ASTM D5470-95
(Test Method)Standard Test Methods for Thermal Transmission Properties of Thin Thermally Conductive Solid Electrical Insulation Materials
Standard Test Methods for Thermal Transmission Properties of Thin Thermally Conductive Solid Electrical Insulation Materials
SCOPE
1.1 This standard covers a test method for measuring thermal impedance of thin electrical insulation materials.
1.2 This test method is useful with either homogeneous or composite thermally conductive sheet material ranging from 0.02 to 10 mm in thickness.
1.3 This test method measures steady-state heat flux through a flat specimen. Calculations are made as if the specimens were homogeneous. In fact, these materials are usually not homogeneous, but the assumption does not detract from the usefulness of the test methods.
1.4 The term "thermal conductivity" applies only to homogeneous materials. Thermally conductive electrical insulating materials are usually heterogeneous since they typically include fillers, binders, reinforcements such as glass fiber mesh, or a layer of polymeric film. To avoid confusion, this test method uses "apparent thermal conductivity" for measurements of both homogeneous and non-homogeneous materials.
1.5 A limitation of using this test method to calculate apparent thermal conductivity is the problem of accurately determining the specimen thickness. To reflect the commercial practice of measuring thickness as manufactured rather than measuring thickness in an assembly, thickness is determined from measurements made at room temperature in accordance with Method C of Test Methods D 374.
1.6 Thermal impedance test data are influenced by contact pressures, specimen surface characteristics, and the existence of alternate paths for heat transmission which are not through the specimen. This test method determines thermal conduction properties under a specific set of conditions (including a 50°C average test temperature) which may not agree exactly with the conditions in an application. As a result, the degree of correlation between this test method and any particular application needs to be determined.
1.7 The values stated in SI units are to be regarded as standard.
1.8 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.
Note 1—Earlier versions of this document included a Method B (the Roiseland Method). This method is now deleted because of a lack of general support.
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Designation: D 5470 – 95 An American National Standard
Standard Test Methods for
Thermal Transmission Properties of Thin Thermally
Conductive Solid Electrical Insulation Materials
This standard is issued under the fixed designation D 5470; 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 (e) 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 safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
1.1 This standard covers test methods for measuring thermal
priate safety and health practices and determine the applica-
impedance of thin electrical insulation materials.
bility of regulatory limitations prior to use.
1.2 These test methods are useful with either homogeneous
or composite thermally conductive sheet material ranging from
0.02 to 10 mm thickness.
2. Referenced Documents
1.3 The test methods measure steady-state heat flux through
2.1 ASTM Standards:
a flat specimen. Calculations are made as if the specimens were
D 374 Test Methods for Thickness of Solid Electrical Insu-
homogeneous. In fact, these materials are usually not homo-
lation
geneous, but the assumption does not detract from the useful-
E 691 Practice for Conducting an Interlaboratory Study to
ness of the test methods.
Determine the Precision of a Test Method
1.4 The term “thermal conductivity” applies only to homo-
E 1225 Test Method for Thermal Conductivity of Solids by
geneous materials. Thermally conductive electrical insulating
Means of the Guarded-Comparative-Longitudinal Heat
materials are usually heterogeneous since they typically in-
Flow Technique
clude fillers, binders, reinforcements such as glass fiber mesh,
2.2 Military Specification:
or a layer of polymeric film. To avoid confusion, this standard
MIL-I-49456A Insulation Sheet, Electrical, Silicone Rub-
uses “apparent thermal conductivity” for measurements of both
ber, Thermally Conductive, Fiberglass Reinforced
homogeneous and non-homogeneous materials.
1.5 A limitation of using these test methods to calculate
3. Terminology
apparent thermal conductivity is the problem of accurately
3.1 Definitions of Terms Specific to This Standard:
determining the specimen thickness. To reflect the commercial
3.1.1 average temperature (of a surface), n— the area-
practice of measuring thickness as manufactured rather than
weighted mean temperature.
measuring thickness in an assembly, thickness is determined
3.1.2 composite, n—a material made up of distinct parts
from measurements made at room temperature in accordance
which contribute, either proportionally or synergistically, to the
with Method C of Test Methods D 374.
properties of the combination.
1.6 Thermal impedance test data are influenced by contact
3.1.3 heater/sensor, n—an assembly consisting of electri-
pressures, specimen surface characteristics, and the existence
cally insulated wire-wound coils, one for applying a measured
of alternate paths for heat transmission which are not through
quantity of heat energy into the assembly and the second used
the specimen. These test methods determine thermal conduc-
to sense the temperature in the assembly.
tion properties under a specific set of conditions (including a
3.1.4 homogeneous material, n—a material in which rel-
50°C average test temperature) which may not agree exactly
evant properties are not a function of the position within the
with the conditions in an application. As a result, the degree of
material.
correlation between these methods and any particular applica-
3.1.5 thermal conductivity (l), n—the time rate of heat flow,
tion needs to be determined.
under steady conditions, through unit area, per unit temperature
1.7 The values stated in SI units are to be regarded as
gradient in the direction perpendicular to the area.
standard.
3.1.6 thermal impedance (u), n—the total opposition that an
1.8 This standard does not purport to address all of the
assembly (material, material interfaces) presents to the flow of
heat.
These test methods are under the jurisdiction of ASTM Committee D-9 on
Electrical and Electronic Insulating Materials and are the direct responsibility of Annual Book of ASTM Standards, Vol 10.01.
Subcommittee D09.19 on Dielectric Sheet and Roll Products. Annual Book of ASTM Standards, Vol 14.02.
Current edition approved Sept. 10, 1995. Published November 1995. Originally Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700
published as D 5470 – 93. Last previous edition D 5470 – 93. Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 5470
3.1.7 thermal interfacial impedance (contact resistance), thermocouples for temperature sensing. The use of these test
n—the temperature difference required to product a unit of heat methods avoids problems of measurement due to non-uniform
flux at the contact planes between the specimen surfaces and pressures, surface conditions, or techniques used to assemble
the hot and cold surfaces in contact with the specimen under electronic equipment.
test. The symbol for contact resistance is R . 5.3 In effect, the test methods assume that specimen layers
I
3.1.8 thermal resistivity, n—the reciprocal of thermal con- coalesce and that there is no effective interfacial resistance
ductivity. Under steady-state conditions, the temperature gra- between layers. The slope of the plot of thermal impedance
dient, in the direction perpendicular to the isothermal surface against cumulative thickness permits the determination of
per unit of heat flux. thermal conductivity without regard to thermal interfacial
3.2 Symbols:Symbols Used in This Standard: impedance.
3.2.1 l5 thermal conductivity, watt per metre-K. 5.4 These test methods are approved for use by the Depart-
3.2.2 T 5 temperature of hot surface in contact with a ment of Defense, and are included in Military Specification
A
specimen, K. MIL-I-49456A.
3.2.3 T 5 temperature of hot surface of a specimen, K.
B
TEST METHOD A—GUARDED HEATER METHOD
3.2.4 T 5 temperature of cold surface of a specimen, K.
C
3.2.5 T 5 temperature of cold surface in contact with a
D
6. Apparatus
specimen, K.
6.1 General features are shown in Fig. 1 and Fig. 2. The
3.2.6 A 5 area of a specimen, m .
3.2.7 X 5 thickness of specimen, m.
3.2.8 Q 5 time rate of heat flow, W or J/s.
3.2.9 q 5 heat flux, or time rate of heat flow per unit area,
W/m .
3.2.10 a5 temperature coefficient of electrical resistance
for the heater/sensor wire.
3.2.11 I 5 electrical current, A.
3.2.12 u5 thermal impedance, temperature difference per
unit of heat flux, (K·m )/W.
4. Summary of Test Methods
4.1 In Test Method A (a modification of Test Method
E 1225) a specimen is sandwiched between two metal masses,
compressed and supplied with a measured amount of heat
energy. At equilibrium, temperatures are measured and a
thermal impedance is calculated. The thermal impedance and
thickness are used to compute apparent thermal conductivity.
4.2 Test Method B (Roiseland Heater/Sensor Method) uti-
lizes a pair of heater/sensor elements having large area relative
to a small specimen thickness, which reduces edge effects to a
negligible value. A Wheatstone Bridge is used to obtain
temperature differentials. The current is passed through both
heaters, causing a temperature rise in both sensors. The sensors
form two legs of a Wheatstone Bridge, and the bridge output
corresponds to the temperature difference. Specimens (more
than one layer) are placed between heat sinks of relatively large
thermal mass and strong enough to resist deformation when
placed in a press frame and subjected to the specified pressures.
Thermal impedances are determined and apparent thermal
conductivity is calculated.
5. Significance and Use
5.1 These test methods measure the thermal transmission
properties of low modulus (deformable) dielectric materials.
These materials are used to aid heat transfer in electrical and
electronic applications.
NOTE 1—These test methods are useful with high modulus materials if
layers of low modulus materials are combined with test specimens to
exclude air from test interfaces.
5.2 These test methods are especially useful for generating
thermal data on specimens that are too thin to be fitted with FIG. 1 Guarded Heater with Reference Calorimeter
D 5470
6.6 The press is capable of transmitting the specified force
to the test fixture through a free-floating spherical seat attach-
ment, to prevent offset loads and uneven pressures on the test
specimen.
6.7 Insulation surrounding the specimen stack, if used, is a
fibrous thermal insulating blanket (see 8.8).
7. Test Specimens for Test Method A
7.1 For thermal impedance: Make the specimen from a
piece of the test material, the same area (length and width) as
the metering bars. Unless previously known, and prior to
placement into the assembly, measure the thickness of the
piece in accordance with Method C of Test Methods D 374.
7.2 For apparent thermal conductivity: prepare a sufficient
number of specimens to provide the required number of layers
(see 8.11).
7.3 Specimen conditioning: Unless otherwise specified, test
the specimens in the as-received state. Remove any dirt or
other obvious secondary contamination by a suitable non-
reaction solvent prior to testing. To ensure the removal of
cleaning solvents, use suitable drying procedures after any
cleaning.
8. Procedure for Test Method A
8.1 At room temperature, measure the specimen thickness
in accordance with Method C of Test Methods D 374.
8.2 Center the specimen between the two metre bars.
8.3 Insert the reference calorimeter, if used, between the
lower metre bar and the cooling unit.
8.4 Place the assembled test stack into the press.
8.5 With the press, apply a force to the stack such that 3.0
6 0.1 MPa pressure is applied to the specimen. Maintain this
FIG. 2 Guarded Heater
pressure on the stack for the duration of the test.
NOTE 2—A pressure of 3.0 MPa is adequate to reduce to a negligible
apparatus shown in Fig. 1 uses a reference calorimeter to
level the effects of contact resistance between the specimen and the water
determine rate of heat flow through the specimen. Optionally
bars due to minor surface irregularities.
omit the reference calorimeter (Fig. 2). The rate of heat flow in
the specimen is determined from the electrical power applied to 8.6 Circulate cooling fluid and apply power to the heating
the heater. Smoothly finish all contacting surfaces to within 0.4 element. Maintain the guard heater temperature to within 60.2
μm to approximate a true plane for the metre bars in contact K of the heater temperature.
with the specimen surface. 8.7 Since the pressure may increase during heatup, monitor
6.2 The heater unit (or block) is made of copper or other and adjust the applied force in the press to counteract the
highly conductive material, containing cartridge or similar wire increased pressure on the specimen due to thermal expansion.
wound heaters. It is separated from a surrounding guard heater 8.8 Conduct the testing under conditions that produce an
by a layer of thermal insulation material (epoxy FR-4 or average specimen temperature of 50°C. For measurements
similar) 5 mm thick. The guard heater is also insulated from the made at temperatures above 300 K, it is necessary to apply a
fibrous thermal insulating blanket loosely around the calorim-
press and ensures that all measured energy is transformed to
the upper metre bar. eter sections.
8.9 Record the temperatures of the metre bars and the
6.3 Metre bars are constructed from high thermal conduc-
tivity material having parallel working surfaces. A suitable reference calorimeter at equilibrium. In the absence of a
material of construction is a high purity grade aluminum. reference calorimeter, record the voltage and current applied to
6.4 The reference calorimeter is constructed from a material the heater. Equilibrium is attained when 2 successive sets of
which has a known thermal conductivity over the range of test temperature readings are taken at 15 min intervals and the
temperatures to be used. A recommended material is SRM- differences between the two are less than 60.2 K.
1462 austenitic stainless steel. Test Method E 1225 lists other 8.10 Calculate the mean specimen temperature and the
useful materials of construction. thermal impedance. Label the calculated thermal impedance
6.5 The cooling unit is a metal block cooled by fluid for the single-layer specimen as the “thermal impedance” of
supplied from a constant temperature bath such that the the specimen.
temperature is maintained uniformly within 60.2 K. 8.11 Determine the thermal impedance of multiple layers.
D 5470
Maintain the mean temperature of the multiple-layered speci- against the respective specimen thickness. Plot values of the
mens within 62 K of the single layer specimen temperature by specimen thickness on the x axis and specimen thermal
reducing the heat flux as the number of layers is increased. impedance on the y axis.
9.5.1 The curve is a straight line whose slope is the
9. Calculation for Test Method A
reciprocal of the apparent thermal conductivity. The intercept
9.1 Thermal Impedance:
at zero thickness is the thermal interfacial impedance, R
I
9.1.1 Heat flow using reference calorimeter. Calculate the
specific to the sample, clamping force used, and the clamping
heat flow from the reference calorimeter readings as follows:
surfaces.
9.5.2 As a preferred alternative, compute the slope and the
l 3 A
R
Q 5 3 T 2 T (1)
@ #
5 6
intercept using least mean squares.
d
TEST METHOD B—ROISELAND HEATER/SENSOR
where:
METHOD
Q 5 heat flow, W,
l 5 thermal conductivity of the reference calorim-
R
10. Apparatus
eter material, W/(m·K),
10.1 General—Schematic diagrams of the assembly, instru-
A 5 area of the reference calorimeter, m ,
mentation, and heater/sensor element are shown in Fig. 3, Fig.
T −T 5 temperature difference between thermocouples
5 6
4, and Fig. 5, respectively.
of the reference calorimeter, K, and
d 5 distance between thermocouples in the refer- 10.2 Conditions—The test temperature of 50°C is estab-
lished by placing the assembly in a temperature control
ence calorimeter, m.
9.1.2 Heat flow when not using reference calorimeter. Cal- chamber, or alternatively by pumping controlled temperature
water through channels in
...
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