GB2421639A

GB2421639A - Visually-checkable coupler for an HF strip transmission line

Info

Publication number: GB2421639A
Application number: GB0525291A
Authority: GB
Inventors: Stefan Keith
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2004-12-21
Filing date: 2005-12-09
Publication date: 2006-06-28
Anticipated expiration: 2025-12-09
Also published as: DE102004061403A1; FR2879829B1; GB2421639B; FR2879829A1; ITMI20052401A1; GB0525291D0

Abstract

A coupler 3 for an HF strip transmission line structure 2 is formed on an HF substrate 1. A finger coupler structure 6 is implemented as a thin film structure on a different substrate 4. The arrangement is such that both the finger coupler structure and the join to the strip transmission line structure can be checked visually. This is achieved by virtue of the fact that the finger coupler structure is formed on the top side of the substrate 4, whereas terminal pads 7 are formed on the underside of the substrate 4. The finger coupler structure is electrically connected to the HF strip transmission line structure via the terminal pads. A connecting material such as solder may be used in the area of the side surfaces 8 of the substrate 4 in order to provide an electrical connection. A ceramic substrate may be used.

Description

Coupler for an HF strip transmission line structure and HF strip

transmission line structure with such a coupler

Prior art

The invention concerns a coupler for an HF strip transmission line structure on an HF substrate, with a finger coupler structure which is implemented as a thin film structure on a substrate.

Additionally, the invention concerns an HF strip transmission line structure on an HF substrate with such a coupler.

For direct voltage decoupling of components in HF technology, particularly radar technology, so-called finger couplers or d.c. blocks which are part of the strip transmission line structure are used. The overlapping fingers generate a bandpass characteristic for the HF signal, whereas direct voltage components are blocked. The bandpass characteristic is essential for the functioning of the radar, since it prevents low-frequency parts of the control pulse being passed on.

In German patent application 101 34 685, a coupler of the above-mentioned type is described. It causes d.c.

decoupling and simultaneously has a suitable bandpass characteristic. The known coupler consists of a finger coupler structure, which is implemented as a thin film structure on a silicon substrate. This is mounted in flip-chip technology on the HF substrate, so that the finger coupler structure faces the HF substrate and is connected electrically via metallizations to the strip transmission line tracks of the HF strip transmission line structure.

Use of the known coupler is shown in practice to be problematical in several respects. In the case of reflow soldering, the result is often that the coupler floats, since only the terminal pads facing the HF substrate are wetted with solder paste. This floating results in variations of the air gap between the finger coupler structure and the HF substrate, which in turn affects the HF performance of the component. Checking the soldered joints which are hidden between the coupler and the HF substrate requires expensive investigation. Also, the finger coupler structure can no longer be checked visually after the coupler is mounted, so that for instance short circuits caused by impurities cannot easily be detected.

Advantages of the invention With this invention, a coupler which is of the above- mentioned type and can be produced and mounted simply and economically is proposed, so that both the finger coupler structure and the join to the strip transmission line structure can be checked visually.

According to the invention, the way this is achieved is that the finger coupler structure is formed on the top side of the substrate, and that on the underside of the substrate, terminal pads, which are connected electrically to the finger coupler structure and via which the finger coupler structure can be connected to the HF strip transmission line structure, are formed.

According to the invention, it has been recognised that the finger coupler structure and the terminal pads via which the finger coupler structure and the HF strip transmission line structure are connected electrically do not necessarily have to be arranged on the same main surface of the substrate. Forming the terminal pads on the back of the substrate is shown to be advantageous with respect to the miniaturisation of the coupler according to the invention, since the whole area of the top side of the substrate is now available to the finger coupler structure. Mounting the coupler according to the invention is also simplified by arranging the terminal pads on the underside of the substrate, since it is unnecessary to use flip-chip technology. The coupler according to the invention can simply be placed on the HF substrate, so that the finger coupler structure on the top side of the substrate is accessible and in particular can be checked visually.

The precondition for the functionality of the coupler according to the invention is that the terminal pads on the underside of the substrate are connected electrically to the finger coupler structure on the top side of the substrate. The implementation of this connection depends on both the type of substrate and the nature of the terminal pads.

Use of a ceramic substrate as the substrate is shown to be advantageous, since in this case known techniques for implementation of thin film structures, particularly of the finger coupler structure on the top side but also of the terminal pads on the underside, can be used.

The electrical connection between the finger coupler structure and the terminal pads can be created, for instance, using correspondingly arranged through holes with a conducting coating. In a specially advantageous variant of the coupler according to the invention, the finger coupler structure on the top side of the substrate is connected electrically via an electrically conducting coating of the side surfaces of the substrate to the terminal pads on the underside of the substrate. In this case, the side surfaces of the coupler for mounting on the HF substrate can simply be wetted with an electrically conducting connecting means such as solder.

In this way, floating of the coupler can be effectively prevented. Also, a solder meniscus forms, and then also makes checking the soldered joints possible.

The finger coupler structure, the terminal pads and/or the electrical connection between the finger coupler structure and the terminal pads can be implemented, for instance, in the form of structured metallizations of the appropriate surface of the substrate, which is shown to be specially economical.

As previously mentioned, with this invention an HF strip transmission line structure on an HF substrate with a coupler is also proposed, as described above. The coupler is mounted via its underside on the strip transmission line structure, and connected electrically to the finger coupler structure via the terminal pads.

Drawing As explained above, there are various possibilities for implementing and extending the teaching of this invention advantageously. Reference is made, on the one hand, to the claims following the independent claims, and on the other hand, to the following description of an embodiment of the invention on the basis of the drawing.

The only figure shows the cross-section through an HF strip transmission line structure on an HF substrate with a coupler according to the invention.

Description of the embodiment

The only figure shows an HF substrate 1, on which an HF strip transmission line structure 2 is formed. On the HF substrate 1, above the HF strip transmission line structure 2, a coupler 3 is arranged. In the embodiment shown here, the coupler 3 consists of a ceramic substrate 4 with a metallization 5, which is applied in thin film technology and extends from the top side of the substrate via the shown side surfaces to the underside. This metallization 5 is structured both on the top side of the substrate 4 and on the underside so that on the top side of the substrate 4 it forms a finger coupler structure 6, and on the underside of the substrate 4 terminal pads 7 for the coupler 3 are formed. The metallization of the side surfaces 8 forms an electrical connection between the finger coupler structure 6 on the top side of the substrate 4 and the terminal pads 7 on the underside, so that the finger coupler structure 6 can be connected via the terminal pads 7 to the HF strip transmission line structure 2.

In the embodiment shown here, the terminal pads 7 are not only used for electrical connection of the coupler 3 to the strip transmission line structure 2, but also for mounting the coupler 3 on the HF substrate 1. As the electrically conducting connecting material, here a solder is used, and the metallized side surfaces 8 of the coupler 3 are wetted with it. The only figure illustrates that a solder meniscus 9 has thus formed around the edge of the coupler 3, as is known from standard SMD insertion, and that part of the solder mass has migrated between the terminal pads 7 and the strip transmission line structure 2, so that the coupler 3 is fixed on the HF substrate 1 by the solder mass.

Regarding the layout of the finger coupler structure 6 of the coupler 3 according to the invention, which is described here, the following should also be noted: The dimensions of the fingers here do not only depend on the HF substrate 1 and the line impedance, but also on the properties of the ceramic substrate 4 and the air gap between the HF substrate 1 and the coupler 3. These conditions must be taken into account in the layout of the finger coupler structure 6.

If the coupler according to the invention is to be used in the context of a radar application, e.g. in the case of a radar front end in the motor vehicle field working at a frequency of 24.125 GHz, the length of the fingers is decisive for optimum transmission. The principle is that the higher the effective dielectric constant of the arrangement is, the more the line length is shortened.

Comparison with the coupler which is known from the prior art shows that silicon has an effective dielectric constant of 11.4, whereas the dielectric constant of ceramic is only about 9.9, but that the effective dielectric constant of an arrangement with the coupler 3 according to the invention is significantly higher, since here the ceramic substrate is arranged between the current-carrying tracks - finger coupler structure 6 on the top side of the substrate and HF strip transmission line structure 2. Additionally, the coupler 3 according to the invention makes mounting on the HF substrate 1 possible without floating, so that the air gap between the coupler 3 and the HF substrate 1 is reduced, which additionally increases the effective dielectric constant of the arrangement. In these conditions, the coupler 3 according to the invention can be implemented with a relatively short finger length. Because the terminal pads 7 are arranged on the underside of the substrate, the whole area of the top side of the substrate is available for the finger coupler structure 6. This concept contributes additionally to miniaturisation of the coupler 3 according to the invention.

Claims

Claims 1. Coupler (3) for an HF strip transmission line structure (2) on

an HF substrate (1), with a finger coupler structure (6) which is implemented as a thin film structure on a substrate (4), characterized in that - the finger coupler structure (6) is formed on the top side of the substrate (4), and that - on the underside of the substrate (4), terminal pads (7) are formed, and are connected electrically to the finger coupler structure (6) and can be connected via the finger coupler structure (6) to the HF strip transmission line structure (2)
2. Coupler (3) according to Claim 1, characterized in that a ceramic substrate is used as the substrate (4)
3. Coupler (3) according to one of Claims 1 or 2, characterized in that the terminal pads (7) on the underside of the substrate (4) are also implemented as a thin film structure.
4. Coupler (3) according to one of Claims 1 to 3, characterized in that the finger coupler structure (6) on the top side of the substrate (4) is connected electrically via an electrically conducting coating (8) of the side surfaces of the substrate (4) to the terminal pads (7) on the underside of the substrate (4).
5. Coupler (3) according to one of Claims 1 to 4, characterized in that the finger coupler structure (6), the terminal pads (7) and/or the electrical connection between the finger coupler structure (6) and the terminal pads (7) are implemented in the form of structured metallizations (5) of the appropriate surface of the substrate (4)
6. Coupler substantially as hereinbefore described with reference to the accompanying drawings.
7. HF strip transmission line structure (2) on an HF substrate (1) with a coupler (3) according to one of Claims 1 to 6, the coupler (3) being mounted via its underside on the strip transmission line structure (2), and connected electrically to the finger coupler structure (6) via the terminal pads (7)
8. HF strip transmission line structure (2) according to Claim 7, the coupler (3) being fixed on the HF strip transmission line structure (2) via an electrically conducting connecting material, and the connecting material being arranged in the area of the coated side surfaces (8) of the substrate (4)
9. HF strip transmission line structure (2) according to Claim 8, a solder or adhesive being used as the connecting material.
10. HF strip transmission line substantially as hereinbefore described with reference to the accompanying drawing.
11. Use of a capacitive coupler according to one of Claims 1 to 6 as a bandpass filter for high frequency signals, with simultaneous blocking of direct current parts and low frequency parts, particularly for radar applications.