prEN IEC 63609-2:2026

• Status: Not Published
• Publication Date: 23-Feb-2028
• ICS: 31.180 - Printed circuits and boards
• ICS: 31.190 - Electronic component assemblies
• Technical Committee: CLC/SR 91 - Electronics assembly technology
• Current Stage: 4020 - Enquiry circulated - Enquiry
• Start Date: 18-Sep-2026
• Due Date: 31-Oct-2026
• Completion Date: 18-Sep-2026
• Ref Project: oSIST prEN IEC 63609-2:2026 - Thermal design of electronics assemblies with miniaturised surface mounting devices and related measurements

prEN IEC 63609-2:2026 is a CLC draft aligned with IEC 63609-2, prepared by IEC TC 91: Electronics assembly technology. It specifies how to evaluate the in-plane and thickness thermal conductivities of circuit boards made of polymer composites using representative volume element (RVE) models, finite element analysis (FEA), and verification measurements. It is aimed at people working on the thermal design and testing of electronics assemblies with miniaturised surface mounting devices.

What does prEN IEC 63609-2:2026 specify?

prEN IEC 63609-2:2026 covers a method for circuit boards such as FR-4 and CEM-3 made of polymer composites. Clause 1 sets the scope, Clause 2 lists the normative references, Clause 3 gives the key terms, Clauses 4 to 7 cover apparatus, procedure, verification, and reporting.

The document is organized around modelling, calculation, and measurement. It explains how to build an RVE model, how to run FEA to estimate thermal conductivity, and how to check those results with standard thermal conductivity measurements.

Annex	What it covers
Annex A	Density and specific heat capacity measurements
Annex B	Methods for measuring thermal conductivity
Annex C	Finite element analysis of thermal conductivity using RVE models

What are the key requirements of prEN IEC 63609-2:2026?

The standard focuses on two directions of heat flow in polymer-composite circuit boards: the in-plane direction and the thickness direction. In practice, that means the user must treat the board as anisotropic and evaluate both directions separately.

Modelling inputs and board description

Clause 5.1 requires geometric parameters of the reinforcement, read from cross-section images such as scanning electron microscope photographs. For FR-4, the needed data describe the woven glass-fibre structure; for CEM-3, the method also needs parameters for the non-weave layer. This matters because the RVE model depends on the real fibre geometry, not on a generic board description.

Clause 5.2 requires thermal conductivities of the constituent materials. For analysis, the user must know the properties of the glass fibre and matrix materials, and in CEM-3 the non-weave layer may be treated as a composite matrix. In practice, the quality of the FEA result depends on how well these material properties represent the actual board.

RVE and FEA setup

Clause 5.3 explains how to generate the RVE model from material properties and reinforcement geometry. For FR-4, the warp and weft yarns are homogenised before FEA; for CEM-3, separate RVE models are used for the plain weave and non-weave layers. The practical point is that the board is broken into modelled building blocks before thermal conductivity is calculated.

Clause 5.4 sets the meshing and boundary-condition approach. The RVE model should use conforming tetrahedral meshing, although voxel meshing with brick elements is allowed for complex shapes, and periodic symmetry conditions are applied. Heat flux is applied in the required direction while the opposite face is fixed at a reference temperature, so the user can compute the temperature difference needed for conductivity calculation.

Verification by measurement

Clause 6.1 requires thickness-direction conductivity to be measured with the steady-state method. The procedure calls for at least three specimens, etching of the copper foil when needed, and a graph of thermal resistance versus thickness. The result is then compared with the FEA value, so the modelling method is checked against a direct measurement.

Clause 6.2 requires in-plane conductivity to be measured with the laser flash method on lamellar specimens. The board is first etched when conductor-layer effects need to be removed, then cut into strips and bonded so the in-plane direction becomes the specimen thickness direction. This is important because standard steady-state in-plane testing is difficult for these specimens, so the document uses a practical laser-flash-based workaround.

Reported results

Clause 7 requires the report to record the calculation and measurement date, operator, material identification, software name and version, reinforcement geometry, material conductivities, mesh size in node and element counts, calculated conductivities, tester name and vendor, and measured conductivities. This gives a full trace from raw board description to final conductivity values, which is useful for design reviews, supplier qualification, and audit trails.

What terms does prEN IEC 63609-2:2026 define?

• Representative volume element (RVE) - the smallest model volume that can represent the whole composite material, and the basis for the board-level thermal calculation.
• Finite element analysis (FEA) - a numerical method that divides the board model into finite elements and solves the heat-flow equations on that mesh.
• Thermal conductivity - the property calculated in this document for both the in-plane and thickness directions, used to support thermal design decisions.
• Thermal diffusivity - the quantity obtained in the laser flash method and then used, together with density and specific heat capacity, to calculate thermal conductivity.
• Steady-state method - the measurement method used here for thickness-direction conductivity, based on a stable heat flow through the specimen.
• Laser flash method - the method used here for in-plane conductivity verification on lamellar specimens by measuring the back-face temperature rise after a laser pulse.

Who uses prEN IEC 63609-2:2026?

prEN IEC 63609-2:2026 is used by electronics assembly engineers, thermal design engineers, materials engineers, and test laboratories working with printed circuits and boards. It is also relevant to quality teams that need to compare simulation results with measured values.

In practice, users apply it to board characterization, thermal modelling, specimen preparation, conductivity testing, and validation of FEA results for polymer-composite circuit boards. It is especially relevant where the thermal path through the board is important for miniaturised surface mounting devices.

What changed in prEN IEC 63609-2:2026 from the previous edition?

prEN IEC 63609-2:2026 supersedes 91/2079/CD and 91/2117/CC.

Which standards are used with prEN IEC 63609-2:2026?

• IEC 60194-1 - provides circuit-board and electronic-assembly vocabulary used for the terms in this document.
• IEC 60194-2 - adds further vocabulary for electronic technologies and electronic assembly technologies.
• IEC 63609-1 - the companion part for thermal design guidance where the heat dissipation path to the board is dominant.
• IEC 63609-3 - the companion part for temperature measurement in miniaturised surface mounting devices using thermocouples.

What does the prEN IEC 63609-2:2026 document contain?

Clause 5 gives the modelling and calculation workflow, including reinforcement geometry, constituent material properties, RVE generation, meshing, boundary conditions, and conductivity calculation. Clause 6 gives the verification measurements for both thickness and in-plane directions, including specimen preparation and result comparison.

Annex A provides example density and specific heat capacity measurements. Annex B describes the steady-state and laser flash measurement set-ups and how the thermal conductivity values are obtained from those tests. Annex C shows worked FEA examples for FR-4 and CEM-3, including RVE models, laminate models, measured-versus-analytical comparisons, and the figures used to explain the procedures.

SLOVENSKI STANDARD
01-november-2026
Toplotno načrtovanje elektronskih sklopov z miniaturiziranimi napravami za
površinsko montažo in s tem povezane meritve
Thermal design of electronics assemblies with miniaturised surface mounting devices
and related measurements
Ta slovenski standard je istoveten z: prEN IEC 63609-2:2026
ICS:
31.180 Tiskana vezja (TIV) in tiskane Printed circuits and boards
plošče
31.190 Sestavljeni elektronski Electronic component
elementi assemblies
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

91/2127/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 63609-2 ED1
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2026-09-18 2026-12-11
SUPERSEDES DOCUMENTS:
91/2079/CD, 91/2117/CC
IEC TC 91 : ELECTRONICS ASSEMBLY TECHNOLOGY
SECRETARIAT: SECRETARY:
Japan Mr Osamu IKEDA
OF INTEREST TO THE FOLLOWING COMMITTEES: HORIZONTAL FUNCTION(S):
TC 40,TC 47,SC 47A,SC 47D
ASPECTS CONCERNED:
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft
for Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of
which they are aware and to provide supporting documentation.
Recipients of this document are invited to submit, with their comments, notification of any relevant “In Some
Countries” clauses to be included should this proposal proceed. Recipients are reminded that the CDV stage is
the final stage for submitting ISC clauses. (SEE AC/22/2007 OR NEW GUIDANCE DOC).

TITLE:
Thermal design of electronics assemblies with miniaturised surface mounting devices and
related measurements - Part 2: Method for evaluating thermal conductivity of circuit boards
with polymer composites by using finite element analysis and measurements

PROPOSED STABILITY DATE: 2032
NOTE FROM TC/SC OFFICERS:
download this electronic file, to make a copy and to print out the content for the sole purpose of preparing National
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IEC CDV 63609-2 © IEC 2026
1 CONTENTS
3 FOREWORD . 3
4 INTRODUCTION . 5
5 1 Scope . 6
6 2 Normative references . 6
7 3 Terms and definitions . 6
8 4 Apparatus . 7
9 5 Procedure . 7
10 5.1 Geometric parameters of the reinforcement . 7
11 5.2 Thermal conductivities of constitution materials . 9
12 5.3 Generation of RVE model . 9
13 5.4 Generation of finite element model. 11
14 5.5 Thermal conductivity calculations . 13
15 6 Verification . 13
16 6.1 Thermal conductivity in the thickness direction . 13
17 6.2 Thermal conductivity in the in-plane direction. 14
18 7 Report . 15
19 Annex A (informative) Density and specific heat capacity measurements . 16
20 A.1 Experimental procedures . 16
21 A.2 Results . 16
22 Annex B (informative) Methods for measuring thermal conductivity . 18
23 B.1 General . 18
24 B.2 Steady state method . 18
25 B.3 Laser flash method . 19
26 Annex C (informative) Finite element analysis of thermal conductivity using RVE
27 models . 21
28 C.1 Analytical procedures. 21
29 C.1.1 FR-4 . 21
30 C.1.2 CEM-3 . 21
31 C.2 Experimental procedures . 22
32 C.2.1 Results . 23
34 Figure 1 – cross-section of FR-4 . 8
35 Figure 2 – cross-section of CEM-3 . 9
36 Figure 3 – RVE model of FR-4 . 9
37 Figure 4 – RVE model of warp and weft yarns (unidirectional fibre composite) . 10
38 Figure 5 – RVE model of non-weave layer in CEM-3 . 11
39 Figure 6 – Finite element model of FR-4 . 11
40 Figure 7 – Finite element model of CEM-3 . 13
41 Figure 8 – Relationship between the thermal resistance and thickness of the specimen . 14
42 Figure 9 – Fabrication of lamellar specimens . 15
43 Figure B.1 – Setup for the steady-state method . 18
IEC CDV 63609-2 © IEC 2026
44 Figure B.2 – Setup for the laser flash method . 19
45 Figure B.3 – Temperature rise versus time curve . 20
46 Figure C.1 – Experimental thermal conductivities in the thickness direction of neat FR-
47 4 obtained by steady-state method with single and multiple specimen techniques . 23
48 Figure C.2 – Experimental and analytical thermal conductivities in the thickness
49 direction of neat FR-4 . 24
50 Figure C.3 – Experimental and analytical thermal conductivities in the in-plane
51 direction of neat FR-4 . 24
52 Figure C.4 – Experimental and analytical thermal conductivities in the thickness
53 direction of neat CEM-3 (R1786) . 24
54 Figure C.5 – Experimental and analytical thermal conductivities in the in-plane
55 direction of neat CEM-3 (R1786) . 25
57 Table A.1 – Specific heat capacity and density of CEM-3 CCL and FR-4 CCL . 16
58 Table A.2 – Specific heat capacity and density of neat CEM-3 and neat FR-4 . 17
59 Table C.1 – Thermal conductivity of constitution materials in FR-4. 21
60 Table C.2 – Thermal conductivity of constitution materials in CEM-3. 22
IEC CDV 63609-2 © IEC 2026
80 INTERNATIONAL ELECTROTECHNICAL COMMISSION
81 ____________
83 THERMAL DESIGN OF ELECTRONICS ASSEMBLIES WITH MINIATURISED
84 SURFACE MOUNTING DEVICES AND RELATED MEASUREMENTS
86 Part 2: Method for evaluating thermal conductivity of circuit boards with
87 polymer composites by using finite element analysis and measurements
91 FOREWORD
92 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
93 all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
94 co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
95 in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
96 Publicly Available Specifications (PAS) and Guides (hereafter referred to as "IEC Publication(s)"). Their
97 preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
98 may participate in this preparatory work. International, governmental and non-governmental organizations liaising
99 with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
100 Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
101 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
102 consensus of opinion on the relevant subjects since each technical committee has representation from all
103 interested IEC National Committees.
104 3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
105 Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
106 Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
107 misinterpretation by any end user.
108 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
109 transparently to the maximum extent possible in their national and regional publications. Any divergence between
110 any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
111 5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
112 assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
113 services carried out by independent certification bodies.
114 6) All users should ensure that they have the latest edition of this publication.
115 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
116 members of its technical committees and IEC National Committees for any personal injury, property damage or
117 other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
118 expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
119 Publications.
120 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
121 indispensable for the correct application of this publication.
122 9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
123 patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
124 respect thereof. As of the date of publication of this document, IEC had received notice of (a) patent(s), which
125 may be required to implement this document. However, implementers are cautioned that this may not represent
126 the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
127 shall not be held responsible for identifying any or all such patent rights.
128 IEC 63609-2 has been prepared by IEC technical committee 91: Electronics assembly
129 technology. It is an International Standard.
130 The text of this International Standard is based on the following documents:
IEC CDV 63609-2 © IEC 2026
Draft Report on voting
XX/XX/FDIS XX/XX/RVD
132 Full information on the voting for its approval can be found in the report on voting indicated in
133 the above table.
134 The language used for the development of this International Standard is English.
135 This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
136 accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, and the
137 ISO/IEC Directives available at www.iec.ch/members_experts/refdocs. The main document
138 types developed by IEC are described in greater detail at www.iec.ch/publications.
139 A list of all parts in the IEC 63609 series, published under the general title Thermal design of
140 electronics assemblies with miniaturised surface mounting devices and related measurements ,
141 can be found on the IEC website.
142 The committee has decided that the contents of this document will remain unchanged until the
143 stability date indicated on the IEC website under webstore.iec.ch in the data related to the
144 specific document. At this date, the document will be
145 • reconfirmed,
146 • withdrawn,
147 • replaced by a revised edition, or
148 • amended.
IEC CDV 63609-2 © IEC 2026
149 INTRODUCTION
150 In the thermal design of electronic assemblies with miniaturised SMDs considering circuit board
151 heat-dissipation type, it is extremely important to accurately determine the thermal conductivity
152 of the circuit board. However, circuit boards are often polymer composites, and their
153 microstructure makes them heterogeneous, making it difficult to accurately measure their
154 thermal conductivities. Currently, internationally standardised steady-state method and the
155 laser flash method are used to measure the thermal conductivity of materials. The steady -state
156 method can measure the thermal conductivity of polymer composites but requires greater
157 sample thickness for greater accuracy. The laser flash method is often used because of its
158 small sample size, short measurement time and convenience. However, due to the
159 measurement principle of the laser flash method, this method is suitable for homogeneous
160 materials, and when measuring polymer composites with heterogeneous structure, the accuracy
161 of the thermal conductivity obtained may become poor.
162 On the other hand, circuit boards exhibit anisotropy, with different thermal conductivities in the
163 in-plane and thickness directions. While the thermal conductivity in the thickness direction can
164 be easily measured using a standardised steady-state method, the thermal conductivity in the
165 in-plane direction is difficult to measure using a standardised steady -state method due to the
166 difficulty of preparing test specimens.
167 This part of IEC 63609-2 specifies a method for evaluating the in-plane and thickness thermal
168 conductivities of circuit boards with polymer composites using the finite element analysis with
169 representative volume element models and the measurements.
170 IEC 63609 series aim to specify measurements in the thermal design of electronic assemblies.
171 IEC 63609 series consist of the following parts under the general title " THERMAL DESIGN OF
172 ELECTRONICS ASSEMBLIES WITH MINIATURISED SURFACE MOUNTING DEVICES AND
173 RELATED MEASUREMENTS ".
174 IEC 63609-1, Part 1: Guideline for thermal design of applications where the heat dissipation
175 path to the board is dominant
176 IEC 63609-2, Part2: Method for evaluating thermal conductivity of circuit boards with polymer
177 composites by using finite element analysis and measurements
178 IEC 63609-3, Part3: Determination for temperature measurement in miniaturised surface
179 mounting devices using thermocouples
IEC CDV 63609-2 © IEC 2026
185 THERMAL DESIGN OF ELECTRONICS ASSEMBLIES WITH
186 MINIATURISED SURFACE MOUNTING DEVICES AND RELATED
187 MEASUREMENTS
189 Part 2: Method for evaluating thermal conductivity of circuit boards with
190 polymer composites by using finite element analysis and measurements
192 1 Scope
193 This part of IEC 63609-2 specifies the method for evaluating the thermal conductivities in the
194 in-plane and thickness directions of circuit boards such as FR-4 and CEM-3 made of polymer
195 composites.
196 This document applies to circuit boards made of polymer composites with thermal conductivities
-1 -1 -1 -1
197 from 0,1 W·m ·K to 10 W·m ·K and thicknesses from 0,5 mm to 3,0 mm.
198 This document specifies the method for constructing a representative volume element (RVE)
199 model and the method for determining the in-plane and thickness thermal conductivities using
200 finite element analysis with the RVE model.
201 The methods for measuring thermal conductivity used to verify the validity of the results
202 obtained from finite element analysis using the RVE model referred to in this document are the
203 standardised steady-state method and the laser flash method. This document describes the
204 specimen preparation method, the measurement conditions and the measurement procedures.
205 NOTE The trade names of products in this document are examples of a suitable product(s) available commercially.
206 This information is given for the convenience of users of this document and does not constitute an endorsement by
207 IEC of these products.
208 2 Normative references
209 The following documents are referred to in the text in such a way that some or all of their content
210 constitutes requirements of this document. For dated references, only the edition cited applies.
211 For undated references, the latest edition of the referenced document (including any
212 amendments) applies.
213 IEC 60194-1, Electronic assembly, design and circuit boards – Vocabulary – Part 1: Common
214 usage in circuit board and electronic assembly technologies
215 IEC 60194-2, Electronic assembly, design and circuit boards – Vocabulary – Part 2: Common
216 usage in electronic technologies as well as electronic assembly technologies
217 3 Terms and definitions
218 For the purposes of this document, the terms and definitions given in IEC 60194 -1,
219 IEC 60194-2 and the following apply.
220 ISO and IEC maintain terminology databases for use in standardization at the following
221 addresses:
222 • IEC Electropedia: available at https://www.electropedia.org/
223 • ISO Online browsing platform: available at https://www.iso.org/obp
IEC CDV 63609-2 © IEC 2026
224 3.1
225 representative volume element
226 RVE
227 smallest volume that can be evaluated to obtain a value that represents the whole
228 Note1 to entry: In the theory of composite materials, it is also called a unit cell.
229 Note 2 to entry: In the case of a microstructure with periodicity, the RVE can be selected simply, but in the case of
230 a microstructure with randomness, the RVE is much more complex.
231 3.2
232 finite element analysis
233 FEA
234 analytical method that approximates an object with infinite degrees of freedom as a collection
235 of elements (finite elements) with finite degrees of freedom and then solves the equations that
236 hold for this collection.
237 Note 1 to entry; It is a method of mathematically solving a physically rigorous governing equation by replacing it
238 with an approximate equation using the concept of finite elements. The RVE model is divided into elements made up
239 of nodes to create a finite element model, and finite element analysis is carried out using this model under appropriate
240 boundary conditions to obtain the temperature of the nodes and the heat flux within the elements.
242 4 Apparatus
243 a) The computer system used for analysis should be a workstation equipped with a high-speed
244 CPU such as Intel Xeon, 64 GB or more of memory, and a graphics card such as NVIDIA
245 RTX series.
246 b) Large-capacity storage is required for storing analysis data. Storage capacity of 5 TB or
247 more is recommended.
248 c) Software for creating RVE models and conducting finite element analysis includes Digimat-
249 FE (MSC Software), GeoDict (Math2Market GmbH) and so on. Operating systems are
250 Windows 11 (64bit) and Linux.
251 5 Procedure
252 NOTE See Annex C for examples of finite element analysis using the RVE model.
253 5.1 Geometric parameters of the reinforcement
254 To represent the geometry of the reinforcement in the RVE model, the geometric parameters of
255 the reinforcement material are required. For example, in the case of FR-4, the reinforcement
256 has a plain weave structure of glass fibres, so the following parameters are required: the
257 diameter of the glass fibre in the yarn (d), the density of the warp yarn (yarns/cm), the density
258 of the weft yarn (yarns/cm), the width of the yarn (S), the thickness of the yarn (H), the total
259 thickness of the plain weave (h), and the volume fraction of the glass fibre in the plain weave.
260 The geometric parameters of the reinforcement should be read from the photographs obtained
261 by observing the cross-section of the circuit board using a scanning electron microscope as
262 shown in Figure 1.
IEC CDV 63609-2 © IEC 2026
S
h
H
key
1 warp yarn
2 weft yarn
S width of the yarn
H thickness of the yarn
h total thickness of the plain weave
265 Figure 1 – cross-section of FR-4
266 In the case of CEM-3, the reinforcement consists of two layers of a plain weave structure with
267 continuous glass fibres and a layer of a non-weave structure of very long glass fibres as shown
268 in Figure 2, so the following parameters are required: for the plain weave structure, the diameter
269 of the glass fibre in the yarn, the density of the warp yarn (yarns/cm), the density of the weft
270 yarn (yarns/cm), the width of the yarn (S ), the thickness of the yarn (H ), the total thickness of
1 1
271 the plain weave (h1), and the volume fraction of the glass fibre in the plain weave, for the non-
272 weave structure, the volume fraction of the glass fibre in the non-weave and total thickness of
273 the non-weave layer (h2).
h
H
S
h
h
IEC CDV 63609-2 © IEC 2026
key
1 plain weave layer
2 non-weave layer
h total thickness of plain weave layer
h total thickness of non-weave layer
275 Figure 2 – cross-section of CEM-3
276 5.2 Thermal conductivities of constitution materials
277 To perform a finite element analysis of thermal conductivity using the RVE model of a circuit
278 board under the steady state, the thermal conductivities of the constituent materials are
279 required. For example, in the case of FR-4, the thermal conductivity and linear density of the
280 glass fibre as the reinforcement with plain weave structure, and the thermal conductivity of the
281 epoxy resin as the matrix are required.
282 In the case of CEM-3, so the following parameters are required: for plain weave layer, the
283 thermal conductivity and linear density of the glass fibre as the reinforcement and the thermal
284 conductivity of the epoxy resin as the matrix, for non-weave layer, the thermal conductivity of
285 the glass fibre as the reinforcement and the thermal conductivity of the epoxy resin or the
286 composite as the matrix.
287 5.3 Generation of RVE model
288 RVE models are created using the material properties of the constituent materials and the
289 geometric parameters of the reinforcements. Figure 3 shows an RVE model of FR-4 created
290 using Digimat-FE. In Figure 3, the warp yarns are shown in red, the weft yarns in blue, and the
291 epoxy resin in white.
key
1 warp yarn
2 weft yarn
292 Figure 3 – RVE model of FR-4
293 The warp and weft yarns should be made of a homogeneous composite material without
294 considering microstructure with glass fibres and epoxy resin. Based on the rule of mixtures,
295 thermal conductivity in the longitudinal direction of the homogenised warp and weft yarns can
296 be calculated by Formula (1).
IEC CDV 63609-2 © IEC 2026
L
 =VV+ (1− )
(1)
c f f m f
297 where
L
298  is the thermal conductivity in longitudinal direction of the homogenised warp and weft yarns,
c
-1 -1
299 expressed in W·m ·K ;
-1 -1
300  is the thermal conductivity of glass fibre in the yarn, expressed in W·m ·K ;
f
-1 -1
301  is the thermal conductivity of epoxy resin in the yarn, expressed in W·m ·K ;
m
302 is the volume fraction of glass fibre in the yarn.
V
f
303 The thermal conductivity in the transverse direction of the homogenised warp and weft yarns
304 can be obtained by Formula (2).
VV(1− )
ff
=+
(2)
  
c f m
306 where
T
307  is the thermal conductivity in transverse direction of the homogenised warp and weft yarns,
c
-1 -1
308 expressed in W·m ·K
309 For high volume fraction of glass fibre, the thermal conductivity in transverse direction of the
310 homogenised warp and weft yarns can be obtained by Formulas (3) and (4).
T
(+1)+V ( −1)

f
c
=
...