Circuit board bonding method

Abstract: A circuit assembly having a thin and large-area dielectric substrate and an improved earthing condition. To make the assembly, a circuit board comprising the substrate, and a circuit pattern and a metal layer that are formed on respective sides of the substrate. A bath of conductive bonding material (e.g., a low melting point solder) is made inside a tray-like metal chassis of the assembly. The circuit board is floated on the bath and excessive portion of the conductive material is absorbed. A branch circuit for branching a first path into at least two second paths is provided by mainly using impedance transformers but by using fewest possible stub(s). Also, the elements are arranged in symmetry around the axis through the first path. This yields a wide operating frequency band. A waveguide-microstrip line transition that is easy to work and low in transition loss is provided by shaping the wider walls of the ridge waveguide so as to spread out toward the end. Since doing this increases the degree of freedom in design parameters, the width and the height of the ridge can be fitted to that of the microstrip line and that of the waveguide. The above elements is applicable to HF circuits, antennas and communication systems.

Description: The present invention relates to a technique for bonding a circuit board to a metal chassis or case for a high frequency (HF) circuit in a communication device. Recently, as frequency resources in communications technology are running dry, frequency bands available for building a new communications system have been and are shifting to higher bands. In this situation, the government and the people are jointly promoting a development to milliwave and microwave communication systems domestically and internationally. For example, it has been decided that extremely high frequency bands ranging from some GHz to hundreds GHz are assigned as available frequency bands to various communication systems under development for wireless LAN (local area network) and ITS (Intelligent Transport System). Since available frequencies are rising as described above, antennas and HF (high frequency) circuits are desired which satisfactorily work in milliwave and microwave bands. However, design and manufacturing techniques that have been believed to be available may not work satisfactorily with an increase in frequency. For this reason, there is a need for novel design and manufacturing techniques. Problems exist in conjunction with working if an antenna arrangements are to be implemented for milliwave or microwave. With an increase in frequency, dielectric materials available for the dielectric layer 10 is limited to substances lack of a mechanical strength, e.g., ceramics, quartz, silicon, etc. Further, if an antenna that radiates a beam of two degrees in mesial width in a 76 GHz band is to be fabricated, the dielectric substrate 10 for the antenna will be approximately 100 to 300 µm thick and 15 cm long in one side. Bonding such a thin and wide substrate 10 to the chassis 30 often results in a breakage of the dielectric substrate 10. Also, as the frequency increases, the characteristics of the antenna 20 depends strongly on the earthing state of the dielectric substrate 10. For this reason, a sufficient electrical contact is indispensable for the junction of the dielectric substrate 10 and the circuit pattern 20. However, this is hard to be achieved by conventional techniques. US Patent application US-5,366,027 describes a circuit board which is conductively bonded to a metal ground plane in a manner that maintains a highly effective bond even after exposure to soldering. The invention is directed to solving these and other problems and disadvantages of the prior art. It is an aim of the invention to provide a technique of bonding a thin and large-area circuit substrate to a metal layer with a sure and uniform contact but no fear of substrate breakage. It is another aim of the invention to provide a high frequency circuit and an antenna that incorporate an circuit substrate implemented by such a bonding technique. According to an aspect of the invention, a method of bonding a circuit board with a metal plate is provided. The method includes the steps of working the metal plate so as to have a shape that permits a fluid to form a bath in an area including a part where the circuit board is to be bonded; heating the worked metal plate to such a temperature as melt a conductive bonding material; forming a bath of the conductive bonding material in the area of the metal plate; floating the circuit board on the bath; and absorbing excessive portion of the conductive material without applying a force to the dielectric substrate. A circuit assembly according to just-described aspect of the invention is provided with a thin and large-area dielectric substrate with an improved earthing condition. A bonding agent with a low melting point, a low melting point solder, etc. may be used as conductive material. The features and advantages of the present invention will be apparent from the following description of an exemplary embodiment of the invention and the accompanying drawing, in which: FIG. 1 is a schematic diagram showing an exemplary arrangement of an array antenna assembly according to an illustrative embodiment of the invention; FIG. 2 is a schematic sectional view taken along the plane X-Y of FIG. 1; FIG. 3 is a diagram showing an exemplary process of bonding the metal layer 11 of a circuit board to the chassis 130 in accordance with the principles of the invention. Throughout the drawing, the same elements when shown in more than one figure are designated by the same reference numerals. FIG. 1 is a schematic diagram showing an exemplary arrangement of an array antenna assembly 100 according to an illustrative example for understanding the invention. In FIG. 1, the antenna assembly 100 comprises an dielectric substrate 10, a circuit pattern or an array antenna 120 formed by patterning a metal film on the dielectric substrate 10 through photocopying, etching, etc., a chassis 130 that holds the dielectric substrate 10 and serves as the ground, and a waveguide-microstrip line transition 140 formed in the edge of the chassis 130. The circuit pattern 120 serves as a microstrip antenna array. A signal transmitted through a waveguide (not shown) is passed by the transition 140 to a microstrip line coupled to the circuit pattern 120, and further passed by the T branch circuit 50 to the right and the left portions of the array antenna 120. Bonding the Dielectric Substrate FIG. 2 is a schematic sectional view taken along the plane X-Y of FIG. 1. In FIG. 2, the antenna assembly 100 comprises metal layers 120 and 11, the dielectric layer 10, a solder layer 111 and the chassis 130. The metal layer 120 on the dielectric layer 10 comprises a circuit pattern forming a microstrip structure in cooperation with the metal layer 11. The metal layer 120, the dielectric layer 10 and the metal layer 11 constitute a circuit board 120+10+11. According to the invention, the metal layer 11 of the circuit board 120+10+11 is bonded with the chassis 130 by means of a bonding material of a good conductivity, which realizes sufficient earthing and strong support of the circuit board. In this specific embodiment, a lower-melting-point solder is used as the bonding material. It should be noted that the chassis 130 is not a flat plate but has a concavity on one side thereof as shown in FIG. 2. FIG. 3 shows how the metal layer 11is bonded with the chassis 130 in accordance with the principles of the invention. The processing steps are as follows: (1) Put solder 111a in the concavity of the heated chassis 130to make a solder bath 111b. The solder 111a is preferably a low melting point solder with a low melting point. Because if the difference between the rates of expansion for the substrate 10 and the chassis 130is large, heating too much for a higher melting point solder may cause the substrate 10 to break during a cooling process. (2) Float the circuit board 120+10+11 on the solder bath 111b. The circuit board floats of itself due to the surface tension of the solder 111b. (3) Get the metal layer 11 of the circuit board attached to the solder 111b, getting bubbles out from between the metal layer 11and the chassis 130. (4) Remove excessive solder by absorbing solder 111b little by little without applying external force directly to the circuit board. The absorption of solder 111b is achieved by using a dedicated copper fiber. (5) Cool taking a sufficient time such that any undesirable stress will not remain in the resultant assembly. The just-described process enables even a thin and large-area dielectric substrate 10 (i.e., circuit board 120+10+11) from which alone a sufficient mechanical strength can not be expected to be bonded with the chassis 130 without a fear of breaking the substrate 10 (or the circuit board), realizing good earthing. Though the inventive bonding technique has been described in conjunction with an antenna assembly 100 using an array antenna pattern 20, the inventive technique may be applied to any high frequency (HF) circuit assembly with any circuit pattern. It is noted that any suitable conductive bonding agents may be used instead of the solder 111a. In this case, the bonding agents are preferably low in the temperature and small in the ratio of volume change at about the hardening temperature. The inventive bonding technique has been described in conjunction with bonding a circuit board with a chassis. However, the inventive technique is applicable to bonding a thin and large-area circuit board with a metal surface in a concavity or a metal surface fringed with walls. Many widely different embodiments of the present invention may be constructed without departing from the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.

Claims: A method of bonding a metal layer of a board comprising at least a dielectric layer and the metal layer with a metal plate, the method including the steps of: working said metal plate so as to have a shape that permits a fluid to form a bath in an area including a part where said board is to be bonded; heating said worked metal plate to such a temperature as to melt a conductive bonding material; forming a bath of said conductive bonding material in said area of said metal plate; floating said board on said bath; and absorbing an excess portion of said conductive bonding material without applying a force to said board. A method of manufacturing a circuit assembly, including the steps of: preparing a circuit board comprising a dielectric layer, a circuit pattern formed on one side of said dielectric layer and a metal layer formed on the other side of said dielectric layer; said metal layer being bonded to a metal plate according to the method of claim 1. A method according to claim 2, further including the step of removing bubbles from between said metal layer of said dielectric substrate and said metal plate. A method according to claim 1, 2, or 3, further including the step of selecting a bonding agent with a low melting point as said conductive bonding material. A method according to claim 4, wherein said conductive bonding material is a low melting point solder. A circuit assembly manufactured according to a method of claim 2, comprising: said metal plate, whereby said metal layer is bonded with an inside bottom of said metal plate by using said conductive bonding material with substantially no bubbles between said inside bottom and said metal layer. A circuit assembly as defined in claim 6, wherein a bonding agent with a low melting point is used for said conductive bonding material. A circuit assembly as defined in claim 6, wherein a low melting point solder is used for said conductive bonding material. A circuit assembly as defined in claim 6, 7 or 8, wherein said circuit pattern is for a high frequency circuit. A circuit assembly as defined in claim 6, 7 or 8, wherein said circuit pattern is for a high frequency circuit including an array antenna elements.

Inventors: FUJITA SUGURU, SANGAWA USHIO, FUJITA, SUGURU, SANGAWA, USHIO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1999
Main classification: H01P5
IPC codes: H05K7, H01Q13, H01P5, H01P11, H05K3, H05K1, H01P3, H01Q1, H01Q9