WO2004038471A1

WO2004038471A1 - An optical board with electrical and optical wiring layers and a method of its production

Info

Publication number: WO2004038471A1
Application number: PCT/GB2003/004660
Authority: WO
Inventors: Andrew Grant
Original assignee: Terahertz Photonics Ltd
Priority date: 2002-10-28
Filing date: 2003-10-28
Publication date: 2004-05-06
Also published as: GB0225018D0; AU2003276411A1

Abstract

A method for processing an optical board so that it is adapted to receive an optical connector (10), the method comprising: defining a mask (34) on a front side of a substrate; depositing optical material (16) over the mask (34), the optical material defining optical components, such as waveguides; and removing optical material through the mask (34) from a reverse side of the substrate, the reverse side of the substrate carrying a second mask with openings and/or alignments marks, the second mask openings/alignments marks corresponding to the openings in the first mask (34).

Description

AN OPTICAL BOARD WITH ELECTRICAL AND OPTICAL WIRING LAYERS AND A METHOD OF

ITS PRODUCTION

The present invention relates to a method for fabricating an optical board that is adapted to receive or be connected to an optical connector. In particular, the present invention relates to an optical backplane or motherboard that is made using printed circuit board

(PCB) compatible technology.

Traditional electrical based communication systems rely heavily on printed circuit boards, which use metal, typically copper, traces as the information carrying channels. However, in the quest for increasing speed, there is a desire to use optical systems, in which waveguides are used as the information carrying channels. Whilst these provide many potential advantages, there are difficulties that have to be overcome before optical boards can be made in anything close to the quantities that are needed to compete with printed circuit boards. Particular problems arise when trying to form connections between components on optical boards and other such boards or components.

In conventional printed circuit boards, it is relatively straightforward to form connections between current carrying tracks. This can be done merely by defining cross over points between the traces at the appropriate places. Forming connections to other boards is equally straightforward and relies on the insertion of a connector plug into a suitable socket on the board and subsequent physical contact between electric contacts in the socket and the connector. For optical circuits, the situation is more complicated. Typically, signals are redirected or rerouted using mirrors or other beam deflecting elements to deflect the information carrying light from one waveguide into another. The alignment of these elements relative to the waveguides is critical to ensure that optical coupling is optimised. To direct light between optical components on one board to optical components on another such board, optical connecters are used in a similar manner to standard electrical connectors. Typically, these include an array of optical fibres that are held together in a fixed position at a connector head. The end of each fibre is exposed so that light can pass into and out of it. In order to ensure that the fibres are correctly aligned with optical components on the board, the physical location of the optical connector in situ is important. In addition, care has to be taken to ensure that the ends of the waveguides on the board are properly processed to ensure minimal radiation losses and at the same time ensure that light is directed in a well-defined manner from the ends of the waveguides into the optical fibres of the connector.

Many arrangements for mounting optical connectors to optical boards have been proposed. For example, WO 02/31567 describes an arrangement for fitting a connector to an optical backplane assembly. In this, the backplane includes optical vias that are coupled to optical signal traces in the backplane. Positioned in the optical vias are connectors that comprise reflective elements for directing optical signals from the backplane to optical traces in daughterboards. To accurately locate the reflective elements in the vias a process of active alignment is used. A disadvantage of this arrangement is that the active alignment process is sensitive and complicated. Furthermore, it cannot be implemented using standard PCB processing technology. This makes its use for the mass manufacture of backplane assemblies impractical .

An object of the invention is to provide an improved method for mounting or locating an optical connector on a substrate, in particular a printed circuit board with optical functionality.

According to one aspect of the present invention, there is provided a method for mounting or locating an optical connector on a substrate, the method comprising: defining a conformal mask on one side of the substrate; depositing optical material over the conformal mask, the optical material defining optical components, such as waveguides, that are located at a predetermined position relative to the conformal mask; and removing optical material through the conformal mask from a reverse side of the substrate.

The conformal mask is shaped to define an opening that can be used to form a cavity that is sized to receive the optical connector and locate that connector in an aligned position relative to the optical components defined in the optical material. By forming the conformal mask on a front side of the substrate and removing material from the reverse side, a cavity for receiving an optical connector can be accurately defined in the optical material. This cavity defines the longitudinal and transverse position of the connector and simultaneously exposes the ends of the waveguides. In order to ensure that the connector can be passively aligned relative to the ends of the waveguides, the thickness of the various layers within the optical material has to be accurately known. Crucially, these thicknesses can be controlled by the board manufacturer at the optical material deposition stage, unlike, say, the substrate thickness and thickness tolerance. Because material is removed from the reverse side of the substrate, damage to the optical material on the front side can be minimised. The step of defining the conformal mask may involve using lithographic techniques, such as photolithography, to define a lithographically defined photo-resist mask.

An advantage of this is that the location and features of the photo-resist mask can be defined to within 2 to 5μm and, for example, better than 3μm using standard PCB photo-plotters. This means that the location of the connector can be accurately defined. The step of defining may further comprise etching through the lithographically defined photo-resist mask to form the conformal mask on the substrate. The conformal mask may be etched into a copper layer on the substrate.

The step of depositing may comprise using a liquid film deposition technique. The liquid film deposition technique may comprise any one or more of screen-printing or dip-coating or curtain coating.

The optical material may be deposited as a monomer or monomer mixture and the method may further comprise polymerising the monomer or monomer mixture to form a polymer. The monomer or monomer mixture may be acrylate based.

The optical material may include a buffer layer. The buffer layer may be part of the optical material and may act as a cladding layer for confining light in other layers within the optical material. The buffer layer may be provided to form a substantially flat surface. The buffer layer may be provided separately from the optical material . The step of removing may involve mechanically machining at least part of the substrate by for example routing or drilling out material. The step of machining may involve laser machining. Typically, the step of removing involves mechanically machining out the bulk of the material and then laser machining the rest, and in particular laser machining the optical material through the conformal mask.

The substrate may comprise a bulk material, such as glass-reinforced epoxy, with layers of metal formed on each side thereof. The metal may be copper.

According to another aspect of the present invention, there is provided an optical board for carrying or locating an optical connector, the board comprising a substrate; a conformal mask on one side of the substrate; optical material deposited over the mask; and a cavity that is defined by the conformal mask and is adapted to receive the optical connector from a reverse side of the substrate. The cavity may be in-filled after insertion of the connector. The optical material may define waveguides and/or other optical components or elements. The optical material may include cladding layers for confining light within the waveguides. One of the cladding layers may be deposited directly over and in contact with the mask and may additionally serve as a buffer layer for providing a planar surface. The substrate may comprise a bulk material, such as glass-reinforced epoxy, with layers of metal formed on each side thereof. The metal may be copper.

Various aspects of the invention will now be described by way of example only and with reference to the following drawings, of which: β Figure 1 is a cross section through an optical board with an optical connector connected thereto;

Figure 2 is a perspective view of an initial PCB substrate for use in making the optical board of Figure 1;

Figure 3 is a plan view of a reverse side of the substrate of Figure 2, after the formation of alignment or feducial marks;

Figure 4 is a plan view of the reverse side of the substrate of Figure 2, after formation of alignment or feducial marks and additionally windows;

Figure 5 is a plan view of a front of the substrate of Figure 4 after patterning of a conformal mask;

Figure 6 is a more detailed view of the conformal mask of Figure 5;

Figure 7 is a cross-section through the patterned substrate of Figure 6, after application of a buffer layer;

Figure 8 is a cross-section through the substrate of Figure 7 after application of a core layer;

Figure 9 is a plan view of the substrate of Figure, 8 in which waveguides are patterned in the core layer, these being positioned in a pre-determined location relative to the conformal mask; Figure 10 is a cross-section through the substrate of Figure 9 after application of a cladding layer;

Figure 11 is a cross-section through the substrate of Figure 10 after application of subsequent layers;

Figure 12 is a cross-section through the substrate of Figure 11, after material is removed through the conformal mask, thereby to define a cavity for receiving the optical connector; Figure 13 is a perspective view of a cavity that is defined for receiving an optical connector, and

Figure 14 is a cross-section through another board similar to that of Figure 12. Figure 1 shows an optical connector 10 that is located in a cavity 12 that is defined in an optical board 14, such as an optical backplane or motherboard.

The connector 10 shown in Figure 1 is a modified version of the well-known MT connector. It will be understood, however, that any suitable connector could be used. The board 14 includes a core layer 16 in which are defined optical components, and in particular waveguides (not shown) . The MT connector 10 includes an array of optical fibres 18. Each of these is aligned with an end of one of the waveguides.

The present invention is directed to the board 14 and a method for making this in such a manner as to optimise optical coupling, by minimising optical losses and cross talk and ensuring that light emitted from a particular waveguide is directed into the correct fibre. This method involves defining a conformal mask 20 on a front surface of a substrate 22, depositing optical material, including the core 16, over the mask 20, which material defines the optical components, and then selectively removing material from a reverse surface of the substrate 22 and through open parts of the conformal mask 20. In this way, a cavity 12 in the optical material 16 can be defined, this cavity 12 being adapted to receive the optical connector 10 from a reverse side of the board.

In order to make the optical board of Figure 1, it is preferred that the starting material is an initial substrate 22 that is the same as or similar to a standard PCB compatible material. An example of this is shown in Figure 2. This board comprises a bulk substrate 24, typically glass-reinforced epoxy. The thickness of this is not critical. On a front surface of the substrate 24 is a copper layer 26. This is the surface 26 to which the optical material is to be applied. On the other side, that is the reverse side, there is provided a copper layer 28. The accuracy of the thicknesses of the copper layers 26 and 28 are not critical.

Alignment feducials 30 are etched into the reverse side copper layer 28, as shown in Figure 3, in order to allow features to be defined accurately on the reverse side and/or provide a mechanism for allowing other layers, such as electronic layers, to be accurately positioned on the reverse side 28. Optionally, windows 32 are also defined on the reverse side 28 of the substrate at the same time as the feducials. These windows 32 are located so as to correspond to the positions of optical connectors 10 that are to be accurately located relative to the front side of the substrate. The windows 32 have to have smaller dimensions and so define a smaller area than the conformal mask that is to be defined on the front side of the board. At the same time, the windows 32 also have to be larger than any features that are defined in the conformal mask. The reasons for this will be described in more detail later. Typically, the windows 32 are rectangular, as shown in Figure 4. The windows 32 are formed by lithographically defining a pattern consisting of a series of rectangles in a layer of positive photo-resist, developing the photo-resist to expose rectangular areas of copper 28 on the reverse side of the substrate and etching the reverse side using, for example a cupric chloride etch, so that all copper within these windows is removed. It should be noted that the etch process is relatively controllable because the glass-reinforced substrate acts as a stop. At the same time as forming the windows 32, the feducial marks 30 are defined in the copper layer 28.

These 30 can be formed around the edges of the board and additionally around the edges of the windows 32. Once the windows 32 are formed on the reverse side

28 of the substrate, conformal masks 34 are defined on the front side 26. These are shaped so as to define a window that is to be used for forming a cavity for receiving the connector head and exposing the ends of the waveguides. Every mask 34 on the front is optionally in registry with a corresponding window 32 on the reverse side of the substrate. For clarity only a single mask 34 is shown in Figure 5. The overall size of each mask 34 is slightly larger than that of the reverse side windows 32, although, as noted previously, the features defined in the mask 34 have dimensions that are smaller than those of the reverse side windows 32. This can be seen from Figure 5, which shows the outline of the reverse side window as a dotted line. The conformal masks 34 are defined using lithography techniques, such as photolithography. This means that the features of the mask can be defined to within an accuracy of better than, say, 3 microns.

Each mask 34 defines a shape that corresponds to the shape of the connector 10 that is to be received in the board at that location. In the present example, the modified MT connecter 10 has a head portion that has a generally rectangular outer periphery and two connector pins (not shown) on either side of the head for securing the connector 10 to the board. Hence, the mask

34 has to define a rectangular window 38 for use in forming a cavity for receiving the connecter head and a pair of circular openings 40 for use in forming two elongate cavities for receiving the alignment pins on the connector, as shown in Figure 6.

To form the conformal masks 34, the front surface 26 of the substrate is coated with a layer of negative photo-resist. This resist is selectively exposed to light thereby to define a large rectangular portion with a smaller rectangular opening formed within it, as well as two circular openings. At the same time, to ensure that subsequent layers can be correctly aligned with the conformal masks 34, feducial or alignment marks are defined in the photo-resist. These can be sited at predetermined locations on the board, for example, at the edges and/or the corners and/or round the conformal masks 3 . Feducials provided around the edges of the board are used mainly for layer to layer alignment. Feducials provided round the conformal masks 34 are primarily provided for aligning waveguides in the core layer with the conformal masks 34.

Once the areas of the photo-resist that are to define the conformal masks 34 and feducials are exposed, the photo-resist is then developed, leaving various photo-resist based feducial marks and a mesa structure of exposed photo-resist on the front surface, this mesa structure being substantially rectangular with a rectangular opening and two circular opening formed therein. The front side of the substrate is then etched using a cupric chloride etch, or any other suitable copper etchant, to remove all copper that is not covered by the photo-resist. Then the remaining photo-resist is removed, leaving a copper mask 34 having a rectangular opening 38 and two circular openings 40 on the front of the substrate, as shown in Figures 5 and 6, as well as various alignment marks 42.

After formation of the copper mask 34, a buffer layer 44, for example a polymer layer, optionally an acrylate-based monomer mixture, is applied to the front side of the substrate, see Figure 7. This can be deposited in liquid form using any suitable film deposition technique such as screen-printing. Once the layer 44 is applied, it is cured, typically by exposing it to UV radiation, to form a solid, cross-linked acrylate polymer. The function of the buffer layer 44 is two-fold, firstly it acts as a cladding layer for partially confining light in the subsequently defined core layer and secondly it has a planarising effect, so that it provides a substantially flat surface for depositing further layers on. Deposited over the buffer layer 44 is the core layer

46, as shown in Figure 8. As for the buffer layer, this could be an acrylate-based monomer mixture that is deposited in liquid form and cured to form a cross-linked acrylate polymer. The composition of this layer is, however, different to that of the buffer layer and in particular has a slightly higher refractive index. The core layer 46 is used to define optical features and in particular waveguides. In order to define waveguides 48, any suitable lithography technology can be used, for example, laser writing. At this stage the alignment feducials 42 provided around the conformal mask 34 are used to ensure that the waveguides 48 are correctly oriented or aligned relative to the rectangular opening 38 in the conformal mask 34. Once areas of the core layer are selectively exposed, unexposed areas of the core are developed, i.e. dissolved, in a suitable solvent, leaving the waveguides 48. Generally, the waveguides 48 are defined so that they extend substantially perpendicular to the long edges of the rectangular window 38.

Once patterning of the core layer is finished, the waveguides 48 extend over the rectangular opening 38 in the mask 34, as shown in Figure 9. The depth of the waveguides 48 from the reverse side of the conformal mask 34 should be chosen so that when the connector 10 is fitted in place, the ends of the optical fibres are at substantially the same level as the ends of the waveguides 48. Of course, it will be appreciated that this will vary according to the type of connector 10 that is to be used.

Once the optical features are defined, a cladding layer 50 is applied, see Figure 10. This may be made of the same material as the buffer layer. As before the cladding layer 50 can be deposited using any suitable technigue such as screen-printing. The buffer and the cladding layers 44 and 50 respectively together act to confine light in the waveguides 48. The substrate 22 with its copper layers 26 and 28, the buffer layer 44, the core 46 and cladding 50 together define an optical layer. In practice the optical material is typically 150-450 microns thick, although it could be made thicker if desired. The overall thickness of the optical layer is typically l-2mm, depending on the thickness of the substrate and the optical material. On either side of the optical layer, further layers may be applied, for example electronic layers. Various options for doing this exist, but in one example, a prepreg layer 52 is firstly applied. This is adapted to secure together further substrates 54. In this way other devices can be built on and around the optical layer, with its waveguides 48. Optionally, a copper stop 49 may be provided on the front side of the optical layer and in registry with the mask

34, see Figure 1. This acts to stop laser machining of the board extending into subsequent layers.

It should be noted that if other layers are applied to the reverse side of the substrate 22, the windows 32 defined in the reverse side copper layer are in filled, as can be seen from Figure 11. For every subsequent layer on the reverse side of the initial substrate, appropriately sized rectangular windows may be defined in locations that correspond to and are in registry with the windows in the reverse side copper layer 28. The size of these later windows is smaller than the initial windows in the reverse side copper layer to take up the layer to layer registration tolerances and any beam diffraction when light impinges on the window. These windows are not essential, but are useful where laser machining may be used in isolation from mechanical machining to remove material from the reverse side of the board. In the example of Figure 12, a window 56 is defined through the outer copper layer 58.

Once all the required layers are deposited, the board is processed in order to define an opening for receiving the optical connector 10 by machining unwanted material away. This is done from the reverse side of the board, that is, from the side that is remote from the optical material. In the case of the board shown in Figure 12, because windows 32 and 56 are formed through all the copper layers 26 and 28 on the reverse side of this board, this can be done by laser machining the material away quickly, down to the conformal mask 34. As will be appreciated, the laser should be selected so that it is suitable for removing the bulk material, such as the glass-reinforced epoxy and the optical material, but not the copper of the conformal mask 34. Because the reverse side window 32 and later windows 56 have smaller surface areas than that of the conformal mask 34, this means that only material directly behind and in registration with the conformal mask 34 is removed.

Once the bulk of the material is removed, the laser machining is continued to selectively remove parts of the polymer optical material 46, including the waveguides 48, through the openings in the conformal mask 34. Because the laser does not remove copper, this machining leaves the conformal mask 34 substantially unaffected. The continued laser machining causes the formation of two narrow cavities 60 that are adapted to receive the connector pins and a larger rectangular cavity 62 that is adapted to receive and locate accurately the connecter head, as shown in Figure 13. It should be noted that the laser impinging on the reverse side of the conformal mask 34 is limited in size to be smaller than the mask 34, but larger than the features 38 and 40 defined in it, i.e. the rectangular window 38 and the circular openings 40. These features 38,40, therefore, limit the spatial extent of the laser beam and so the laser in effect conforms to the shape of the mask 34. In this way, only the two holes and rectangular window are machined into the optical material by the laser and by the lithographic design of the conformal mask 34 and there is no need to take into account the size and positional accuracy of the incident laser beam. As will be appreciated, formation of the larger cavity 62 fulfils two functions simultaneously, these being the definition of the position of the connector 10 and the preparation of the ends of the waveguides 48. Once the relevant openings 60 and 62 are defined, the connector 10 can be inserted from the reverse side of the board as shown in Figure 1.

Because of the accurate positioning of the rectangular window 38 relative to the waveguides 48, this ensures that the connector 10 is accurately and passively aligned with the ends of the waveguides 48. After insertion of the connector 10, the cavity may be in-filled with, for example, an epoxy resin or any other suitable material.

It should be noted that doing double sided processing and in particular lithography of PCBs can lead to side to side registration errors. By sizing the reverse side windows 32 and/or 56 accurately relative to the conformal mask 34, tolerance issues can be reduced. For example, if the accuracy of the lithography is 10 microns then undersizing the reverse side windows 32 and/or 56 by 15 microns will account for this.

By forming a conformal mask on the side of the substrate that is to carry optical material and then machining through the conformal mask from a reverse side of the substrate, a connector opening or cavity can be accurately and easily defined without causing unnecessary damage to the optical components on the front side of the surface. This opening defines the longitudinal and transverse position of the connector position relative to the waveguides. Because these can be relatively accurately defined, the connector can be passively aligned with the ends of the waveguides. In this way, there is provided a very simple and effective mechanism for correctly positioning the connector on the board.

The method in which the invention is embodied has many advantages. For example, it can be carried out using techniques that are wholly compatible with current

PCB manufacturing technology. This means that it could be readily included in any PCB manufacturing process.

Furthermore, the tolerances required are set at the copper and polymer lithographic stage, not at any postprocessing step. This means that the cavity for receiving the connector can be defined to within a few micrometers accuracy. A further advantage is that connectors can be positioned using standard pick-and- place technology. Hence, in the specific example described above, which uses an adapted version of an MT connector, any tolerance can be accommodated using tapered precision ground MT pins. A yet further advantage of the invention is that it could be extended for use with multiple core layers within the optical material. This could be done by, for example, cleaving a portion of the connector fibres at different levels corresponding to the positions of different core layers in the optical material. A still further advantage is that the board in which the invention is embodied is less susceptible to alignment problems that may arise due to surface roughness on the front side copper layer of the initial PCB. This is because material is removed from a reverse side of the conformal mask 34, and the connecter 10 is inserted into the board from the reverse side. This means that the thickness of the mask 34 or any variations in the front side surface of the mask do not affect the position of the connector relative to the waveguides.

In contrast, in many known arrangements, connectors rest on the front surface of a copper mask, which can cause problems when trying to align the connector with the waveguides.

A still further advantage is that this method eliminates the need to perform direct patterning, such as lithography or laser machining, of the buffer and/or cladding layers 44 and 50, and therefore simplifies the production of the boards as well as avoiding the inaccuracies associated with aligning patterns in different layers of the optical material.

A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. For example, although a specific shape has been described for the conformal mask 34, it will be appreciated that the shape would be defined by the connector. Hence, in practice this will vary according to the particular connectors being used. In addition, although windows 32 are defined in the reverse copper layer 28 in the example described with reference to Figures 1 to 13, it should be noted that these are optional. Rather than forming windows in the reverse copper layer, a mask defining openings that are each in registry with corresponding ones of the conformal masks 34 could be formed on an outer surface of the board. Once this mask is defined, the reverse side removal of material could be carried out by removing the bulk of the material mechanically with an appropriate router/driller to within about 50-100 microns of the front side copper mask, or any other readily achievable depth accuracy, as in Figure 14. Mechanical removal of the bulk material is faster than using laser machining. However, it is not as easily controllable. Hence, once the bulk of the material is mechanically machined away, laser machining is used to remove the rest of the substrate material and prevent damage to the conformal mask 34. Laser machining is then used to machine through the openings 38 and 40 defined in the copper mask 34 and into the front side polymer layers. As before, the laser beam is over-sized, that is it is chosen to have an area or extent that is bigger than the features 38 and 40 defined in the conformal mask 34, while ensuring that it is under-sized relative to the extent of the conformal mask. A board that has been processed in this way is shown in Figure 14. . Alternatively, a smaller beam may be chosen and scanned in a raster, or other appropriate fashion, within the extent of the conformal mask. In this case, the effect is the same. As a yet further alternative, although the hole that is left when the material is removed from the reverse side of the substrate is shown as being substantially perpendicular to the plane of the substrate, it will be appreciated that it could equally be oriented at another angle relative thereto. Also, the connector cavity could be shaped so as to define an angled reflector. Accordingly, the above description of a specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.

Claims

1. A method for mounting or locating an optical connector on an optical board, the method comprising: defining a mask on one side of a substrate; depositing optical material over the mask, the optical material defining optical components, such as waveguides; and removing optical material through the mask from a reverse side of the substrate.

2. A method as claimed in claim 1, wherein the step of defining involves using lithographic techniques, such as photolithography, to define a lithographically defined mask and etching through the lithographically defined mask to form the mask on the substrate.

3. A method as claimed in claim 2, wherein the substrate comprises a bulk material with opposing metal layers, preferably copper, and the mask is etched into the metal layer on the substrate.

4. A method as claimed in any of the preceding claims, wherein the step of depositing comprises using a liquid film deposition technique.

5. A method as claimed in claim 4, wherein the liquid film deposition technique comprises any one or more of screen- printing or dip-coating or curtain coating.

6. A method as claimed in any of the preceding claims, wherein the optical material is deposited as a monomer or monomer mixtureand the method further comprises polymerising the monomer or monomer mixture to form a polymer .

7. A method as claimed in claim 6, wherein the monomer or monomer mixture is acrylate based.

8. A method as claimed in any of the preceding claims, further involving depositing or forming a buffer layer over the mask.

9. A method as claimed in claim 8, wherein the buffer layer is provided to form a substantially flat surface for depositing subsequent layers thereon.

10. A method as claimed in claim 8 or claim 9, wherein the buffer layer is formed directly over and in contact with the mask.

11. A method as claimed in any one of claims 8 to 10, wherein the buffer layer is part of the optical material.

12. A method as claimed in claim 11, wherein the buffer layer acts as a cladding layer for confining light in other layers within the optical material..

13. A method as claimed in any of the preceding claims, wherein the step of removing material involves machining the board.

14. A method as claimed in claim 13, wherein the step of machining involves mechanically machining, by for example routing or drilling out material.

15. A method as claimed in claim 13 or claim 14, wherein the step of machining involves laser machining.

16. An optical board for carrying or locating an optical connector, the board comprising a substrate; a mask on a front side of the substrate; optical material deposited over the mask; and a cavity for receiving the optical connector from a reverse side of the substrate.

17. An optical board as claimed in claim 16, wherein the cavity is in-filled after insertion of the connector.

18. An optical board as claimed in claim 16 or claim 17, wherein the optical material defines waveguides and/or other optical components or elements.

19. An optical board as claimed in any one of claims 16 to 18, wherein the substrate comprises a bulk material, such as glass-reinforced epoxy, with layers of metal formed on each side thereof.

20. An optical board as claimed in claim 19, wherein the metal is copper.

21. A method substantially as described hereinbefore with reference to the accompanying drawings.

22. An optical board substantially as described hereinbefore with reference to the accompanying drawings and as shown in Figure 1 and/or Figure 13 and/or Figure 14.