surface are produced by µ-metal injection moulding (µMIM, microMIM).

In contrast to conventional heat sinks made of copper, in this case the parts are made of copper-tungsten and copper-molybdenum due to their combination of low coefficient of thermal expansion (CTE) and reasonable thermal conductivity.

In this paper the current status of production of micro parts and parts with microstructured surfaces by µ-MIM for thermal management applications are presented. The scope and limitations are outlined with special concern to the materials and the minimal structure size. The implication and advantages of using ultrafine powders are pointed out. Therefore sintering behaviour, microstructure of sintered parts and characteristic properties as density, CTE and thermal conductivity are shown.

Introduction

Micro channel heat sinks for laser diodes are currently manufactured by single copper sheets, which are joined together. Copper is used due to its good thermal conductivity and easy machinability. However, these heat sinks have two essential disadvantages. Firstly, there is a big difference in thermal expansion (CTE Cu: 17 ppm/K, GaAs: 6,7 ppm/K) of the heat sink and the mounted laser bar. This leads to mechanical stress during mounting as well as operating and decreases the lifetime of laser diodes.

In addition, due to the required cooling efficiency (ca. 5000 kW/m²) the flow rate of the cooling liquid must be very high (ca. 0,5l/min) which causes corrosion of copper [1].

Therefore the adjustment of CTE is an important objective concerning the development of high power laser diodes. Currently there are two cost intensive approaches to achieve this adjustment. One common approach is mounting the laser bar on a CTE adjusted submount, which is soldered to a conventional micro channel heat sink. Another way of adjusting the CTE is using copper and molybdenum layers in a sandwich construction combining the good machinability and the excellent thermal conductivity of copper with the low thermal expansion of molybdenum. The CTE can be adapted to the laser bar by varying the thicknesses of the individual layers, this was calculated in advance by means of thermal simulation. Both systems are too expensive for an implementation in the market.

The new approach is to manufacture heat sinks by micro metal-injection-moulding (µ-MIM), which allows an economic mass-production of complex micro near net shape parts. A large number of different metals and alloys are suitable for µ-MIM. Especially for mass production (>1000 parts) µ-MIM is a low cost process, which reduces part costs considerably. The main goal is to use the opportunities µ-MIM offers. That means to produce complex parts, which have an adjusted CTE in this case to gallium arsenide. Therefore a material is needed, which combines good thermal conductivity behaviour with a low coefficient of thermal expansion. Tungsten and molybdenum combine a moderate thermal conductivity with a low CTE and had shown, in combination with copper, sufficient properties for application as submounts.

Further sections of this article include:

- Properties of tungsten and molybdenum copper alloys
- Experimental procedure
- Results and discussions
- Conclusion and outlook
- Acknowledgements
- References

Figures and Tables:

Fig. 1 Thermal conductivity over CTE for different materials
Fig. 2 Particle size distribution and SEM picture of fine W powder
Fig. 3 Particle size distribution and SEM picture of fine Mo powder
Fig. 4 Particle size distribution and SEM picture of fine Cu powder
Fig. 5 Particle size distribution and SEM picture of ultra fine Cu powder
Fig. 6 Comparison of copper-tungsten 80:20 with two different copper powders and varied sintering program
Fig. 7 Comparison of copper-molybdenum 70:30 and 75:25 with two different copper powders and varied sintering program a,b
Fig. 8 Comparison of copper-tungsten 80:20 with two different copper powders by sintering program b. Left side fine Cu powder; right side ultra fine Cu powder
Figs. 9 and 10 MoCu 70:30 compacts with fine Cu powder (sintering program b)
Figs. 11 and 12 MoCu 70:30 compacts with ultra fine Cu powder (sintering program b)
Figs. 13-16 3D analysis with white-light interferometry on copper-tungsten 80:20 samples fine powder

Table 1 CTE of copper-tungsten for different ratios of copper
Table 2 Sintering programs copper-tungsten
Table 3 CTE- measurement for WCu at 200°C
Table 4 Thermal conductivity of sintered WCu and MoCu parts