Abstract:
Method for manufacturing a multilayer microwave board in one operation. Individually etched layers of a laminate, such as duroid, are bonded to form a package and drilled after which the resulting holes are plated. Next, the superfluous connections are removed by drilling during which process each stripline-coax transition buried in the package is tuned by a correct selection of the drill diameter and the hole depth.

Description:
This Appln is a con&#39;t of Ser. No. 08/389,404 filed Feb. 16, 1995, ABN. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for manufacturing a multilayer microwave board in which individual layers of plated laminate can be provided with a conductor pattern and in which, in a repeated process, individual layers or semimanufactures are subsequently bonded or provided with holes, which holes are then plated for realizing connections between the separate conductor patterns. 
     The invention also relates to a multilayer microwave board comprising a bonded package of laminate provided with conductors and plated layers and with connections between the conductors in the form of plated through-holes. 
     The invention furthermore relates to a method for tuning a multilayer board provided with blind or buried vias. 
     Both multilayer microwave boards and related manufacturing methods are well-known in the art. Nevertheless, their field of application is limited owing to the expensive and time-consuming production and the high ratio of rejects. Moreover, several restrictions apply, particularly with respect to the realization of connections between the various conductor patterns, the so-called vias. Various types of connections that are possible in the manufacturing of conventional multilayer printed circuit boards, cannot, or only at a high ratio of rejects, be realized. Especially the production of blind vias, buried vias and sequential blind vias presents difficulties and the manufacturing of interlocked vias is even considered to be impossible. Also microwave multilayers comprising more than three layers are practically non-existent. 
     SUMMARY OF THE INVENTION 
     The present method according to the invention is aimed at obviating these difficulties by the application of a simple and excellently reproducible method and is characterised in that the multilayer microwave board can be produced in one operation during which all layers are simultaneously bonded together to form a multilayer, which multilayer is subsequently provided with holes, which holes are then plated after which any excess plating is removed. 
     The multilayer microwave board according to the invention is characterised in that the holes penetrate the entire package and are at least substantially plated between the conductor patterns to be connected. 
     The method for tuning a multilayer microwave board manufactured according to a method well-known in the art is characterised in that after the actual production of the board a hole is drilled to at least one buried via in the board between a stripline and a plated hole, such that this connection is at least substantially ohmic in nature. 
     According to the known method, individual layers of plated laminate are provided with a conductor pattern and, where necessary, drilled after which the resulting holes are plated. These layers are then bonded under high pressure and temperature conditions. By repeatedly bonding the thus obtained semimanufactures and again drilling and plating the resulting holes, it is possible to realize more complex products. 
     Products realized on the basis of said known method are characterised by a high ratio of rejects, particularly since connections between the conductor pattern and plated holes fail at the high temperature and pressure required for the bonding process. This is particularly due to fluctuations in coefficients of expansion between metal parts and the laminate, although the high pressure and temperature also render the laminate viscous as a result of which any asymmetry surrounding a plated through-hole may cause a crack. 
     In the new, inventive method this problem is not experienced. It is further characterised in that the holes are provided by drilling right through the bonded package. Subsequently the holes are fully plated in a way known in the art after which any excess plating is mechanically removed, for instance by means of a drill. 
     A most favorable embodiment of the method is characterised in that a diameter of the drill is selected depending on the type of laminate and on the diameter of the plated hole in order to obtain optimized wave properties of the connection between the conductor pattern and the plating at that position. A favorable selection would for instance be a diameter of approximately 1.15 times the diameter of the plated hole. 
     A further favorable embodiment of the method is characterised in that the excess plating, depending on the type of laminate and the diameter of the plated hole, is removed in axial direction such that the wave properties of the connection between the conductor pattern and the plating at that position are optimized. It may then be effected that the connection at that position is at least substantially purely ohmic. A favorable implementation of this method, using a standard type of laminate of for instance {fraction (3/100)} inch is characterised in that a volume of excess plating is retained in axial direction at a length of 10-50 μm. 
     Since excess plating is mechanically removed in a multilayer microwave board according to the invention, the board is further characterised in that the holes, where the plating has been mechanically removed, have a larger diameter than the plated holes. 
     Depending on the type of laminate and the diameter of the plated hole, the larger diameter can be again selected such to yield favorable wave properties of the connection between the conductor pattern and the plating at that position, using a standard laminate of {fraction (3/100)} inch, for instance approximately 1.15 times the diameter of the plated hole. 
     A still further favorable embodiment of the multilayer microwave board according to the invention is furthermore characterised in that, depending on the type of laminate and the diameter of the plated hole, the plating between the conductor patterns to be connected extends at least slightly further in axial direction than merely the distance between these conductor patterns in order to obtain optimized wave properties of the connection at that position, for instance over 10-50 μm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The method of manufacture and the multilayer microwave board according to the invention will now be described with reference to the following figures, of which: 
     FIGS. 1A,  1 B and  1 C schematically represent the conventional construction of a multilayer microwave board provided with a blind via; 
     FIGS. 2A and 2B schematically represent the construction according to the invention of a multilayer microwave board provided with a blind via. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A,  1 B and  1 C show the various steps in a possible manufacturing process for a multilayer microwave board according to the state of the art. In FIG. 1A, laminate  1 , for instance duroid, is provided with a conductor pattern  2  and a ground plane  3 . Laminate  4  is also provided with a conductor pattern  5  and a ground plane  6 . A connection with favorable microwave properties shall be effected between conductor pattern  2  and conductor pattern  5 . The ground planes are thereto provided with circular recesses  7 ,  8 , usually obtained by means of etching. Subsequently, ground planes  3 ,  6  are bonded together after which a through-hole  9  is drilled through the package, which is then provided with a plating according to a method known in the art, as shown in FIG.  1 B. As shown in Fig. 1C, laminate  10  provided with ground plane  11  and laminate  12  provided with ground plane  13  are then bonded to the package. These bonding treatments that take place under high pressure and temperature conditions, may impair the connection between conductor patterns  2  and  5 . Particularly in case of complex multilayer microwave boards incorporating many connections and requiring many operations, a faulty connection is likely to occur. The connection thus effected between conductor patterns  2  and  5  is moreover found to possess less favorable wave properties. The transition from a stripline mode in conductor pattern  2  to a coaxial mode between plated hole  9  and recesses  7 ,  8  and subsequently in a stripline mode in conductor pattern  5  is found to cause reflections. Even a careful selection of all contributory components, such as hole diameter, plating thickness and diameter of recesses  7 ,  8 , still yields a minimum reflection of −20 dB, which limits the range of application of the multilayer microwave board. 
     The known method of manufacture allows several variations that are of minor relevance. It is for instance possible to leave out ground plane  3  or ground plane  6  or to replace laminate  10  or laminate  12  by a thick adhesive layer as well-known in the art. This does not substantially change the complexity of the process, the ratio of rejects and the microwave properties. 
     FIGS. 2A and 2B show the simplified method of manufacture of a multilayer microwave board according to the invention. In FIG. 2A, laminate  1  is provided with conductor pattern  2  and ground plane  3 , and laminate  4  is provided with conductor pattern  5  and ground plane  6 , and laminate  10  is provided with ground plane  11  and laminate  12  provided with ground plane  13 . All layers are simultaneously bonded. In view of the connections between conductor patterns  2  and  5  having not yet been effected, this bonded construction is risk-free. After the bonding process, through-hole  9  is drilled through the entire package and is plated as shown in FIG.  2 B. Subsequently, the superfluous parts  15 ,  16  of the plating are mechanically removed. Milling, spark machining or etching are acceptable alternatives for drilling. This enables all connections in a multilayer microwave board to be realized in one single operation comprising etching, bonding, drilling, plating and machining, which keeps down the costs and the ratio of rejects. 
     The mechanical removing of excess plating entails two options for tuning the connection between the conductor pattern and the plating; one option is to select the thickness of the drill, the second option entails the possibility to retain any excess plating, strictly speaking. This choice shall be made dependent on the type and thickness of the laminate and is to a minor extent dependent on the microwave frequency and the bonding process. Individual tuning will, however, not be required; once said parameters are known, the single determination of the tuning parameters enables an improvement of the reflection properties of the transitions of at least 10 dB, as a result of which the reflection decreases to assume a value of less than −30 dB. When using a standard laminate of {fraction (3/100)} inch, this can usually be directly achieved by using a drill having a diameter of approximately 1.15 times the diameter of the plated hole and by retaining in axial direction 10-50 μm of the excess plating. If too much plating is retained, the transition will be capacitive in nature, if too much plating is removed, the transition will be inductive in nature. This rule makes it easy to quickly obtain the desired result for each type of laminate. 
     With boards based on conventional design, it is likewise possible to tune the transition between the conductor pattern and the plated hole and to obtain an improvement of at least 10 dB. Since during the plating process some material is formed at the conductor pattern around the hole, the transition is capacitive in nature. By making a hole into the package shown in FIG. 1C, for instance by drilling, right through ground plane  11  and partly through laminate  10  in the direction of the plated hole  9 , it is possible to effect a tuning of the transition, through which the transition becomes substantially ohmic in nature. Also in this case, individual tuning is not required and can be determined once on the basis of the laminate, the process and the frequency used. 
     A via is often surrounded by grounding pins. These are vias that make contact with all ground planes in order to facilitate a transition from the stripline mode in the conductor pattern to the coaxial mode between plated hole  9  and recesses  7 ,  8  and to prevent the occurrence of false modes. During manufacturing, these grounding pins can also be drilled and plated in the one operation after the adhesive bonding process.