This document discusses the design of heat sinks for semiconductor devices. It describes the evolution of heat sink designs from simple uniform designs in the early 1990s to more complex designs with features like short fins, longitudinal fins, and combined pin fins to improve convection cooling. The key factors in heat sink design are thermal conductivity, cooling surface area, material and manufacturing costs. Designs must consider parameters like air temperature, velocity, duct size, and power dissipation. Optimization of factors like fin geometry, number of fins, and base thickness can improve thermal performance while meeting other constraints.

1. Design of Heat Sinks
P M V Subbarao
Mechanical Engineering Department
IIT Delhi
Success Based on Cooling Challenges …….

2. 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

3. 1990’s 486 Chip Heat Sink

4. Forced Convection with Short Fins

5. guided air flux down the longitudinal fins

6. Combined two kinds of the pin fins

7. 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.

8. • 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.

9. 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.

10. • 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

11. Steps in Design of Forced Convection Heat Sinks
 Analytical modeling
 Maximization of heat dissipation
 Least-material optimization
 Design for manufacture

12. 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
int
mod R
q
T
T case
base 


b
s d
b
Tbase
Rint

13. • The maximum allowable heat sink resistance, Rmax, is then given
by
mod
,
max,
q
T
T
R in
air
base
th


 
fin
fin
fin
base A
N
A
h
R



1
sink
heat
1
.



fin
fin
fin
N
N
W
S
d
•The thermal resistance of the heat sink is given by
• The gap, S, between the fins may be determined from

14. Constant air velocity

15. Constant volumetric flow rate

16. 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.

17. 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.

18. Fan Characteristic Curves

19. Effect of Changing A Fan

20. Pressure Drop Curves

21. Effect of number of fins and fin height

22. Thermal Resistance

23. Junction Temperature Vs Number of Fins

24. 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.

25. 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.

26. 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.