WO1999057951A1

WO1999057951A1 - A printed circuit board and a method of processing printed circuit boards

Info

Publication number: WO1999057951A1
Application number: PCT/SE1999/000669
Authority: WO
Inventors: Per Ligander
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1998-04-30
Filing date: 1999-04-23
Publication date: 1999-11-11
Also published as: SE9801528D0; SE9801528L; SE518269C2; AU4301199A

Abstract

The present invention relates to a printed board, and, more specifically, to cooling of an electric component (819) mounted on said board. The printed board is comprised of thin laminates and enables microvias to be provided. A recess that extends down to a cooling plane (12) in the printed board is formed at the place where the component shall be mounted. The width of the recess is much greater than the width of a microvia. The recess is filled with copper, therewith forming a cooling via (56). The cooling via functions to conduct heat away from a component (810) soldered on the cooling via (56) to the underlying cooling plane (12).

Description

A PRINTED CIRCUIT BOARD AND A METHOD OF PROCESSING PRINTED

CIRCUIT BOARDS

FIELD OF INVENTION

The present invention relates to a printed circuit board for electronic components, and more particularly to a method and to an arrangement for cooling a board mounted component.

DETAILED DESCRIPTION OF THE BACKGROUND ART

A printed circuit board is provided with a cooling plane for leading heat away from board-mounted electric components, so as to avoid power dissipation. The cooling plane constitutes one of several board layers. The components are normally mounted on the surface of the board. Each component is connected to the cooling plane through the medium of one cooling via, normally through the medium of several cooling vias .

A cooling via is created by making through the board a hole that also perforates the cooling plane. The hole will have a diameter in the order of 0.3 mm. The inner walls of the hole against the printed board are then copper plated, so as to form said cooling via. The copper surface of said cooling via will therewith have a cylindrical shape.

The component is mounted by soldering it against one or more cooling vias. In the soldering process, the hole present in the centre of the cylindrical body of the cooling via is partially filled by solder paste that runs down into the via. Gas bubbles are generated in the solder paste during the soldering process and it may be difficult to expel these gas bubbles when the component is mounted over the cooling via. One drawback with this known technology is that cooling of the component is not sufficiently effective for all applications .

In the electronic industry, developments are proceeding rapidly towards progressively more advanced products in progressively smaller sizes. One aim is to enable integrated circuits to be made smaller while, at the same time, accommodating more gates . The capacity of an integrated circuit is proportional to the number of gates it contains. Another aim is to enable integrated circuits and other electrical components to be mounted closer together.

The latest technique for mounting electric circuits in close proximity to one another, or for increasing packing density, includes printed circuit boards that can be used with microvia technology. In microvia technology, the printed boards are built-up from thin laminates and holes for electric vias are not drilled mechanically in the board. The electric vias, so-called microvias, have a diameter in the order of 100 μm or less. The thickness of a dielectric layer in a printed board that includes microvias is about 50 μm.

Printed boards intended for microvia techniques are given different names, depending on how the board has been manufactured and/or which dielectric material has been used.

One designation is the BUM board (Built Up Multilayer) , while another technique is designated the SBU technique (SBU = Sequential Build Up) , in which the board layers are built-up sequentially. Each dielectric layer of the board is lacquered and cured before a new layer is applied.

In another method of manufacture, the board layers are pressed together in one step. The dielectric used with such printed boards is normally RC foil. An article headed "Build-up laminates used in high density applications" published in the magazine "Electronic Packaging & Production", August 1997, refers to cooling of components mounted with the aid of microvia technology The article gives as an example cooling of a mounted integrated circuit with 24 cooling vias beneath the circuit.

SUMMARY OF THE INVENTION

The present invention addresses the problem of cooling board- mounted electric components with the aid of microvia technology. Because microvia technology enables components to be packed more densely, more heat is generated per unit of surface area than when components are mounted traditionally. Furthermore, power dissipation in the form of heat from individually mounted components is often high in the case of printed circuit boards with which microvia technology has been applied. One reason is because surface mounted integrated circuits have high computing capacities. Another reason is because in the case of mobile telephones for instance, there are components which operate at high radio transmission powers. The effective useful life of such components will be shortened if heat cannot be conducted away. A further problem is that cooling must be relatively inexpensive to achieve and must be adaptable to methods of mass-producing printed circuit boards and of mounting components thereon.

An object of the present invention is thus to provide means for leading heat effectively away from a board-mounted electric component .

The problem is solved in accordance with the present invention by perforating the various board layers within a relatively small surface area down to the cooling plane so as to form a cooling via This perforated area is then plated with a heat-conductive material and the perforation filled so as to form said cooling via. This results in the formation of a cooling area on the surface plane, which together with the cooling via functions to conduct heat away from a component that has been soldered above the cooling area. Gas evacuation passageways which lead from the interior of the cooling area to its outer limit are formed in said cooling area. A component is then soldered on the cooling area. Gas generated in the soldering process is conducted away from the cooling area by the gas passageways .

Depending on the material from which the printed circuit board is built-up, the board layer is perforated from a surface plane down to the cooling plane solely with the aid of laser light or photolithographically .

If laser light is used, there is burned a large number of recesses each having a diameter of about 100 μm. The recesses are formed in such close proximity with one another as to form together a wider recess . The recesses are formed in both dielectric and metallic layers.

When a photolithographic process is used, a recess is formed down to the cooling plane. The recess is substantially wider than a microvia in the surface plane. A photolithographic process can only be used to perforate dielectric layers, and consequently any metallic layers present between the cooling plane and the surface plane shall already have been perforated to provide the cooling via prior to the board layers having been pressed together.

The recess is completely filled with heat-conductive material in the following plating process and a cooling area is formed in the surface plane. Gas passageways which lead from the interior of the surface to its outer limit are provided preferably by etching. These passageways are intended for the evacuation of any gas that is generated as the component is soldered to the cooling area.

In one embodiment of the invention, the printed circuit board is built-up with further layers on top of the earlier surface plane after having created a cooling area. Pads for establishing electrical connections with the component are also created in the new surface plane of the board.

The component may be glued to the cooling area as an alternative to soldering, therewith obviating the need for a gas passageway.

The invention affords effective cooling of the component, by virtue of the fact that a large cooling body lies against the surface of the component (and because the distance between the cooling plane and the component is short) . The cooling body is comprised of the cooling plane together with the cooling mass accommodated in the perforation.

A further advantage is that there is no danger of cooling being impaired as a result of gas between the component and the cooling body. Any gas generated in the soldering process will be led away by the gas passageways.

A further advantage is that the impedance can be kept high between the component conductors and the earth plane. This is possible when the component is recessed in its mounting surface, therewith enabling the component conductors to be spaced further from the earth plane.

The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross-sectional view of an untreated printed circuit board to which a dielectric layer has been applied.

Figure 2 is a cross-sectional view of an untreated printed circuit board produced by pressing.

Figure 3 is a cross-sectional view of the board shown in Figure 1 and shows the presence of apertures in the dielectric layer.

Figure 4 is a cross-sectional view of the same board as that shown in Figure 2, but with apertures in the dielectric layer.

Figure 5 is a cross-sectional view of a printed circuit board which includes a cooling area and electrical connection pads and which may be either the board shown in Figure 3 or the board shown in Figure 4.

Figure 6 illustrates part of the surface of the printed circuit board illustrated in Figure 5.

Figure 7 illustrates the printed circuit board of Figure 5 with the cooling area filled with solder.

Figure 8 illustrates the printed circuit board of Figure 5 on which components have been surface-mounted.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figures 1 and 2 are respective sectioned views of a multilayer printed circuit board. Each layer forms a plane in the board, and at least one of the layers is comprised of electric conductors. The conductor layer is unable to completely cover a plane, because the conductors must be separated electrically.

Neither of the boards shown in Figure 1 or in Figure 2, nor yet in any other Figure referred to, is drawn to scale.

The printed boards are constructed in accordance with so- called microvia technology. Microvia technology enables components to be packed very densely on a printed board. An electric via, a so-called microvia, in a printed board intended for microvia technology will have a diameter of about 100 μm or smaller, and the dielectric layers will have a thickness of about 50 μm or smaller.

The first printed board 10 shown in Figure 1 has nearest the first surface 1 a dielectric layer 11 which has been applied to an underlying copper layer and then hardened.

The second circuit board shown in Figure 2 is produced by pressing all layers together in one single step. The dielectric layer 21 of this circuit board is comprised of RC foil (Resin Coated Foil) .

Both boards 10, 20 include a cooling plane 12 beneath the dielectric layer, said cooling plane being comprised of copper foil.

The layer located beneath the cooling plane 12 is comprised of a carrier 13, which functions to strengthen the board and give rigidity thereto.

The carrier 13 is followed by a metal layer 14. The metal layer 14 is not a coherent layer, but consists of a number of electrically separated conductors. The other printed circuit board 20 shown in Figure 2 has nearest the first surface plane 1 a copper layer 24 which lies on top of the dielectric layer 21.

The top layer of the first printed circuit board 10 is the dielectric layer 11. The first surface 1 on which components are surface-mounted is located on the dielectric layer 11.

Figure 3 shows the printed board 10 subsequent to having formed two recesses 15 therein. Figure 4 shows the printed board 20 provided with two recesses 15, in which a cooling via shall be formed. The recesses 15 extend from the surface plane down to the cooling plane 12. The area of respective recesses in the surface plane is typically half the area of the abutment surface of the component to be mounted over the cooling via. The width 31 of the recess and the ultimate cooling via is much greater than the width of the microvia, e.g. at least four-five times as wide. This width is also much greater than the depth of the recess .

The recesses 15 are formed in the first board 10 either by a photolithographic process or with the aid of laser light. The photolithographic process is well known to the person skilled in this art. Briefly, it involves placing a mask on the circuit board 10 and then irradiating the board with light. Parts of the dielectric are therewith removed chemically to form the recesses 15, depending on whether irradiation has taken place or not .

The copper layer 24 and the dielectric layer 21 of the printed board 20 are perforated with the aid of laser irradiation. Each irradiation of the layers 24, 21 results in a recess whose width is typical of a microvia. The large recesses shown in Figure 4 have been formed by irradiating the layers at a large number of locations in close proximity of one another such that the small recesses thus formed will merge together to form the recesses 15.

Figure 5 is a cross-sectional view of a finished printed board 50 provided with two cooling vias 56 and component connecting pads 58.

The finished printed board 50 has been constructed by panel- plating the first surface 1 of the first board 10 and the second board 20 with copper. The copper plating has a thickness of 30 μm in the illustrated case. The recesses 15 were filled completely with copper in the plating process, therewith forming a cooling via 56 in each recess 15. A shallow hollow or depression 59, however, was formed across the recess 15.

Subsequent to plating said first surface 1, the first and the second circuit boards 10, 20 were etched and excess copper removed in the surface plane. Copper intended for pads 58 and cooling area 57 has been left on the surface.

Figure 6 illustrates a small part of the first surface 1 of the finished printed board. This illustrated part of said first surface includes one of the cooling areas 57. The cooling area 57 is intended for mounting of a component. A row of pads 58 is found on two sides of the cooling area 57. A corresponding leg of the mounted component shall be connected electrically to each of these pads 58.

As will be seen, the depression or hollow 59 is in the centre of the cooling area 57 and gas passages 60 are provided in that part of the cooling area 57 that lies around the depression 59. The gas passages 60 extend from the depression 59 to the outer limit of the cooling area 57. The gas passages 60 have the form of grooves in said surface and have a depth which is equal to the thickness of the plating, or slightly smaller, i.e. a depth in the order of 20-50 μm. The width of the gas passages 60 will preferably be greater than their depth, in order to facilitate the flow of gas therethrough. The width of the passages will be in the order of 200-300 μm. The gas passages 60 have been etched into the cooling area 57.

Figure 7 shows the finished printed board 50 with solder paste 710 applied to the cooling areas 57 prior to mounting the components. The solder paste 710 has a semi-liquid state in Figure 7 and contains resins that are vaporised when the paste 710 is heated.

Figure 8 shows the finished printed board 50 with two components 810 mounted thereon. The component 810 is fastened to its corresponding cooling area 57 through the medium of a solder joint 820. Figure 8 also shows that the legs 830 of the components 810 are connected to corresponding connection pads 58. The connection between legs 830 and connection pads is normally a solder connection.

A printed board 50 that includes surface-mounted components 810, i.e. an assembled board, as shown in Figure 8 is called a circuit board, here referenced 840.

When the circuit board assembly 840 operates electronically, power losses in the form of heat are experienced in the circuits 810. This heat is conducted from the circuits to the cooling plane 12 by means of the cooling via 56. The heat is distributed over the whole of the cooling plane 12. The inventive cooling via 56 provides good thermal conductivity, by virtue of a large cooling mass being connected to the component 810. The distance between the component and the cooling plane 12 is short and any gas bubbles that are generated will have no deleterious effect on the thermal conductivity of the solder joint 820 between the component 810 and the cooling area 57.

In addition to component cooling, it is also important in a number of applications to maintain a high impedance between an earth plane in the printed board assembly and the component conductors in the surface plane. Circuits that operate with frequencies around 900 MHz, as in mobile telephony, are examples of such components .

There will now be described a printed board that has good cooling properties and provides a high impedance between conductors and earth plane.

Figure 9 illustrates a third printed board 90. The sole difference between the third printed board 90 and the finished printed board 50 is that the third printed board 90 lacks pads 58. The third printed board includes cooling vias 56 created in the same way as the vias in the finished printed board 50.

The third printed board 90 shall be furnished with a further layer, such as to obtain a greater distance between the surface plane and the cooling plane 12. In the Figure 9 embodiment, the cooling plane 12 also constitutes an earth plane. Alternatively, the earth plane may be comprised of a separate plane although the principle of increasing the distance between surface plane and earth plane is the same. This greater distance between the earth plane and the surface plane also increases the impedance between the earth plane and conductors in the surface plane.

Figure 10 illustrates a number of layers pressed on the first surface 1 of the third printed board. Located nearest the dielectric layer 11 is a non-flow prepreg layer 101. Prepreg is a term of the art describing woven glass fabric dipped in epoxy resin. The epoxy resin in the prepreg is semi-hardened although it will become molten as pressure is applied to the layers of the third printed board 90 and thereafter harden completely. Non-flow prepreg is a term of the art which describes prepreg that will not flow when pressure is applied to the layers of the third printed board 90. The non-flow prepreg layer 101 has formed therein holes for the cooling vias. A copper layer 102 for electric conductors is provided on top of the non-flow prepreg layer. A carrier layer 103 is provided on top of the copper layer 102. The carrier layer is followed by a dielectric layer 104, in this case an RC foil, which, in turn, is followed by a copper top layer 105.

Figure 11 illustrates the layers that are pressed together in the case when the dielectric shall be applied as a varnish instead of being pressed on the third printed board 90. The same layers as those referred to in Figure 10, with the exception of the last two layers, the dielectric and copper layers, are pressed on the third printed board 90. Varnish or lacquer is then applied to obtain the dielectric layer 104 and cured said dielectric layer then plated with a top copper layer 105.

These mutually joined layers that incorporate cooling vias form the laminate 120 shown in Figure 12. The uppermost copper layer is etched and copper that shall not form connection pads removed therewith. Wells are cut in the laminate, to expose the cooling vias.

Figure 13 shows the finished printed board 130 with the cooling vias 56 sunken in wells 132. The pads 58 are shown on the uppermost surface 131, adjacent the wells 132.

Figure 14 illustrate components 810 that are let partially into the wells 132 and onto the cooling vias 56. This recessed mounting of the components means that the distance between pad 58 and the component capsule will be shorter than in other cases. This enables the legs 830 that connect the component to the capsule to be made shorter than the legs 830 in the Figure 8 embodiment. One advantage with short legs is that the legs will have less affect on inductance.

The aforedescribed component 810 conveniently has a metallised surface for abutment with the cooling area 57. Legs on the component 810 are connected electrically to pads on the printed board by soldering. This is a typical method of electrically connecting integrated circuits and analog components. However, other methods are available. One example is found in a BGA component (Board Grid Array) . A BGA component has a vector or a matrix of balls beneath it, where each element is an electric connection point. Another name is Chip Scale Packaging (CSP) . Similar to the BGA component, a CSP component includes a matrix of electric connections, although the capsule of a GSP is slightly smaller than a BGA. Another type of integrated circuit is a Flip Chip circuit. A Flip Chip has no capsule around the semiconductor material. Electric conductors are connected directly to corresponding points on the semiconductor material.

A BGA and CSP component is cooled in accordance with the inventive method, by making the cooling area 57 smaller than the surface of the BGA component that lies in abutment with the printed board. The vector or the matrix for electrical connection of the component is accommodated adjacent the cooling area 57.

Cooling of a Flip Chip is effected by pacifying an area of the surface, wherewith no electrical potential within said area will affect the circuit. The pacified area is connected to the cooling area 57. As in the case of the BGA component, room is made for the cooling area 57 adjacent the electrical connections . The cooling via has a smaller width 31 when the cooling area 57 shall not cover the whole of the surface with which the component abuts the finished printed board 50, 130. The width of a recess, and therewith the width 31 of the cooling via, is normally of the same size as the surface with which the component abuts the printed board, or one or more tenths thereof. Thus, the width 31 of the cooling via is much greater than the diameter of a microvia for electrical connection. If the recess (15) has a square cross-section, the length of its sides will be at least five times the diameter of a microvia.

The aforedescribed plating was effected in the form of a so- called panel plating over the entire surface 1, whereafter excess copper on the surface 1 was etched away. Another conceivable method is so-called pattern plating, wherewith copper is applied solely where it has a function to fulfil. The pattern-plated surface shall be comprised of dielectric. No subsequent etching is required.

In the aforedescribed embodiments, copper is applied to the surfaces of the recesses and to the surface 1 of the printed boards 10, 20. Copper has good thermal conductivity in relation to its price. Aluminium and gold are other conceivable plating materials for use in the cooling plane 12.

The components are soldered to the cooling area 57 in the case of the described embodiments. This method of mounting components is the most usual method and provides satisfactory thermal conductivity properties. The generation of gas bubbles in the solder joints is also avoided when the method is combined with the inventive gas passages . The larger the surfaces to be joined together, the greater the problem of gas bubbles in solder joints. Although problems associated with gas generation are known in conjunction with soldering processes, the problem is relatively small when the soldering process is intended to electrically connect a component. Gas is also able to diffuse laterally, since the joint is small.

The component 810 can be glued to the cooling area 57, as an alternative to soldering said component. This avoids problems relating to gas formation and obviates the need for gas passages 60. One difficulty associated with gluing is that of obtaining good thermal conductivity.

Claims

1. A printed circuit board (50) having two surfaces of which at least a first surface (1) is intended for mounting at least one electric component (810) and is provided with microvias, wherein the printed circuit board (50) includes: a plurality of layers of which at least one is a cooling plane (12) and at least one is a dielectric layer (11, 21) located between said cooling plane (12) and said first surface (1) ; and at least one cooling area (57) adjacent a cooling via (56) of given width, said cooling via containing thermally conductive material in contact with said cooling plane (12), characterised in that the width of said cooling via (56) is substantially greater than a microvia; and in that the cooling via is completely filled with said thermally conductive material.

2. A printed board according to Claim 1, characterised in that the width of said cooling via is at least equal to the width of five microvias .

3. A printed board according to Claim 1 or 2 , characterised in that the cooling area (57) includes gas passages (60) that extend from the interior of the cooling area to its outer limitation.

4. A printed board according to Claim 3 , characterised in that said gas passages (60) have been etched in the cooling area (57 ) .

5. A printed board according to Claim 1, 2 or 3, characterised in that said cooling area (57) is adapted for cooling abutment with a surface of an electric component (810) .

6. A printed board according to Claim 1, 2 or 3, characterised in that said cooling mass is comprised of copper .

7. A printed board according to Claim 1, 2 or 3, characterised in that said cooling plane is comprised of copper .

8. A printed board according to Claim 1, 2 or 3, characterised by a recess (15) in the layer of said plurality of layers that is located between said first surface (1) and said cooling plane (12), said recess (15) being intended for said cooling via (56) and being formed by a plurality of mutually merging recesses formed with the aid of laser light.

9. A printed board according to Claim 1 or 2 , characterised by a recess (15) in said at least one dielectric layer (11, 12) between the cooling plane (12) and the first surface (1), said recess (15) being formed photolithographically .

10. A printed board (130) that comprises: at least one microvia; a plurality of layers of which at least one is a cooling plane (12) and at least one is a dielectric layer (11, 21, 104) ; and at least one cooling area (57) which is connected to said cooling plane by means of at least one cooling via (56) that includes thermally conductive material; and in that said cooling area (57) is sunken relative to a surface plane (131) in said printed board (130), characterised in that the width of said cooling via (56) is much greater than the width of said microvia; and in that said cooling via (56) is completely filled with thermally conductive material.

11. A printed board according to Claim 10, characterised in that the width of said cooling via corresponds at least to the width of five microvias.

12. A method of providing a printed board (50, 130) with a cooling area (57) that functions to conduct heat away from a mounted component (810), said printed board (50, 130) comprising a plurality of layers of which one is a cooling plane (12) and at least one of which is a dielectric layer (11, 21) between said cooling plane (12) and a first surface (1), characterised by the steps of: a) perforating those of said layers that are located between the cooling plane (12) and said first surface (1) such as to form a recess (15) whose width is much greater than the width of a microvia; and b) plating the recess (15) , wherewith said recess (15) is essentially filled with thermally conductive material, and wherewith a cooling area (12) is formed on said first surface

(1) ΓÇó

13. A method according to Claim 12, in which said recess is given a width that corresponds at least to the width of five microvias .

14. A method according to Claim 12 or 13, in which said cooling plane and said thermally conductive material are comprised of copper.

15. A method according to Claim 12 or 13 , in which said surfaces are perforated by irradiating the surfaces with laser light, and in which a plurality of narrow mutually merging smaller recesses are formed which together form said recess (15) .

16. A method according to Claim 12 or 13, in which said surfaces are perforated photolithographically .

17. A method according to Claim 12, in which said plating is applied by panel-plating the whole of said first surface (1) .

18. A method according to Claim 17, in which said plating process is followed by an etching process in which thermally conductive material is removed from around said cooling area (57) .

19. A method according to Claim 12, in which said plating is applied by a patterned plating process.

20. A method according to any one of Claims 12, 13, 14, 17, 18 or 19 comprising the steps of: c) etching the cooling area gas passages that lead from the interior of said cooling area to its outer limitation.