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Simulation Used To Optimize Heat Sink Design
April 12, 2005

A major Asian engineering consulting organization used thermal simulation to optimize the design of a heat sink used in a parallel optical transmitter, dramatically reducing engineering time and costs. The goal of the consulting project was to maintain the case temperature at or below 70C by natural convection cooling (i.e. without using a fan). The greatest design challenge was creating a heat sink for the driver integrated circuit (IC), which has a very high heat flux. Engineers modeled and simulated six different heat sink designs using Flotherm cooling simulation software from Flomerics. The results of the simulations gave the engineers all the information they needed to achieve an optimized heat sink design. The heat transfer path between the laser array and driver IC were also improved by optimizing the tradeoff between thermal and electrical performance. Thermal simulation saved a substantial amount of time and money on this project and the consulting organization has achieved similar results in many other applications.

The project described here was performed for the manufacturer of a 12 channel pluggable parallel optical transmitter based on vertical cavity surface emitting laser (VCSEL) technology. The module can be used for applications up to a data rate of 2.5 Gbps per channel or an aggregate data rate of 30 Gbps with 12 independent optical signals for a transmission distance of up to 300 meters. It mainly consists of a laser array of 12 VCSELs with a wavelength of 850 nm. Module temperatures are critical because excessive temperatures may increase the emitted wavelength and increase threshold current density, which may damage the module, decrease output of a laser; and broaden the spectral line width which affects its speed. The heat flux of the IC is very high at 24 W/cm2. There are two main design issues: 1) designing a heat sink that will keep the module case temperature below 70C, and 2) optimizing the heat transfer path to minimize the effect of the IC power on the temperature of the laser array.

An engineer working for the consulting organization modeled the heat sink design in Flotherm based on natural convection and normal ambient temperature. He tried a number of different potential designs and viewed the simulation results for airflow, pressure and temperature throughout the module. Based on the results, he quickly iterated to an optimized design. The engineer also studied different separations between the IC and the laser array in order to understand the tradeoff between thermal and electrical performance. Increasing the separation improves thermal performance but reduces electrical performance. He designed a cavity and thermal pad under the laser array to improve thermal performance. When the design was completed, prototypes were built and the experimental results agreed very well with the simulation. Temperatures on the finished product are maintained under 70C with natural convection and the optimized heat sink. The project was completed in substantially less time and at a lower cost than could have been achieved using conventional build-and-test methods.
For more information, visit Flomerics’ Web site at

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