U.S. patent application number 09/176679 was filed with the patent office on 2001-11-15 for coaxial cable, method for manufacturing a coaxial cable, and wireless communication device.
Invention is credited to LIPPONEN, MARKKU.
Application Number | 20010040051 09/176679 |
Document ID | / |
Family ID | 8549774 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010040051 |
Kind Code |
A1 |
LIPPONEN, MARKKU |
November 15, 2001 |
COAXIAL CABLE, METHOD FOR MANUFACTURING A COAXIAL CABLE, AND
WIRELESS COMMUNICATION DEVICE
Abstract
The invention relates to a coaxial conductor comprising an inner
conductor (S), an outer conductor (U) encasing the inner conductor
(S) at least partly, and a dielectric (E) placed between the same.
The coaxial conductor (K) is formed in a multi-layer circuit board
(M) primarily by means of vias (2a-2h) and strip conductors
(1a-1i). According to an embodiment of the coaxial conductor of the
invention, the inner conductor (S) is formed substantially parallel
to the board layers (3a-3e) of the multi-layer circuit board (M),
the inner conductor (S) is formed of at least one strip conductor
(1a, 1b) or at least one electroconductive via (2a) or a
combination of the same, and the outer conductor (U) is formed of
at least four electroconductive vias (2b-2h) and at least two strip
conductors (1c-1i). The dielectric (E) is at least partly formed of
the material of the board layers (3a-3e). The invention relates
also to a method for manufacturing a coaxial conductor. The
invention relates further to a wireless communication device
comprising at least one multi-layer circuit board (M) and at least
one coaxial conductor.
Inventors: |
LIPPONEN, MARKKU; (TAMPERE,
FI) |
Correspondence
Address: |
CLARENCE A GREEN
PERMAN & GREEN
425 POST ROAD
FAIRFIELD
CT
06430
|
Family ID: |
8549774 |
Appl. No.: |
09/176679 |
Filed: |
October 21, 1998 |
Current U.S.
Class: |
174/262 ;
333/246 |
Current CPC
Class: |
H05K 3/429 20130101;
H05K 1/0219 20130101; H05K 2201/09618 20130101; H05K 2201/09809
20130101; Y10T 29/49126 20150115; H01P 3/06 20130101; H05K
2203/0733 20130101; H05K 3/4069 20130101; H01P 3/088 20130101; Y10T
29/49162 20150115; Y10T 29/49123 20150115 |
Class at
Publication: |
174/262 ;
333/246 |
International
Class: |
H05K 001/11; H01B
012/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 22, 1997 |
FI |
974022 |
Claims
1. Coaxial conductor comprising an inner conductor (S), an outer
conductor (U) encasing the inner conductor (S) at least partly, and
a dielectric (E) placed between the same, characterized in that the
coaxial conductor (K) is formed in a multi-layer circuit board (M)
primarily by means of vias (2a-2h) and strip conductors
(1a-1i).
2. Coaxial conductor according to claim 1, characterized in that
the inner conductor (S) is formed substantially parallel to the
board layers (3a-3e) of the multi-layer circuit board (M).
3. Coaxial conductor according to claim 1 or 2, characterized in
that the inner conductor (S) is formed of at least one strip
conductor (1a, 1b) or at least one electroconductive via (2a) or a
combination of the same, and that the outer conductor (U) is formed
of at least four electroconductive vias (2b-2h) and at least two
strip conductors (1c-1i).
4. Coaxial conductor according to any of the claims 1 to 3,
characterized in that the dielectric (E) is at least partly formed
of the material of the board layers (3a-3e).
5. Method for manufacturing a coaxial conductor, wherein the
coaxial conductor (K) comprises an inner conductor (S), an outer
conductor (U) encasing the inner conductor (S) at least partly, and
a dielectric (E) placed between the same, characterized in that the
coaxial conductor (K) is formed in a multi-layer circuit board (M)
primarily by means of vias (2a-2h) and strip conductors
(1a-1i).
6. Method according to claim 5, characterized in that the inner
conductor (S) is formed substantially parallel to the board layers
(3a-3e) of the multi-layer circuit board (M).
7. Method according to claim 5 or 6, characterized in that the
inner conductor (S) is formed of at least one strip conductor (1a,
1b) or at least one electroconductive via (2a) or a combination of
the same, and that the outer conductor (U) is formed of at least
four electroconductive vias (2b-2h) and at least two strip
conductors (1c-1i).
8. Method according to any of the claims 5 to 7, characterized in
that the dielectric (E) is at least partly formed of the material
of the board layers (3a-3e).
9. Wireless communication device comprising at least one
multi-layer circuit board (M) and at least one coaxial conductor
(K), wherein the coaxial conductor (K) comprises an inner conductor
(S), an outer conductor (U) encasing the inner conductor (S) at
least partly, and a dielectric (E) placed between the same,
characterized in that the coaxial conductor (K) is formed in a
multi-layer circuit board (M) primarily by means of vias (2a-2h)
and strip conductors (1a-1i).
10. Wireless communication device according to claim 9,
characterized in that the inner conductor (S) is formed of at least
one strip conductor (1a, 1b) or at least one electroconductive via
(2a) or a combination of the same, and that the outer conductor (U)
is formed of at least four electroconductive vias (2b-2h) and at
least two strip conductors (1c-1i).
11. Multi-layer circuit board which comprises at least a first
board layer comprising a lower surface and an upper surface, and at
least a second board layer comprising a lower surface and an upper
surface, and said multi-layer circuit board embedding a coaxial
conductor comprising an inner conductor, an outer conductor
enclosing the inner conductor at least partly, as well as a
dielectric placed between said inner conductor and outer conductor,
said multi-layer circuit board being manufactured by a method which
comprises the following steps: embedding a first via in the first
board layer and a second via at a distance from the same, to form
part of said outer conductor, wherein said first board layer forms
part of said dielectric between said first and second vias,
embedding a third via in the second board layer and a fourth via at
a distance from the same, to form part of said outer conductor,
wherein said second board layer forms part of said dielectric
between said third and fourth vias, embedding a first strip
conductor on part of the lower surface of said first board layer,
between said first and second vias, or on part of the upper surface
of said second board layer, between said third and fourth vias, to
form said inner conductor, embedding a second strip conductor on
part of the upper surface of said first board layer, to form part
of said outer conductor, wherein an electric coupling is formed by
means of said second strip conductor between said first and second
vias, embedding a third strip conductor on part of the lower
surface of said second board layer, to form part of said outer
conductor, wherein an electric coupling is formed by means of said
third strip conductor between said third and fourth vias,
connecting said first and second board layers to each other in a
way that said first via and third via have an electric contact to
each other, that said second and fourth via have an electric
contact to each other, and that said first strip conductor is left
between said first and second board layers.
12. Multi-layer circuit board according to claim 11, wherein part
of the upper surface or lower surface of any of said board layers
embeds a strip conductor which has an electric contact to a via
embedding in said board layer.
13. Multi-layer circuit board according to claim 11, wherein at
least one via embedded in any of said board layers is formed
elongated by engraving, cutting, incising, or alternatively by
boring several consecutive borings in said board layer.
14. Multi-layer circuit board which comprises at least a first
board layer comprising a lower surface and an upper surface, and at
least a second board layer comprising a lower surface and an upper
surface, and at least a third board layer comprising a lower
surface and an upper surface, and said multi-layer circuit board
embedding a coaxial conductor comprising an inner conductor, an
outer conductor enclosing the inner conductor at least partly, as
well as a dielectric placed between said inner conductor and outer
conductor, said multi-layer circuit board being manufactured by a
method which comprises the following steps: embedding a first via
in the first board layer and a second via at a distance from the
same, to form part of said outer conductor, wherein said first
board layer forms part of said dielectric between said first and
second vias, embedding a third via in the second board layer and a
fourth via at a distance from the same, to form part of said outer
conductor, as well as embedding a fifth via between said third and
fourth vias, to form said inner conductor, wherein said second
board layer forms part of said dielectric between said third and
fourth vias as well as between said fifth and fourth vias,
embedding a sixth via in the third board layer and a seventh via at
a distance from the same, to form part of said outer conductor,
wherein said third layer forms part of said dielectric between said
sixth and seventh vias, embedding a first strip conductor on part
of the upper surface of said first board layer, to form said outer
conductor, wherein an electric coupling is formed by means of said
first strip conductor between said first and second vias, embedding
a second strip conductor on part of the lower surface of said third
board layer, to form part of said outer conductor, wherein an
electric coupling is formed by means of said second strip conductor
between said sixth and seventh vias, connecting said first, second
and third board layers to each other in a way that said first,
third and sixth vias have an electric contact to each other, that
said second, fourth and seventh via have an electric contact to
each other, and that said fifth strip conductor is left between
said first and third board layers.
Description
[0001] The present invention relates to a multi-layer circuit board
which comprises at least a first board layer comprising a lower
surface and an upper surface, and at least a second board layer
comprising a lower surface and an upper surface, and said
multi-layer circuit board embedding a coaxial conductor comprising
an inner conductor, an outer conductor enclosing the inner
conductor at least partly, as well as a dielectric placed between
said inner conductor and outer conductor.
[0002] The selection of various transmission lines for different
uses for transferring electric signals from one place to another
depends on the properties of the transmission lines, such as
attenuation caused by the conductors, frequency band of the signal
to be transferred, power capacity and size of the conductors.
Compared with other known transmission lines, coaxial conductors
have the advantage that they are suitable for broadband
radio-frequency signals, even microwave-frequency signals. Other
transmission lines include a dielectrically isolated wire, a twin
wire, and a conductor comprising several twin wires. A special
advantage of a coaxial conductor is excellent interference
suppression properties due to its structure. Thus, electromagnetic
fields outside the coaxial conductor disturb the signals to be
transferred in the coaxial conductor less than in other
transmission lines, in which such fields induce interfering
signals.
[0003] According to prior art, wireless communication devices, such
as mobile phones, use a coaxial conductor formed of a coaxial
conductor for the transmission of a radio-frequency signal from
components placed on a circuit board, such as integrated circuits
(IC), to an antenna. A coaxial cable can be used also for the
transmission of sensitive signals, such as audio signals, on a
circuit board from one place to another. A requirement for
elimination of disturbances in data transmission by wireless
communication devices is also the interference shielding capacity
of transmission lines and, particularly in the case of
microwave-frequency signals, also the shielding of other
components, such as those placed on a circuit board, from the
electromagnetic field of the signal.
[0004] The signal to be transferred in the transmission line can
also itself induce a disturbing electromagnetic field. By using a
coaxial conductor and a coaxial conductor in the transmission of
the signal, it is also possible to shield other components and
conductors from disturbances caused by the signal.
[0005] In a coaxial conductor, the electric circuit for
transferring a signal from one place to another consists of an
inner conductor and an outer conductor enclosing the inner
conductor, wherein these are placed coaxially. The inner conductor
is usually a wire with a circular cross-section. The outer
conductor is usually cylindrical, and there is a dielectric between
the inner conductor and the outer conductor. The outer conductor is
usually coupled to the ground potential, and the function of the
outer conductor is to provide the coaxial conductor with sufficient
electromagnetic shielding, wherein the best result is achieved with
a continuous and rigid tubular structure. The inner conductor and
the outer conductor are made of a conductive material, usually
copper. The dielectric filling the open space between the
conductors functions as a mechanical support of the conductors, and
it also contributes to the electric properties of the coaxial
conductor, such as attenuation of the signal to be transferred. In
the manufacture of coaxial cables, the outer conductor is further
coated with a jacket which is usually made of a polymeric material
and serves the purposes of functioning as a dielectric and
protecting the coaxial conductor from mechanical wearing and
environmental conditions. In coaxial cables, also twisted band
layers or stranded wires are used as the outer conductor, wherein
the cable can be easily bent.
[0006] Thanks to its excellent properties, the coaxial cable is
widely used, but several manual work stages must be conducted when
connecting the coaxial cable to a circuit board. The work stages
may include soldering of the ends of the coaxial cable onto the
surface or connectors of the circuit board, which will require more
and more precision and time, particularly in view of the circuit
boards and the components to be placed on the circuit board
becoming smaller. Smaller circuit boards will also require precise
placement of the coaxial cable, wherein the cable must also be
bent, if necessary. However, a minimum to the radius at bend of the
coaxial cable will be set by possible damage of the outer
conductor, wherein the coaxial cable will emit at the damaged
portion and thus cause disturbances. Small bending radii will
damage also the inner conductor and the dielectric, changing the
electric properties of the coaxial cable. Due to its size and large
bending radii, usually about 5 to 8 times the outer diameter of the
coaxial cable, coaxial cables require a large space on the circuit
board.
[0007] In the manufacture of mobile phones, the different
components and coaxial cables are fixed on the circuit board by
means of a soldering paste, and soldering is carried out first by
heating the circuit board in an oven, e.g. at 270 degrees, wherein
the soldering paste melts. After this, the circuit board is cooled
down, wherein the final solid soldering joints are made. A
considerable disadvantage, however, is the fact that it is also
possible that in the oven, the coaxial cable is wholly or partly
released from the soldering paste, due to the different curling
directions of the coaxial cable and the circuit board when their
material is heated. Defective products increase the manufacturing
costs or malfunction of the products during their use.
[0008] A known method for manufacturing a coaxial conductor on a
circuit board is disclosed in the patent publication SE 462 194.
The principle of the invention presented in the publication is that
a long groove is cut through at least two board layers on a circuit
board, particularly a multi-layer circuit board, the groove
extending from a strip conductor functioning as a first ground
potential to a strip conductor functioning as a second ground
potential. In the next step, the groove is filled to establish a
contact between the strip conductors and to build an outer
conductor. After this, the circuit board is compressed in a press
to make the material used in the filling to spread in the
groove.
[0009] According to the publication SE 462 194, the cutting and
compression of the circuit board are conducted in separate
operations and phases, which, however, increases considerably the
time consumed in the manufacture and thus also the costs of the
circuit board. A further problem is that the precise control of the
cutting depth is very difficult, because the thickness of the strip
conductors can be as small as 17 micrometers. Furthermore, the
cutting is complicated by the fact that the circuit board must be
positioned very carefully and without clearances to avoid lateral
displacement. Moreover, the thickness of different board layers can
vary in different circuit boards due to manufacturing techniques,
so that it is very difficult to control the cutting depth. In
addition to this, one should note that when the knife mentioned in
the publication is used for cutting, the circuit board is subjected
to considerable forces and its damage is very probable with normal
circuit board materials.
[0010] Patent publication U.S. Pat. No. 4,673,904 discloses a
method for manufacturing a coaxial conductor by superimposing on a
circuit board. According to the publication, the outer conductor
and the inner conductor, as well as the dielectric therebetween,
are formed by superimposing on a board consisting of a copper layer
and a dielectric. Because of its expensiveness, the method
presented is only suitable for special uses, because it is very
difficult to spread, smooth and control the thickness of the
dielectric material placed between the conductors and to be formed
outside the coaxial conductor, which increases considerably the
work stages needed in the process and the materials to be handled.
The method is not suitable for use on circuit boards which comprise
also other wiring, because thus said wirings and particularly the
dielectric layer of the circuit board must be manufactured in the
same way as the coaxial conductor itself. The result is a
considerable extension in the duration of the manufacture of the
circuit board. Furthermore, the way of forming the dielectric layer
differs to a great extent from the conventional technique of
manufacturing circuit boards.
[0011] Furthermore, it should be noted that the size of the coaxial
conductor of the publication U.S. Pat. No. 4,673,904 is limited
significantly by the thickness of the conductor to be superimposed
and also the size of other structures of the circuit board, because
the number of layers, and simultaneously also the manufacturing
time, increases with the increasing size of the conductor. For this
reason, the circuit board should be made thin, whereby its strength
is not sufficient e.g. for supporting components and preventing
buckling. Typically, circuit board layers have a thickness of
100-150 .mu.m. Moreover, it is obvious that with an increasing
number of layers and a decreasing size of the conductor, the
positioning of the circuit board must be conducted particularly
accurately and carefully, which increases further the manufacturing
costs and time.
[0012] It is a purpose of this invention to eliminate the
above-mentioned drawbacks and particularly to present a structure
for a coaxial conductor and a method for manufacturing a coaxial
conductor particularly on a multi-layer circuit board. The
invention is based on the idea that the coaxial conductor is formed
in connection with the manufacture of other wirings and vias on the
circuit board, preferably simultaneously. Furthermore, the
invention is based on the idea that the coaxial conductor can be
formed by applying simple vias and strip conductors which are known
as such. Moreover, the invention is based on the idea that the
structural elements required for manufacturing the coaxial
conductor are formed in different board layers of the circuit
board, and that the board layers are subsequently combined to form
the coaxial conductor and, at the same time, the entire circuit
board.
[0013] The most significant advantage of the invention is that the
coaxial conductor can be formed in the board layers of the circuit
board simultaneously with other wirings. Finally, the coaxial
conductor is composed when the different board layers of the
multi-layer circuit board are connected e.g. by pressing to form
the entire circuit board. Thus, the manufacture of the conductor
requires no separate work stages, equipment or tools. For
manufacturing the conductor, use is made of vias and strip
conductors embedded in different layers, these being typical
structures to be placed on a circuit board; consequently, the
technique is inexpensive and fast. In the simplest way, the vias of
the coaxial conductor can be formed by borings which are either
filled in or coated. Another advantage of the invention is that by
the technique of the invention, it is possible to construct large
conductors, having a thick dielectric layer, in a fast and simple
way.
[0014] A remarkable advantage of the coaxial conductor of the
invention is that it can be integrated on a multi-layer circuit
board, wherein the coaxial conductor can be manufactured in
connection with the manufacture of other conductor patterns of the
circuit board. Thus, it is possible to avoid a separate
installation of the coaxial conductor on the circuit board. Because
the coaxial conductor can be integrated in the multi-layer circuit
board, space is simultaneously released for components to be
installed on the surface of the circuit board. By means of the
invention, it is possible to eliminate problems arising e.g. in the
manufacture of mobile phones, and the reliability of the products
is improved. By means of the invention, the coupling of the coaxial
conductor and the conductor patterns placed between the board
layers of the multi-layer circuit board to each other will be very
simple, wherein the number of solderings required can be reduced. A
remarkable advantage is also the fact that changes in the direction
of the coaxial conductor of the invention can be made without
limits set by bending radii. Further, branching of the coaxial
conductor can be implemented in a very simple manner, avoiding the
need for separate connectors and manual work stages. As a result,
the manufacturing costs of a device comprising a coaxial conductor
according to the invention are reduced in comparison with those of
prior art.
[0015] In the following, the invention will be described in more
detail with reference to the appended drawings, in which
[0016] FIG. 1a shows a coaxial conductor of prior art, partly cut
open and seen from the side,
[0017] FIG. 1b shows a coaxial conductor of prior art seen from the
end,
[0018] FIG. 2 shows a wireless communication device of prior art
seen from above and a detail of the placement of a coaxial
conductor of prior art in connection with a circuit board, in an
axonometric view,
[0019] FIG. 3 shows an advantageous embodiment of the coaxial
conductor of the invention in an axonometric view,
[0020] FIGS. 4 to 7 show other embodiments of the coaxial conductor
of the invention in axonometric views, and
[0021] FIG. 8 shows an advantageous embodiment of the method of the
invention in a schematic view.
[0022] With reference to FIGS. 1a and 1b, a coaxial conductor KK of
prior art comprises a coaxial conductor K consisting of an inner
conductor S which is a wire with a usually circular cross-section,
and an outer conductor U. The outer conductor U, which is usually
cylindrical and has a circular cross-section, is placed to enclose
the inner conductor S substantially coaxially with the same.
Between the inner conductor S and the outer conductor U, there is a
dielectric E. With reference to FIG. 1a, the coaxial conductor KK
is shown with the nested layers exposed for clarity. The inner
conductor S and the outer conductor U are made of a conductive
material, usually copper. The dielectric E filling the space
between the inner conductor S and the outer conductor U acts as a
mechanical support for the coaxial conductor K and affects also the
electric properties of the coaxial conductor K. A very common
material for the dielectric E is poly(tetrafluoroethylene), i.e.,
PTFE plastic which can be used within a wide range of temperature.
When manufacturing coaxial conductors KK, the outer conductor U is
still coated with a jacket V made of a plastic material, usually
poly(vinyl chloride), i.e., PVC plastic, or PE plastic, and having
the purpose of protecting the coaxial conductor K from mechanical
wearing and environmental conditions. In coaxial conductors KK,
also twisted or stranded layers are used as the outer conductor U,
made of copper wire or copper strip. Other possible materials for
the outer conductor U are copper-coated aluminium and tin-coated
copper wire.
[0023] FIG. 2 shows a wireless communication device according to
prior art, a mobile phone MS, a multi-layer circuit board M being
placed (arrow N1) in the mobile phone MS. The multi-layer circuit
board M is shown in an axonometric view, and its measurements are
exaggerated for clarity. By means of strip conductors, this
multi-layer circuit board M is provided with conductor patterns
R1-R6 for transferring signals from one place to another. These
conductor patterns R1-R6 can be connected with each other by means
of vias and conductor patterns placed between the board layers of
the multi-layer circuit board M. It is obvious that the shape of
the multi-layer circuit board M of the mobile station MS can
deviate from that shown in FIG. 2, and there are usually
components, such as integrated circuits (IC), placed on the
multi-layer circuit board M for implementing the functions of the
mobile phone. The couplings between the components are implemented
by means of the conductor patterns. As shown in FIG. 2, there is
also a coaxial conductor KK placed on the multi-layer circuit board
M, for the purpose of transferring the radio-frequency signal
between the conductor patterns R1 and R2.
[0024] According to prior art, printed circuit boards (PCB) are
used as setting-up pieces, particularly in electronics. Printed
conductor patterns on PCB boards are used at least partly to
replace separate wires for coupling components electrically. The
material of the conductor patterns is usually copper film. A very
common material for manufacturing PCB boards is glass fibre
reinforced epoxide resin, and in demanding microwave-frequency
applications, also glass fibre reinforced PTFE plastic is used. One
alternative material for PCB boards is aramid fibre reinforced
epoxide resin. According to prior art, printed boards are either
single-sided, wherein the conductor patterns are placed on only one
side of the PCB board, or double-sided, wherein the conductor
patterns are placed on both sides of the PCB board. There are PCB
boards available, with at least one side fully coated with copper
e.g. by vaporizing, wherein the conductor patterns can be made by
etching. For accomplishing an electric contact between conductor
patterns placed on different sides, they can be connected by means
of through holes made at the conductor patterns, wherein the walls
of the through holes are coated with copper. Coating is usually
carried out by electrolysis. The connection can also be made by
means of filled-in vias, wherein the hole bored in the circuit
board is filled in with a conductive copper paste. The copper paste
contains e.g. copper powder, epoxy resin and a hardener. The hole
is filled in by pressing by using the thick film technique known as
such. The via hole can be made also by using a laser beam.
[0025] Multi-layer circuit boards may comprise two or several board
layers combined e.g. in a clamp by means of heat. Between the board
layers, conductor patterns are placed, whose electrical contact
with the components fixed on the PCB board is implemented by vias
penetrating through one or several board layers. A via penetrating
through several board layers can be made in a way that the via is
made separately in each board layer and the board layers are
subsequently combined, or in a way that the via is made first after
combining the required board layers. In a multi-layer circuit board
comprising e.g. two board layers, the layers of the conductor
patterns can be placed on the bottom surface and the upper surface
of the circuit board and between the two board layers.
[0026] With reference to the FIGS. 3. to 7, a coaxial conductor K
of the invention comprises an inner conductor S, an outer conductor
U enclosing the inner conductor S, and a dielectric E placed
between these. For example in FIG. 3, the coaxial conductor K
according to an advantageous embodiment of the invention is formed
in the multi-layer circuit board M by means of elongated,
conductive vias 2a-2g and strip-like conductors 1a-1h. The coaxial
conductor K is made primarily by means of the vias and the
strip-like conductors, but the coaxial conductor K comprises
further e.g. a dielectric E consisting of the material of the
multi-layer circuit board M. A via refers in this description to a
via arranged through one board layer of the multi-layer circuit
board, wherein for example in FIG. 3, the outer conductor U
comprises six vias 2b-2g and the multi-layer circuit board M
comprises three board layers 3a-3c. The via can be cylindrical, as
shown in FIG. 7, or elongated, as shown in FIG. 3. Strip-like
conductors or strip conductors refer in this description also to a
conductor placed between the vias, such as the four strip
conductors 1e-1h of the outer conductor U in FIG. 3. For clarity,
FIGS. 3 to 7 show only that part of the multi-layer circuit board M
which is essential for the invention, but it is obvious that the
coaxial conductor K can be made also for a larger PCB board, most
advantageously in connection with manufacturing the other conductor
patterns. Thus, the length of the coaxial conductor K can be
substantially greater than its width, and it is obvious that the
head and the tail of the coaxial conductor K can be placed in the
inner part of the circuit board instead of the edge plane, as shown
e.g. in FIG. 3. Similarly, it is obvious that the shape of the vias
varies according to the way of manufacturing, wherein they can be
e.g. conical. The vias can be hole-like or elongated filled-in or
coated through holes. When making coated through holes, a strip
conductor, e.g. strip conductors 1a and 1b in FIG. 3, can be made
simultaneously when coating the through hole by methods known as
such. Thus, it is possible that e.g. a strip conductor 1e placed
between the vias 2b and 2d consists of two layers, the first of
which being fixed on the lower surface of the board layer 3a and
the second on the upper surface of the board layer 3c. The
elongated vias can be replaced by providing the board layer with
holes close to each other in a row, whereby these holes are further
filled in or coated and their electrical contact is implemented by
means of strip conductors. In this case, however, holes are formed
in the outer conductor, through which particularly
microwave-frequency inteference signals can propagate. With
reference to FIGS. 3 to 7, it is also obvious that the width of
each strip conductor can vary in the directions of their length and
width, but they are shown having substantially equal widths for
simplicity.
[0027] However, the size of the openings formed in the outer
conductor can be optimized for the signal propagating in the
conductor, particularly when the frequency of the signal is
constant or varies within a range. Thus, with the frequency used,
the size of the opening is dimensioned to differ from the size
which is most advantageous in view of propagation of the
signal.
[0028] With reference to FIG. 3, according to an advantageous
embodiment of the coaxial conductor of the invention, the coaxial
conductor K comprises strip conductors 1a-1h, wherein the strip
conductors 1e and 1g are placed between the vias 2b, 2d and 2f, and
wherein the strip conductors 1f and 1h are placed between the vias
2c, 2e and 2g. These strip conductors are used to secure the
formation and maintenance of electric contacts of the vias placed
in the different board layers 3a-3c during the manufacture of the
multi-layer circuit board M and the conductor patterns. A known
problem in the manufacture of vias is the fact that the holes are
filled in only partly or that air-filled cavities come up in
connection with filling in the holes. The inner conductor S is
placed between the board layers 3b and 3c of the multi-layer
circuit board M, wherein the inner conductor S is substantially
parallel to the other conductor patterns of the multi-layer circuit
board M. The inner conductor S consists primarily of an elongated
via 2a. The inner conductor S comprises also strip conductors 1a
and 1b, but it is obvious that the via 2a can solely form the inner
conductor S. In another embodiment of the coaxial conductor K, with
reference to FIGS. 4 and 5, the inner conductor S consists of one
strip conductor 1a and the outer conductor U consists of four
conductive vias 2d-2g and four strip conductors 1c-1f. Also in this
embodiment, the inner conductor S Is substantially parallel to the
planar board layers 3b and 3c. With reference to FIG. 3, in the
cross-sectional plane perpendicular to the longitudinal direction
of the inner conductor S, the vias 2b-2g and the strip conductors
1c-1h constitute an advantageously closed circular outer conductor
U enclosing the inner conductor S. The dielectric E consists in
this case of that part of the material of the multi-layer circuit
board M which is left between the outer conductor U and the inner
conductor S. According to this embodiment, the dielectric E
consists of three board layers 3a-3c. In case of several board
layers, it is possible that the dielectric E consists partly of
air-filled cavities formed by holes made in the board layers.
[0029] FIGS. 4 to 7 show other alternative embodiments of the
coaxial conductor according to the invention. By means of the
embodiment shown in FIGS. 3 to 7, it is possible to replace the
coaxial conductor KK placed on the multi-layer circuit board M of
the mobile phone MS of prior art, shown in FIG. 2, or to replace
any interfering or interference sensitive normal routing of the
circuit board. In comparison with the embodiment shown in FIG. 3,
the board layer 3a is missing from the multi-layer circuit board M
in FIG. 4. It should be noted that e.g. vias 2f and 2d on top of
each other can be formed also simultaneously first after combining
the board layers 3b and 3c, wherein it will not be necessary to
form the strip conductor le. This applies also to the strip
conductors 1e-1h of FIG. 3. FIG. 5 shows an embodiment in which, in
comparison with the embodiment of FIG. 4, the board layers 3d and
3e are added, wherein the coaxial conductor K is left fully inside
the multi-layer circuit board M. It is also possible that one of
these board layers 3d and 3e is missing, wherein the broad strip
conductor 1c or 1d is visible on the surface of the multi-layer
circuit board M. According to the same principle, with reference to
FIG. 3, it is possible to add a new board layer on the upper and
lower surfaces of the multi-layer circuit board M. When the
multi-layer circuit board M comprises several board layers 3b-3e as
in FIG. 5, it is possible that the inner conductors S of different
coaxial conductors K placed between the different board layers
3b-3e must be coupled to each other for transferring the signal.
Thus, the vias can be used for coupling both the inner conductors S
and the outer conductors U. Coupling the inner conductor S to the
conductor pattern made on the multi-layer circuit board M is
carried out easily with the strip conductor 1a connected with the
inner conductor S by extending it over the end plane of the coaxial
conductor K and by using vias, if necessary.
[0030] With reference to FIG. 6, the portion of the outer conductor
U placed in the board layer 3d is formed by several vias placed
next to each other, their electrical contact being secured with
wide strip conductors 1d and 1i. Further, the portion of the outer
conductor U placed in the board layer 3e is formed by means of one
wide elongated via 2h. As shown in FIG. 7, by arranging the vias of
the outer conductor U placed in different board layers in a
circular form and forming their electrical contact with strip
conductors, a coaxial conductor K with its inner conductor S is
accomplished, extending substantially perpendicular to the board
layers. Thus, it is obvious that the outer conductor U can be
formed of curved elongated vias or single circular vias.
[0031] An advantageous embodiment of the method of the invention
for manufacturing a coaxial conductor is illustrated in FIG. 8. It
shows the manufacture of a coaxial conductor K of FIG. 3 in a
schematic view. For clarity, in steps A to I of FIG. 8, the
multi-layer circuit board M and the different board layers 3a-3c
are shown only in those parts that are essential for the invention.
Similarly, it is obvious that the relative dimensions of the strip
conductors and the vias can deviate from those shown. According to
step A of FIG. 8, the manufacture of the multi-layer circuit board
M is started with the board layers 3a-3c. In the next step B, the
board layers 3a-3c are provided with elongated grooves 4a-4g, for
example by engraving, milling, or cutting with a laser beam, for
making the vias 2a-2g. When adjacent grooves, such as the elongated
grooves 4a and 4b in FIG. 8, are very long, elongated tongues are
formed in the board layer which may be supported by means of bridge
structures, if necessary. These bridge structures are formed e.g.
in a way that a single long groove is replaced by two shorter
successive grooves. However, the result will be that an opening is
formed in the outer conductor U of the coaxial conductor K. In the
next step C, the elongated grooves 4a-4g are filled in with a
conductive copper paste, to make conductive vias 2a-2g. In the next
step D, copper films 5a and 5b are placed on both sides of the
board layer 3a and fixed to the board layer 3a either by vaporizing
or e.g. by pressing. At the same time, an electric contact is
formed between the copper films 5a and 5b fixed on the surfaces of
the board layer 3a as well as the conductive vias 2a-2c made in the
elongated grooves 4a-4g, as shown in step E.
[0032] The conductor patterns, such as strip-like conductors, are
made in the copper films 5a and 5b by methods known as such. In the
following, one such method will be described, wherein e.g. the
copper film 5a is coated with a substance sensitive to ultraviolet
(UV) light, i.e. a photoresist. After this, the photoresist layer
is exposed to UV light through a mask layer. On the exposed areas
of the photoresist, a photochemical change takes place, leaving a
copy of the conductor pattern of the mask layer in the photoresist.
The exposed photoresist layer is developed chemically, wherein the
photoresist will be removed from areas other than the exposed
areas. Next, the board layer 3a is immersed in a corrosive bath, so
that the copper film 5a of the board layer 3a is dissolved in the
bath from other areas than those covered by the photoresist layer.
Finally, the photoresist is removed from the surface of the board
layer 3a e.g. by a solvent. The result is a board layer 3a
according to step F, provided with the strip conductors 1a, 1b and
the via 2a for forming the inner conductor S, as well as the strip
conductors 1e-1h and the vias 2b, 2c for forming the outer
conductor U. The strip conductors 1a, 1b and 1e-1h are necessary
particularly when conductor patterns are formed on the upper and
lower surfaces of the board layer 3a. At the same time, these strip
conductors formed at this step protect the vias 2a-2c during the
etching. In the next step G, the board layer 3a of step F, the
board layers 3b and 3c of step C (arrows Ga and Gb), and copper
films 5c and 5d are combined, wherein combining in a clamp will
result in a multi-layer circuit board M formed of board layers
3a-3c according to step H. The copper films 5c and 5d of the
multi-layer circuit board M can be treated with a photoresist,
exposed and etched in the way presented above, whereby it is
possible to form also other conductor patterns on the surface of
the multi-layer circuit board M in connection with the strip
conductors 1c and 1d of step 1. Conductor patterns can be formed
between the board layers 3a-3c at step E. In connection with
manufacturing the coaxial conductor K, also the required conductor
patterns are formed e.g. for coupling the inner conductor S to
other conductor patterns and thus for transferring the signal.
[0033] An advantageous embodiment of the method of the invention as
described above can also be applied in forming other coaxial
conductors K shown in FIGS. 4 to 7, within the scope of the claims.
The required steps will be, however, obvious for a man skilled in
the art on the basis of the above description, wherein its detailed
description will be unnecessary in this context. It is also obvious
that the number of board layers is not limited to the five board
layers 3a-3e shown in FIG. 6. Similarly, it is possible that the
inner conductor S comprises vias extending to at least two board
layers also when the inner conductor S is parallel to the board
layers. Further, in case of several board layers, the cross-section
of the outer conductor U of the coaxial conductor K can be formed
more circular than that shown in FIGS. 3 to 6, wherein the
conductive vias placed on top of each other in different board
layers are arranged in stepwise positions in relation to each
other.
[0034] It will be appreciated by a man skilled in the art that the
invention is not limited solely to the embodiments presented above
but it can be modified within the scope of the claims.
* * * * *