The invention relates to the cooling of electronic circuit boards plugged
5 into a cabinet or backplane, also called drawer or rack. The invention is of
particular use for electronic equipment items, in particular the power supply
boards, whose components heat up significantly and which require a rapid
discharging of the heat. The invention is particularly advantageous for
embedded electronic equipment items, in particular cooled by fanned air.
10 -There are, for example, computers comprising several electronic
circuit boards arranged parallel to one another in the cabinet. In order to
ensure the maintenance of the equipment item, the various boards are
dismountable. The mounting and the dismounting of the boards is done by
translation thereof in grooves or runners of the cabinet.
15 Printed circuit board is the term given to a bare printed circuit board,
that is to say one without electronic components and with no heat sink
device, and an electronic circuit board is a printed circuit board equipped with
a heat sink and one or more electronic components. A printed circuit board
comprises or is an assembly of a plurality of conductive layers separated in
20 pairs by a layer of insulating material or an assembly of at least one
conductive layer and one insulating layer. The conductive layers each
comprise conductive tracks. They are, for example, obtained by etching. In
operation, the components of the electronic circuit board can dissipate a lot
of heat. This heat has to be discharged in order to maintain a suitable
25 operating temperature that does not exceed an acceptable maximum value.
Several solutions have been considered for the cooling of electronic
equipment items: cooling by thermal convection and cooling by thermal
conduction.
Thermal convection consists in placing a hot body in contact with a
30 fluid, preferably set in motion relative to the board so as to improve the heat
exchange between the hot components and the cold fluid.
Thermal conduction is a thermal transfer mode resulting from a
temperature difference betweelli!two zones of one and the same medium or
between two different media in contact.
35 One known solution for the cooling of electronic circuit boards consists
in equipping the printed circuit board with a thermal drain in the form of a
2
plate produced in a material with strong thermal conductivity, such as copper
or aluminium for example. The plate has substantially the same surface area
as the printed circuit board and is parallel thereto. This plate is fixed on to the
face of the printed circuit board opposite that receiving the electronic
5 components. It makes it possible to conduct the heat to be discharged from
the printed circuit board to a heat exchanger positioned in the cabinet and
pressed against the thermal drain, for example in the form of channels or of
plates in which a coolant circulates. It is then necessary to connect these
channels or plates to a cooling network externalto the equipment item.
10 This solution is suited to applications equipped with such a cooling
network. On the other hand, this type of solution proves to be insufficiently
effective for the applications in which the cabinets are not equipped with a
cooling network and which are generally equipped only with a fan.
To correct this drawback, it is conventional practice to screw a heat
15 sink comprising cooling fins on to the face of the thermal drain opposite the
printed circuit board. The heat sink, through its fins, increases the contact
surface between the electronic circuit board and the fluid in contact with the
electronic circuit board, for example air. It makes it possible to improve the
convection cooling performance levels. A solution of this type is represented
20 in Figure 1 in which can be seen the electronic circuit board 100 comprising a
printed circuit board 101 comprising two faces 101 a and 101 b, a drain 102
fixed to the printed circuit board 101 (and more specifically to the second
face 101 b of the printed circuit board 101 a), a component 1000 fixed to the
printed circuit board 101 (and more specifically soldered on to the first face
25 101 a), a heat sink 104 comprising cooling fins 105 and a base 106 that is thin
relative to the thermal drain. The heat sink 104 is fixed to the drain 102 and
to the printed 1:ircuiHroard 101 by means of a plurality of screw 107 - nut 108
systems, typically at least ten or so, only one of which is visible in Figure 1,
the screw 107 passing through the drain 102 and the printed circuit board
30 101, and a layer of thermal grease 109 interposed between the heat sink 104
and the drain 102 and ensuring a good heat exchange between the heat sink
104 and the drain 102 by filling the interstices between these two elements.
The applicant has found that this type of board presents a certain
number of drawbacks. The grease is made up of a mixture of two phases
35 comprising a liquid phase (oil) which is a grease and a solid phase
3
comprising conductive particles usually of silver. These two phases separate
in time and with temperature increases which causes the energy dissipation
performance levels of the heat sink plus drain system to be diminished and
can limit the performance levels of the other boards or equipment items
5 inserted into the cabinet, even degrade their performance levels. The fixing of
the heat sink 104 to the thermal drain 102 by means of the screw-nut
systems 107-108 considerably limits the useful surface of the printed circuit
board. It is in fact necessary to provide an adequate isolation zone between
each screw-nut system and the compooents on the face 101a of the printed
10 circuit board receiving the components.
Moreover, the operations of fixing of the heat sink on to the thermal
drain and of application of the thermal grease are difficult operations that
have to be performed manually, which are consequently costly.
Furthermore, the screw-nut systems disrupt the flow of air on the heat
15 sink side which causes the control of the cooling of the electronic plate to be
reduced.
Finally, this type of solution exhibits a significant thickness.
The aim of the invention is to correct all or part of the abovementioned
drawbacks.
20 To this end, the subject of the invention is an electronic circuit board
comprising an assembly of a printed circuit board comprising a first face
receiving at least one electronic component, and of a heat sink, the printed
circuit board and the heat sink being stacked in a stacking direction, said
heat sink being fixed to the printed circuit board on a second face opposite
25 the first face, said heat sink comprising a base, in the form of a plate, and
reliefs extending from a flat surface of said base, the reliefs being intended to
increase the wntact surface between the heat sink and a flow of air relative
to the contact surface between the base and the flow of air, the base being
interposed between the printed circuit board and the reliefs in the stacking
30 direction. The heat sink is fixed directly to the printed circuit board by gluing
only and said heat sink is of a single piece. Advantageously, only an
adhesive layer separates the base and the printed circuit board, the adhesive
layer comprising a glue extending continually from the base to the printed
circuit board.
35
4
Advantageously, the adhesive layer is a film of glue.
Advantageously, the adhesive layer . comprises glue-impregnated
fibres.
Advantageously, the adhesive layer is an electrical insulator.
5 Advantageously, the glue is a thermosetting glue.
Advantageously, the heat sink is made of metal.
Advantageously, the adhesive layer is less than or equal to
200 micrometres and preferably between 100 micrometres and
200 micrometres.
10 Advantageously, the heat sink has a thickness of between 5 mm and
20mm.
Advantageously, the printed circuit board has a thickness of between
0.1 mm and 1 mm.
Advantageously, the heat sink extends substantially over all the useful
15 surface of the printed circuit board.
Advantageously, the heat sink is fixed to the printed circuit board by
gluing by means of a film of glue interposed between the base the printed
circuit board in the stacking direction.
The invention relates also to a method for manufacturing an electronic
20 circuit board comprising:
25
a stacking step in which the heat sink is positioned such that
the base is interposed between the reliefs and the second face
of the printed circuit board and in which an adhesive layer is
arranged between the second face of the printed circuit board
and the base, the adhesive layer comprising a glue extending
continually over all the thickness of the glue in the stacking
·direction,
an assembly step consisting in assembling the heat sink and
the printed circuit board in which the stacking obtained is
30 subjected to a hot compression.
Advantageously, the assembly step is performed by means of a press
comprising a tool comprising a portion configured and arranged relative to
the heat sink so as to come to bear on the surface from which the reliefs
extend and extending between the reliefs.
35 Advantageously, the assembly step is performed by means of a press
5
comprising a tool comprising a portion configured and arranged relative to
the heat sink so as to come to bear on all of the surface from which the
reliefs extend and extending between the reliefs.
Advantageously, the assembly step is performed by means of a press
5 comprising a tool comprising a portion configured and arranged relative to
the heat sink so as to come to bear on the reliefs in the direction z.
Advantageously, the printed circuit board is multilayer and in which the
assembly step is a step of assembly of a plurality of layers of the printed
circuit board.
1 o Advantageously, the assembly of the heat sink and of the printed
circuit board is performed by polymerization of the glue.
Other features and advantages of the invention will become apparent
on reading the following detailed description, given as a nonlimiting example
15 and with reference to the attached drawings in which:
- Figure 1, already described, schematically represents, in cross
section, an electronic circuit board of the prior art,
- Figure 2 schematically represents a cabinet receiving an
electronic circuit board according to the invention on which the electronic
20 components and the heat sink have not been represented for greater clarity,
- Figure 3 schematically represents a cross section of an
electronic circuit board according to the invention,
- Figure 4 represents a block diagram of the steps of the method
according to the invention,
25 - Figure 5 schematically represents a press used in the method
according to the invention.
From one figure to another, the same elements are identified by the
same references.
30 Figure 2 represents a cabinet 90 comprising a plurality of runners 91
extending longitudinally parallel to one another. The cabinet 90 receives an
electronic circuit board 1 according to the invention. The electronic circuit
board 1 is held in the frame 92 of the cabinet 90 in runners 91 at right angles
to the backplane.
35 Figure 3 represents, in perspective, a cross section of an electronic
circuit board according to the invention along a plane parallel to the stacking
direction.
The electronic circuit board 1 comprises an assembly of a printed
circuit board 2 and a heat sink 3. The printed circuit board 2 is of the type
5 comprising a plUrality of individual layers not represented in Figure 1. It
comprises at least one conductive layer and at least one insulating layer.
Each conductive layer comprises electrically conductive tracks. These tracks
are conventionally made of copper. As a variant, the tracks are produced in a
material other than copper. The printed circuit board can be of the
10 single-sided type (comprising one conductive layer and one insulating layer),
double-sided type (comprising two conductive layers separated by one
insulating layer), or multilayer type (comprising at least three conductive
layers separated in pairs by one insulating layer). The conductive layers are
for example formed from layers of copper. These layers of copper, or
15 individual conductive layers of the printed circuit board are for example
etched so as to form tracks. An insulating layer should be understood to be
an electrically insulating layer and a conductive layer should be understood
to be an electrically conductive layer.
The printed circuit board 2 comprises a first face 4 intended to receive
20 one or more electronic components. Just one electronic component 5 can be
seen in Figure 3. The first face 4 can receive several electronic components.
All the electronic components fixed on to the printed circuit board 2 are
received by the first face 4. In other words, each electronic component is
fixed, preferably soldered, on to the first face 4.
25 The printed circuit board 2 and the heat sink 3 are stacked in a
stacking direction z. The individual layers, not represented, of the printed
circuit board 2 are also 'Stacked in the stacking direction z. The heat sink 3 is
fixed to the printed circuit board on a second face 6 of the printed circuit
board 2. The second face 6 is opposite the first face 4. The first and second
30 faces are parallel to one another and at right angles to the stacking direction
z.
The heat sink 3 extends substantially over all the useful surface of the
printed circuit board. In other words, the heat sink can extend over all the
surface of the printed circuit board or else, as a variant, extend over the
35 useful surface of the printed circuit board extending between two runners
1
holding the printed circuit board in the cabinet without extending into the two
runners.
The heat sink 3 comprises cooling fins 7 and a base 8. The base 8 is
interposed between the cooling fins 7 and the printed circuit board 2 in the
5 stacking direction z. The heat sink 3 has a profile in the form of a comb in the
plane of the figure. The base 8 ensures a thermal drain function making it
possible to discharge the heat transmitted by the components to the printed
circuit board 2 from the second face 6 of the printed circuit board 2 to the fins
7.
10 The base 8 has the form of a plate. The plate extends according to its
thickness e, parallel to the stacking direction z. It is pressed against the
printed circuit board. The base 8 is thick in relation to the printed circuit board
2. The printed circuit board 2 typically has a thickness of between 0.10 and
1 mm. The base 8 has a thickness at least equal to 2 mm. Typically, the base
15 8 has a thickness of between 2 and 5 mm. However, this range is not limiting
in as much as the dimensions of the base and of the cooling fins are
determined as a function of the thickness of the printed circuit board, as a
function of the thickness of the electronic circuit board desired and as a
function of the desired thermal performance levels.
20 In a particular embodiment, the heat sink 3 ensures a printed circuit
board 2 stiffener function. In this case, the base 8 has a stiffness greater than
that of the printed circuit board. As a variant, the printed circuit board has a
stiffness at least equal to the base of the heat sink.
The cooling fins 7 make it possible to improve the discharge of the
25 heat (dissipated by the printed circuit board 2 because of the heating up of
the components) by convection relative to a heat sink in the form of a plate
by increasing the 'COntact surface between the heat sink and the fluid in
contact therewith relative to the contact surface between the base 8 and the
flow of air, that is to say the free surface of the base. In other words, a heat
30 sink comprising cooling fins, that is to say formed by the base provided with
the cooling fins, presents with the air a contact surface greater than that of a
heat sink comprising only the base. Each fin 7 has the form of a plate
extending longitudinally in a direction parallel to a direction at right angles to
the plane of Figure 3 and having, in the plane of Figure 3, the form of a rod
35 extending longitudinally, in the stacking direction z, from the base 8 to a free
8
end 10 according. The fins 7 extend from a flat surface Sa of the base
opposite the printed circuit board 2 in the stacking direction z. The flat
surface Sa is discontinuous. It is formed by a plurality of portions separated
by the fins. The fins 7 have a height H typically at least equal to 2 mm and
5 typically between 2 and 15 mm. As for the base, this range is not limiting
since the dimensions of the base depend on several parameters as
explained previously. The plane of Figure 3 is a plane containing the stacking
direction and at right angles to the longitudinal direction of the fins. The
height of the fins H of the fins is the direction of the fins in the stacking
10 direction z.
The fins 7 are separated in pairs by trenches 12. The trenches 12
extend longitudinally parallel to the longitudinal direction of the fins. The
trenches have a U-section in the plane of Figure 3. The bottom of each U is
formed by a portion of the surface Sa of the base S opposite the printed
15 circuit board 2 and the arms of each U are formed by two adjacent fins 7. The
section of the fins is constant In other words, they have dimensions that are
constant along the direction in which they extend longitudinally (direction at
right angles to the plane of Figure 3). Consequently, the trenches also have a
constant section.
20 According to the invention, the heat sink 3 is of a single piece. In other
words, the fins 7 and the base S are formed in a single piece. That means
that the heat sink 3 is produced in a single material. This material can be
homogenous. As a variant, it is composite.
This part is preferably, but not necessarily, solid, as opposed to
25 hollow. The heat sink can be produced in a material exhibiting a strong
thermal conductivity. It is for example produced in metal, preferably in
aluminium, or in ·copper. According to the invention, the heat sink is fixed
directly to the printed circuit board 2 only by gluing. In other words, the heat
sink 3 is fixed to the printed circuit board by means of a film of glue 9
30 interposed between the heat sink and the printed circuit board 2. More
specifically, the film of glue 9 is interposed between the base S and the
printed circuit board 2. "Heat sink fixed directly to the printed circuit board by
gluing" should be understood to mean that the printed circuit board and the
heat sink are separated only by a film of glue 9.
35 More generally, according to the invention the heat sink is fixed to the
9
printed circuit board by means of an adhesive layer. The printed circuit board
and the sink are separated only by this adhesive layer.
Advantageously, the adhesive layer comprises a glue which extends
continually from the printed circuit board to the base.
5 The adhesive layer can be a glue, that is to say a film of glue. As a
variant, the adhesive layer is a fibrous layer comprising glue-impregnated
fibres. The fibres are, in a non limiting example, glass fibres.
In this type of adhesive layer, the glue passes advantageously
between the fibres such that it extends continually from the printed circuit
10 board to the base. The fibrous layer can be a glue-impregnated fabric, in
other words it comprises glue-impregnated woven fibres. As a variant, the
fibrous layer is a glue-impregnated mat. This type of adhesive layer offers the
advantage of being easy to apply.
Preferably but not necessarily, the glue is in contact with the printed
15 circuit board and the heat sink substantially over all the surface of the heat
sink.
Advantageously, the gluing of the heat sink to the printed circuit board
is performed by polymerization of the glue. The adhesive layer then
comprises polymers.
20 The film of glue 9, or more generally the adhesive layer, is an electrical
insulator. In other words, this layer is produced in an electrically insulating
material. More specifically, it is produced in one or more materials which is
(are) electrically insulated. That makes it possible to avoid the formation of a
short-circuit between the printed circuit board (in particular with the layer of
25 the printed circuit board which is facing the film of glue) and the heat sink
conventionally forming the mechanical ground.
In the -case ·of a fibrous layer, the glue and the fibres are formed in
electrically insulating materials. In the case of the film of glue, the glue is an
electrical insulator.
30 The film of glue, or more generally the adhesive layer, has a thickness
less than or equal to 200 micrometres. It preferably has a minimum thickness
of 100 micrometres and typically between 100 and 200 micrometres. It
makes it possible to effectively compensate for the surface defects of the
base and of the heat sink.
35 The glue is, preferably but not necessarily, thermosetting. The
10
adhesive layer extends over all the surface of the base 8 facing the printed
circuit board. These features are linked to the assembly method used and
described hereinafter in the text. The reduced thickness of the film of glue
allows a very effective heat transfer between the printed circuit board and the
5 heat sink 3. Moreover, the adhesive layer makes it possible to limit the risks
of formation of air bubbles by penetrating into the unevennesses of the two
surfaces. It can be used to fix together large surfaces and at low temperature
making it possible to preserve the integrity of the printed circuit board.
The glue is preferably produced based on epoxy resin or is of the
10 acrylic glue type. As a variant, in a nonlimiting manner, the glue is a
polyurethane, cyanoacrylate, elastomer or silicone glue.
The fact that the heat sink, ensuring the dual function of discharging of
the heat by thermal convection (fins) and thermal conduction (drain or base)
15 is of a single piece and fixed only by gluing to the printed circuit board
provides a certain number of advantages. It can be fixed in a single
non-manual step of gluing to the printed circuit board, which limits the cost of
the assembly of the electronic circuit board and makes it possible to offer a
surface for locating the electronic components 5 that is significant on the side
20 of the first face 4 of the printed circuit board 2. The heat sink 3 has a single
interface with the printed circuit board 2 (film of glue 9 or adhesive layer)
which means that the thermal transfer between these two elements is better
than when the heat sink is of the type comprising a base and a heat sink
linked together by screws and separated by a thermal grease. This
25 increasing of the performance levels in terms of thermal energy dissipation
makes it possible, for a given printed circuit board, given electronic
components and ·-given performance levels in terms of discharging of the
heat, to reduce the thickness of the heat sink. The thickness of the heat sink
is given by the thickness of the base 8 and the height H of the cooling fins 7.
30 The invention typically makes it possible to produce electronic circuit boards
that can go into the space allotted for a single electronic circuit board in a
rack or drawer according to the VME64 standard while obtaining the
performance levels in terms of heat dissipation that are desired, in particular
for a power supply board which was not possible with the devices of the prior
35 art. Typically, the heat sink has a thickness of between 5 and 20 mm in the
11
stacking direction which makes it possible to produce electronic circuit
boards that have a thickness E less than or equal to 20.32 mm in the
stacking direction corresponding to the thickness allotted to an electronic
circuit board in the VME standard. The electronic circuit board according to
5 the invention has a limited weight
Moreover, the invention makes it possible to assemble the heat sink in
a single assembly step which can be the step of assembly of the printed
circuit board in the case of a multilayer printed circuit board. In this last case,
the fixing of the heat sink on to the printed circuit board is not an additional
10 assembly step subsequent to the step of assembly of the printed circuit
board.
The invention has been described with reference to an embodiment in
which the heat sink comprises cooling fins. These fins are reliefs, that is to
say structures protruding on a flat surface 8a of the base 8. The reliefs are
15 intended to increase the contact surface between the heat sink 3 and a flow
of air relative to the contact surface between the base 8 (without relief) and
the flow of air. This embodiment is non limiting. An embodiment is envisaged
in which the reliefs take the form of pins, that is to say posts or rods
extending longitudinally in the stacking direction. The pins extend
20 longitudinally in the stacking direction z from the base 8 to a free end. As a
variant, the heat sink comprises at least one cooling fin and at least one pin.
More generally, the invention relates to an electronic circuit board in
which the heat sink comprises a base 8 as described previously and reliefs
on the surface of said base, the reliefs being intended to increase the contact
25 surface between the heat sink and a flow of air relative to the contact surface
between the base and the flow of air. The reliefs extend from a flat surface 8a
of the base in ih-e··stacking direction z. More specifically, these reliefs extend
from the flat surface 8a which is the surface of the base opposite the printed
circuit board in the stacking direction z. The flat surface 8a extends between
30 the reliefs. The flat surface 8a is discontinuous when the reliefs are fins and
continuous when the reliefs are pins. The reliefs are for example, but in a
non limiting manner, pins or fins.
35
Everything that has been stated previously with the cooling fins is also
valid with the pins and, generally, with the reliefs on the surface of the base.
12
The invention relates also to a method for manufacturing an electronic
circuit board 1 according to the invention.
Figure 4 represents a block diagram of the steps of the method
5 according to the invention.
10
15
The method for manufacturing the electronic circuit board according to
the invention comprises:
a step of stacking 50, in the stacking direction, comprising a
first step 50a consisting in placing the heat sink 3 such that the
base 8 is interposed between the reliefs which are for example
fins 7 and the second face 6 of the printed circuit board 2 and in
arranging a layer of glue 9, or more generally the adhesive
layer, between the second face 6 of the printed circuit board
and the base 8,
an assembly step 51 of the heat sink 3 and of the printed circuit
board 2 during which the stacking obtained in the stacking step
50 is subjected to a hot compression.
The assembly step 51 is performed so as to assemble the base to the
20 printed circuit board by gluing by means of the adhesive layer comprising the
glue. In the assembly step 51, the printed circuit board is advantageously
glued to the heat sink by polymerization of the glue.
Advantageously, the glue hardens during the assembly step. It is
liquid, pasty or viscous when the adhesive layer is applied during the
25 stacking phase 50.
During the stacking step 50, an adhesive layer is deposited comprising
a glue extem:ling ·=ntinuaily ·over all the thickness of the adhesive layer, in
the stacking direction.
30 In the stacking step 50, the heat sink 3 and the printed circuit board 2
or the layers intended to form the printed circuit board 2 and the heat sink are
stacked in the stacking direction z so as to obtain a stacking 11. In the
stacking step 50, the layer of glue 9 or the adhesive layer is applied to the
second face 6 of the printed circuit board 2 and/or to the face of the base 8
35 facing the second face 6 of the printed circuit board 2. In the assembly step
13
51, the compression is performed in the stacking direction z. Hot
compression should be understood to mean a step of compression of the
stacking during which the stacking obtained is heated up.
The electronic components 5 are then added to the printed circuit
5 board 2 and more specifically to the second face of the printed circuit board.
Advantageously, the tracks are etched prior to the step 50.
Advantageously, the printed circuit board 2 is a multilayer printed
circuit board. The stacking step 50 comprises a second step SOb of stacking
of a plurality of layers intended to form the multilayer circuitin the stacking
10 direction z. These layers are double-sided assemblies.
The step of assembly 51 of the heat sink 3 and of the printed circuit
board 2 is then advantageously a step of assembly of the printed circuit
board 2, that is to say a step of assembly of the double-sided assemblies
intended to form the multilayer printed circuit board, that is to say a step of
15 assembly of a plurality of layers of the printed circuit board. The assembly
step 51 is performed by pressing the stacking obtained in the step 50 in the
stacking direction. It is a step called lamination. As a variant, the printed
circuit board is multilayer but the layers or some of the layers of the printed
circuit board are assembled before the assembly step 51. Such is the case,
20 for example, when layers have to be interconnected with one another. They
are assembled prior to the step of assembly of the heat sink with the printed
circuit board.
Figure 5 schematically represents a press 40 making it possible to
perform the assembly step 51 of the method according to the invention. In
25 this embodiment, the reliefs are fins.
The press 40 comprises two tools 41, 42. The stacking is performed
between these 1:wo tools 41, 42. Each tool 41, 42 consists of a plate a face
41 a, 42a positioned facing the stacking 11. The press comprises a load
take-up tool 43. The tool is adjacent to the heat sink so as to come to bear on
30 it during the pressing. The tool 43 comprises a portion 44 having a form
substantially complementing the part of the heat sink 3 opposite the printed
circuit board 2 in the direction z. In other words, the portion 44 of the tool 43
has a form substantially complementing the part of the heat sink formed by
the cooling fins 7 and the surface Sa of the base 8 from which they extend. In
35 the assembly step 51, the heat sink 3 and the tool 43 cooperate such that the
14
portion 44 substantially forms the die of the part of the heat sink 3 opposite
the printed circuit board, that is to say of the part of the heat sink 3 formed by
the cooling fins 7 and by the surface of the base from which they extend. In
other words, the portion 44 of the tool 43 substantially matches the form of
5 the cooling fins? and of the surface 8a from which they extend.
In other words, the tool 43 has a portion 44 intended to cooperate with
the heat sink. This portion 44 has fins of the tool 44a separated in pairs by
trenches 44b of the tool 44b. The trenches of the tool 44a and the fins of the
tool 44b are configured and arranged relative to one another such that the
10 portion 44 has a form substantially complementing that formed by the fins 7
and the trenches 12 of the heat sink 3. In the nonlimiting embodiment of
Figures 3 and 5, the fins 7 have the same dimensions in the plane of Figure 3
and are evenly spaced apart, in a direction at right angles to the stacking
direction z in the plane of Figure 3, and separated by trenches 12 all having
15 the same dimensions in the plane of Figure 3. The fins of the tool 44a
consequently have a section substantially identical to that of the trenches 12
and the trenches of the tool have dimensions substantially identical to those
of the fins 7.
In the step 51, the tool 43 and the heat sink 3 are positioned relative to
20 one another such that the fins of the tool 44a penetrate into the trenches 12
and the fins 7 penetrate into the trenches of the tool 44a. The tool then
comes to bear on all of the surface 8a from which the fins 7 extend and on
the fins 7 in the pressing step.
The tools 41, 42, 43 are for example parts made of steel or of
25 aluminium. The tool 43 is preferably produced in a material exhibiting a
thermal expansion coefficient substantially identical to that of the heat sink 3
between ambienti:emperature and the heating temperature.
The tools 41, 42, 43 are then transferred in the compartments of the
plate press 40. The plates 45, 46 can be brought together and separated
30 from one another using a device that is not represented, for example of the
type comprising cylinder actuators. The tools 41, 42, 43 are compressed, in
the stacking direction z, by bringing the two plates 45, 46 closer to one
another in the stacking direction z. A heating circuit that is not represented
ensures the heating of the tools 41, 42, 43 or 41, 43 using a heating circuit at
35 least partially incorporated in the plates 45, 46 so as to heat up the stack 11
15
obtained in the step 50.
Advantageously, a deformable mat 47, 48 is interposed between the
tool 41 and the layers 20a, 20b intended to form the printed circuit board 2
and between the tool 42 and the tool 43.
5 Advantageously, the pressure applied to the stacking 11 in the
assembly step 51 is typically between 30 and 40 bar and the stacking is
heated up to a temperature typically lying between 120 and 180°C. The
temperature applied depends on the polymerization temperature of the glue
used. The pressure and the temperature are applied for a duration typically
10 lying between 1 h30 and 3 h30. This duration depends on the polymerization
cycle of the glue used.
In the embodiment of Figure 5, a plurality of layers 20a, 20b intended
to form the printed circuit board 2 are stacked in the direction z. These layers
are typically double-sided. A layer of glue 21 is interposed between these two
15 layers, that is to say applied to at least one of the two layers facing one
another.
The form of the tool 43 makes it possible to produce a powerful fixing
of the heat sink 3 on to the printed circuit board 2 without deformation of the
cooling fins 7 and without warping the assembly formed by the printed circuit
20 board 2 and the heat sink 3. It makes it possible to apply a substantially
uniform pressure over all the surface of the heat sink in a plane at right
angles to the stacking direction which ensures a gluing of the heat sink on to
the printed circuit board over all the surface of the face of the heat sink
situated facing the printed circuit board. It makes it possible to avoid the
25 formation of air bubbles at the interface between the heat sink 3 and the
printed circuit board 2. That makes it possible to limit the risks of
delamination of the electronic circuit board and thus a diminishing of its
performance level over time under the effect of temperature changes.
More generally, the assembly step 51 is performed by means of a
30 press comprising a tool 42 comprising a portion 44 configured so as to exhibit
a form substantially complementing the part of the heat sink 3 formed by the
reliefs 7 and by the surface 8a, of the base 8, from which they extend.
Moreover, during this step, the heat sink 3 and the tool 42 are arranged so as
to cooperate in such a way that the portion 44 of the second tool 42
35 substantially forms the die of the part of the heat sink 3 formed by the reliefs
16
7 and by the surface Sa, of the baseS, from which they extend.
A portion 44 of the tool 43 substantially forms the die of the part of the
heat sink 3 formed by the reliefs 7 (or fins) and by the surface Sa of the base
S from which the reliefs extend should be understood to mean that the
5 portion 44 has a form complementing the reliefs 7 and the surface Sa within
operating play tolerances. The operating play tolerance is defined so as to
avoid jams between the tool 43 and the heat sink 3 during the pressing, that
is to say during the step 51, and so that the tool 43 comes to bear on all of
the surface of the reliefs and of the surface Sa extending between the reliefs.
10 In other words, in the assembly step 51, the heat sink and the tool 43 are
deformed so as to fill the play tolerances between them. The tool is
configured and positioned relative to the heat sink so as to come to bear on
all of the surface Sa and on the reliefs 7 in the direction z.
In a variant, the portion 44 of the tool 43 forms, in the step 51,
15 substantially the die of the at least one hollow part 12 extending between the
reliefs 7 and being delimited by the surface Sa. In other words, the portion 44
is configured and arranged relative to the heat sink 3 so as to have at least
one relief substantially complementing the at least one hollow part extending
between the reliefs 7. This complementarity is produced to within an
20 operating play tolerance so as to avoid jams between the tool 43 and the
heat sink 3 during the pressing, that is to say in the step 51 and such that the
tool 43 comes to bear on all of the surface Sa extending between the reliefs.
Consequently, the portion 44 is configured and arranged relative to the heat
sink, in the step 51, so as to come to bear on all of the surface Sa from which
25 the reliefs 7 extend. In a less advantageous variant, the form of the portion
44 is defined such that it comes to bear on a part of the surface Sa.
In the case of a heat sink with pins, the portion of the tool has a relief
extending continually between the pins and in the case of a heat sink 3 with
fins 7, the portion 44 comprises several reliefs (or fins) 44a substantially
30 complementing the grooves 12. For example, the height h of the reliefs of the
tool 43 is greater than that of the reliefs 7 of the heat sink and defined such
that the play tolerance between the tool and the reliefs is not taken up in the
pressing step. That makes it possible to avoid having the fins 7, the most
fragile part of the heat sink, support the pressing force. That makes it
35 possible to achieve a good uniformity of gluing on the surface of the heat sink
between the reliefs 7.
The method according to the inventioo makes it possible to obtain an
electronic circuit board in which the film of glue or more generally the
adhesive layer has a thickness typically of between 100 and
5 200 micrometres. That makes it possible to guarantee a good heat transfer
efficiency between the heat sink 3 and the printed circuit board 2. Moreover,
the glue overflows around the heat sink in a plane at right angles to the
stacking direction z. This overflowing is due to a creeping of the glue in the
step 51. Moreover, the glue is advantageously polymerized.
10 The tool 43 described can be used even when the printed circuit board
15
is not multilayer.
The ranges given in the present patent application are preferential
ranges but they are nonlimiting, the measurements, durations, can be
situated outside these ranges.

CLAIMS
1. Electronic circuit board (1) comprising an assembly of a printed circuit
board (2) comprising a first face (4) receiving at least one electronic
component, and of a heat sink (3), the printed circuit board (2) and the heat
5 sink (3) being stacked in a stacking direction (z), said heat sink (3) being
fixed to the printed circuit board on a second face (6) opposite the first face
(4), said heat sink (3) comprising a base (8), in the form of a plate, and
reliefs (7) extending from a flat surface (Sa) of said base (8), the reliefs (7)
being intended to increase the contact surface between the heat sink (3)
10 and a flow of air relative to the contact surface between the base (8) and
the flow of air, the base (8) being interposed between the printed circuit
board (2) and the reliefs (7) in the stacking direction (z), characterized in
that the heat sink (3) is fixed to the printed circuit board (2) by gluing only
such that only an adhesive layer (9) separates the base (8) and the printed
15 circuit board (2), the adhesive layer comprising a glue extending continually
from the base (8) to the printed circuit board (2), and in that said heat sink
(3) is of a single piece.
2. Electronic circuit board according to the preceding claim, in which the
20 adhesive layer is a film of glue.
3. Electronic circuit board according to Claim 1, in which the adhesive
layer comprises glue-impregnated fibres.
25 4. Electronic circuit board according to any one of the preceding claims, in
which the adhesive layer is an electrical insulator.
30
5. Electronic circuit board according to any one of the preceding claims, in
which the glue is a thermosetting glue.
6. Electronic circuit board according to any one of the preceding claims, in
which the heat sink is made of metal.
19
7. Electronic circuit board according to any one of the preceding claims, in
which the adhesive layer is less than or equal to 200 micrometres.
8. Electronic circuit board according to the preceding claim, in which the
5 adhesive layer has a thickness of between 100 micrometres and
200 micrometres.
10
9. Electronic circuit board according to any one of the preceding claims, in
which the heat sink has a thickness of between 5 mm and 20 mm.
10. Electronic circuit board according to any one of the preceding claims, in
which the printed circuit board has a thickness of between 0.1 mm and
1 mm.
15 11. Electronic circuit board according to any one of the preceding claims, in
which the heat sink (3) extends substantially over all the useful surface of
the printed circuit board (2).
12. Method for manufacturing an electronic circuit board according to any
20 one of the preceding claims, characterized in that it comprises:
25
30
a step of stacking (50) in the stacking direction, in which the
heat sink (3) is positioned such that the base (8) is interposed
between the reliefs (7) and the second face (6) of the printed
circuit board (2) and in which an adhesive layer (9) is arranged
between the second face (6) of the printed circuit board and the
base (8), the adhesive layer comprising a glue extending
·continually over all the thickness of the glue in the stacking
direction,
an assembly step (51) consisting in assembling the heat sink
(3) and the printed circuit board (2) in which the stacking
obtained is subjected to a hot compression.
13. Method for manufacturing an electronic circuit board according to the
preceding claim, in which the assembly step (51) is performed by means of
35 a press comprising a tool (42) comprising a portion (44) configured and
20
arranged relative to the heat sink (3) so as to come to bear on the surface
(8a) from which the reliefs (7) extend and extending between the reliefs (7).
14. Method for manufacturing an electronic circuit board according to the preceding claim, in which the assembly step (51) is performed by means of
a press comprising a tool (42) comprising a portion (44) configured and
arranged relative to the heat sink (3) so as to come to bear on all of the
surface (Sa) from which the reliefs (7) extend and extending between the
reliefs (7).
10
15. Method for manufacturing an electronic circuit board according to any
one of Claims 1312 to 13, in which the assembly step (51) is performed by
means of a press comprising a tool (42) comprising a portion (44)
configured and arranged relative to the heat sink (3) so as to come to bear
15 on the reliefs (7) in the direction z.
16. Method for manufacturing an electronic circuit board according to any
one of Claims 12 to 15, in which the printed circuit board (2) is multilayer
and in which the assembly step is a step of assembly of a plurality of layers
20 of the printed circuit board 17. Method according to any one of Claims 12 to 16, in which the assembly
of the heat sink (3) and of the printed circuit board (2) is performed by
polymerization of the glue.