|Fig. 1:Plot of CPU transistor counts against dates of introduction. Note the logarithmic vertical scale; the line corresponds to exponential growth with transistor count doubling every two years. (Source: Wikimedia Commons)|
The semiconductor industry has been thriving on the improvement in process technology and fabrication processes.  A new node, a smaller channel length device decreases the transistor size and facilitates in more chips to be accommodated in a similar sized PCB. This also grows the need for efficient heat dissipation. Recent developments include mechanisms such as micro heat exchanger systems. This active cooling approach requires the integration of microfluidic components near the main heat sources of the electronic devices. Despite the investigation of several micro-cooling configurations, their commercial utilization by the electronic industry is rather limited due to complex fabrication and integration methods.
The increase in heat flux at the chip and module level with each new generation of mainframe computer technology resulted in a continuing cooling challenge throughout the years.  The development of liquid immersion, conduction, and air-cooling technologies, the Liquid Encapsulated Module (LEM) provides an early example of an IBM direct immersion cooling technology developed to utilize pool boiling to cool chips on an MCM.  The LEM was designed to cool a 90 mm × 90 mm ceramic substrate carrying 100 integrated circuit chips mounted within a sealed module cooling assembly containing a fluorocarbon liquid. Boiling of the fluorocarbon liquid at the chip surface provided heat transfer coefficients (1700-5700 W/m2-K) with which to meet chip cooling requirements. The LEM cooling technology provided the capability to support chip heat fluxes of 15 to 20 W/m2 and module heat loads up to 300 W. Thermal management has become a major limitation for the electronic industry. The utilization of extremely small transistors at high operating frequencies( GHz) generate a significant amount of heat which is exceeding the capacity of conventional heat removal techniques. The lack of heat dissipation yields higher operating temperatures that increase the risk of electrical failures in the device. This concern is not only problematic for the electronic industry but also the design of aerospace structures. For many applications, air-cooling mechanisms are not sufficient or simply impossible and other cooling technologies have to be used. Fluid cooling (e.g., water) offers a thermal conductivity and a specific heat capacity 25 and 4 times superior than air, respectively. The passive or active circulation of liquid could be used to transfer the heat from a specific location to another location where heat dissipation becomes more effective. For example, the heat generated by all the main electronic components of a computer could be transferred to a fluid and then transported to a single heat dissipation system, reducing the number of fans while lowering the noise level. Thus, electronic manufacturers have an increasing interest in passive micro heat pipe, and active micro heat exchanger technologies since they enable the creation of compact and efficient heat removal systems located close to the heat source. [4,5] The cooling of electronic components using micro heat exchangers is a promising approach. [6-9] A micro heat exchanger is an active system where the heat is transferred to a fluid circulating inside a micro-channel (i.e., channel with a hydraulic diameter smaller than 1 mm). The system is usually composed of a pump for fluid circulation, a heat source, a heat sink and a network of micro-channels. The cooling efficiency of the micro heat exchanger strongly depends on the fluid flow rate, the thermal properties of the cooling fluid, the hydraulic diameter of the micro-channel and the surface and the distance of the micro-channel network to the heat sources. In addition, the utilization of integrated micro-channels inside a printed circuit board (PCB) will enable a tight coupling with the heat source while minimizing the thermal resistance. Exposed paddle packages (i.e., packaging technique where metallic die paddles are exposed on the die) exhibit better thermal characteristics compared to other packaging techniques but the heat flux absorbed by the ground planes is not removed. This heat flux in the board may lead to an increase of temperature over time leading to malfunctions or failures of the device. Thus, the heat removal of conventional PCB such as copper/FR4 requires enhancement in order to facilitate the fabrication of high power electronic devices.
In this paper I have sought to provide a background describing the challenge of cooling high performance electronic packages such as are used in mainframe computers. Almost since the birth of electronic computer technology heat flux has been increasing, and we may expect this trend to continue into the next century. In addition, a new dimension has been added to the cooling challenge by the requirement to reduce operating temperatures to achieve enhanced speed. With the continued demand of improved cooling technology for enhancing the performance and reliability of CMOS applications, thermoelectric cooling may be considered a potential candidate for meeting some of the challenge.
© Shiv Agarwal. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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