FR2560731A1 - Thermally conducting multilayer printed card - Google Patents

Abstract

The card 1 comprises at least one internal conducting layer 13 and plated-through holes conventionally connecting power-supply pins of electronic-component packages 2 to earth. It is characterised by first thermally conducting plated-through holes 19 filled with solder 7, which are distributed at least beneath the packages 2 and in intimate thermal contact with the packages and the internal layer 13. The layer drains the heat coming from the packages towards a radiator 3 fixed to a double-sided metallised edge 11R, 17R of the card, pierced by plated-through holes 19R. A copper plate 5 and a strip 6, preferably made of plastic ceramic, may be interposed between each package 2 and the card 1 in order to improve heat transfer to the layer.

Description

MULTI-LAYER PRINTED CARD
The present invention relates generally to the problems of dissipating the heat dissipated by electronic components installed on a multilayer printed board. More particularly, the invention relates to thermal conduction in a multilayer printed board comprising a first internal conductive layer connected to ground and metallized holes connecting first supply terminals of housings of electronic components to the first internal conductive layer, the boxes being fixed on the card.

 The object of the invention is to add substantial modifications to such a known multilayer printed board, in order to conduct the heat coming from the housings of electronic components inside the board towards a cooling radiator and or distribute this heat. by diffusing localized heating, without modifying the general internal structure of the card and considerably increasing the cost of the card.

 To this end, a multilayer printed board is characterized in that it comprises first heat-conductive metallized holes filled with solder, distributed at least under the housings and in intimate thermal contact with the housings and the first internal conductive layer.

 Thus, the first conductive layer already existing in a known multilayer card, for connecting the terminals of the electronic components to ground, is reused, according to the invention, to conduct the heat emanating from electronic components. The substantial modifications to the card consist essentially of the perforation of the first heat-conductive metallized holes under and around the implantation areas of the electronic component housings and the filling of solder. These two operations can be carried out by automatic machines, which only slightly increases the cost of the card.

 The printed card can be provided with coplanar internal conductive ribbons for connecting second supply terminals of the electronic components to at least one voltage source. In this case, the invention provides a second internal conductive layer coplanar with the internal conductive tapes, isolated therefrom and in intimate thermal contact with the first thermally conductive holes.

The second layer increases the heat drainage capacity of the card, without increasing the thickness of the card.

 As will be seen below, the invention provides, at the interface between the housings of electronic components' and the thermally conductive holes, means for improving heat transfer to the internal conductive layers and therefore reducing the operating temperature of each bolier.

 The internal layers arranged accordingly drain the calories towards a static radiator fixed to a double-sided metallized edge of the card. The radiator receives the calories through second metallic heat-conducting holes connecting the internal conductive layers to parts of the radiator enclosing the metallized edge.

The invention is less effective than a cold source device with circulation of water to the same extent that the cold source has a higher thermal impedance. However, the invention makes it possible to economically solve certain thermal problems in equipment the installed power of which does not justify forced removal of the calories. Efficiency is directly linked to optimization of the cold source, such as a radiator, and to rules for installing components
specific electronics such as
- distribution of the main thermal sources as close as possible
from the cold spring
- layout of conductive tapes arranged for reduction
the thermal impedance of internal layers.

Other advantages and characteristics of the invention
will appear more clearly on reading the description
following of several preferred embodiments of the invention in
reference to the corresponding attached drawings, in which
- Fig. 1 is a sectional view of part of a multilayer printed card according to the invention, supporting an integrated circuit box and bordered by a radiator
- Fig. 2 is a top view at the level of a first internal conductive layer connected to earth, for the card part shown in FIG. 1
- Fig. 3 is a top view at the level of a second internal conductive layer and internal conductive supply tapes for the card part shown in FIG. 1; and
- Fig. 4 is a view of the upper face of the card part shown in FIG. 1, supporting the integrated circuit box.

As shown in Fig. 1, a double-sided and multilayer printed card 1 according to the invention comprises four electrically conductive layers of copper -or more-, parallel to the faces of the card and separated by three insulating layers of epoxy resin. A first external conductive layer is disposed on a first face of the card, such as the underside of the card 1. The first external conductive layer consists of conductive tapes 10 connecting terminals of electronic components supported by the second face of the card, here the upper face of the card, and by electrical and thermal conductors 11 and 11R. A first internal conductive layer is embedded in the card 1 and is separated from the first external conductive layer 10-11 by an insulating layer 12. The first internal conductive layer consists of an electrical and thermal conductive layer 13 extending over the entire surface of the card 1 and connecting first supply terminals of the electronic components to ground. A second internal conductive layer is also embedded in the card 1 above the layer 13 and separated from the layer 13 by a second insulating layer 14. The second internal conductive layer consists of conductive tapes 15+, visible on the
Fig. 3, connecting second supply terminals of the electronic components to positive terminals of at least one voltage source, and by an electric and thermal conductive layer 16 extending over the rest of the card and also connecting the first terminals d supply of electronic components to ground. A second external conductive layer is deposited on the upper face of the card 1 and is constituted by conductive zones 17 and 17R extending at least below the dissipating electronic components and a radiator heel.

 One of the electronic components supported by the printed circuit board 1 is represented in the form of a parallelepipedal integrated circuit casing 2 in FIGS. 1 and 4. The boot 2 is of the DIL type ("dual-in-line package" in English) and comprises two rows of conductive pins 20 protruding under the longitudinal sides of the boot 2. Two neighboring pins in one of the rows are spaced at a standard pitch p = 2.54 mm. The pins 20 pass through metallized holes 18 in the card 1, filled with solder 7. A pin 20 constitutes a first supply terminal iu integrated circuit 2 and is connected to the internal conductive layers 13 and 16 through a metallized hole 18, as shown in Figs. 2, 3 and 4. Another pin 20+ constitutes a second supply terminal of the integrated circuit 2 and is connected to a conductive tape 15+ through a metallized hole 18+. As shown in Figs. 2 and 3 of the insulating material of the card, such as epoxy resin, isolates the metallized hole 18+ with respect to layers 13 and 16 and insulates the conductive tape 15+ with respect to layer 16. The other pins 20 of the shoemaker integrated circuit 2 are connected to conductive strips 10 through metallized holes 18 isolated from layers 13 and 16 by cylindrical spares made of insulating material.

 At least under the integrated circuit case 2, metallized holes 19 are made in the card 1. For example three holes 19 are equally spaced between two pins 20 arranged opposite each other on the longitudinal sides of the housing and four holes 19 are equally distributed between two neighboring pairs of facing pins.

However, preferably, the metallized holes 19 are distributed as a function of the hot spots of the shovel 12 and therefore of the distribution of the electronic component module (s) contained in the shell 2. The metallized holes 19 are in contact with the internal layers 13 and 16 and an external conductive area 17 underlying the housing 2, as well as with conductive pads forming the conductors 11 on the underside of the card. The holes 19 are used to conduct the heat emanating from the bolier 2 to the layers 13 and 16 constituting a composite thermal drain internal to the card 1.

 The heat drained by layers 13 and 16 is removed by a static radiator 3. As shown in Figs. 1 and 4, the radiator is fixed at one of the edges of the card 1, here the double-sided metallized edge on the right. The radiator 3 has appropriate dimensions and shapes and can be in the conventional form of an extruded aluminum profile. The radiator comprises a rectangular flat plate 30 perpendicular to the card 1 and at the edge thereof and a rectangular heel 31 parallel to the card and covering the metallized upper face of the edge. The radiator plate 30 is provided with fins 32, here four in number, in order to radiate the heat captured by the radiator. A bar made of a good thermal conductive material 33 having dimensions similar to those of the heel 31 of the radiator 3 is arranged under the metallized underside of the card 1 at the edge. The edge of the card is firmly clamped by the heel 31 and the bar 33 by means of clamping screws 34 passing through the heel 31 and the card 1 and screwed into the bar 33. The tightening of the card 1 between the heel 31 and the strip 33 is high in order to reduce any discontinuities between the perfectly polished surfaces of the heel 31 and of the strip 33 in contact with the faces of the card 1 and thus reduce the thermal resistance of contact.

 The transfer of heat from the internal conductive layers 13 and 16 to the radiator 3 is carried out through metallized holes 19R similar to the holes 19, formed in the edge of the card between the heel 31 and the bar 33. According to the embodiment illustrated, four rows of holes 19R are made parallel to the edge of the card 1. The holes 19R can be staggered. Conductive zones 11R and 17R are provided on the lower and upper faces of the card and connect the metallized holes 19R to the opposite faces of the bar 33 and the heel 31.

 The card 1 also supports intermediate means between the upper face of the card 1 and the housing 2 to distribute the heat emanating from the housing over a surface of the card larger than that of the housing and therefore increase the thermal conductivity between the housing 2 and the thermal conductive holes 19. The means for distributing the heat are in the form of a rectangular copper plate 5 shown in FIGS. 1 and 4. The plate 5 extends axially under the housing 2 between the pins 20 and is longer than the 'housing in order to distribute the heat transmitted by the housing over a large metallized area 17 of the upper face of the covered card entirely by the plate 5.

Typically, the thermal conductivity of the wafer 5 is of the order of 400 W / m. 0C, which gives the wafer 5 the role of a real thermal shunt. The plate 5 can extend to the radiator 3 and be fixed thereto to further improve the heat transfer balance.

The thermal contact put the wafer 5 and the thermal drainage layers 13 and 16 embedded in the card is obtained by plugging the holes 19 with solder 7. The holes 19 are filled with solder 6 by passing a wave of solder under the underside of the card i. The passage to the wave also makes it possible to weld the pins 20 of the. housing in the metallized holes 18 of the card. In order to evacuate the air in the holes 19 during the passage of the weld wave and to prevent the formation of air bubbles in the holes which adversely affect the thermal conductivity, the underside of the plate 5 is striated. As shown in the
Fig. 1, at least one transverse streak 50 opens into each hole 19. The streaks are obtained by electro-erosion for example.

During the welding, the plate 5 is pressed against the upper face of the card, and the solder 7 fills the holes 19 almost completely up to the grooves 50 overhanging the holes and welds the plate 5 to the card 1. The filling of the holes 19 by welding 'thus lowers the thermal resistance between the wafer 5 and the layers 13 and 16.

 Between the underside of the housing 2 and the plate 5 are provided means for reducing the thermal contact resistance between the housing 2 and the plate 5 and therefore between the housing 2 and the thermal drainage layers 13 and 16. The means for reducing the thermal contact resistance are in the form of a flexible strip 6 made of plastic ceramic, preferably made of charged silicone. The strip 6 is placed under the housing 2 between the two rows of pins 20 and on the plate 5.

 The thickness of the strip is small, of the order of 0.25 mm, compared to the thickness of the wafer 5 of the order of 1 mm.

As shown in Fig. 4, the strip 6 is covered by the housing 2 and has a width at most equal to the width of the housing 2 but at least equal to the width of the plate 5. The strip 6 has a length at least equal to the length of the housing 2 and can overflow on each side of the housing. The strip 6 is here glued to the upper face of the plate 5. The housing 2 is pressed against the strip 6 during welding so that the housing is in intimate contact with the strip 6. The low expansion of the strip 6 under the 'effect of heat from the housing 2 gives perfect adhesion of the housing to the strip. This avoids any discontinuity of contact between the housing and the strip, and in particular, the presence of air pockets between the housing 2 and the strip 6 which would cause an increase in thermal resistance.

 According to another variant, the strip 6 can be a soft indium strip or be replaced by a silicone grease.

 Thus, thanks to the interface formed by the strip 6, the wafer 5, the weld 7 and the holes 19, the major part of the calories emanating from the housing 2 is transferred to the layers 13 and 16 #. These latter layers embedded in the card 1 transfer the calories to the radiator 3 while reducing heat dissipation.

 The thickness of the thermally conductive layers 13 and 16 is generally greater than the thickness of the ribbons 15+ and 10 located in and on prior art multilayer printed circuit boards, in order to drain a high amount of thermal energy. . Typically, the thickness of layers 13 and 16 is of the order of 2 to 4 times the standard thickness of 35 µm.

 Although the invention is described above for DIL type boxes, a card 1 according to the invention can support flat boxes of electronic component of the "flat pack" type.

 Furthermore, a printed card according to the invention may comprise several internal layers similar to layer 13 or a solid internal drain.

Claims (12)

RESELL ICATIONS  1 - Multilayer printed board (1) comprising a first internal conductive layer (13) connected to ground and metallized holes (18) connecting first supply terminals (20) of electronic component housings (2) to the first layer internal conductive (13), the boxes being fixed on the card (1), characterized in that it comprises first heat-conductive metallized holes (19) filled with solder (7), distributed at least under the boxes (2) and in intimate thermal contact with the boxes (2) and the first internal conductive layer (13 ~).  2 - Printed card according to claim 1 comprising internal coplanar conductive ribbons (15+) and other metallized holes (18+) for connecting second supply terminals (20+) of the electronic component boxes (2) to at least one voltage source, characterized in that it comprises a second internal conductive layer (16) coplanar with said internal conductive tapes (15+), isolated therefrom and in intimate thermal contact with the first thermally conductive holes (19) .  3 - Printed card according to claim 2, characterized in that the internal conductive layers (13, 16) are thicker than the internal conductive tapes (15+) and external conductive tapes (10) of the card (1).  4 - Printed card according to any one of claims 1 to 3, characterized in that it comprises an edge covered with two external conductive zones (bed, 17R) on the two faces of the card (1), second holes metallized thermally conductive (19R) connecting the internal conductive layers (13, 16) to the external conductive areas (llR, 17R), and metallic heat dissipation means (3) comprising two parts (31, 33) enclosing said edge and covering said two external conductive zones (bed, 17R).  5 - Printed map. according to any one of claims 1 to 4, characterized in that it comprises an external conductive zone (17) underlying each housing (2).  6 - Printed card according to any one of claims 1 to 5, characterized in that it comprises means (5, 17) between each housing (2) and a face of the card (1) supporting the housing (2 ) to distribute #the heat emanating from the housing over a surface of the card that is larger than the face of the housing facing the card, said means for distributing the heat being in intimate thermal contact with first metallized thermally conductive holes (19).  7 - Printed card according to claim 6, characterized in that the means for distributing the heat comprise a conductive plate (5), preferably made of copper, having a larger surface than the face of the housing (2) facing the card (1) and secured to the card by the solder (7) filling the first thermally conductive holes (19) closed by the plate (5).  8 - Printed card according to claim 5 and to claim 6 or 7, characterized in that the external conductive area (17) underlying each housing (2) is covered by the plate - (5) and has a surface larger than the face of the box opposite the #card (1).  9 - Printed card according to claim 7 or 8, characterized in that the face of the plate (5) opposite the card (5) has ridges (30) opening outwards and at the mouth of the first thermally conductive holes (19) closed by the plate (5).  10 - Printed card according to any one of claims 7 to 9, characterized in that it comprises means (6) for reducing the resistance of thermal contact between the housing (2) and the wafer (5).  11 - Printed card according to claim 10, characterized in that the means for reducing the thermal contact resistance comprise a flexible strip (6), preferably made of plastic ceramic such as charged silicone, or a soft strip, preferably made of indium , bonded to the plate (5) and in intimate thermal contact with the face of the housing (2) facing the card (1).  12 - Printed card according to claim 11, characterized in that the lamella (6) is replaced by a layer of grease preferably silicone.