TRANSITION BETWEEN ASYMMETRIC STRIPLINE AND MICROSTRIP IN CAVITY

Abstract: The present invention relates to a multilayer printed circuit board arrangement which results in better matching between a stripline (9) and a microstrip (4) in a cavity (6). The solution comprises the use of an asymmetric stripline (9) where the electric field is tied primarily to the lower earth plane (10). This results in good matching at the transition to the microstrip (4), whose field is tied to the lower earth plane (10).

Description: TRANSITION BETWEEN ASYMMETRIC STRIPLINE AND MICROSTRIP IN CAVITY FIELD OF INVENTION The present invention relates to a multilayer printed circuit board and more specifically to a transition between a symmetric stripline and microstrip in the cavity of such a board . BACKGROUND OF THE INVENTION Many different sorts of multilayer printed circuit boards are known to the art. LTCC (Low Temperature Co-fired Ceramic) will be used hereinafter as an example, although it will be understood that the invention can also be applied in other types of multilayer printed circuit boards. Briefly, multilayer printed circuit boards are manufactured in the following way. There is obtained on the basis of a printed circuit board design a drawing that contains necessary information, such as the number of layers, the appearance and dimensions of the patterns on the various layers, the locations at which different layers shall contact one another, and so on. Each layer per se is rolled out from a ceramic mass to a predetermined thickness on a plastic film; this is a so-called tape. Different patterns are punched from these tapes in accordance with the design, these including the outer edges of the board, the marks that are later used to match the layers together, and holes for binding different layers together with so-called vias . Subsequent to configuring the layers, the via holes are filled with a suitable conductive material. The patterns are then printed on each of the layers. A common method in this respect is to use screen-printing to correctly position the conductors. These conductors may consist of gold, silver or some other suitable conductive material. When the patterns are in place, the various layers are placed one upon the other until all layers are in position. The whole of the printed circuit board is then placed under pressure, inserted into an oven and baked immediately (Co- fired) at a relatively low temperature, 700-800 degrees centigrade (Low Temperature) , wherewith the ceramic mass is sintered and transformed to a ceramic. Subsequent to this curing or hardening process, it is usual to speak of layers instead of tapes. In the case of applications for high frequency signals, particularly within the microwave field, it is not always possible to use traditional conductors, since this would result in unacceptable losses and disturbances . A normal requirement in the case of microwave signals is the presence of an earth plane above or beneath a conductor, this earth plane following the conductor. When a conductor only has an earth plane on one side it is called a microstrip. These strips are normally arranged so that they have the printed circuit board on one side and air or another dielectric on the other side. In other cases, it is desirable that the conductor is surrounded by both an upper and a lower earth plane, this conductor then being called a stripline. When the distances between a stripline and the earth planes are the same on both sides of the conductor, it is said that the stripline is symmetrical. When the distances are different, an asymmetric stripline is obtained. Although symmetric striplines are he most common, there are occasions when an asymmetric stripline is preferred. One advantage afforded by striplines is that radiation from the conductors is small when, e.g., transmitting signals in the microwave range in so-called stripline-mode, which is one reason why such signals are often transmitted in this way. Microstrips and striplines can be easily provided in multilayer printed circuit boards, and are consequently often used to this end. In order to enable conductors to be surrounded by earth planes, conductor planes and earth planes are normally disposed alternately in the printed circuit board. It is possible to mount chips, for instance an MMIC (Monolithic Microwave Integrated Circuit) , directly in a multilayer printed circuit board. This is achieved by placing the chip on the earth plane in a cavity and connecting the chip to the nearest signal carrying layers with the aid of so- called bonding wires. This is shown in Figure 1, in which the chip has been connected to a pair of microstrips in a known manner. The transition between the symmetric striplines and the microstrips, however, is not fully satisfactory, since the electric field in the strip line is tied just as strongly to the upper earth plane as to the lower earth plane. This means that when the upper earth plane suddenly disappears, it becomes "heavy going" for the conductor and thus results in a poor match. This matching problem does not apply to signals that have sufficiently low frequencies. On the other hand, the problem does arise in the case of high frequency signals, for instance RF signals. The reason why these signals are nevertheless still transmitted in so-called stripline mode is because it reduces radiation from the conductors, among other things. SUMMARY OF THE INVENTION The present invention addresses the problem of improving matching in a transition between a stripline and a microstrip in a cavity, preferably in a multilayer printed circuit board that has chips mounted therein. One object of the present invention is thus to provide a transition between stripline and microstrip in a cavity such that good matching will be obtained. In brief, the present invention relates to an arrangement in which microstrip and stripline are asymmetrical instead of symmetrical. The electric field in the stripline will thus essentially be undisturbed downstream of the transition to the microstrip, since the field is tied to the adjacent earth plane . The inventive arrangement is characterised by the features set forth in the accompanying Claim 1. One advantage afforded by the solution to said problem is that transitions between stripline and microstrip in a cavity can be made simpler and with good matching. This enables multilayer printed circuit boards which have components mounted therein to be made more effective. The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a cut-away side view of a multilayer printed circuit board that has a known transition between stripline and microstrip. Figure 2 is a view similar to that of Figure 1 and shows the transition effected with the inventive arrangement. DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 is a cut-away side view of a multilayer printed circuit board. The reference numeral 1 identifies a number of layers, which need not necessarily have different thicknesses, on which a conductor 2, 3, 10 of some sort is normally mounted. These conductors 2, 3, 10 may, for instance, form an earth plane 2, 10 or a stripline 3, in this case a symmetrical stripline. A cavity 6 extends through the upper earth plane 2 and through a number of layers 1 down to the underlying earth plane 10. The cavity 6 narrows on at least one side of the stripline 3, which means that the stripline 3 transits m a microstrip 4 (that only has layers 1 (tape) on one side) The region m which the microstrip 4 is located is called a bonding shelf 5. Furthest down m the cavity 6 is a chip 7, which is normally connected directly to the earth plane 10. This chip 7 is connected to the microstrips 4 on the bonding shelves 5 by means of a number of so-called bonding wires 8. The electric field m the symmetrical stripline 3 is connected equally harshly to both earth layers 2, 10, since the vertical distances between stripline 3 and respective earth planes 2 and 10 n Figure 1 are essentially equal to one another. The microstrip 4 m the cavity 6 lacks the upper earth plane 2 and its field is therefore tied to the bottom earth plane 10 This results in a mismatch in the transition between the stripline 3 and the microstrip 4, which causes the performance of the- entire board to be lower than it would otherwise be if the stripline 3 had been less dependent on the upper earth plane 2. Figure 2 is a view similar to the view of Figure 1 and shows ' the inventive transition. Similar to Figure 1, there is shown a number of layers 1, two earth planes 2, 10, a cavity 6 with microstrips 4 on bonding shelves 5 and a chip 7 connected to the microstrips 4 by means of bonding wires 8. In the case of the illustrated inventive arrangement, the conductor includes an asymmetric stripline. This is achieved by arranging the stripline 9 so that the layer 1 between said stripline and the upper earthplane 2 is thicker than the layer 1 between the stripline 9 and the lower earth plane 10. The distance di is thus greater than the distance d 2 in Figure 2. Matching to the microstrip 4 in the cavity 6 is thus improved by virtue of the fact that the electric field is essentially tied to the lower earth plane 10 in this case, since the electric field of the microstrip is also tied to the lower earth plane 10. The quality of the match is determined by the quotient q=d 2 /dχ between the distances d_ and d 2 from the stripline 9 to the upper earth plane 2, d_, and from the earth plane 10, d 2 , respectively. In the case of the symmetrical transition shown in Figure 1, where distances di and d 2 are the same, the quotient q=l. On the other hand, in the case of the asymmetric transition in Figure 2, where the distances d_ and d 2 differ from each other, the aforesaid quotient will be q<l. Generally speaking, the smaller the quotient q, i.e. the greater the difference between the distances to respective earth planes, the better the match in the transition. However, the quotient q cannot be made arbitrarily small. Firstly, the stripline 9 must be disposed on a layer 1. The smaller the desired quotient q the more layers that are required, or the layers must be made thicker. The number of layers is normally limited by the thickness of the printed circuit board for design reasons and should not exceed a given value, or for economic reasons, since cost increases with the number of tapes (layers) . Secondly, the losses in the transmission through the stripline 9 become greater with increasing asymmetry, which often results in the quotient q being limited in a downward sense. Thus, the quotient q is normally a compromise between economy, board thickness, stripline losses and transition matching. It is therefore not possible to give a general optimal value of the quotient q. Because, in the inventive arrangement, matching between stripline 9 and microstrip 4 in said transition is improved, the signals pass more easily between the stripline and microstrip. This increases the effectiveness of the entire printed circuit board. It will be understood that the invention is not restricted to the aforedescribed and illustrated embodiments thereof, and that modifications can be made within the scope of the accompanying Claims .

Claims: CLAIM 1. A transition between a stripline (9) and a microstrip (4) on a multilayer printed circuit board, wherein the board includes at least two layers (1) and at least two earth planes (2 and 10 respectively) , and wherein the board includes a cavity (6) which extends through at least two layers (1), characterised in that said stripline (9) is arranged asymmetrically between said two earth planes (2 and 10) such that the distance (di) to one earth plane (2) will be greater than a corresponding distance (d 2 ) to the other earth plane (10) .

Inventors: ALBINSSON BJOERN, HARJU THOMAS, ALBINSSON, BJOERN, HARJU, THOMAS
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 2000
Main classification: H05K1
IPC codes: H01L23, H01P5, H05K1, H05K3