Abstract:
A printed circuit board for an electronic circuit, especially for the ultra-high frequencies located in the GHz range that comprises at least one conductor layer, which is arranged on top of an insulating layer and which is flatly joined to said insulating layer. Improved mechanical, thermal and electrical properties are attained by virtue of the fact that the insulating layer is a thin glass layer.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to a printed circuit board, a method for producing such a printed circuit board, and a layer composite material for such a printed circuit board. 
   2. Prior Art 
   In present-day electronic circuit technology concerning, in particular, computer technology and the communication and processing of data in communications technology, development is tending toward an ever greater integration and packing density and toward ever higher clock frequencies, which are normally already in the GHz range. Under these circumstances, increasing importance is being attached to the printed circuit boards or printed circuits in the construction of functionally reliable and, at the same time, cost-effective circuits. On the one hand, such a printed circuit board, in particular for relatively large series, must be simple and cost-effective to produce and process (cut, drill, populate, etc.). On the other hand, the printed circuit board must have a sufficient thermal and mechanical stability and the best possible thermal conductivity, because a considerable amount of heat is generally to be expected owing to the high integration density and high frequencies during operation. Furthermore, it is desirable for the printed circuit board to be adapted to the ubiquitously used semiconductor components (made of Si or else GaAs) with regard to thermal expansion, because this enables direct mounting of the semiconductor chips on the printed circuit board (Direct Chip Attach DCA) with all its advantages. 
   However, the dielectric properties of the insulation material used within the printed circuit board are particularly important with regard to the high frequencies. Thus, a printed circuit board used in the extremely high frequency range should have an insulating layer between the conductor layers with the lowest possible (relative) permittivity ε r  and a small dielectric loss factor tanδ in order to keep down the losses that increase with the frequency. 
   Finally, at the high clock rates and with very fine conductor tracks, it is becoming more and more important for the printed circuit board material to be distinguished by a high degree of homogeneity in the dielectric and a high uniformity in the external dimensions (small thickness fluctuations, etc.) because otherwise, in adjacent regions of the printed circuit board, undesirable propagation time differences are produced during the signal propagation and impair the functionality of the circuit constructed therewith. 
   All the requirements presented are satisfied only poorly or not at all by conventional epoxide-based substrate materials. Therefore, various proposals have already been made in the past in respect of using, for single or multilayer printed circuit boards, insulating intermediate layers made of a sintered glass ceramic which, at the same time, have good dielectric properties and are adapted to GaAs circuits, for example, in terms of their thermal expansion coefficient (see e.g. U.S. Pat. No. 6,017,642). However, such sintered glass ceramic substrates are complicated to produce and, as ceramic plates, have only a limited mechanical strength, so that they allow the realization of, in particular, thin single-layer printed circuit boards only with difficulty. 
   Another proposal relates to the use of a “glass paper”—produced from glass fibers—as dielectric for printed circuit boards (JP-A-9208252). Although such a fiber material should be less at risk of fracture compared with the glass ceramic, the irregular fibrous structure of the material results in a local inhomogeneity in the dielectric properties, which can lead to the abovementioned propagation time problems with high line densities and at high frequencies. 
   Finally, a whole class of substrate materials for printed circuit boards is known which comprise fluoropolymers filled with additives (ceramic particles, glass fibers) (see e.g. U.S. Pat. No. 5,149,590). Although such materials, which are commercially available for example from the US company Rogers Corp. under the designations RT/DUROID 5870–5880 and RO3000, exhibit relatively good and homogeneous dielectric properties and are therefore well suited to extremely high frequency applications, such a material is comparatively expensive on account of the complicated production and, moreover, has an unfavorable thermal expansion coefficient which is significantly higher than that of silicon. 
   SUMMARY OF THE INVENTION 
   Therefore, it is an object of the invention to provide a printed circuit board which avoids the disadvantages of known printed circuit boards and, in conjunction with comparatively simple and cost-effective production, is distinguished by very good mechanical and thermal properties, has very good dielectric properties and can thus be used up to extremely high frequencies, and is optimally adapted to the customarily used Si semiconductor chips in terms of its thermal expansion. Furthermore, it is an object of the invention to specify a method for producing such a printed circuit board. 
   The heart of the invention consists in providing a thin glass layer, as is known principally from the technology of liquid crystal displays (LCDs), as dielectric in the printed circuit board below the at least one conductor layer comprising conductor tracks. Even with a small thickness, such a thin glass layer has a good mechanical stability, has very advantageous dielectric and thermal properties and is distinguished by a high optical quality, which is manifested in particular in a high homogeneity of the material and a high degree of planarity with small evenness and thickness deviations. The use of a thin glass layer as dielectric makes it possible to satisfy all the requirements which are made of printed circuit boards for electronic circuits with a high integration density and extremely high frequencies. 
   A preferred refinement of the invention is characterized in that a respective conductor layer is arranged on both sides of the thin glass layer and connected to the thin glass layer in planar fashion. In this case, one conductor layer or both conductor layers may be structured, i.e. comprise individual conductor tracks. Such a thin glass layer provided with a conductor layer on both sides has the advantage over the one which is coated on one side that an electronic circuit realized therewith is provided with an unambiguously defined volume which is predominantly filled with the thin glass dielectric and has correspondingly favorable properties. 
   Preferably, the thin glass layer is pulled from the melt and comprises a modified borosilicate glass, the thin glass layer having a thickness in the range from a few μm to a few mm, preferably in the range between 30 μm and 1.1 mm, and having, at 1 MHz, a relative permittivity ε r  of between 6 and 7, preferably of about 6.2, and a dielectric loss factor tanδ of about 9×10 −4  and having, for temperatures of between 20 and 300° C., a thermal expansion coefficient α 20-300  of between 4×10 −6 K −1  and 8×10 −6 K −1 , preferably of about 4.5×10 −6 K −1 . 
   In an advantageous manner, the conductor layers in each case comprise a metal foil, preferably a Cu metal foil, have a thickness of between 5 and 50 μm, preferably of 18 or 35 μm, and are adhesively bonded to the thin glass layers in each case by means of a connecting layer, the connecting layers essentially comprising a resin. Such resin-coated Cu foils (Resin Coated Foil or RCF) are known from the technology of HDI circuits, i.e. printed circuit boards with high integration densities. 
   Preferably, the connecting layers are additionally provided with a silane as adhesion promoter. Furthermore, it is advantageous if the surfaces of the thin glass layers are pretreated in order to improve the adhesion. 
   In the simplest case, the printed circuit board comprises an individual thin glass layer which is adhesively bonded to metal foils on both sides. Given a small thickness of the thin glass layer, this results in a thin, flexible printed circuit board having very good electrical and thermal properties. 
   However, it is also conceivable for the printed circuit board to comprise, in a stack one above the other, a plurality of thin glass layers adhesively bonded to metal foils and thus to form a multilayer printed circuit board which may also be provided with plated-through holes in the customary manner. 
   Equally, it is conceivable for the printed circuit board to comprise, besides a thin glass layer, at least one further insulating plate made of a different insulating material. The combination of conductor layer(s) and thin glass layer can thus advantageously be integrated into a printed circuit board of a conventional type. 
   Further embodiments emerge from the dependent claims. 

   
     BRIEF EXPLANATION OF THE FIGURES 
     The invention will be explained in more detail below using exemplary embodiments in connection with the drawing, in which 
       FIG. 1  shows, in a sectional illustration, the construction of an unstructured printed circuit board in accordance with a first preferred exemplary embodiment of the invention; 
       FIG. 2  shows the printed circuit board from  FIG. 1  after the (two-sided) structuring of the conductor layers and with a semiconductor chip which, by way of example, is applied directly by means of the DCA method; 
       FIG. 3  shows, in a sectional illustration, the construction of a (structured) printed circuit board in accordance with a second preferred exemplary embodiment of the invention; 
       FIG. 4  shows, in a sectional illustration, the construction of a (structured) multilayer printed circuit board in accordance with a third preferred exemplary embodiment of the invention; 
       FIGS. 5   a  and  5   b  show a method for producing a layer composite material for a printed circuit board according to  FIG. 1  in accordance with a preferred exemplary embodiment of the invention; and 
       FIGS. 6   a  – 6   f  show a method for producing a printed circuit board according to  FIG. 3  in accordance with another preferred exemplary embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention will be explained below using examples in which a thin glass layer is provided with a structured or unstructured conductor layer in each case on both sides. However, it is also conceivable, in principle, within the scope of the invention to provide a conductor layer only on one side of the thin glass layer. The construction and the method for producing a printed circuit board or a layer composite material with such a thin glass layer “occupied” on one side emerge analogously from the explanations below. 
     FIG. 1  represents, in a sectional illustration, the construction of an unstructured printed circuit board in accordance with a first preferred exemplary embodiment of the invention. In this case, the printed circuit board  10  comprises an individual thin glass layer  13 , a conductor layer  11  and  15  respectively being arranged on the two sides of said thin glass layer. The conductor layers  11 ,  15  are formed by Cu metal foils which are adhesively bonded to the thin glass layer  13  in planar fashion by means of an adhesive connecting layer  12  and  14 , respectively. 
   Thin glass layers of the kind provided in the context of the invention and used to achieve the advantages according to the invention are known from the prior art (see e.g. EP-A1-0 972 632 or DE-A1-198 10 325). They have thicknesses of between about 30 μm and 2 mm, are pulled from the melt, and are distinguished by a good mechanical and optical quality and excellent thermal and electrical properties. Such thin glass layers are produced commercially on a relatively large scale for electrooptical displays (e.g. LCDs) and their price is therefore comparatively favorable. 
   Suitable thin glass layers or thin glasses are produced and offered by the company Schott Glas, Mainz (Del.) under the type designations AF 45 and D 263 T. 
   The thin glass AF 45 is a modified borosilicate glass with a high proportion of BaO and Al 2 O 3 . It is normally in sizes of up to 440 mm×360 mm and is distinguished by the following properties: 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               AF 45 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Thickness (standard): 
               50 μm to 1900 μm 
             
             
                 
               Transformation point T g : 
               662° C. 
             
             
                 
               Thermal expansion coefficient a 20–300 : 
               4.5 × 10 −6 K −1   
             
             
                 
               Rel. permittivity ε r  (at 1 MHz) 
               6.2 
             
             
                 
               Dielectric loss factor tanδ: 
                 9 × 10 −4   
             
             
                 
                 
             
           
        
       
     
   
   The thin glass D 263 T is a borosilicate glass produced from very pure starting materials. It is likewise available in sizes up to 440 mm×360 mm and is distinguished by the following properties: 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               D 263 T 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Thickness (standard): 
               30 μm to 1100 μm 
             
             
                 
               Transformation point T g : 
               557° C. 
             
             
                 
               Thermal expansion coefficient a 20–300 : 
               7.2 × 10 −6 K −1   
             
             
                 
               Rel. permittivity ε r  (at 1 MHz): 
               6.7 
             
             
                 
               Dielectric loss factor tanδ: 
                61 × 10 −4   
             
             
                 
                 
             
           
        
       
     
   
   The two thin glass layers or thin glasses are particularly well suited to use in a printed circuit board according to the invention. 
   The production of a suitable layer composite material for the (unstructured) printed circuit board  10  in accordance with  FIG. 1  is preferably effected in the manner illustrated in  FIG. 5 : what is used as a starting point is a thin glass layer  13 , whose surfaces are firstly freed of adhereing moisture (water) and subsequently pretreated—e.g. by means of a corona discharge or the like—in order to obtain a good, permanent adhesion during the subsequent adhesive bonding. In order to produce the conductor layers  11 ,  15  on the thin glass layer, use is made of resin-coated metal foils (Resin Coated Foils RCFs)  28 ,  29  made of Cu, a connecting layer  12  and  14  respectively being applied to the adhesive bonding side thereof ( FIG. 5   a ). The connecting layers  12 ,  14  contain a resin which, expediently, is partly cured or prereacted. Such resin-coated Cu metal foils are known from the technology of large scale integrated (HDI) circuits. 
   In the context of the invention, particularly suitable resin-coated foils are offered for example by the company Isola AG(Del.) under the designation ISOFOIL 160 and RCC. The (ISOFOIL 160) foils have a thickness of the copper foils of. 18 μm or 35 μm, for example, and are provided with a 75 μm resin layer which is prereacted (so-called B stage coating). 
   The resin-coated foils  28 ,  29  are packed, with the additional use of a silane layer for adhesion promotion (in this respect, see e.g. U.S. Pat. No. 5,149,590), with the pretreated central thin glass layer  13  to form a stack. The stack is then adhesively bonded under the action of heat and pressure ( FIG. 5   b ). A layer composite material or an (unstructured) printed circuit board  10  in accordance with  FIG. 1  is obtained as a result. The conductor layers  11 ,  15  can then optionally be structured using known methods from printed circuit board production, in order to produce as required specific conductor tracks (or conductor areas)  16 ,  17  ( FIG. 2 ). By way of example, SMD devices or the like can then be soldered or conductively bonded onto the structured printed circuit board  10  in accordance with  FIG. 2 . A particularly advantageous type of mounting is made possible by virtue of the thermal expansion coefficients of the thin glass layer  13  which are adapted to the customary semiconductor materials: thus, in accordance with  FIG. 2 , a semiconductor chip  18  provided with corresponding connection contacts  19  can be mounted directly on the printed circuit board  10  (so-called Direct Chip Attach DCA). As a result of this, even higher packing densities can be achieved in conjunction with simplified mounting and high reliability. 
   In addition to the simple printed circuit board  10  from  FIGS. 1 ,  2  and  5  which comprises only one thin glass layer  13  and two conductor layers  11  and  15  and can therefore be made particularly thin and flexible, it is also possible to realize in an analogous manner multilayer printed circuit boards  30  which comprise, in a stack one above the other, a plurality of thin glass layers  13 ,  13 ′ and  13 ″ with conductor layers  34  and  35  lying in between and outer conductor layers  31 ,  38  ( FIG. 4 ). In this case too, each conductor layer is provided with a corresponding connecting layer  32 ,  33 ,  36 ,  37  which adhesively bonds the conductor layer to the respective thin glass layer. In this case, the inner conductor layers  34 ,  35  are expediently embedded in an associated connecting layer  33  and  36 , respectively. It goes without saying that such a multilayer printed circuit board  30  may also be provided with plated-through holes (not shown in  FIG. 4 ) which are produced in a manner known per se and connect conductor tracks in different conductor layers to one another. 
   Another possibility within the scope of the invention consists in integrating a thin glass layer that is provided with a conductor layer on one side or on both sides into a printed circuit board of a conventional type. Thus, the example illustrated in  FIG. 3  shows a printed circuit board  20  in which a central thin glass layer  13  with conductor layers  23 ,  24  and conductor tracks  27 ,  27 ′ bonded on both sides is arranged between two insulating plates  21 ,  26  of a conventional type (e.g. based on epoxy resin or polytetrafluoroethylene) and adhesively bonded to said insulating plates. Connecting layers  22 ,  25  once again serve here for adhesive bonding, the conductor layers  23 ,  24  being embedded in said connecting layers. 
   The production of such a “hybrid” printed circuit board  20  with conventional insulating plates and thin glass layers is illustrated in various steps in  FIGS. 6(   a )–( f ): the starting point is one of the insulating plates, namely the insulating plate  21 , onto which a first conductor layer  23  is bonded in a conventional manner by means of a first connecting layer  39  and is then structured ( FIG. 6   a ). The first connecting layer  39  is then filled up whilst covering the first conductor layer to form the final connecting layer  22  ( FIG. 6   b ). The structure  21 ,  22 ,  23  thus prepared is thereupon adhesively bonded on one side to a correspondingly pretreated thin glass layer  13  ( FIG. 6   c ). A further structure  24 ,  25 ,  26  can then be bonded onto the free side of the thin glass layer  13  ( FIGS. 6   e,f ), which structure comprises the other insulating plate  26  provided with a second conductor layer  24 . In this case, the second conductor layer  24  is bonded onto the insulating plate  26  by means of a second connecting layer  40 , and subsequently structured ( FIG. 6   d ), and the second connecting layer  40  is then filled up to form the final connecting layer  25  ( FIG. 6   e ). In this exemplary embodiment too, it is conceivable to use resin-coated Cu metal foils to produce the conductor layers  23  and  24 . 
   Overall, the invention yields a printed circuit board which is distinguished by the following properties and advantages:
         The printed circuit boards can be made very thin without losses in mechanical stability   A direct mounting of semiconductor chips is possible by virtue of the adaptation of the thermal expansion coefficients of the central dielectric (thin glass) to the customary semiconductor materials   The optical quality, homogeneity and uniformity in the dimensions of the thin glass material avoid disturbing influences on the signal propagation, in particular at high frequencies   The comparatively good thermal conductivity of the thin glass layer facilitates the dissipation of heat and thus allows higher integration densities   The small relative permittivity and the small dielectric loss factor of the thin glass material enable the circuit arranged on the printed circuit board to have very high frequencies   The thin glass layers are commercially available in outstanding quality and at favorable prices from series production and therefore limit the costs of the printed circuit boards   The thin glass layers can be worked and processed well in the context of printed circuit board production; in particular, known methods of printed circuit board production can be used without difficulty   Multilayer printed circuit boards can be produced without difficulty, which boards can be made even more compact on account of the small thickness of the thin glass layers   Even finer conductor track structures are possible on account of the high degree of homogeneity and uniformity of the dielectric.       

   The layer composite material produced by the method according to the invention is particularly well suited as starting material for the printed circuit board. However, it is also conceivable to use this material in other applications. 
   LIST OF REFERENCE SYMBOLS 
   
       
         10 ,  20 ,  30  Printed circuit board 
         11 ,  15  Conductor layer (Cu metal foil) 
         12 ,  14  Connecting layer 
         13 ,  13 ′,  13 ″ Thin glass layer 
         16 ,  17  Conductor track 
         18  Semiconductor chip 
         19  Connection contact (semiconductor chip) 
         21 ,  26  Insulating plate (e.g. epoxy) 
         22 ,  25  Connecting layer 
         23 ,  24  Conductor layer (Cu metal foil) 
         27 ,  27 ′ Conductor track 
         28 ,  29  Resin-coated metal foil (Cu) 
         31 ,  34 ,  35 ,  38  Conductor track 
         32 ,  33 ,  36 ,  37  Connecting layer 
         39 ,  40  Connecting layer