ASTM D5470-06(2011)
(Test Method)Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials
Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials
SIGNIFICANCE AND USE
This standard measures the steady state thermal impedance of electrical insulating materials used to enhance heat transfer in electrical and electronic applications. This standard is especially useful for measuring thermal transmission properties of specimens that are either too thin or have insufficient mechanical stability to allow placement of temperature sensors in the specimen as in Test Method E1225.
This standard imposes an idealized heat flow pattern and specifies an average specimen test temperature. The thermal impedances thus measured cannot be directly applied to most practical applications where these required uniform, parallel heat conduction conditions do not exist.
This standard is useful for measuring the thermal impedance of the following material types.
Type I—Viscous liquids that exhibit unlimited deformation when a stress is applied. These include liquid compounds such as greases, pastes, and phase change materials. These materials exhibit no evidence of elastic behavior or the tendency to return to initial shape after deflection stresses are removed.
Type II—Viscoelastic solids where stresses of deformation are ultimately balanced by internal material stresses thus limiting further deformation. Examples include gels, soft, and hard rubbers. These materials exhibit linear elastic properties with significant deflection relative to material thickness.
Type III—Elastic solids which exhibit negligible deflection. Examples include ceramics, metals, and some types of plastics.
The apparent thermal conductivity of a specimen can be calculated from the measured thermal impedance and measured specimen thickness if the interfacial thermal resistance is insignificantly small (nominally less than 1 %) compared to the thermal resistance of the specimen.
The apparent thermal conductivity of a sample material can be accurately determined by excluding the interfacial thermal resistance. This is accomplished by measuring the thermal impedance of di...
SCOPE
1.1 This standard covers a test method for measurement of thermal impedance and calculation of an apparent thermal conductivity for thermally conductive electrical insulation materials ranging from liquid compounds to hard solid materials.
1.2 The term “thermal conductivity” applies only to homogeneous materials. Thermally conductive electrical insulating materials are usually heterogeneous and to avoid confusion this test method uses “apparent thermal conductivity” for determining thermal transmission properties of both homogeneous and heterogeneous materials.
1.3 The values stated in SI units are to be regarded as standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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An American National Standard
Designation:D5470–06 (Reapproved 2011)
Standard Test Method for
Thermal Transmission Properties of Thermally Conductive
Electrical Insulation Materials
This standard is issued under the fixed designation D5470; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope* 3.1.1 apparent thermal conductivity (l), n—the time rate of
heat flow, under steady conditions, through unit area of a
1.1 This standard covers a test method for measurement of
heterogeneous material, per unit temperature gradient in the
thermal impedance and calculation of an apparent thermal
direction perpendicular to the area.
conductivity for thermally conductive electrical insulation
3.1.2 average temperature (of a surface), n—the area-
materials ranging from liquid compounds to hard solid mate-
weighted mean temperature.
rials.
3.1.3 composite, n—a material made up of distinct parts
1.2 The term “thermal conductivity” applies only to homo-
whichcontribute,eitherproportionallyorsynergistically,tothe
geneous materials. Thermally conductive electrical insulating
properties of the combination.
materialsareusuallyheterogeneousandtoavoidconfusionthis
3.1.4 homogeneous material, n—a material in which rel-
testmethoduses“apparentthermalconductivity”fordetermin-
evant properties are not a function of the position within the
ing thermal transmission properties of both homogeneous and
material.
heterogeneous materials.
3.1.5 thermal impedance (u), n—the total opposition that an
1.3 The values stated in SI units are to be regarded as
assembly (material, material interfaces) presents to the flow of
standard.
heat.
1.4 This standard does not purport to address all of the
3.1.6 thermal interfacial resistance (contact resistance),
safety concerns, if any, associated with its use. It is the
n—the temperature difference required to produce a unit of
responsibility of the user of this standard to establish appro-
heat flux at the contact planes between the specimen surfaces
priate safety and health practices and determine the applica-
and the hot and cold surfaces in contact with the specimen
bility of regulatory limitations prior to use.
under test. The symbol for contact resistance is R .
I
2. Referenced Documents 3.1.7 thermal resistivity, n—the reciprocal of thermal con-
ductivity. Under steady-state conditions, the temperature gra-
2.1 ASTM Standards:
dient, in the direction perpendicular to the isothermal surface
D374 Test Methods for Thickness of Solid Electrical Insu-
per unit of heat flux.
lation
3.2 Symbols Used in This Standard:
E691 Practice for Conducting an Interlaboratory Study to
3.2.1 l = apparent thermal conductivity, W/m·K.
Determine the Precision of a Test Method
3.2.2 A = area of a specimen, m .
E1225 Test Method for Thermal Conductivity of Solids by
3.2.3 d = thickness of specimen, m.
Means of the Guarded-Comparative-Longitudinal Heat
3.2.4 Q = time rate of heat flow, W or J/s.
Flow Technique
3.2.5 q = heat flux, or time rate of heat flow per unit area,
3. Terminology
W/m .
3.2.6 u = thermal impedance, temperature difference per
3.1 Definitions of Terms Specific to This Standard:
unit of heat flux, (K·m )/W.
This test method is under the jurisdiction of ASTM Committee D09 on 4. Summary of Test Method
Electrical and Electronic Insulating Materials and is the direct responsibility of
4.1 This standard is based on idealized heat conduction
Subcommittee D09.19 on Dielectric Sheet and Roll Products.
between two parallel, isothermal surfaces separated by a test
Current edition approved April 1, 2011. Published April 2011. Originally
approved in 1993. Last previous edition approved in 2006 as D5470 – 06. DOI:
specimen of uniform thickness. The thermal gradient imposed
10.1520/D5470-06R11.
on the specimen by the temperature difference between the two
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contacting surfaces causes the heat flow through the specimen.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
*A Summary of Changes section appears at the end of this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D5470–06 (2011)
This heat flow is perpendicular to the test surfaces and is practical applications where these required uniform, parallel
uniform across the surfaces with no lateral heat spreading. heat conduction conditions do not exist.
4.2 The measurements required by this standard when using 5.3 This standard is useful for measuring the thermal
two meter bars are: impedance of the following material types.
T = hotter temperature of the hot meter bar, K, 5.3.1 Type I—Viscous liquids that exhibit unlimited defor-
T = colder temperature of the hot meter bar, K, mation when a stress is applied. These include liquid com-
T = hotter temperature of the cold meter bar, K, pounds such as greases, pastes, and phase change materials.
T = colder temperature of the cold meter bar, K, These materials exhibit no evidence of elastic behavior or the
A = area of the test surfaces, m , and tendency to return to initial shape after deflection stresses are
d = specimen thickness, m. removed.
4.3 Based on the idealized test configuration, measurements 5.3.2 Type II—Viscoelastic solids where stresses of defor-
are taken to compute the following parameters: mation are ultimately balanced by internal material stresses
T = the temperature of the hotter isothermal surface, K, thus limiting further deformation. Examples include gels, soft,
H
T = the temperature of the colder isothermal surface, K, and hard rubbers. These materials exhibit linear elastic prop-
C
Q = the heat flow rate between the two isothermal surfaces, erties with significant deflection relative to material thickness.
W, 5.3.3 Type III—Elastic solids which exhibit negligible de-
thermal impedance = the temperature difference between the flection.Examplesincludeceramics,metals,andsometypesof
two isothermal surfaces divided by the heat flux through them, plastics.
K·m /W, and 5.4 The apparent thermal conductivity of a specimen can be
apparent thermal conductivity = calculated from a plot of calculated from the measured thermal impedance and mea-
specimen thermal impedance versus thickness, W/m·K. sured specimen thickness if the interfacial thermal resistance is
4.4 Interfacial thermal resistance exists between the speci- insignificantlysmall(nominallylessthan1 %)comparedtothe
men and the test surfaces. These contact resistances are thermal resistance of the specimen.
included in the specimen thermal impedance computation. 5.4.1 The apparent thermal conductivity of a sample mate-
Contactresistancevarieswidelydependingonthenatureofthe rial can be accurately determined by excluding the interfacial
specimen surface and the mechanical pressure applied to the thermal resistance. This is accomplished by measuring the
specimenbythetestsurfaces.Theclampingpressureappliedto thermal impedance of different thicknesses of the material
the specimen should therefore be measured and recorded as a under test and plotting thermal impedance versus thickness.
secondary measurement required for the method except in the The inverse of the slope of the resulting straight line is the
case of fluidic samples (Type I, see section 5.3.1) where the apparent thermal conductivity. The intercept at zero thickness
applied pressure is insignificant. The computation for thermal is the sum of the contact resistances at the two surfaces.
impedance is comprised of the sum of the specimen thermal 5.4.2 The contact resistance can be reduced by applying
resistance plus the interfacial thermal resistance. thermal grease or oil to the test surfaces of rigid test specimens
4.5 Calculationofapparentthermalconductivityrequiresan (Type III).
accurate determination of the specimen thickness under test.
TEST METHOD
Different means can be used to control, monitor, and measure
the test specimen thickness depending on the material type.
6. Apparatus
4.5.1 The test specimen thickness under test can be con-
6.1 The general features of an apparatus that meets the
trolled with shims or mechanical stops if the dimension of the
requirements of this method are shown in Figs. 1 and 2. This
specimen can change during the test.
apparatus imposes the required test conditions and accom-
4.5.2 The test specimen thickness can be monitored under
plishes the required measurements. It should be considered to
test with an in situ thickness measurement if the dimension of
be one possible engineering solution, not a uniquely exclusive
the specimen can change during the test.
implementation.
4.5.3 The test specimen thickness can be measured as
6.2 The test surfaces are to be smooth within 0.4 microns
manufactured at room temperature in accordance with Test
and parallel to within 5 microns.
Methods D374 Test Method C if it exhibits negligible com-
6.3 Theheatsourcesareeitherelectricalheatersortempera-
pression deflection.
ture controlled fluid circulators. Typical electrical heaters are
5. Significance and Use
made by embedding wire wound cartridge heaters in a highly
5.1 This standard measures the steady state thermal imped- conductive metal block. Circulated fluid heaters consist of a
ance of electrical insulating materials used to enhance heat metal block heat exchanger through which a controlled tem-
transfer in electrical and electronic applications. This standard perature fluid is circulated to provide the required heat flow as
is especially useful for measuring thermal transmission prop- well as temperature control.
erties of specimens that are either too thin or have insufficient 6.4 Heat flow through the specimen can be measured with
mechanical stability to allow placement of temperature sensors meter bars regardless of the type of heater used.
in the specimen as in Test Method E1225. 6.4.1 Electrical heaters offer convenient measurement of the
5.2 Thisstandardimposesanidealizedheatflowpatternand heating power generated but must be combined with a guard
specifies an average specimen test temperature. The thermal heater and high quality insulation to limit heat leakage away
impedances thus measured cannot be directly applied to most from the primary flow through the specimen.
D5470–06 (2011)
FIG. 2 Guarded Heater Test Stack
FIG. 1 Test Stack Using the Meter Bars as Calorimeters
for both the hot side and cold side meter bars. Surface
6.4.2 Heat flow meter bars can be constructed from high
temperaturescanalsobemeasuredwiththermocouplesthatare
conductivity materials with well documented thermal conduc-
located in extreme proximity to the surfaces although this can
tivity within the temperature range of interest.The temperature
bemechanicallydifficulttoachieve.Meterbarscanbeusedfor
sensitivity of thermal conductivity must be considered for
both heat flow and surface temperature measurement or for
accurate heat flow measurement. The thermal conductivity of
exclusively one of these functions.
the bar material is recommended to be greater than 50 W/m·K.
6.6 Thecoolingunitiscommonlyimplementedwithametal
6.4.3 Guard heaters are comprised of heated shields around
block cooled by temperature controlled circulating fluid with a
the primary heat source to eliminate heat leakage to the
temperature stability of 60.2 K.
environment. Guard heaters are insulated from the heat source
6.7 The contact pressure on the specimen can be controlled
and maintained at a temperature within 60.2 K of the heater.
and maintained in a variety of ways, including linear actuators,
This effectively reduces the heat leakage from the primary
lead screws, pneumatics, and hydraulics. The desired range of
heater by nullifying the temperature difference across the
forces must be applied to the test fixture in a direction that is
insulation. Insulation between the guard heater and the heat
perpendicular to the test surfaces and maintains the parallelism
source should be at least the equivalent of one 5 mm layer of
and alignment of the surfaces.
FR-4 epoxy material.
6.4.4 Iftheheatflowmeterbarsareusedonboththehotand
7. Preparation of Test Specimens
cold surfaces, guard heaters and thermal insulation is not
required and the heat flow through the test specimen is 7.1 The material type will dictate the method for controlling
computed as the average heat flow through both meter bars. specimenthickness.Inallcases,preparespecimensofthesame
6.5 Meter bars can also be used to determine the tempera- area as the contacting test surfaces. If the test surfaces are not
ture of the test surfaces by extrapolating the linear array of of equal size, prepare the specimen equal to the dimension of
meter bar temperatures to the test surfaces. This can be done the smaller test surface.
D5470–06 (2011)
7.1.1 Type I—Use shims or mechanical stops to control the Alternatively, screws or linear actuators can be used to control
thickness of the specimen between the test surfaces. Spacer thespecimenthicknessundertestforeasilydeformableTypeII
beads of the desired diameter can also be used in approxi- materials.
mately 2 % volumetric ratio and thoroughly mixed into the
8.3.3 Type III materials require enough pressure to exclude
sample prior to being applied to the test surfaces.
excess thermal grease from the interface and to flatten speci-
7.1.2 Type II—Use an adjustable clamping pressure to
mensthat arenot flat.Thiscanbeaslowas0.69MPa(100psi)
deflectthetestspecimenby5 %ofitsuncompressedthickness.
for flat specimens with low viscosity thermal grease or as high
This represents a trade-off between lower surface contact
as3.4MPa(500psi)fornon-flatspecimensorwhenusinghigh
resistance and excessive sample deflection.
viscosity thermal grease.
7.1.3 Type III—Measurethesamplethicknessinaccordance
8.4 Record the temperatures of the meter bars and the
with Test Method C of Test Methods D374.
voltage and current applied to electrical heaters at equilibrium.
7.2 Prepare specimens from material that is in original,
Equilibrium is attained when, at constant power, 2 sets of
as-manufactured condition or as noted otherwise. Remove any
temperature readings taken at 5 minute intervals differ by less
contamination and dirt particles. Do not use solvent th
...
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