As consumers continue to demand smaller, faster speeds, a formidable challenge has arisen in addressing heat dissipation from ever-increasing density printed circuit boards (PCBs). As stacked microprocessors and logic cells reach the GHz operating frequency range, cost-effective thermal management has perhaps become the highest priority issue for engineers in design, packaging and materials.
The current trend to fabricate 3D ICs for higher functional densities further increases the difficulty of thermal management. Simulation results show that a 10°C rise in temperature doubles the thermal density of a 3D IC chip and reduces performance by more than a third.
Microprocessor Challenges The International Technology Roadmap for Semiconductors (ITRS) forecasts that interconnect traces in hard-to-cool areas within microprocessors will consume up to 80 percent of the chip’s power over the next three years. Thermal Design Power (TDP) is a measure of a microprocessor’s ability to dissipate heat. It defines the heat released by the processor at maximum load and the corresponding case temperature.
The TDPs of the latest microprocessors from Intel and AMD range from 32W to 140W. As the operating frequency of microprocessors increases, this number will continue to rise.
Large data centers with hundreds of computer servers are particularly vulnerable to cooling problems. According to some estimates, a server’s cooling fan, which can consume up to 15 percent of the power, has actually become a sizable heat source in and on the server. Additionally, data center cooling costs can account for about 40% to 50% of data center power consumption. All these facts place higher demands on local and remote temperature sensing and fan control.
Thermal management challenges become even more daunting when it comes to mounting PCBs containing multi-core processors. While each processor core in a processor array may consume less power (and thus dissipate less heat) than a single-core processor, the net effect on mainframe computer servers is an increase in computer systems in the data center more heat dissipation. In short, run more processor cores on a given area of PCB board.
Another thorny IC thermal management issue involves hot spots on the chip package. The heat flux can be as high as 1000W/cm2, which is a difficult state to track.
The PCB plays an important role in thermal management and therefore requires a thermal design layout. Design engineers should place high-power components as far apart as possible from each other. Additionally, these high-power components should be kept as far away from the corners of the PCB as possible, which will help maximize the PCB area around the power components and speed up heat dissipation.
It is common practice to solder exposed power pads to PCBs. In general, exposed pad-type power pads can conduct about 80% of the heat generated through the bottom of the IC package and into the PCB. The remaining heat will be dissipated from the package sides and leads.
Thermal Helpers Design engineers can now turn to a number of improved thermal management products for assistance. These products include heat sinks, heat pipes and fans for active and passive convection, radiation and conduction cooling. Even the way the chips are mounted on the PCB is interconnected to help mitigate thermal issues.
For example, the common exposed pad method used to interconnect IC chips to PCBs can increase thermal issues. When soldering exposed paths to a PCB, heat can quickly escape the package and into the board, where it is then dissipated through the various layers of the board and into the surrounding air.
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