GB2280547A

GB2280547A - Printed circuit board with integral strip-line structure

Info

Publication number: GB2280547A
Application number: GB9313751A
Authority: GB
Inventors: David William Bowman
Original assignee: Quantel Ltd
Current assignee: Quantel Ltd
Priority date: 1993-07-02
Filing date: 1993-07-02
Publication date: 1995-02-01
Anticipated expiration: 2013-07-02
Also published as: GB9313751D0; GB2280547B

Abstract

A printed circuit board in the farm of an integral strip-line structure comprises two spaced apart conducting ribbons 7 having therebetween a layer of stiff dielectric material 13 flanked on opposite sides with respective layers 17 of curable dielectric material on the outermost surfaces of which the ribbons are respectively mounted. Respective further layers 3, 5 of stiff dielectric material are provided on the outermost surfaces of the layers of curable material, conductive tracks 11 being supported on the stiff dielectric and/or curable layers 3, 5, 13, 15 or 17 and spaced laterally from the ribbons. Further provided are ground plates 9 disposed on the outermost surfaces respectively of the further layers 3, 5 and facing the ribbons. Heat and pressure is applied to the structure to cure the curable layers. The structure enables connection together of the ribbons exclusively at corresponding ends which simplifies calculation of the characteristic impedance of the structure. <IMAGE>

Description

Printed Circuit Board with Integral StrizLine Structure This invention relates to printed circuit boards (p c b's) and more particularly to such a board having an integral strip-line structure. Strip-line structures are provided in p c b's and act as connecting means for connecting the circuit elements mounted on the board. To this end a strip-line comprises a conductive ribbon located between layers of dielectric material on outer surfaces of which are provided common ground plate conductors so located in relation to the ribbon as to exhibit a predetermined characteristic impedance.

Traditionally, the characteristic impedance of the strip-line was determined by the dimensions of the ribbon conductor, the spacing between the ribbon conductor and the adjacent ground plates and the dielectric constant of the laminate material separating the ribbon conductor and the ground plates. Using standard p c b geometries and generally available inexpensive conductor and dielectric materials to make a p c b with an integral strip-line structure, of 1/16 inches nominal thickness, it was not possible to achieve a characteristic impedance of more than 50 ohms.

In our co-pending United Kingdom application No. 8927412.0, the contents of which are incorporated herein by reference, there is disclosed a p c b having an integral strip-line the characteristic impedance of which is substantially increased above 50 ohms by controlling the effective area of the ground plates which are opposed to the conductive ribbon by forming suitably sized apertures in the ground plates. This has the same effect as reducing the dielectric constant of the laminate material separating the ribbon and the ground plates without the need to employ relatively expensive dielectric material.

In practice the strip-line geometry is incorporated into the p c b design as a small part of a larger design and the latter may require complex multi-layer structure and layout. A multi-layer p c b is made up using many interleaved thin stiff layers of dielectric material each having etched copper laminated on one or opposite sides thereof and layers of uncured materials separating the thin stiff layers. This complex sandwich is formed into a single multi-layered laminate sheet when heat and pressure are applied to 'cure' the uncured materials which act as the glue between the layers.

Due to the many layers of laminate and curable material used in the construction of a multilayer p c b, it is very difficult accurately to control the spacing between the strip-line signal conductor and the strip-line ground plates. In order that the strip-line characteristic impedance be maintained within +/- 5% of a prescribed value, the spacing between stripline signal conductor and the ground plates must be accurate and repeatable.

It is an object of the present invention to provide a p c b with integral strip-line structure in which the control of the spacing between the strip-line and ground plates is improved whilst allowing the p c b to be made with many layer and thickness variations.

The present invention consists in a printed circuit board having an integral strip-line structure and comprising two spaced apart facing conducting ribbons, layer means disposed between said ribbons and having one or more layers of stiff dielectric material each of which is flanked on opposite sides thereof with respective layers of curable dielectric material, the conductive ribbons being disposed respectively at outer surfaces of outermost layers of said curable material, respective further layers of stiff dielectric material flanking outer surfaces of said outermost layers of curable material, conductive track means supported on said layer means and/or said further layers and spaced laterally of said ribbons, and ground plates disposed on outer surfaces of said further layers in facing relationship with said ribbons, said curable material being cured by heat and pressure applied to said structure.

In use, the strip-line ribbon conductors are connected only at their ends and therefore are disposed in parallel and, for the purpose of calculating the characteristic impedance, can be considered as a single conductor of twice the ribbon thickness. Also, the spacing between the ground plates can be taken as the sum of the spacing between each ribbon and the ground plate adjacent thereto since, provided the conductive ribbons are connected only at their corresponding ends, the layers between the ribbons play no significant part in the characteristics impedance calculation. The value of the strip-line characteristic impedance is accordingly independent of the curing under heat and pressure of the layers of curable material of the structure.

Advantageously, the ground plates include apertures which at least partly are disposed in facing relation relationship to the adjacent one of said ribbons and which are of dimensions which impart a predetermined characteristic impedance to the strip-line structure.

Preferably, the conductive ribbons are connected together only at corresponding ends thereof.

The invention will now be described, by way of example, with reference to the accompanying drawing which is a cross-sectional view of a p c b with integral strip-line structure according to the invention.

In the drawing a p c b generally indicated by the reference 1 comprises outer layers 3 and 5 of stiff dielectric material, suitably glass reinforced plastics (g r p). On opposite sides of each of the layers 3 and 5 are respectively provided, in facing relationship, a conductive ribbon 7 and a ground plate 9. Laterally of the ribbon and ground plate are disposed on opposite sides as the case may be of the layer 3 or 5, conductive tracks 11 of the p c b. Between the layers 3 and 5 are disposed a further layer 13 of stiff dielectric material having, if desired, further tracks 11 on one or opposite sides thereof and between which and the respective layers 3 and 5 are layers 15 and 17 of curable dielectric material. The layer 13 and layers 15 and 17, suitably, may comprise glass reinforced plastic. If desired, also, additional layers 13 can be provided between the layers 3 and 5 in which case each layer 13 is flanked on opposite sides by respective curable layers such as the layers 15 and 17.

The completed assembly is finally subjected to heat and pressure to cure and consolidate the layers of curable material.

In use, the ribbons 7 are electrically connected to one another only at their corresponding ends, such connections being provided either by the p c b manufacture or the user. Also, the ground plates 9 are formed with apertures 19 for control, as explained earlier, of the characteristic impedance of the strip-line. Further the width of the ribbons 7 as compared with that of the structure of our co-pending application No. 8927412.0 is reduced by 30%.

As explained earlier, the described structure is one in which, because the ribbons 7 are connected at their ends, effectively eliminates from the calculation of characteristic impedance the dimensions of the layers of curable and stiff dielectrie material between the ribbons. Accordingly, this calculation involves apart from dimensions of the ribbons and ground plates only the fixed dimensions of the layers 3 and 5.

Those skilled in the art, will appreciate that many changes, as for example, to the number and disposition of the tracks 11, can be made in the described embodiment of the invention without departing from the invention concept disclosed.

Claims

Claims:

1. A printed circuit board having an integral strip-line structure and comprising two spaced apart facing conducting ribbons, layer means disposed between said ribbons and having one or more layers of stiff dielectric material each of which is flanked on opposite sides thereof with respective layers of curable dielectric material, the conductive ribbons being disposed respectively at outer surfaces of outermost layers of said curable material, respective further layers of stiff dielectric material flanking outer surfaces of said outermost layers of curable material, conductive track means supported on said layer means and/or said further layers and spaced laterally of said ribbons, and ground plates disposed on outer surfaces of said further layers in facing relationship with said ribbons, said curable material being cured by heat and pressure applied to said structure.

2. A printed circuit board as claimed in claim 1, wherein said ground plates include apertures which at least partly are disposed in facing relation relationship to the adjacent one of said ribbons and which are of dimensions which impart a predetermined characteristic impedance to the strip-line structure.

3. A printed circuit board as claimed in claim 1 or claim 2, wherein the conductive ribbons are connected together only at corresponding ends thereof.

4. A printed circuit board as claimed in any preceeding claim, wherein said layers of stiff and of curable dielectric material comprise glass reinforced plastics.

5. A strip line comprising an integral, multi-layered structure in which a conducting ribbon is supported between two conducting planes by dielectric material and wherein the conducting ribbon comprises first and second ribbon components spaced apart from one another in facing relationship by dielectric material.

6. A printed circuit board having an integral strip line structure as claimed in claim 5.

7. A printed circuit board substantially as herein described with reference to the accompanying drawing.