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Posted in heat, heat sink, Heat Sink Attach, heat sink design, Heat Sink Material, Heat Sinks, Heat Spreaders, Manufacturing, materials, semiconductor, sink, Thermal Design, thermal engineering, thermal management, Uncategorized
Tagged Advanced Thermal Solutions, education, electronics, electronics cooling, engineering, heat sink, heat sink attach, heat sink design, mechanical engineer, thermal analysis, thermal characterization, thermal management, thermal research
One of the basic concepts of electronics cooling is effective transfer of heat from semiconductor devices to the ambient using heat sinks or other cooling technologies. The effectiveness of this approach depends on a systems total thermal resistance, which is composed of discrete thermal resistances on the path of heat from the source to the ambient. One of these resistances is spreading resistance.
Spreading resistance occurs whenever a small heat source comes in contact with the base of a larger heat sink. The heat does not distribute uniformly through the heat sink base, and consequently does not transfer efficiently to the fins for convective cooling. Figure 1 shows a CFdesign® simulation solution for such an occurrence. The spreading resistance phenomenon is shown by how the heat travels through the center of a heat sink base causing a large temperature gradient between the center and edges of the heat sink.
Figure 1: CFdesign solution showing temperature distribution
at the base of a heat sink
Spreading resistance is an increasingly important issue in thermal management as microelectronic packages become more powerful and compact and larger heat sinks are required to cool these devices. In high heat flux applications, spreading resistance can comprise 60 to 70% of the total thermal resistance.
A good estimate of spreading resistance is required to manage heat effectively using conventional air-cooled heat sinks. There have been a number of theoretical and experimental studies to estimate spreading resistance. Two of the most notable methods belong to Yovanovich et al. [1] and to Gordon N. Ellison [2].
While these extensive studies cover all aspects of spreading resistance, they involve cumbersome infinite series and complicated coefficient terms. (click the link to read the rest of our article here on our electronics cooling blog)
Posted in CFD, Thermal Analysis, thermal management, Thermal Science
Tagged CFD, thermal management, thermal research, thermal resistance
Thermal Conductivity: A measure of the ability of a material to transfer heat. Given two surfaces on either side of a material with a temperature difference between them, the thermal conductivity is the heat energy transferred per unit time and per unit surface area, divided by the temperature difference [1].
Thermal conductivity is a bulk property that describes the ability of a material to transfer heat. In the following equation, thermal conductivity is the proportionality factor k. The distance of heat transfer is defined as †x, which is perpendicular to area A. The rate of heat transferred through the material is Q, from temperature T1 to temperature T2, when T1>T2 [2].
The path of heat from the die to the outside environment is a complicated process that must be understood when designing a thermal solution. In the past, many devices were able to operate without requiring an external cooling device like a heat sink. In these devices, the conduction resistance from the die to the board needed to be optimized, as the primary heat transfer path was into the PCB. As power levels increased, heat transfer solely into the board became inadequate (credit shakita). Much of the heat is now dissipated directly into the environment through the top surface of the component. In these new higher-powered devices, low junction-to-case resistance is important, as is the design of the attached heat sink.
Today we begin a two-part series on Thermal Coupling in Electronics Cooling. In part 1 we’ll cover what thermal coupling is and how the coupling effect works.
Thermal coupling is the interrelationship among the three primary modes of heat transfer: conduction, convection and radiation. Each of these modes is common in electronics cooling and thermal engineers must understand how they can be used together to lower the junction temperature of hot electronic components.
To further explore heat transfer types, a simple virtual test was performed using CFdesign software [1]. A block of material was modeled and subjected to a prescribed heat load. The block was cooled via convection (air flow over the block) and radiation heat transfer. Different block materials were modeled to understand how their inherent thermal conductivity affected overall heat transfer. Each of the test cases was plotted on a graph to show the coupling effects of the various modes of heat transfer.
The test featured a 60 mm x 60 mm block of solid material set in a 250 mm x 25 mm tunnel (or duct). A 10 mm x 10 mm heat source was applied to the blocks base. Figure 2 shows a schematic of the thermal resistance network for this case. The schematic, Figure 1, is a one-dimensional representation of the heat transfer path with the convective, radiative and conductive resistances clearly shown.
Figure 1. Network Model for a Solid Block with Heat Source
This model shows that heat must first flow through the solid block via conduction. It can then be dissipated to the wall of the tunnel by radiation or carried away in the fluid (air flow) by convection. In effect, the block is thermally coupled to the tunnel walls and to the air passing through the tunnel.
The total convective resistance in the network is equal to the sum of the convective resistances from the surface of the block to the fluid (Rconvcf), and from the fluid to the walls of the tunnel (Rconvfw). It is defined in Equation 1 below.
Rconv = Rconvcf+ Rconvfw (1)
The total conductive resistance is equal to the sum of the through-plane conduction for the block and the spreading resistance or in-plane conduction through the block. This is defined in Equation 2:
Rcond = Rcond + Rsp (2)
The radiation resistance (Rrad) is defined from the surface of the block to the walls of the tunnel.
In part 2 we’ll explore the coupling effect of radiation, conduction and convection.
Got a question on part 2 already or maybe part 1 from today? Contact us and lets see how ATS thermal engineers can make your next project a success! Email us at ats-hq@qats.com , call us at 781-769-2800 or visit our Design Services
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References
1. CFdesign® Software, Blue Ridge Numerics, Inc.
Posted in Engineering, Thermal Design, thermal management
Tagged engineering, thermal design, thermal research
On Thursday, April 28, 2011 2:00 PM – 3:00 PM EDT, ATS will be holding a free webinar, “How to Properly Measure Temperature within Electronic Systems and Analyze the Results”
Attendees will become familiar with the importance of temperature measurement in electronic systems. They will learn about the essential instruments and the locations within a system where testing should be conducted.  The webinar presenter will also discuss how to analyze the temperature data as part of a complete thermal analysis.
To register, click to our registration site at this link: https://www2.gotomeeting.com/register/952923650
For the Chinese Mandarin presentation of our webinar please register at http://bit.ly/ATS_temp_mesaurement_Mandarin