Thermal Design of Semiconductor Components in Electronic EquipmentSummary

Key Points of This Article

・Thermal design is, in a nutshell, designing semiconductor components such that temperatures remain within the maximum rating Tjmax.

・If thermal design is not performed thoroughly in the design stage, problems may be discovered in the prototyping stage or before mass production.

・The closer a countermeasure is to mass production, the greater the amounts of time and money needed for its implementation, and delays in product shipment may result in lost opportunities.

・In the worst case, problems occur in the marketplace, culminating in recalls and loss of trust. Thermal design is vitally important.

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・As recent engineering trends, much emphasis has been placed on miniaturization, performance enhancement, and design properties.

・Because of these trends, heat generation has increased and heat dissipation has worsened, so that thermal design has become more difficult.

・It is important to examine whether existing criteria for evaluation of thermal designs can accommodate these current engineering trends.

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・In addition to thermal design satisfying current requirements and the establishment of evaluation criteria, mutual understanding of thermal design is also necessary for thermal design optimization.

・Moreover, it is essential that engineers “come to grips with” thermal design.

・By improving design quality, reductions both in use of manpower and in costs are possible.

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・Thermal resistance is a numerical expression of the difficulty of propagation of heat.

・As symbols, Rth and θ (theta) are used; units are ℃/W （K/W）.

・Thermal resistance can be considered to be roughly analogous to electrical resistance.

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・There are three modes of heat transmission: conduction, convection, and emission (radiation).

・In an example of an IC mounted on a printed circuit board, the source of heat generation is the IC chip. And the heat is transmitted by conduction to the package, lead frame, die attach pads, and the printed circuit board. Then the heat is transmitted from the surfaces of the printed circuit board and the IC package to the ambient atmosphere by convection and emission.

・If the thermal resistances of these paths and the power dissipation of the IC are known, then the thermal Ohm’s law can be used to calculate the temperature difference between T_A and T_J.

・Thermal design involves reducing thermal resistances of heat dissipation paths from a chip to the atmosphere.

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・In thermal conduction, thermal resistance can be thought of as similar to the electrical sheet resistance of an electrical conductor.

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・A fluid is something that flows, whether a gas, a liquid, or the like.

・Convection is a phenomenon of heat movement in which a fluid that has received heat moves, thereby transferring heat.

・Natural convection is flow driven by the buoyancy that arises from temperature differences in the fluid.

・Forced convection is flow driven by external factors, such as a fan or a pump.

・Thermal resistance in convection is the reciprocal of the product of the convective heat transfer coefficient hm and the surface area A of the object emitting heat.

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・In thermal emission, heat movement occurs due to electromagnetic waves; the mechanism differs from those of conduction and convection, in which heat movement occurs by means of molecules.

・Thermal resistance in emission is the reciprocal of the product of the radiative heat transfer coefficient and the surface area of the heat-emitting body.

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・Thermal resistance data is acquired in conformation with standard specifications; ordinarily the standard specifications are clearly stated as well.

・Among the JEDEC standards, the following are the main two standards relating to heat.

• －JESD51 series：Includes nearly all standards relating to heat in packages for ICs and other components
• －JESD15 series：Standardizes thermal resistance models used in simulations

・Environments for measurement of thermal resistance are stipulated in JESD51-2A.

・The boards used to measure thermal resistance are stipulated in JESD51-3/5/7.

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・Thermal resistance data is generally presented on IC data sheets and the like, but there may be differences in the information given depending on the type of IC and the manufacturer.

・Thermal resistances differ considerably depending on PCB conditions. Measurement conditions must always be confirmed.

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・Thermal resistance and thermal characterization parameters are defined in JESD51, which is a JEDEC standard.

・There are basic applications for the different thermal resistances and thermal characterization parameters; the relevant thermal resistances and thermal characterization parameters are used in calculations.

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・It is deemed difficult to study thermal design in terms of θ_JA starting from the conditions of packaging used in recent devices.

・It is difficult to provide a definition of T_A that can be used uniformly for all recent hardware. Instead, separate definitions are sought.

・Actual measurements of T_A in equipment with high measurement densities are extremely difficult.

・In recent years, estimation of T_J using T_T and Ψ_JT, which are relatively easy to measure, has come into widespread use.

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・It is essentially not possible to estimate T_J using θ_JA while the device is installed in actual equipment.

・Ψ_JT is different depending on the mounting conditions, but with an understanding of the PCB being used and the mounted state, it can be used for T_J estimation with the device installed in actual equipment.

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・There are two methods for estimating T_J: using T_A and θ_JA, or using T_T and Ψ_JT.

・In both calculations, the IC power consumption P is necessary.

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・To estimate T_J using the thermal resistance θ_JA, the values of the power consumption P and of T_A are necessary.

・T_J is found from calculation equations, and is confirmed to be within T_JMAX.

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・For estimation of the thermal resistance Ψ_JT using T_J, the power consumption P and the temperature T_T at the center of the top surface of the IC package in the state of actual use are required.

・Actual measurement of T_T is necessary.

・T_J is found from a calculation equation, and is confirmed to be less than T_JMAX.

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・When a transient increase in power consumption is anticipated, the peak T_J in the transient state is sought.

・The transient thermal resistance is used for the thermal resistance when determining the temperature rise in a transient state.

・The peak T_J in the transient state should be checked to confirm that it does not exceed T_{J MAX}.

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・When heat-generating ICs are mounted close together, thermal interference occurs, inducing rises in temperature.

・The required θ_JA is found from the allowed maximum T_J, and the resulting necessary heat dissipation area is estimated.

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・When measuring the surface temperatures of mounted semiconductor components, often the most practical approach is to use a thermocouple.

・In this article, type K thermocouples in class 1, and AWG 38 wiring, are used.

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・Methods for fastening the tip (junction) of a thermocouple to the package of an IC or other device include ① using polyimide (PI) tape or the like, and ② using an epoxy adhesive.

・The JEDEC standards recommend methods using an epoxy adhesive.

・Apart from the method of attaching the thermocouple tip, the arrangement of the wiring also affects the measurement results, and so the wiring itself is run along the surface of a heat source.

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・The thermocouple mounting position is also important for accurate measurement of T_T.

・Even millimeter-unit shifts can result in temperature differences.

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・The tip of a thermocouple must not be twisted.

・In order to minimize heat dissipation from the thermocouple, the junction is welded.

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・When measuring the surface temperature of a semiconductor component, the smaller the package of the component, the greater is the influence of heat dissipation from the thermocouple, so caution must be exercised.

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・When using hand soldering to mount components on the surface of an evaluation board, thermal resistance may be high.

・When mounting components on a board where heat-related evaluations are necessary, as a rule reflow mounting is used for surface-mounted components.

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・The greater the number of layers in a multilayer PCB, the greater is the influence of the copper foil thickness on the thermal resistance.

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・The greater the number of layers in a multilayer PCB, the lower the thermal resistance will be.

・The effect of vias in reducing thermal resistance is considerable; rather than increasing the number of layers in a PCB, it is more effective to add vias.

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・It is most effective to position vias as close to heat sources as possible.

・If vias are installed on the periphery rather than directly below, lateral (horizontal) paths are added to the heat dissipation route, and the thermal resistance rises.

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・Thermal resistance changes depending on IC mounting positions, even for PCBs with the same number of layers.

・When an IC is mounted at the edge of a PCB, the heat dissipation region is effectively reduced in size.

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・In a natural air-cooled environment, thermal resistance differs according to whether a PCB is horizontal or vertical.

・If a PCB is oriented vertically, convective heat transfer is increased, and the thermal resistance drops.

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・In order to reduce man-hours at all stages from design through to mass production, front-loading of thermal design is making headway.

・In order to promote front-loading, accurate thermal simulations are of key importance.