Selecting materials with characteristics well suited for efficient thermal management is only part of the task of maintaining manageable temperatures for high-power devices such as power transistors and laser diodes; mechanical layout and design is often essential for achieving an effective heat flow away from a heat source. A number of factors must be considered in the design of a cooling structure, including the minimum or maximum physical dimensions required for a particular application, the expected operating lifetime, the cost, the required T between the inlet and outlet of a cooling structure, and even the type of liquid and flow rate to be used in a liquid-filled cooler.
MCCs are heat-exchanging structures that also provide electrical connections to active devices such as laser diodes. Multiple MCCs based on thermally conductive materials such as copper or copper/ceramic combinations, are used in arrays of laser diodes, to minimize thermal effects. The multiple MCCs may be arranged in serial or parallel configurations, with inlet and output holes in each MCC to enable the flow of a cooling fluid (Figure 4). In a series configuration, cooling occurs from the inlet to the outlet of an MCC. Heat builds towards the outlet, as the thermal energy of each laser diode adds together. In a parallel cooling configuration, heat flows in a direction crosswise to the inlet and outlet of an MCC. To minimize unwanted electrical connections, cooling fluids with low electrical conductivity, such as deionized (DE) water, are typically used to cool the MCCs.
This combination of water and copper, while it provides a low-thermal-resistance path for the flow of heat away from the laser diode, also has long-term effects that are detrimental to copper. Water, which is often referred to as “the universal solvent,” has oxidation effects which can wear down copper over time and shorten the lifetime of a copper-based MCC, with or without electrical potential applied to the MCC. Due to the effects of corrosion and erosion, the copper walls of an MCC containing the cooling fluid can break down into microparticles which can move throughout the fluid and deposit on other surfaces within the MCC, even eventually clogging the flow of fluid through the MCC. By adjusting the pH of the cooling fluid and by strict particle filtering control, it is possible to minimize the effects of a cooling fluid on the deterioration of an MCC's copper.
In addition, the choice of fluid velocity or flow rate of the cooling fluid can determine the rate of deterioration of the copper walls within an MCC's liquid cooling chamber. The flow rate is a tradeoff between achieving improved thermal dissipation at higher flow rates and causing faster deterioration of the cooler's internal copper walls. Quite simply, the greater the flow rate, the greater the flow of heat away from the source, but also the greater the amount of copper corrosion. Although MCCs are designed for typical liquid velocity as high as 5 mL/s, flow rates of 2 mL/s or less will result in less corrosion of copper surfaces exposed to the flowing fluid.
Testing and Simulating
As with many electronic designs, development of an effective MCC for high-power laser diodes involves a weighing of various tradeoffs including size, cost, complexity, and operating lifetime. Measurements can help in the analysis of the cooling requirements of a particular application, while 3D electromagnetic (EM) simulation software can greatly speed the optimization of possible MCC configurations for high-power-density cooling applications.
In addition, by reinforcing the simulation power and accuracy of the software with models based on measurements using a specialized test setup with pressure sensors to precisely measure the flow rates of cooling liquids through different MCC structures (Figure 5), it is possible to efficiently develop optimum cooling solutions based on custom requirements, without the time and expense of trial-and-error prototyping.