SELF-ALIGNED TRANSITION BETWEEN A TRANSMISSION LINE AND A MODULE

Abstract: The invention relates to a method for forming a self-aligned transition between a transmission line (1) and a module (2) to be connected to said transmission line (1). The invention equally relates to such a module (2), to such a transmission line (1), to an intermediate element (5) possibly to be positioned between such a module (2) and such a transmission line (1), and finally to a system comprising such a transition. In order to enable a simple and precise connection of a transmission line (1) with some other module (2), it is proposed to use matching solder particles (8) and soldering pads on module (2) and transmission line (1) or intermediate element (5) respectively, and to apply a reflow treatment to the joined elements (1) and (2) or (5) and (2), thereby forming a self-aligning connection. The invention is to be applied particularly to waveguide-to-microstrip transitions.

Description: Self-aligned transition between a transmission line and a module FIELD OF THE INVENTION The invention relates to a method for forming a self- aligned transition between a transmission line and a module to be connected to said transmission line. The invention equally relates to such a module, to such a transmission line, to an intermediate element possibly to be placed between such a module and such a transmission line, and finally to a system comprising such a transition. BACKGROUND OF THE INVENTION Transitions between transmission lines and other modules are employed in diverse systems. A common connection is, e.g., a waveguide-to-microstrip transition. Usually, the requirements on the precision of the connection are high, since even small deviations will degrade the performance resulting in a considerable insertion loss. Especially waveguide-to-microstrip transitions are extremely sensitive to geometry errors. Simulations show that a length difference of only 50 microns in a microstrip can result in an insertion loss increase of 0,5 dB to 1,2 dB. In practice, transmission lines and other modules are connected mechanically to provide the needed transition. To this end, screwing, soldering or similar methods are employed. Such methods lead, however, to rather inaccurate results and the tolerances can be quite large so that the requirements cannot be met satisfactorily. To connect transmission lines to other modules mechanically also constitutes an extremely difficult and time- consuming assembly phase. In addition, special tools, like registration devices and CAE tools, are required. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a method permitting a simple and precise connection of a transmission line with to some other module, to provide a module that can be connected easily and precisely to a transmission line, to provide an intermediate element that can be used for an easy and precise connection of a module to a transmission line and to provide a system comprising the transition of a transmission line to a module formed by a precise connection. This object is reached on the one hand by a method for forming a self-aligned transition between a transmission line and a module to be connected to said transmission line, comprising the steps of claim 1 or 2. On the other hand, the object is reached by a module destined to form a self-aligned transition with a transmission line presenting the features of claim 5, an intermediate element presenting the features of claim 10 and a transmission line destined to form a self-aligned transition with a module presenting the features of claim 11 or 12. Finally, the object of the invention is solved by a system presenting the features of claim 14 or 15. The invention proceeds from the idea that a precise connection between a transmission line and another module can be realised by taking advantage of the behaviour of solder particles under reflow treatment. It is known to those skilled in the art that solder balls and other solder particles are self-aligning under reflow treatment due to surface tension phenomena in the solder material. Accordingly, under reflow treatment, slight misalignment is corrected by the surface tension forces, making it unnecessary to use any additional procedures for alignment. When using a reflow soldering of solder particles for transitions between transmission lines and modules, as proposed by the invention, a very precise connection of transmission line and module can be achieved, thereby reducing considerably the resulting insertion losses. The proposed method and the corresponding module, transmission line, intermediate element and system are also less expensive than those known in the state of the art, since no extra tools and work phases are required besides the simple reflow treatment, which should be performed also with the methods according to the state of the art . Two basic alternatives are proposed for the method, the module, the transmission line and the system according to the invention, both alternatives being based on the above described principle. According to the first alternative, module and transmission line are connected or destined to be connected directly to each other. According to the second alternative, module and transmission line are connected or destined to be connected to each other via an intermediate element, for example a printed circuit board. The second alternative has the advantage of a facilitated application of the soldering pads and of a facilitated connection by reflow soldering. Advantageous embodiments of the invention can be taken from the subclaims . In a preferred embodiment of the method according to the invention, the soldering pads are provided by using a solder mask, in particular by using a photolitographic pattering process. The employment of such a process results in a particularly good accuracy when connecting the module and the transmission line or the intermediate element respectively by reflow soldering. If an intermediate element is employed, this element can be fixed to the transmission line mechanically, -for example by screws. With the method and the elements proposed, any module - system module or single chip - can be attached with good precision to a transmission line. The invention is aimed especially at attaching a microstrip, like a stripline, a coplanar waveguide or similar, or a T/R (transmitter / receiver) or similar module to a transmission line, since these connections require a particular accuracy of positioning. If the transmission line concerned is a waveguide, it can have any cross-section required, e.g. a rectangular, a ridged or a circular cross-section. The solder particles provided on the module can be of Ball Grid Array (BGA) type, of Column Grid Array (CGA) type, or even of flip-chip type, if only a single chip is attached, instead of a whole system module. BRIEF DESCRIPTION OF THE FIGURES In the following, the invention is explained in more detail with reference to drawings, of which Fig. 1 schematically shows a first embodiment of the invention and Fig. 2 schematically shows a second embodiment of the. invention. DETAILED DESCRIPTION OF THE INVENTION Figure 1 illustrates the principle of the invention. To this end, it shows a sectional view of the elements of a system according to the invention before being connected. A partly depicted rectangular waveguide 1 forms the transmission line of the system. At its open end 3, the waveguide 1 has an enlarged rim 4, on which an intermediate element 5 with a middle-opening has been fixed. Above the open end 3 of the waveguide 1, a simple microstrip structure 2 is depicted. This microstrip 2 represents a system module that is to be connected to the waveguide 1. The microstrip 2 has an aperture 6 in its ground plane 7, by which it is to be coupled with the open end 3 of the waveguide 1 in order to form a transition. The microstrip 2 is provided at its ground plane 7 with solder balls 8 in a Ball Grid Array (BGA) or a similar type of joining technique, the solder balls 8 being arranged closely around the aperture 6 in the ground plane 7. On top of the intermediate element 5 there are formed soldering pads (not shown) corresponding to the arrangement of solder balls 8 by using a solder mask. The solder mask definition is done using photolitographic patterning. Following the situation depicted in Fig.l, the microstrip 2 is placed onto the waveguide 1 in a way that the solder balls 8 come to rest on the soldering pads of the intermediate element 5. After that, a reflow soldering process is carried out, combining the microstrip 2 with the intermediate element 5 and thereby with the waveguide 1. By this procedure, the waveguide 1 is attached to the microstrip 2 with very good accuracy. Even if the original placement of the microstrip 2 on the intermediate element 5 has been slightly inaccurate, the soldering process will set the microstrip 2 to exactly the right location, because of the surface tension of the solder material. The second embodiment shown in Figure 2 represents a more complex system according to the invention with a whole system module being connected to a waveguide to form a waveguide-to-microstrip transition. Corresponding parts are referred to with the same reference signs as in Fig. 1. At the bottom of Fig. 2, a waveguide 1 made up of aluminium is depicted. The waveguide 1 is plated with an inert metal such as gold, in order to maintain a low-loss characteristic . A FR-4 (a thermoset glassfiber-epoxy laminate) printed circuit board 5 comprising a middle-opening forms the intermediate element of the system and is attached to the rim 4 of the open end 3 of the waveguide 1 with screws 9. The rim 4 of the waveguide 1, the printed circuit board 5 and the holes for the screws in both of them are designed to guarantee a defined relative position between the waveguide 1 and the board 5 when fixed. On top of the FR- 4 printed wiring board 5 there is attached a ceramic module 2 with a kovar lid 10 by BGA-type solder balls 8. An aperture 6 in the ground plane 7 of the module 2 is positioned exactly above the opening of the waveguide 1 at its open end 3. The system- was assembled as follows: The ceramic module 2 was provided with BGA-type solder balls 8 and the FR-4 printed wiring board 5 with a corresponding arrangement of soldering pads. Thereupon, the FR-4 printed wiring board 5 was soldered by reflow with the ceramic module 2. Only after that, the whole entity made up of module 2 and printed wiring board 5 was placed on top of the waveguide structure 1. Finally, the FR-4 printed wiring board 5 combined with the ceramic module 2 was attached mechanically to the waveguide structure 1 with screws 9.

Claims: Claims 1. Method for forming a self-aligned transition between a transmission line (1) and a module (2) to be connected to said transmission line (1) , comprising the steps of: a) providing the module (2) with an arrangement of solder particles (8) in an area designated for the connection of said module (2) to the transmission line (1) ; b) providing the rim (4) of an open end (3) of the transmission line (1) designated for the connection to the module (2) with an arrangement of soldering pads corresponding to the arrangement of solder particles (8) on the module (2); c) placing the module (2) with its solder particles (8) onto the soldering pads and connecting the module (2) to the rim (4) of the open end (3) of the transmission line (1) by reflow soldering. 2. Method for forming a self-aligned transition between a transmission line (1) and a module (2) to be connected to said transmission line (1) , comprising the steps of: a) providing the module (2) with an arrangement of solder particles (8) in an area designated for the connection of said module (2) to the transmission line (1) ; b) providing an intermediate element (5) fixed to or suited to be fixed to the rim (4) of an open end (3) of the transmission line (1) designated for the connection to the module (2) with an arrangement of soldering pads corresponding to the arrangement of solder particles (8) on the module (2) ; c) placing the module (2) with its solder particles (8) onto the soldering pads and connecting the module (2) to the intermediate element (5) by reflow soldering; and d) in case the intermediate element (5) is not fixed to the transmission line (1) , fixing the intermediate element (5) with its face opposite to the soldering pads to the transmission line (1) . 3. Method according to claim 2, c h a r a c t e r i s e d i n t h a t in step d) the intermediate element (5) is fixed to the transmission line (1) mechanically, in particular by screwing. 4. Method according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t in step b) the soldering pads are provided by using a solder mask, in particular by using a photolitographic pattering process. 5. Module (2) destined to form a self-aligned transition with a transmission line (1) and provided with an arrangement of solder particles (8) that are suited to match and to be soldered by reflow to soldering pads arranged on the rim (4) of an open end (3) of a transmission line (1) designated for connection with a module (2) or on an intermediate element (5) fixed to or suited to be fixed to such a rim (4) of a transmission line (1). 6. Module (2) according to claim 5, c h a r a c t e r i s e d i n being a microstrip, in particular a strip line or a coplanar waveguide, with an aperture (6) in the ground plane for forming a transition to a transmission line (1), the solder particles (8) being arranged to surround said aperture (6). 7. Module (2) according to claim 5, c h a r a c t e r i s e d i n being a Transmitter/Receiver (T/R) module. 8. Module (2) according to one of claims 5 to 7, c h a r a c t e r i s e d i n being a system module or a single chip. 9. Module according to one of claims 5 to 8 , c h a r a c t e r i s e d i n t h a t the solder particles (8) are of a Ball Grid Array (BGA) type, of a Column Grid Array (CGA) type, or of a flip-chip type. 10. Intermediate element (5) fixed to or suited to be fixed to the rim (4) of an open end (3) of a transmission line (1) designated for connection to a module (2) and comprising an arrangement of soldering pads suited to match and to be soldered by reflow with solder particles (8) arranged on a module (2) to be connected to said transmission line (1) , forming a self-aligned transition. 11. Transmission line (1) to which an intermediate element (5) according to claim 10 is fixed or suited to be fixed by screws (9) , the transmission line (1) being destined to form a self-aligned transition with a module (2) . 12. Transmission line (1) destined to form a self- aligned transition with a module (2) and comprising on a rim (4) of an open end (3) designated for connection to such a module (2) an arrangement of soldering pads suited to match and to be soldered by reflow with solder particles (8) arranged on such a module (2) . 13. Transmission line (1) according to claim 11 or 12, c h a r a c t e r i s e d i n being a waveguide. 14. System comprising a transmission line (1) according to one of claims 12 or 13 connected by reflow soldering to a module (2) according to one of claims 5 to 9. 15. System comprising a transmission line (1) according to one of claims 11 or 13 connected by reflow soldering via an intermediate element (5) according to claim 10 to a module (2) according to one of claims 5 to 9.

Inventors: ARSLAN ALI NADIR, JOKIO KALLE, KOIVISTO MARKKU, SAARIKOSKI MIKKO, SALMELA OLLI, ARSLAN, ALI, NADIR, JOKIO, KALLE, KOIVISTO, MARKKU, SAARIKOSKI, MIKKO, SALMELA, OLLI
Applicants: ARSLAN ALI NADIR, JOKIO KALLE, KOIVISTO MARKKU, NOKIA CORP, SAARIKOSKI MIKKO, SALMELA OLLI, ARSLAN, ALI, NADIR, JOKIO, KALLE, KOIVISTO, MARKKU, NOKIA CORPORATION, SAARIKOSKI, MIKKO, SALMELA, OLLI
Application year: 2000
Main classification: H01P5
IPC codes: H01P5, H05K1, H05K3