Abstract:
The present invention relates to a device and a method at a printed board for obtaining good transmission qualities in transmission conductors on a predetermined area ( 10 ) of the printed board ( 11 ). A separate component ( 1 ) for signal transmission comprises a conductor ( 5 ). The component ( 1 ) is mounted, with the conductor facing the printed board ( 11 ), over the area ( 10 ) of the printd board, which requires good transmissions qualities, whereby an air gap (L) is obtained between the conductor ( 5 ) and the printed board ( 11 ). Soldering joints ( 21 ) connect each one of the outer parts ( 7   a,    7   b ) of the conductor ( 5 ) of the component ( 1 ) to corresponding pattern conductors ( 17   a,    17   b ) on the printed board ( 11 ). The thickness of the soldering connections and the thickness of the pattern conductors form the air gap (L) be the conductor ( 5 ) and the printed board ( 11 ). In an alternative embodiment according to the invention, a groove ( 23 ) is milled out of the printed board ( 11 ) under the conductor ( 5 ), obtaining an enlarged air gap between the conductor ( 5 ) and the printed board ( 1 ).

Description:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 9702687-6 filed in Sweden on Jul. 11, 1997; the entire content of which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a device and a method at printed boards for use in electronics systems. 
     BACKGROUND OF THE INVENTION 
     A simple printed board according to prior art consists of a number of layers, wherein the lowest layer is termed support in the text below. The support is made of a suitable electrically conductive material, for instance brass, copper or aluminium. Transmission conductors, also termed conductors below, are etched out of an electrically conductive conductive pattern layer of the printed board, wherein a layer of dielectric material separates the conductive pattern layer frown the support. The conductors connect components on the printed board electrically to each other as desired. 
     An electromagnetic field occurs in a known manner between the conductors and the support of the printed board, wherein part the electromagnetic field is lost in the dielectric. This is specifically related to printed board assemblys used at high frequencies above 1 GHz, as energy losses can be significant and large amounts of heat is liberated in the conductors. 
     Examples of said problems with losses in the dielectric are at long transports of high frequency signals in the conductors, and communication between modules, so-called MCM (Multi Chip Modules). The modules, working independently at high frequencies, are mounted on a common so-called mother board. This board is made of a cheaper material, because the electronics on the mother board operates at low frequencies. 
     One known method for minimizing said losses occurring in the printed board assembly is to use a dielectric with good high frequency qualities. 
     A disadvantage with this method is that these dielectrics are expensive to use in the printed board assembly. 
     Another method according to prior art is to mill out a groove with air of the dielectric between the conductor and the support. The dielectric field from the conductor then passes through the air groove, implying remarkably decreased losses of the conductor compared to a solid dielectric. 
     A disadvantage with the method above is that it is complicated and requires long time to perform. 
     U.S. Pat. No. 3,904,997 discloses a conductor attached to a dielectric material. An electrically conductive ground plane, comprising a channel, has contact with the dielectric, so the conductor is enclosed in the channel with an air gap to the ground plane. 
     In U.S. Pat. No. 2,800,634 a method for the losses of a wave guide for instance on a printed board assembly operating at high frequencies is disclosed. Thereby, an air gap is employed between a ground plane and the wave guide, wherein a layer of dielectric material of the printed board is arranged to the ground plane and builds up the air gap between the ground plane and the conductor. 
     SUMMARY OF THE INVENTION 
     One problem that is solved by the invention is to obtain good transmission qualities in a non-expensive manner of conductors in a predetermined area of a printed board, in particular employed at high frequencies. 
     Thus, the object of the present invention is to obtain good transmission qualities in a non-expensive manner for transmission conductors at a predetermined area of printed boards. 
     To solve this problem, the present invention employs mounting of a separate component for signal transmission on a predetermined area where good transmission qualities on the printed board is required. An area of the component, facing the printed board, comprises a conductor, wherein air gap is obtained between the conductor and the printed board. The produced printed board with its component has an air gap which is well adapted to the conductor. 
     In more detail, the problem is solved as follows. The component comprises a layer of dielectric material, plated with an electrically conductive layer, and the conductor is etched out of this electrically conductive layer. 
     The printed board comprises a support of an electrically conductive material, wherein one layer of dielectric is attached to the support. Conductor patterns are etched out of an electrically conductive pattern layer on the dielectric layer. The printed board is on a predetermined area, requiring good transmission qualities, provided with a mounting surface for the component. 
     Subsequently, the component is mounts with the conductor facing the printed board, to the mounting surface of the printed board. Electrically conductive attachment joints, also termed attachment joints below, for instance soldering joints or joints of electrically conductive glue, connects each one of the outer parts of the conductor with corresponding pattern conductor on the printed board. The thickness of the attachment joints achieve herein an air gap between the conductor and the printed board. 
     According to another alternative embodiment of the invention, a groove is milled out of the dielectric layer of the printed board above, wherein the groove is arranged under the conductor, obtaining an enlarged air between the conductor and the printed board. 
     If yet better transmission qualities a required of an area of the printed board, the dielectric layer of the component can consist of a dielectric with good high frequency qualities. 
     An advantage of the present invention is that it is simple and non-expensive to use, since well-known standard materials can be employed. Only the component must if required be made of a more expensive material with good high frequency qualities. 
     An other advantage of the present invention is that good transmission qualities easily can be obtained at predetermined local places or areas of the printed board. 
     The invention will now be described in more detail illustrated by preferred embodiments of the invention and with reference to accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a schematical view from below of a component comprising a conductor of the invention, 
     FIG. 2 illustrates a schematical view from above of a printed board of the invention, 
     FIG. 3 a  illustrates a schematical view from above of the component mounted on the printed board of the invention, 
     FIG. 3 b  illustrates a schematical view from above of yet another embodiment when the component is mounted on the printed board of the invention, 
     FIG. 4 illustrates a cross-section A—A of FIG. 3 a  of the component mounted on the printed board of the invention. 
     FIG. 5 illustrates a cross-section A—A of FIG. 3 a  of the component mounted on the printed board of an alternative embodiment of the invention. 
     FIG. 6 illustrates a schematical cross-section of a dielectric layer plated with an electrically conductive layer of the invention, and 
     FIG. 7 illustrates a schematical cross section of the printed board of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following examples, the invention will be further described with reference to FIGS. 1-4. 
     FIG. 1 illustrates a view from below of a component  1  according to the present invention. The component  1  comprises a dielectric layer  3 , a conductor  5  and two mechanical attachment points  7 . 
     The dielectric layer  3  is made of a non-conductive material, for instance glass epoxy FR 4 . The dielectric layer  3  is normally made of the same material as below described intermediate layer of a printed board. 
     The conductor  5  and the attachment points  7  are etched on a first side of the component  1  out of a conductive layer of or instance copper, initially covering the entire dielectric layer  3 . 
     According to the present examples, the conductor  5  is arranged in the middle of the first side of the component, as is illustrated in FIG. 1, but also other arrangements are possible. Likewise, the conductor  5  does not have to be straight but can be inclined or have more complicated shape. 
     The conductor  5  according to the present example extends between a first edge k 1  of the component  1  to an opposite second edge k 2  of the component  1 . The outer parts of the conductor  5  consist of a first attachment surface  7   a,  adjacent to the first edge k 1 , and a second attachment surface, adjacent to the second edge k 2  of the component  1 . These are shown in the figure. 
     The figure shows an example of the arrangement of the attachment points  7  on the first side of the component, wherein the attachment points  7  are symmetrically arranged around the conductor  5 . The number of attachment points  7  is not limited to two, as is disclosed according to the present example, but also more or less attachment point  7  can be employed. 
     FIG. 2 illustrates a view from above of a local area  10  of a printed board  11  according to the invention, said local area  10  requiring good transmission qualities. One example of such an local area is between two high frequency MCM (Multi Chip Modules), which are mounted on a common mother board, made of cheaper material. 
     Just for clarity reasons, only a part of the printed board  11  is showed in FIG. 2, as in all figures. 
     The printed board  11  comprises a support  13  (shown in FIG.  4 ), an intermediate layer  15 , two pattern conductors  17   a,    17   b  and two attachment surfaces  19 . 
     The support is made of an electrically conductive material, for instance brass, copper or aluminium. 
     The intermediate layer  15  is made of a dielectric material, for instance glass epoxy FR 4 . 
     The pattern conductors  17   a,    17   b  and the attachment surfaces  19  are etched out of a first side of the printed board  11  out of a conductive pattern layer, made of an electrically conductive material, such as for instance copper, initially covering the entire intermediate layer  15 . 
     In the present examples, good transmission qualities are to be obtained of the local area  10  of the printed board  11 , wherein the pattern conductors  17   a,    17   b  on the printed board  11  are etched so they are adjacent to the local area  10 . 
     The component  1  above is dimensioned so that when it is arranged over the local area  10  on the printed board  11 , the first attachment surface  7   a  and the second attachment surface  7   b  of the conductor should overlap the local area adjacent ends of the pattern conductors  17   a,    17   b  of the printed board  11 . 
     Thus, the distance between the ends of the pattern conductors, designated a 1  in the figure, on the printed board  11  is less that the length of the conductor of the component  1  according to the present example. 
     FIG. 2 illustrates an example of the arrangement of the pattern conductors  17   a,    17   b  and the attachment surfaces  19  on the first side of the printed board. The number of pattern conductors  17   a,    17   b  and attached surfaces  19  is not limited to the number which is illustrated in the figure. 
     In FIG. 3 is illustrated a view from above the component  1 , disclosed above, mounted with the conductor  5  facing the printed board  11  over the local area  10  of the printed board  11 . 
     The attachment points  7  of the component  1  are fixed by for instance soldering joints or electrically conductive glue to the attachment surfaces  19  on the printed board  11 . Likewise, the first attachment surface  7   a  of the conductor and the second attachment surface  7   b  are fixed by soldering joints  21  to the overlapping ends of corresponding pattern conductor  17   a,    17   b.    
     An alternative to the examples above is illustrated in FIG. 3 b.  Here, the pattern conductors  17   a,    17   b  are etched out of the conductive pattern of the first side of the printed board so the pattern conductors  17   a,    17  extend parallel to an edge of the local area  10 . 
     The component  1  is mounted with the conductor  5  facing the printed board  11  over the local area  10  on the printed board  11 , wherein the first attachment surface  72  of the conductor and the second attachment surface  7   b  overlap a part each of the to the local area  10  adjacent pattern conductors  17   a,    17   b.    
     Correspondingly as above, the first attachment surface  7   a  and the second attachment surface  7   b  are fixed by soldering joints  21 , or alternatively electrically conductive glue to the overlapping part of corresponding pattern conductor  17   a,    17   b.    
     A cross-section A—A of FIG. 3 a  of the component  1  mounted facing the printed board  11  is shown in FIG.  4 . The soldering joints  21  connects the conductor  5  of the component  1  to corresponding pattern conductor  17   a,    17   b  of the printed board  11 . As is illustrated in the figure, the thickness of the soldering joints and the thickness of the pattern conductors form an air gap L between the surface of the conductor, facing the printed board  11 , and the surface of the intermediate layer  15  of the printed board  11 . The height of the air gap L, designated h in FIG. 4, can be varied as required by varying the thickness of the soldering joint  21 . 
     For instance the height h of the air gap of 145 μm is obtained if each soldering joint  21  has a thickness of 100 μm and the pattern conductors have a thickness of 45 μm. 
     Furthermore, as shown in FIGS. 2 and 4, the air gap has a uniform width that is equal to a predetermined distance of the gap between the ends of the first and second conductors, the air gap also has uniform height and width dimensions between the component and the printed board. 
     An electro magnetic field  2 , designated by dashed lines in the figure, occurs in a known way between the conductor  5  and the support  13 . The air gap L between the conductor  5  and the intermediate layer  15 , accomplish that the losses due to the field decrease between the conductor  5  and the support  13 . Thereby the conductor  5  according to the present invention has better transmission qualities than a conductor of a solid dielectric. 
     The air gap L will accomplish that also the heat liberated from the conductor  5  decreases. The invention is in particular applicable at local areas on printed boards used at high frequencies as the energy losses in a solid dielectric can be severe and large amounts of heat is liberated in the pattern conductors. 
     The different thickness of the layers of the component  1  and the printed board  11  are for reasons of clarity, enlarged in FIG. 4, as well as in all figures. 
     FIG. 5 illustrates an alternative embodiment of the invention according to previous examples, wherein a groove  23  in the intermediate layer  15  of the printed board  11  is employed for obtaining a larger height h of the air gap L. 
     Correspondingly, as disclosed above and in FIG. 4, the component  1  is mounted over the local area  10  of the printed board  11  and the conductor  5  is connected by the soldering joints  21  to the pattern conductors  17   a,    17   b.    
     The groove  23  is milled out of the local area  10  of the printed board  11  through the intermediate layer  15 , wherein the height of the groove  23  is designated H in the figure. The height H of the groove  23  can be varied as required. 
     Thus, the height h of the air gap L between the surface of the conductor, facing the groove  23  and the surface of the groove, facing the conductor  5 , according to the present example is the sum of the height H of the groove, the thickness of the soldering joints and the thickness of the pattern conductors. 
     Thus, according to the present examples a larger layer of air is obtained, i. e., the air gap L, and a smaller layer of dielectric, i. e., the intermediate layer  15 , between the conductor  5  and the support  13  than in the previous examples. 
     In the following examples, a method of the invention will be described with reference to the above disclosed example and FIGS. 6-7, and above disclosed FIGS. 1,  2 ,  4  and  5 . 
     According to the method of the invention the component  1  and a mounting surface for the component  1  on the predetermined local area  10 , as described above, will be produced. Furthermore, the component  1  will be mounted over the local area  10  on the printed board  11 , rendering the local area  10  by the component  1  good transmission qualifies. 
     The method for production the component  1  is disclosed below with reference to FIG.  6  and FIG.  1 . 
     FIG. 6 illustrates a board  1   b  comprising a dielectric layer  3   b  plated with an electrically conductive layer  5   b.    
     The dielectric layer  3   b  is made of a non-conductive material, for instance glass epoxy FR 4  and the dielectric layer  3   b  is as disclosed above normally made of the same material as above disclosed intermediate layer  15  of the printed board  11  for minimizing problems occurring with the elongation between the component  1  and the printed board  11 . 
     The method is started by etching out the conductor  5  using a mask and the attachment points  7 , as disclosed above with reference to FIG. 1, of the conductive layer  5   b  of the board  1   b.  Thereby, the component  1  is obtained as is illustrated in FIG.  1 . 
     Also other methods can be used for etching out the conductor  5  and the attachment points  7 , such as etching with photo resist or pattern plating. 
     The conductor  5  extends according to the present examples as disclosed above, between the first edge k 1  and the second edge k 2  of the component  1  and the attachment points  7  are arranged on each side of the conductor  5 . 
     The method for production of the mounting surface for the component  1  on the predetermined local area  10  is disclosed below with reference to FIG.  7  and FIG.  2 . 
     FIG. 7 illustrates a cross-section of the printed board  11  as disclosed above with the intermediate layer  15  fixed to the support  13 , wherein the intermediate layer  15  is plated with an electrically conductive pattern layer  17 . The pattern layer  17  can for instance consist of copper. 
     The method is started for instance using a mask to etch out the pattern conductors  17   a,    17   b  and the attachment surfaces  19 , of the pattern layer  17 , as disclosed above with reference to FIG.  2 . The pattern conductors  17   a,    17   b  are etched out to be adjacent to the local area  10 . See also FIG.  2 . 
     The method of mounting the component  1  over the local area  10  on the printed board  11  is described below with reference to above-described FIG. 4 and 5. 
     The method is initiated by application of soldering paste on the mechanical attachment points  7 , the first attachment surface  7   a  and on the second attachment surface  7   b  of the component  1 . Subsequently, the component  1  is arranged with the conductor  5  facing the printed board  11  over the local area  10  of the printed board  11 . 
     By heating, the attachment points  7  of the component  1  are soldered firmly to the attachment surfaces  19  on the printed board  11 . Also, the first attachment surface  7   a  and the second attachment surface  7   b,  as disclosed above, are soldered, fly to the overlapping end of corresponding pattern conductor  17   a,    17   b  on the printed board  11 , as is illustrated in FIG.  4 . 
     The by the soldering produced soldering joints  21  between the first attachment area  7   a  and corresponding pattern conductor  17  and the between the second attachment surface  7   b  and corresponding pattern conductor  17   b  connect similarly as disclosed above the conductor  5  of the component  1  to corresponding pattern conductor  17   a,    17   b  of the printed board  11 . 
     Thereby, the air gap L is formed between the surface of the conductor, facing the printed board  11 , and the local area  10  of the printed board  11 , as disclosed with reference to FIG.  4 . 
     The groove  23 , described above with reference to FIG. 5, can also, according to the method of the invention, be milled out of the local area  10  of the printed board  11  through the intermediate layer  15  to obtain a higher height h of the air gap L. In this case, the groove  23  is milled out before the component  1  is mounted on the printed board  11 .