1. Thermal Management Introduction EBB 526 – Electronic Packaging
Assoc. Prof. Dr. Azizan Aziz
EBB 526 – Electronic Packaging
Introduction
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2. Why Thermal Management
Thermal management encompasses the technology of the generation and control of heat in electronics circuit
Heat is an unavoidable by-product of every electronic device and circuit
Heat is detrimental to performance and reliability
Size Reduction,increase performance and placing more functions make thermal management a priority in the design cycle in order to maintain system performance and reliability
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3. How and where is heat generated
There are three fundamental questions to be considered in thermal management :
How and where is heat generated
Many sources of heat in electronic circuit all of which must be considered
How is the temperature at a given point in the circuit determined
Thermal analysis is not an exact science. Heat conduction is reasonably well defined but most thermal models relating to radiation and convection are empirically determined for all but the simplest of geometries
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4. How is heat removed
The generated heat must be carried away from the device and dissipated safely
Requires design and construction of an efficient heat path from the device through the mounting surface(s) to the outside world
Can be a simple clip-on heat sink or as complicated as thermoelectric cooler attached to finned heat sink with circulating liquid nitrogen.
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5. Thermal Effects on Electronic Circuits
Both the performance and reliability of electronic circuits are strongly influenced by temperature
Exposure to temperature above the maximum operating and storage temperature might lead to failure to perform certain specification or complete failure.
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6. The detrimental effect to excessive temperature may classified into 3 types
Failure Mode
Characteristic
Soft failures
Circuit continues to operate but does not meet specification when the temperature is elevated beyond the maximum operating temperature.
Circuits return to normal operation when temperature is lowered
Failure is due to change in component parameters with temperature
Hard failures (short term)
Circuit doesn not operate
Circuit may or may not return to normal operation when temperature is lowered
Failure is likely due to component or interconnection breakdown, but may also be due to changes in component parameters with temperature.
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7. The detrimental effect to excessive temperature may classified into 3 types (continue)
Failure Mode
Characteristic
Hard failures (long term)
Circuit dos not operate at any temperature
Failure irreversible
Failure may be caused by corrosion, intermetallic formation or similar phenomena
Failure may also be caused by mechanical stresses due to differences in temperature coefficient of expansion (TCE) between a component and the substrate
Soft failures happen as a result of the tendency of the parameters of both active and passive components to exhibit a degree of sensitivity to temperature
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8. As the temperature increases, the cumulatives effects of component parameter drift may eventually cause he circuit output to deviate from the specification
Hard failures (short term) may occur as a result component overload because of excessive heat or breakdown of component attach or packaging materials.
Hard failures (long term) due a variety of reasons such as corrosion,chemical reactions, intermetallic compound formation (all accelarated by elevated temp).or to mechanical stresses because of CTE mismatch
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9. To maximise circuit performance and reliability, it is important
to be able to predict the maximum operating and storage temperatures
To design and package electronic circuits which minimize the heat generated during operation
To minimize circuit temperature by employing adequate thermal management
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10. Temperature Effects on Passive Components
Passive components are:
- Resistors
-capacitors (Ceramic chip,MOS,tantalum solid electrolytic)
-Inductors and Transformers
The fundamental materials properties such as conductivity, permittivity or permeability which determine the parameters of passive components exhibit a dependence on temperature to a greater or lesser degree
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11. These changes may be +ve or –ve, linear or nonlinear,permanent or temporary,
May be due to a variety of factors including physical changes in the materials or variation in the lattice structure.
Conductivity vs temp
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12. Dissipation factor vs.temperature
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13. Temperature Effects on Active Semiconductors Devices
Active components are:
- Diodes
- Bipolar Transistors
- Field Effect Transistor
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14. Thermal Properties
Selecting the right materials can be a key element of practicing good thermal management
Thermal conductivity, specific heat, CTE, together with density,modulus of elasticity and tensile strength are among the properties that has to be evaluate
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15. Thermal Conductivity
Thermal conductivity,k is defined by time rate of thermal energy (heat) transfer as it conducts between opposite faces of a cubic volume of materials and the temperature varies by 1 K
Thermal conductivity indicates the efficiency of a material when heat flows flows from cooler to warmer region is defined as
(1)
Where q = heat flux in W/m2
k = thermal conductivity in W/m-K
dT/dx = temperature gradient, K/m in steady state
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16. Lower k values – poor thermal performance
-ve sign denotes heat flows from areas of higher temperature to areas of lower temp
Lower k values – poor thermal performance
Measurement of k –direct or indirect methods
ASTM direct method C while E indirect using thermal pulse or heat flash
Thermal conductivity can be calculated from the thermal diffusivity,density and specific heat capacity
(2)
Where λ = thermal conductivity in W/m-K
ρ = density g/m3
α = thermal diffusivity in m2/s
CP = spesific heat capacity (W-s)/(g-K)
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17. Thermal Capacity and Specific Heat
Materials can absorb energy in the form of heat and release it
Thermal capacity is defined as the amount of thermal energy required required to raise the temperature of one mole of material by 1 kelvin
(3)
where C=heat capacity in W-s/mol-K
Q= energy in W-s
T = temperature in K
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18. Thermal Coefficient of Expansion (TCE)
TCE is a result of asymmetrical changes in the interatomic spacing of atoms due to the increased energy content in the form of heat
Most metals and ceramics exhibits a linear,isotropic relationship in the temperature range of interest while polymers are anisotropic in nature due to behaviors above and below Tg
TCE values for polymers are several times larger above Tg than below it
TCE is defined as
(4)
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19. T1 = initial temperature T2= final temperature
where α =TCE, 1/oC
T1 = initial temperature
T2= final temperature
L (T1) = length at initial temperature
L(T2) = length at final temperature
α is often multiplied by 106 with corresponding units of parts per million length change per kelvin, ppm/K
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20. Modulus of Elasticity (E)
Modulus of elasticity is the measure of rigidity or stiffness of a material, which can be described as the plastic deformation under load
E, is the ratio of stress below the proportional limit,compared to the corresponding strain and can be represented by the slope of the initial stress-strain curve.
E can be reprsented by
(5)
where σ = stress in psi or N/m2
E = modulus of elasticity in psi or N/m2
e = strain in m/m, the net elongation
The units of E are generally listed as GPa
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21. Tensile Strength
Tensile strength is the amount of stress applied to stretch a material to its breaking point
Both tensile strength and E depend upon the purity depend upon the purity and processing of seemingly equivalent materials
Scan a tensile-strain curve
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22. END
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