Thermal Management Introduction EBB 526 – Electronic Packaging Assoc. Prof. Dr. Azizan Aziz EBB 526 – Electronic Packaging Introduction 14 August 2007

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 14 August 2007

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 14 August 2007

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. 14 August 2007

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. 14 August 2007

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. 14 August 2007

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 14 August 2007

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 14 August 2007

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 14 August 2007

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 14 August 2007

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 14 August 2007

Dissipation factor vs.temperature 14 August 2007

Temperature Effects on Active Semiconductors Devices Active components are: - Diodes - Bipolar Transistors - Field Effect Transistor 14 August 2007

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 14 August 2007

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 14 August 2007

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 C408-88 while E1461-92 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) 14 August 2007

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 14 August 2007

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) 14 August 2007

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 14 August 2007

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 14 August 2007

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 14 August 2007

END 14 August 2007