Chemistry and interfacial mechanics of a phase change material on metal surfaces

Filling up of the micro voids with thermal interface material will maximize the heat flow path and hence minimizing thermal contact resistance.. Thermal Resistance TR Measurements Surfac

Chapter 1

Introduction

1.1 Thermal Management in IC Packaging

Heat dissipation in IC packages is becoming very vital with chips getting smaller and running at ever increasing speeds Intel® microprocessors advanced from having 1.5-micron lines on INTEL386TM [1] to 0.13-micron in the current Pentium® 4 [2] within

a space of 7 years The International Technology Roadmap for the Semiconductor (ITRS) is continually studying the trend in circuitry lines, which is also called the technology node, reduction The figure below is an adaptation from the ITRS reports [3-5] The thinner these lines go to, the denser the circuitry will be; leading to extensive increases in heat generation in microprocessor packages

0.00 0.05 0.10 0.15 0.20

Figure 1.1: Trend in technology node over the years

The increase in heat flow from IC chips is also apparent in the trend of power dissipation for high-performance chips reported by ITRS as shown in the following figure Increasing amount of power dissipated has to be offset by the efficient cooling

100 150 200 250 300

Figure 1.2: Power dissipation and maximum junction temperature in IC chips

A major challenge in the semiconductor field is, therefore, the ability to manage the heat in the IC chips without compromising on the performance of the device This management of thermal energy is very crucial because heat has many detrimental effects on the device It has been reported [6] that failure rates have near exponential dependence on device temperature When a certain upper critical temperature is reached, important parts of the device may cease to function Furthermore, temperature cycles that result from switching on and off the device can also cause problems, even if the operating temperature does not hit the upper critical temperature This leads to another problem, the reliability of the device It is important to dissipate the heat gradually so that the temperature of the device will not reach its ceiling maximum and minimum values This will reduce the cycling temperature (the difference between the maximum and the minimum) The figure below shows how a gradual cycling reduces the maximum cycling temperature

Normal cycling Gradual cycling

Figure 1.3: Reduced maximum temperature due to gradual cycling

1.2 Thermal Interface Material

IC chips are packaged to provide protection for the intricate chips There are many layers in a packaged chip with every layer performing crucial roles in ensuring proper functioning Below is a schematic, not drawn to scale, of a typical packaged IC chip

Figure 1.4: Schematic diagram of a packaged chip

When the chip is powered up, the die gets hot and heat must effectively be transferred out to prevent overheating Heat spreaders are incorporated to aid the spreading of heat from the small die to the big heat sink and eventually to the ambient Improvements on substrates and heat sinks, which are the junction of the actual IC and the ambient, can constructively affect thermal dissipation It is of equal great advantage that interfacial heat path to the heat sink be improved The layers in the package create more interfaces for the heat to pass and these solid interfaces become the bottleneck of the heat transfer Solid surfaces, not being atomically smooth and straight, may be warped and rough The following figure shows how 3 different surfaces and mated with an atomically smooth surface

Figure 1.5: Schematic diagram of surfaces (a) Perfectly mated surface (b) Warped surface (c) Rough Surface

As seen, the warped and rough surface that are common in the surfaces used in the IC packages, create pockets of air trapped in the interface These poorly mated surfaces cause a rise in contact resistance As a result of the impedance, there will be a temperature drop across the interface This phenomenon is discussed in Chapter 2

Air gaps

Thermal interface material

Figure 1.6: Utilizing thermal interface material to fill air voids

In today’s IC packages, thermal interface materials (TIM) are utilized to fill up the micro air voids, as seen in the above figure The conductivity of the thermal interface

is not the only deliberation because the ability of these materials to conform to the micro roughness at the interfaces is just as essential Filling up of the micro voids with thermal interface material will maximize the heat flow path and hence minimizing thermal contact resistance The use of a solid, for instance, copper, is therefore not a feasible choice for a TIM Commonly used TIM in the semiconductor industry include thermal grease, polymeric adhesives and phase change materials (PCMs) [7] PCM have, in majority cases, replaced grease [8] This is because they are cleaner to use and provide much greater resistance to the liquid state being pumped out of the interface, unlike thermal greases that are prone to pump out [9]

1.3 Overview of Research

The research is a collaboration between the Department of Chemistry, National University of Singapore and Honeywell Electronic Materials, Honeywell (S) Pte Ltd This research focuses on its thermal management using a commercial Thermal Interface Material (TIM), which is a Phase Change Material (PCM), produced by Honeywell The diagrams in the following figure illustrate the focus of the research

Thermal Resistance (TR) Measurements Surface Characterization PCM Characterization

(a) Areas of interest within a packaged chip

Chemistry and Interfacial Mechanics of a Phase Change Material on Metal Surfaces

Interaction of PCM on Metal Surfaces

Surface Characterization

Thermal Resistance Measurement Setup

PCM Characterization

(b) Classification of focus areas Figure 1.7: Illustration of research scope

The initial stage of the work is focused in three areas: PCM characterization, design and construction of thermal resistance measurement setup and surface characterization of metals The PCM and metal surfaces were characterized to understand their basic properties The thermal resistance measurement setup can then

be utilized to understand how the PCM material on metal surfaces perform with varying application methods like temperature and pressure, process conditions, assembly conditions and reliability tests These studies would enable better understanding of the performance of the PCM as TIM on metal surfaces

Chapter 2

Theory

2.1 Thermal Test Methods and Standards

There are various ways by which a packaged chip is tested in the area of thermal management The following is an account of thermal conduction followed by a short review of the thermal test methods that are employed

Conduction is the exchange of internal energy from one body to another, or from one part of a body to another part The exchange is the transfer of kinetic energy of the molecules by direct collision or the drift of free electrons, without an appreciable displacement of the matter comprising the body Convection, on the other hand, is the heat transfer in a fluid by the mixing of one portion of the fluid with another portion due to gross movements Radiation is the heat transfer via electromagnetic radiation emmited by a thermally excited body A medium is not always necesary for heat transfer by radiation

The primary mode of transfer of interest in this project is conduction The relationship governing conduction is Fourier’s law [2] The following is a demonstration of the derivation of the law, relied on works by Chapman [1], Bar Cohen and Kraus [2]

Considering a thin plate of material (Figure 2.1) with cross-sectional area, A and thickness, dx Let each of the two sides be uniformly maintained at temperature T1

and T2 over the surface.This law indicates that the rate of heat flow by conduction through a material, q is proportional to the cross-sectional area of the material, A , and the temperature gradient dT/dx

P

x S

Figure 2.2: Schematic of a thin plate of material within a homogeneous and isotropic

solid Selecting a point P on surface S, within the solid, we have an area δA, which is part

of the surface S containing P, and having a thickness δx in the direction normal to the

surface at P If the difference between the temperature of the back face of the plate and its front face is δT, and if δA is chosen small enough so that δT is essentially uniform over it, the rate of heat flow across the plate, δq is

δq = -k δA

x

Tδ

δ

where the negative sign shows that heat flow is taken to be positive if δT is negative

in the direction of increasing x, the normal displacement The heat flow per unit area known as the heat flux, f, can be calculated using δq/δA When δA → 0, the heat flux through the thickness δx, becomes

f = dA

dq

= -kx

Tδ

dT

The above equation is Fourier’s conduction law after the French mathematician who first made the an extensive analysis of heat conduction It states that the flux of heat conducted (energy per unit time per unit area) across a surface is proportional to the temperature gradient taken in a direction normal to the surface at the point under consideration

The total rate of heat transferred across the finite surface S would be

dx

dTk

of material (Figure 2.3) Assuming that there are no heat source or sink in within a fixed area of material,

q x+dx

q G generated within volume

qy

q z+dz

q x

Figure 2.3: Differential for the derivation of the general heat conduction equation

As energy cannot be created or destroyed but can be transformed from one form to another, adding the amount of heat going into the subvolume (qin) and the amount of heat generated within it (qG) equates to total amount of heat coming out of the subvolume (qout) and heat stored within it (qs)

qin + qG = qout + qs

or

qin - qout + qG = qs (eqn 2.8)

Considering the x direction, heat entering the subvolume, qx is

qx = -k A

x

Tδ

δ = (-k

x

Tδ

δ

−δ

δ+

δ

The difference between the amount of heat entering and leaving the subvolume in the

x direction is, therefore,

x

Tk

δδ

y ⎜⎜⎝⎛ δδ ⎟⎟⎠⎞δ

δδ

δδ

δ

+

z

Tk

δδ

δ

dx dy dz (eqn 2.14) x

Tk

δδδ

The heat transmitted out of the subvolume is related to the heat stored within the subvolume can come from an electrical or electronic heat dissipation or from a chemical reaction With the volumetric rate of heat generation, qi

dt

dT

where u is the internal energy, c is the specific heat capacity, dm = ρ dx dy dz, where

ρ is the density of the material,

dt

du = ρ c

t

Tδ

Tk

δ

+

y

Tk

δδ

δ

+

z

Tk

δδ

δ

+ qi = ρ c

t

Tδ

δ (eqn 2.18) where the common dx dy dz terms have been cancelled

Assuming that k, c and ρ are independent of temperature, direction, and time, a general equation of heat conduction can be obtained:

δ + 2

2

z

Tδ

δ + k

qg = α

1t

Tδ

δ + 22y

Tδ

δ + 22z

Tδ

δ + k

qg = 0 (eqn 2.20)

With an absence of heat sources, 2

2

x

Tδ

δ + 2

2

y

Tδ

δ + 2

2

z

Tδ

δ =0 (eqn 2.21)

The general equation of heat conduction requires the values of the thermal conductivity, specific heat and thermal diffusivity of the material to be known Heat conduction path through changes in physical dimensions involves spreading resistance and that across interfaces involves contact resistance Thermal resistance is

a more rigorous physical property, compared to thermal conductivity It indicates the ease at which heat can be conducted with the consideration of physical size of the material These physical properties will be explained in some details in the sub-

sections that follow

2.2 Physical Properties of Materials

In most, if not all heat conduction mathematical analyses, at least one of the following physical properties is essential

• Thermal conductivity

• Specific heat capacity

on the chemical composition, the phase, the structure and the temperature and pressure the material is subjected to Thermal conductivity was introduced in Fourier’s Law [2]

where q = amount of heat transferred

k = thermal conductivity of the material

A = cross-section area of material

L

T

∆

= temperature gradient per unit length of the material

A higher thermal conductivity shows a greater ability of the material to conduct heat

A commonly used unit of thermal conductivity is W/mK

2.2.2 Specific Heat Capacity

Specific heat capacity of a material, on the other hand, is the change in temperature

of the material with the amount of energy stored in it Specific heat capacity is used

in the general equation of heat conduction (eqn 1.19)

From the first law of thermodynamics [10], the changes in enthalpy at constant pressure and changes in internal energy at constant volume in reversible processes represent heat transferred to and from a system Taking the amount of heat transferred per unit temperature difference during a reversible, constant pressure process as cp, specific heat capacity at constant pressure is

p p

where h = specific enthalpy

u = specific internal energy

T = temperature

subscripts p and v = differentiation done at constant pressure and volume,

respectively

2.2.4 Spreading Resistance

Not all thermal conduction takes place across media of the same cross-sectional areas More often than not, conduction takes place through a solid or across an interface with different cross-sectional area In an electronic package, for example,

heat is conducted through many layers of different cross-sectional areas The figure

below illustrates the example

Figure 2.4: Schematic diagram of heat flow in a packaged chip

In this type of cases, the spreading resistance becomes a concern This spreading

phenomenon has been discussed [6, 11-13] Mathematical analyses have been done to

predict the spreading resistance The work of Song, Lee and Au [12] displays

equations that have verification with experimental examples 23 equations were

involved in the predictions of the spreading resistance These equations will not be

further elaborated, as it is not in the scope of this study

2.2.5 Thermal Resistance and Thermal Impedance

Thermal resistance and contact resistance are, like thermal conductivity, measures of

how well heat is conducted Thermal conductivity, of homogeneous materials, is

independent of physical dimensions while thermal resistance and contact resistance

are very much dependent on the physical dimensions of the material Thermal

resistance is [11] the temperature gradient caused by a unit of heat flow through a

material of a given size From Fourier’s Law (equation 2.2)

θ = A

k

L q

q = amount of heat transferred

A = cross-section area of material

k = thermal conductivity of the material

L = length of the material

From equation 2.26, it can be seen that thermal resistance of a homogeneous material

is proportional to the distance of heat travelled

Thermal impedance, on the other hand, is the product of the thermal resistance and the cross-sectional area

Thermal impedance = θ.A =

k

L A q

∆.where θ = thermal resistance

∆T= temperature gradient

q = amount of heat transferred

A = cross-section area of material

k = thermal conductivity of the material

L = length of the material

2.2.6 Contact Resistance

When heat conduction involves heat travel through 2 solids in contact, an additional

temperature gradient exists together with the normal temperature gradient A

significant temperature drop will be observed due to the interface between the two

solids The phenomena that causes this is contact resistance [11] No matter how well

a surface is polished, surface irregularities still exist Intimate solid contact will

therefore inevitably be accompanied with pockets of air These pockets of air are very

much non-conductive relative to the solid in contact and the heat travel through the

air, consequently, becomes the bottleneck of the conduction path This gives rise to

the extra temperature drop

hot cool

Temperature

Distance

x x x

x x x

interface

Figure 2.5: Temperature profile of two solids in contactContact resistance can be accounted for to a certain extent Mathematical treatments

are discussed in works of Kraus & Bar-Cohen [6] The accountability of contact

resistance is dependent on many parameters These parameters are complex, given

the numerous uncertainties present in real surfaces The parameters of concern

include: [6]

ƒ Number of intimate contacts

ƒ Shape of contact points: circular, elliptic, band, or rectangular

ƒ Size and arrangements of contact points

ƒ Geometry of contacting surfaces with regard to roughness and waviness

ƒ Average thickness and fluid (gas, liquid or vacuum) of void space

ƒ Pressure and conductivity of void space

ƒ Hardness of contacting surfaces

ƒ Average temperature of interface

ƒ Contact pressure and contact history of surface

ƒ Duration of contact with regard to relaxation effects

ƒ Vibrational and directional effects

ƒ Contact cleanliness

Filling the air gaps with conductive and compliant materials, like thermal grease, and using a very high contact pressure normally reduces contact resistance Advances in materials technology have seen some other materials fill the air gaps and one such material is the phase change material under study in the present work

Thermal test methods for IC chips such as microprocessors, therefore, may involve any of the above mentioned physical properties It is of special interest in this project that the thermal resistance be measured

2.3 Methods of Thermal Resistance Measurements

In the electronics industry, a range of thermal resistance is measured The measurements of thermal resistance can be carried on individual components of the chip, like the substrate, die or interface material; or the entire chip itself Typically, the thermal performance of a packaged chip is measured by the ability of the package

to dissipate the heat that is produced within it to the ambient A number of methods have been used [15] but most of these methods are concerned with the determination

of thermal resistance of the entire chip The types of methods are:

• Optical

• Chemical

• Physical

• Electrical

The optical method involves IR scanning of the chip surface This method can only

be used on un-encapsulated chips as it presents temperature profile on the surface The optical method is time consuming and costly

Chemical method of measuring thermal resistance, on the other hand, requires the chip surface to be coated with a thin layer of temperature – indicating chemical like liquid crystal material Similar to the optical method, the chemical method provides the temperature profile of the surface and therefore cannot be used on encapsulated chips There are also a number of disadvantages in using the chemical method Significant expertise is required in selecting and applying the chemical Although

chemical application is more economical compared to the optical method, it may pose resolution and contamination problem; and demands a lot of time

The third method, which is the physical method, makes use of direct attachment of very small temperature-measuring devices (like thermocouples) on the chip surface This technique is relatively inexpensive but placing a temperature-measuring device

on the surface, without affecting the heat source, is a challenge Resolution will, likely, be compromised Like the two previous methods, the physical method needs a lot of time and cannot be used on encapsulated chips

The fourth and last approach to thermal measurement is the electrical method This measurement mode uses pre-calibrated temperature sensitive parameter of the chip The procedure is the fastest, in comparison with the other 3 methods The level of proficiency required to obtain accurate and repeatable results is minimal Encapsulated chips can have thermal resistance measured using this method This method, however, has even lower resolution than chemical and optical methods, and the set-up is moderately expensive This method is widely used and has been discussed in several papers [16-21]

A major part of this project, however, focuses on the thermal resistance of a commercial thermal interface material (TIM) As the name implies, TIM is a material sandwiched between a chip and a heat spreader and/or a heat sink Details of thermal interface materials are discussed in Section 1.2 At first glance, none of the 4 mentioned methods could be used to measure the thermal resistance of TIM Measurements methodologies have to be simulated that of an actual package and

therefore the TIM has to be sandwiched The optical, chemical and physical methods require the surface of the material of interest to be exposed Electrical method, on the other hand, requires the TIM to be circuited into a device The thermal resistance of the TIM is therefore, difficult to obtain using those methods

Methods, using standards (MIL-I-49456A, JESD15, ASTM D5470-95 and ASTM E1530-99) as guidelines, have been created The ASTM methods have very stringent requirements to suppress the contact resistance contributions to the measured values For example, the contact requirement for ASTM D5470 is a force of 3.0 ± 0.1 MPa These values are very high compared to the normal pressure used in semiconductor packaging, which is less than 0.3 MPa The surface smoothness requirement for the ASTM method is also very stringent The contact surfaces have to be smoothly finished to within 0.4 µm A reasonably accepted smoothness by the industry for the surfaces for the measurements is below 25.4µm

It is crucial that the development for a thermal resistance measurement set-up be continued Some companies have built their own set-up with adjustments deemed necessary, to the standards No commercial set-up is available in the market and a basic set-up must, therefore, first be built and developed for this study

The thermal resistance measurements carried out in some laboratories [22-27] concentrated on the bulk properties of the materials Physical properties like thermal conductivity and thermal resistance of the material can be obtained The experimental methodologies involve sandwiching the material of interest with metal blocks Temperature drops across the metal blocks are then measured Due to the

sandwiching of the TIM, contact resistance becomes a crucial issue It can be anticipated to have influence of sizeable proportions to the measured thermal resistance The ASTM D5470 is the most widely used

Thermal resistance values are important in the process of selecting the right material

of the right thickness for a particular application The work by Rauch [27] exhibits how thermal resistance measurements aid in material selection

One other aspect, which is the effects of different surfaces on the thermal performance, however, has not been touched upon It is the main aim of this study to investigate the effects of different metal surfaces on the thermal resistance of the TIM The following chapters will look into the building and development of a set –up for this study Rigorous validations have been planned to verify the working ability and integrity of the built system

H H Force

T1 T2 T3 T4

Insulator

Insulator

Insulator Guard Heater

Heater Upper metal bar

Lower metal bar Sample

Cooling Unit

T5 T6

Reference Calorimeter

Figure 3.1: Schematic of thermal measurement apparatus according to ASTM D5470

General features of the standard include:

1 The sample is placed between two metals bars with high thermal conductivity and smooth finish within 0.4 µm to the approximate true plane

2 A cooling unit (comprised of one of the metal bar) with constant temperature bath maintained uniformly at ± 0.2°C

3 A force of 3.0 ± 0.1 MPa pressing the stack to minimize the contribution of contact resistance in the measurements

4 Upon attaining equilibrium, the temperatures at the bars are taken and the thermal resistance can be calculated using the following equations:

θ = (TA-TD) / q (eqn 3.1) where:

A = Area of reference calorimeter

λ = thermal conductivity of the reference calorimeter

T5-T6 = temperature difference between thermocouples of reference calorimeter

For setup with no reference calorimeter:

where:

I = electrical potential applied to the heater

V = electrical current flow in the heater

The temperature on the surface contacting the sample, TA and TD can be derived

from the following:

)TT(d

dT

A

B 2

)TT(d

dT

C

D 3

where:

TA = temperature of the upper metal bar surface in contact with the sample

TD = temperature of the lower metal bar surface in contact with the sample

dA = distance between temperature sensors in the upper metal bar

dB = distance from the lower sensor to the lower surface of the upper metal bar

dC = distance between temperature sensors in the lower metal bar

dD = distance from the upper sensor to the upper surface of the lower metal bar

The features of the ASTM are normally modified [22-23, 28-33] to suit the provisions of the semiconductor industry The requirements for the smoothness of the metal bars and the applied force are too stringent and are not practical for the applications in the industry These requirements are added in the ASTM to suppress the contribution of contact resistance in the measurements The smoothness criterion

is very difficult to achieve given the heavy usage of the apparatus and the applied force is too great for the industry to comply with Furthermore, the measurements at each high pressures would not be meaningful in the industry The normal applied force is 0.2-0.3 psi but the required pressure in the standard is 3.0 ± 0.1 MPa A setup has been designed and built based on these issues and the industry specific requirements

3.2 Instrument Design and Calculations

3.2.1 Physical Design

The set up consists primarily of a test stand, 2 intermediate blocks, a temperature and

a pressure control system as well as a data acquisition system A schematic diagram, not drawn to scale, and an actual photograph of the setup is as shown in Figure 3.2a and 3.2b respectively The test stand is made of stainless steel There is an opening in the test stand to insert the intermediate blocks The size of the opening is determined

by the extent of screw length after the different blocks and sample, are assembled

The temperature control system is made up of an aluminium heater jacket to house a 200W-cartridge heater and a water cooler jacket that is connected to an 8005 Polyscience water circulator A Teflon jacket to prevent excessive heat loss insulates the heater jacket Type-T thermocouples with 0.6mm diameter and accuracy of +0.1oC are used to monitor the temperature of the heater A 2132 Eurotherm Controls temperature controller controls the cartridge heater with reference to a thermocouple The pressure applied on the stack is monitored using a Honeywell universal

controller, UDC 1000 via a Data Instruments SC500 load cell The application of pressure can be done via the manual screw press or automatically using the actuator

Universal controller

Terminal block

Figure 3.2a: Schematic of the thermal resistance measurement setup

Figure 3.2b: Photograph of the thermal resistance measurement setup

Three t of two

to maximize the accuracy The calculation of the thermal resistance is, as a result, slightly modified The figure below is an example of the temperature profile measured

emperature measurements are taken at each Aluminium block instead

40 45 50 55 60 65 70 75 80

Distance from sample (cm)

y = -18.271x + 55.677

55.5 56.0 56.5 57.0 57.5 58.0 58.5 59.0 59.5 60.0

41.0 41.5 42.0 42.5 43.0 43.5 44.0 44.5 45.0

Figure 3.3: Graph of temperature drop across the intermediate blocks

As soon as the cartridge heater is switched on, a temperature junction within the intermediate blocks occur The sample of interest (or often referred to as the interface material) gets heated and cooled by the aluminium blocks above and below it respectively The thermal resistance value can then be calculated using the following

where

θ = thermal resistance of sample

∆T = temperature drop across the sample (difference in the two intercepts A & B)

q = amount of heat transferred to the sample

The amount of heat, q, transferred to the interface is calculated from the equation

kAl = thermal conductivity of aluminium

AAl = cross-sectional area of the aluminium block

The virtual instrument has the following main features:

1 The temperature readings from the six thermocouples are recorded and displayed

in a waveform with digital indicators The ASTM D5470 states that equilibrium is reached when the difference between two successive sets of temperature readings, taken at 15 minutes intervals, are less than ±0.2oC Indicators of the equilibrium are placed in the program for an online monitor of the equilibrium status

2 The calculations for the thermal resistance measurement and the amount of heat supplied to the sample are done to good approximate values as the analyses proceed

3 Every temperature data point from the analyses can be saved into a spreadsheet with specified frequency

4 The temperature data were then processed using the LINEST worksheet function

to calculate the amount of heat and the thermal resistance

(a) Front panel of the virtual instrument

3.3 Calibration Methodology

Calibration experiments are carried out to verify how reproducible, robust and rugged the setup is In place of a PCM, brass and Teflon calibration blocks of 25.4mm (1 inch) in diameter of various lengths were made The calibration experiments are classified in the following areas

1) Effect of interface material

2) Effect of ambient temperature

3) Effect of water bath temperature

4) Effect of thermal grease at aluminium / sample interfaces

5) Effect of air movement

6) Effect of operator

3.3.1 Effect of interface materials

Two types of materials with very different values of thermal conductivities were studied These two materials were made into blocks with different lengths but the same diameter size as the aluminium blocks As much as possible, the smoothness of these blocks was kept below 25µm

Apart from the absolute value of the thermal resistance of the material at the particular thickness, this set of experiments enables the values of contact resistance and effective thermal conductivity of the material to be obtained

The thermal conductivity of the material can be determined from at least 2 measurements of the thermal impedance, at the same temperature and contact

pressure Thermal impedance is the product of thermal resistance and the cross section area From equation 3.8, the reciprocal of the thermal resistance vs thickness plot will yield the thermal conductivity of the material of interest

k

L q

q = amount of heat transferred

A = cross-section area of material

k = thermal conductivity of the material

L = length of the material

The typical unit used for thermal resistance measurements is oC/W

3.3.2 Effect of ambient temperature

The ambient temperature may affect the equilibration of the experiment For this set

of experiments, the air conditioner was set to the lowest temperature (18oC), to a temperature it is normally operating (21oC), the highest temperature (29oC)

3.3.3 Effect of water bath temperature

Similar to changing the cartridge heater temperature, changing the water bath temperature determines the amount of heat flow through the intermediate blocks The difference is that while the heater temperature is increased, the water bath will get

heated up as well until equilibrium is achieved When the water bath is heated, on the other hand, the heater temperature is fixed at 65oC This temperature was chosen because there will be an appreciable temperature drop across the sample and the blocks will not be too hot to be handled

3.3.4 Effect of thermal grease at aluminium / sample interfaces

Irregular surfaces give rise to contact resistance This set experiment is similar to the set of experiments involving the study of the effect of interface materials but only brass blocks were used Thermal grease was applied at the brass/aluminium interface

to minimize contact resistance and hence increase the amount of heat supplied to the interface material, the brass block

3.3.5 Effect of air movement

This study analyses the sensitivity of the thermal measurements to air movement around the set up A fan was placed at 0.5 meters and 2 meters away from the set up Various fan speeds as well as swinging motions were used to vary the air movements

3.3.6 Effect of operator

The precision of this setup is tested in this study 3 different operators independently performed measurements of the same sample for 4 times The nature of the sample and other details of this study are proprietary to the industrial partner The results are

then tabulated and statistically analysed using Gage Repeatability and Reproducibility (GR&R)

3.4 Calibration Results and Discussions

3.4.1 Effect of Interface materials

The thermal resistance of the 2 interface materials tested has 1 – 2 orders of magnitude difference Teflon, a good insulator has much higher thermal resistance than brass, which is a good conductor of heat The figures below show the results to the experiments

(b) Thermal Impedance and amount of heat transferred to Teflon of varying thickness

Figure 3.5: Graphs of thermal resistance and amount of heat transferred through

interface materials with different thickness

The thermal resistance of the interface material increases with increasing thickness The contact resistance of the material is obtained from the thermal resistance-intercept The brass block has a contact resistance value of 7.70x10-5 Km2/W and Teflon 3.00x10-3 Km2/W The much higher contact resistance of Teflon can be attributed to the very low thermal conductivity of the material The effective thermal conductivity, on the other hand, is calculated from the reciprocal of the best fit linear trend line of the thermal resistance vs thickness graph The brass block has an effective thermal conductivity of 166 W/mK and Teflon 1.00 W/mK The reported values of bulk thermal conductivities of brass and Teflon are 120 W/mK [34] and 0.25 W/mK [34] respectively Theoretically, the effective conductivity is lower than that of the bulk value On the contrary, the observed experimental values of the effective thermal conductivities are much higher than the reported bulk conductivities

Thermal conductivities, very similar to thermal resistance, can be affected by many factors They include physical state and chemical composition of the material, temperature and pressure The condition of the experiment from which the reported value was taken is not specified The reported brass conductivity has composition 70% copper and 30% zinc The brass used, on the other hand, has a composition of 57.5% copper and 42.5% of other metals mainly zinc The variation in the value can result from the difference in the composition For Teflon, no mention of the material grade mentioned Teflon, being a soft material may have their properties altered by the firm screwing, resulting in the much higher conductivity

It has been recently reported [47] that proving the accuracy of thermal resistance measurement setup based on the ASTM D5470 is problem Continuous effort is being done to look for a suitable standard material For the current work, the accuracy

of the setup is validated using reported data from the industrial partner The results are shown in Section 6.3

3.4.2 Effect of ambient temperature

The effect of ambient temperature on both the amount of heat transferred through the interface and the thermal resistance is insignificant It did not make any considerable difference if the experiment was carried out with the air conditioner turned on to the lowest or highest possible temperature It can therefore be ascertained the normal air conditioning will not affect the temperature readings