Design of Heat Sinks

P M V Subbarao

Mechanical Engineering Department

IIT Delhi

Success Based on Cooling Challenges …….

Semiconductor Heat Sinks

• Thermal Physics of Semiconductor Heat Sinks (SHS) is an interesting and urgent topics of semiconductor devices cooling.

• While in the long ago passed years of easy-happy Semiconductor Heat Sinks solutions.

• Those were practically of the single uniform design as, for example, this one for the 486X chipset from the early 90s.

• The continuous more and more stiff and demanding interest in the heat to be evacuated from the electronic devices, more effectively, brought up the newer styles for SHS.

• Specially designed forced convection along the short fins or

• Guided directed air flux down the longitudinal fins

1990’s 486 Chip Heat Sink

Forced Convection with Short Fins

guided air flux down the longitudinal fins

Combined two kinds of the pin fins

Types of Heat Sinks• Thermal conductivity and cooling surface area must always be weighed

against material and manufacturing costs. • Most heat sinks are made from aluminum, because of its low thermal

resistance, light weight, and low cost. • Copper is also used for heat sinks; although lower in thermal resistance than

aluminum, its cost and greater weight make it less desirable for many applications.

• Stamped heat sinks : made from a single sheet of metal, which is cut and bent to give the desired thermal properties.

• Extruded heat sinks : are very cost effective and provide good thermal performance.

• Many basic shapes can be provided off-the-shelf. • Heat Sink Castings : provide a cost-effective solution for high-volume, stable

applications. • Bonded-fin heat sinks : are made by bonding fins, fabricated from sheet

metal or through extrusion, to an extruded base.• This process increases the surface area over a similar extruded piece,

reducing the thermal resistance by A 1/2to 2/3.

• Folded-fin Heat Sinks : are bonded-fin assemblies with complex fin shapes.

• By folding the fins over themselves, these assemblies provide a large surface area in a confined space.

Geometrical & Thermal Design Constraints

In forced convection, the critical parameters for heat sink design include local upstream air temperature (Ta) and velocity (V)

The airflow duct cross-sectional area (Wduct, Hduct) play an important role in heat sink design.

• The most important factor for the airflow available is the maximum volume the heat sink can occupy.

• Wmax, Lmax, and Hmax

• The design must accommodate the component specifications, such as the power dissipated (P) and the maximum junction temperature (Tjmax).

• The maximum heat sink weight, also play a role in the design.

• Another important factor during the design phase is the maximum allowable cost of the thermal solution

Steps in Design of Forced Convection Heat Sinks

Analytical modeling Maximization of heat dissipation Least-material optimization Design for manufacture

Tcase

Design Calculations for Fin Arrays – Thermal Resistance

• In order to select the appropriate heat sink, the thermal designer must

• first determine the maximum allowable heat sink thermal resistance.

• To do this it is necessary to know the

• maximum allowable module case temperature, Tcase,

• the module power dissipation, qmod, and

• the thermal resistance at the module-to-heat sink interface, Rint.

• The maximum allowable temperature at the heat sink attachment surface, Tbase, is

given by

intmod RqTT casebase

b

s

b

TbaseRint

• The maximum allowable heat sink resistance, Rmax, is then given by

mod

,max, q

TTR inairbase

th

finfinfinbase ANAhR

1sinkheat

1

.

fin

finfin

N

NWS

•The thermal resistance of the heat sink is given by

• The gap, S, between the fins may be determined from

Constant air velocity

Constant volumetric flow rate

Heat Sink Pressure Drop

• To determine the air flow rate it is necessary to estimate the heat sink pressure drop as a function of flow rate and match it to a curve of fan pressure drop versus flow rate.

• A method to do this, using equations presented here.

• As in the previous article, the heat sink geometry and nomenclature used is that shown Figure 1.

Cooling Medium

• By adding a fan to any heat sink it changes from passive cooling, using ambient airflow, to active cooling.

• Adding a fan to a typical heat sink almost always improves thermal performance.

• A 50 mm square heat sink with 12 mm hight, for example, has a thermal resistance of 5 °C/W with natural convection. Adding a fan drops this resistance to 1.2 °C/W.

• Most demanding applications use liquid cooling in place of air cooling further improves heat sink performance.

• To dissipate 1000 watts of heat with a 10 °C temperature rise would take thousands of times less volumetric water flow than airflow.

• Liquid cooling can dissipate more heat with considerably less flow volume,

• maintain better temperature consistency, and • do it with less local acoustic noise.

Fan Characteristic Curves

Effect of Changing A Fan

Pressure Drop Curves

Effect of number of fins and fin height

Thermal Resistance

Junction Temperature Vs Number of Fins

Closure

• a fan with a different fan curve is employed, the flow rates will change and the optimum heat sink design point may change as well.

• The important point is that to determine how a heat sink will perform in a given application both its heat transfer and pressure drop characteristics must be considered in concert with the pressure-flow characteristics of the fan that will be used.

• It should also be noted that an underlying assumption is that all the flow delivered by the fan is forced to go through the channels formed between the heat sink fins.

• Unfortunately this is often not the case and much of the air flow delivered by the fan will take the flow path of least resistance bypassing the heat sink.

• Under such circumstances the amount of flow bypass must be estimated in order to determine the heat sink performance.

Design Optimization

• One modify several independent variables to improve heat sink performance with respect to your design criteria.

• A larger heat sink surface area, for example, will improve cooling, but may increase the cost and lead-time.

• Using the entire volume available will provide maximum cooling, but if this provides more cooling than necessary, you can reduce the heat sink volume to reduce its cost and its weight.

• Increasing the base thickness distributes heat more uniformly to the fins if the package is smaller than the heat sink, but increases weight.

• The interface material can have a significant affect on assembly costs as well as on thermal resistance.

• Thicker fins, on the other hand, provide more structural integrity and may be easier to manufacture, but increase the weight for a given thermal resistance.

Design Optimization

• A heat sink that cools adequately fulfills only part of the objective.

• Optimizing the design creates the best available heat sink solution for the application and benefits the overall system design.

• Some advantages of optimization include: • minimization of thermal wake effects: the impact of heat sinks

on downstream components • accommodation of changes in system CFM due to additional

heat sink pressure drop • weight reduction to pass shock and vibration tests • elimination of extra heat sink support (additional holes and real

estate in the PCB) • reduction of costs gained from use of the same heat sink for

multiple components and use of off-the-shelf catalog parts.