Staying cool with nano soldering
Diode lasers take up minimal space. For their performance to improve further, developers urgently need new solutions for heat dissipation.
Sparks spray out. Their source is the box on the robotic arm. Barely bigger than the shoe box, it contains a complete, highly energy-efficient diode laser system.
13 project partners in the BrightLas research project had this vision in mind when they came together in 2013 to produce compact diode lasers which would allow metals and plastics to be welded, soldered, hardened, drilled and cut—ideally without conversion into solids and without transmission in fibers. In order to make them suitable for industrial use, they wanted to increase the power of diodes in the 808 nm wavelength range by 50 percent, while achieving an operating life of 25,000 hours and a wall-plug efficiency of 60 percent.
The targets were too ambitious. The sticking point: heat management. Their plan to increase power while minimizing component size failed mainly because the planned high-performance heat sinks based on copper and diamond expanded excessively during operation. The systems were unable to cope with the resulting mechanical stresses. The problem of heat dissipation isn't new. A decade ago, companies including Osram, Jenoptik, Rofin and Dilas worked together on a project entitled TRUST, where they systematically investigated what causes failures and the premature drop in power of high-performance diode lasers. Their conclusion: “The properties of the semiconductor material play a secondary role.” In actual fact, the reliability of the lasers depends on their installation, the power level and the cooling temperature. Thermal and electro-optical stresses proved particularly critical during endurance tests.
Pinning hopes on nano soldering
Given the tiny dimensions of the laser diodes, the search for more effective heat sinks goes hand in hand with the need to improve soldering procedures during assembly. When it comes to connecting semiconductor diodes and heat sinks, high-performance soldering techniques which achieve maximum precision at the lowest possible temperature are required. Nanotechnology offers one possible solution. On a nano scale, precious metals such as gold and silver can be worked with at temperatures well below their normal melting point but, after cooling, still exhibit their usual material properties. The Universities of Darmstadt and Aschaffenburg have worked on a project with arteos GmbH in order to show the potential of nano-soldering using gold. The gold can be worked at temperatures above 40 degrees and, after cooling, has the usual melting point of gold (1,064 degrees).
During the BrightLas project, researchers from the TU Berlin were able to take advantage of the high thermal and electrical conductivity of silver for laser diodes. And the results were so impressive that they are now marketing their nano silver pastes via their own start-up Nano-Join. BrightLas Spokesman and Lumics Managing Director Nils Kirstaedter is very impressed with the new solution: “We would like to use these silver pastes as standard, since we could then make heat sinks three times smaller.” The key to this is a silver precursor which becomes soft when heated and releases nano silver particles. The paste can be applied using a pressureless sintering process and produces dense, adherent layers of silver at temperatures between 230 degrees and 250 degrees.
New diode lasers in the kW range
The start-up is not the only result of the research project. Lumics has also laid the technical foundation for a new 1.6 kW module, and project partner DirectPhotonics has developed a 500 W module with integrated electronics as a building block for multi-kW systems. Through their work on the project, Scansonic MI and Lumics have also made significant contributions toward a laser hardening head system for soldering and welding fine metal sheets with direct diodes. At LASER World of PHOTONICS 2015, project partner SITEC already presented a first industrial system equipped with the new technology. The continuation can be seen in Munich in 2017.
Image source: Lumics, Scansonic