The field of electronic packaging continues to grow at an amazing rate. The electronic packaging engineer requires analytical skills, a foundation in mechanical engineering, and access to the latest developments in the electronics field. The emphasis may change from project to project, and from company to company, yet some constants should continue into the foreseeable future. One of these is the emphasis on thermal design.
Thermal analysis of electronic equipment is becoming one of the primary aspects of many packaging jobs. An up-front commitment to CFD (Computational Fluid Dynamics) software code, FEA (Finite Element Analysis) software, is the result of realizing that the thermal problems will only get worse. As the size of the electronic circuit is reduced, speed is increased. As the power of these systems increases and the space allotted to them diminishes, heat flux or density (heat per unit area, W/m^2 ) has spiraled. While air cooling is still used extensively, advanced heat transfer techniques using exotic synthetic liquids are becoming more popular, allowing even smaller systems to be designed.
Electronic devices produce heat as a by-product of normal operation. When electrical current flows through a semiconductor or a passive device, a portion of the power is dissipated as heat energy. Besides the damage that excess heat can cause, it also increases the movement of free electrons in a semiconductor, which can cause an increase in signal noise. The primary focus of this book is to examine various ways to reduce the temperature of a semiconductor, or group of semiconductors. If we do not allow the heat to dissipate, the device junction temperature will exceed the maximum safe operating temperature specified by the manufacturer. When a device exceeds the specified temperature, semiconductor performance, life, and reliability are tremendously reduced, as shown in figure. The basic objective, then, is to hold the junction temperature below the maximum temperature specified by the semiconductor manufacturer. Nature transfers heat in threeNature transfers heat in three ways, convection, conduction, and radiation.
Thermal analysis of electronic equipment is becoming one of the primary aspects of many packaging jobs. An up-front commitment to CFD (Computational Fluid Dynamics) software code, FEA (Finite Element Analysis) software, is the result of realizing that the thermal problems will only get worse. As the size of the electronic circuit is reduced, speed is increased. As the power of these systems increases and the space allotted to them diminishes, heat flux or density (heat per unit area, W/m^2 ) has spiraled. While air cooling is still used extensively, advanced heat transfer techniques using exotic synthetic liquids are becoming more popular, allowing even smaller systems to be designed.
Electronic devices produce heat as a by-product of normal operation. When electrical current flows through a semiconductor or a passive device, a portion of the power is dissipated as heat energy. Besides the damage that excess heat can cause, it also increases the movement of free electrons in a semiconductor, which can cause an increase in signal noise. The primary focus of this book is to examine various ways to reduce the temperature of a semiconductor, or group of semiconductors. If we do not allow the heat to dissipate, the device junction temperature will exceed the maximum safe operating temperature specified by the manufacturer. When a device exceeds the specified temperature, semiconductor performance, life, and reliability are tremendously reduced, as shown in figure. The basic objective, then, is to hold the junction temperature below the maximum temperature specified by the semiconductor manufacturer. Nature transfers heat in threeNature transfers heat in three ways, convection, conduction, and radiation.
Modes of Heat Transfer
ConvectionThe basic relationship of convection from a hot object to a fluid coolant presumes a linear dependence on the temperature rise along the surface of the solid, known as Newtonian cooling
$$\ q_c =h_c A_s (T_s - T_m)$$
$\ q_c$ =convective heat flow rate from the surface (W)
$\ A_s$ =surface area for heat transfer ($\ m^2$)
$\ T_s$=surface temperature (°C)
$\ T_m$=coolant media temperature (°C)
$\ h_c$=coefficient of convective heat transfer ($\dfrac{W}{m^2}$)
Conduction
In many applications, we use conduction to draw heat away from a device so that convection can cool the conductive surface, such as in an air-cooled heat sink. For a one-dimensional system, the following relation governs conductive heat transfer:
$\ T_2$ is found by
where
$\ q_r$ = amount of heat transferred by radiation (W)
$\epsilon$ = emissivity of the radiating surface (highly reflective = 0, highly absorptive 1.0)
$\sigma$ = Stefan-Boltzmann constant ($\ 5.67 \times 10^8 W/ \ m^2 K^4$)
$\ F_{1,2}$ = shape factor between surface area of body 1 and body 2
A = surface area of radiation ($\ m^2)
$\ T_1$ = surface temperature of body 1 (K)
$\ T_2$ = surface temperature of body 2 (K)
Thermal Resistance
The semiconductor junction temperature depends on the sum of the thermal resistances between the device junction and the ambient environment, which is the ultimate heat sink. Figure shows a simplified view of the thermal resistance :
$\ A_s$ =surface area for heat transfer ($\ m^2$)
$\ T_s$=surface temperature (°C)
$\ T_m$=coolant media temperature (°C)
$\ h_c$=coefficient of convective heat transfer ($\dfrac{W}{m^2}$)
Conduction
In many applications, we use conduction to draw heat away from a device so that convection can cool the conductive surface, such as in an air-cooled heat sink. For a one-dimensional system, the following relation governs conductive heat transfer:
$$\ q_c = -k A_c \dfrac{ \Delta T}{L}$$
Radiation is the only mode of heat transfer that can occur through a vacuum and is dependent on the temperature of the radiating surface. Although researchers do not yet understand all of the physical mechanisms of radiative heat transfer, it appears to be the result of electromagnetic waves and photonic motion. The quantity of heat transferred by radiation between two bodies having temperatures of $\ T_1$ andwhere:
$\ q_c$ = heat flow rate (W)k = thermal conductivity of the material (W/m K)
$\ A_c$ = cross-sectional area for heat transfer ($\ m^2$ )
$\Delta T$ = temperature differential (°C)
L = length of heat transfer (m)
Radiation
$\ q_c$ = heat flow rate (W)k = thermal conductivity of the material (W/m K)
$\ A_c$ = cross-sectional area for heat transfer ($\ m^2$ )
$\Delta T$ = temperature differential (°C)
L = length of heat transfer (m)
Radiation
$\ T_2$ is found by
$$\ q_r = \epsilon \sigma F_{1,2} A ( T^4_1 - T^4_2 )$$
where
$\ q_r$ = amount of heat transferred by radiation (W)
$\epsilon$ = emissivity of the radiating surface (highly reflective = 0, highly absorptive 1.0)
$\sigma$ = Stefan-Boltzmann constant ($\ 5.67 \times 10^8 W/ \ m^2 K^4$)
$\ F_{1,2}$ = shape factor between surface area of body 1 and body 2
A = surface area of radiation ($\ m^2)
$\ T_1$ = surface temperature of body 1 (K)
$\ T_2$ = surface temperature of body 2 (K)
Thermal Resistance
The semiconductor junction temperature depends on the sum of the thermal resistances between the device junction and the ambient environment, which is the ultimate heat sink. Figure shows a simplified view of the thermal resistance :
$\theta _{tot} = \theta _{jc} + \theta _{cs} + \theta _{sa}$
where:
$\theta _{tot}$ = total thermal resistance (K/W)
$\theta_{jc}$ = junction to case thermal resistance (K/W)
$\theta_{cs}$ = case to heat sink thermal resistance (K/W)
$\theta_{sa}$ = heat sink to ambient air thermal resistance (K/W)
the fact that heat generation and dissipation in for instance power devices are certainly not negligible effects. As both the power handling capability of power electronic devices and the number of very large scale integration (VLSI) components on a single chip increase, the interest in device heating and in how to reduce it is expected to become progressively larger.
$\theta _{tot}$ = total thermal resistance (K/W)
$\theta_{jc}$ = junction to case thermal resistance (K/W)
$\theta_{cs}$ = case to heat sink thermal resistance (K/W)
$\theta_{sa}$ = heat sink to ambient air thermal resistance (K/W)
Heat Generation in Semi-Conductors
With increasing complexity of semiconductor devices the need for accurate simulations increases as well. Isothermal device simulations in one, two, and three dimensions are performed routinely and the physical models entering such simulations are constantly improved and also extended to new materials. The interest in non-isothermal simulations, however, has been considerably lower,despitethe fact that heat generation and dissipation in for instance power devices are certainly not negligible effects. As both the power handling capability of power electronic devices and the number of very large scale integration (VLSI) components on a single chip increase, the interest in device heating and in how to reduce it is expected to become progressively larger.
