Multilayer through hole connections

A method of providing a through-hole connection between conductive layers of a multilayer circuit board assembly whose thickness is liable to expansion and contraction is described. The method includes inserting into through-holes in adjacent boards a tubular flexible conductive member and expanding that member in a position intermediate its ends to engage the through-holes. The boards are then urged together to further expand the tubular member so as to retain that member in the through-holes. Typically the conductive member comprises a tubular braid.

This invention relates to an interconnection technique for through holes in 
multilayer boards, particularly though not exclusively to such boards used 
in a receiver antenna. 
Multilayer boards with plated-through holes are known. The through hole is 
conventionally plated with copper by an electroless plating technique well 
known in the industry, to form an interconnection between the various 
conductive layers in the assembled boards. 
We have found however that if the boards are liable to expand or contract 
in thickness due to for example a large temperature coefficient of linear 
expansion in that direction then discontinuities can occur in the plated 
holes. 
One known solution to the problem is the `C-link`. This comprises a 
C-shaped metal link which is connected to the top and bottom layers and 
which flexes with thickness change of the boards, but cannot be connected 
to an intermediate layer without a platform within the hole to solder an 
intermediate portion of the link and so its use is restricted either to 
those application where an intermediate connection is not required or to a 
more elaborate platformed arrangement for intermediate connection. Also 
good accessibility is required and this is not always available and for 
some applications the impedance is too high and too variable. 
Another solution is the so-called "fuzzball". This comprises a ball of fine 
wire compressed into the through hole. For connection to the outer layer 
end caps are required covering the ends of the hole. This is a 
complication, and the resistance is lower than the C-link but a lower 
resistance still would be preferable for some applications. 
GB patent 1086093 discloses a technique for connecting between terminal 
circuit elements using a flexible electrical conductor on a resilient 
central member. This overcomes the problem of expansion and contraction in 
the thickness of the printed circuit boards, but does not enable 
connection to an intermediate circuit element. 
It is an object of the present invention to provide a simple solution to 
the problem of intermediate connection which is easy to manufacture, cheap 
and effective. 
According to the present invention there is provided a through-hole 
connection in a multilayer board assembly whose thickness is liable to 
change significantly in use thereof, said connection comprising a hollow 
tubular flexible electrically conductive member connected to three 
conductive layers of said assembly, the conductive member having an 
expanded region at a location intermediate its ends and connected to an 
intermediate one of said layers at its expanded region. 
According to another aspect of the present invention there is provided a 
method of making a through hole connection between three conductive layers 
of a multilayer board whose thickness is liable to expansion and 
contraction, comprising assembling at least two boards together with 
through holes or channels aligned, inserting a tubular flexible conductive 
member which fits in the holes, expanding the member at a location 
intermediate its ends and connecting the expanded region to an 
intermediate one of said layers.

The dual band antenna illustrated comprises two crossed slot antenna 
superimposed on a common ground plane 10. FIG. 1 is somewhat schematic and 
ground plane 10 is a cladding under board 12 (FIG. 2). The first crossed 
slot antenna layer is formed of a set of four patches 11a-11d having 
effective lengths .lambda..sub.o.sup.(1) /4 arranged in rotation so that 
their radiating edges form the crossed slot structure. The patches 11a-11d 
are formed by etching copper cladding carried on one face of a sheet 12 of 
dielectric material the other face of which carries the common ground 
plane as a copper cladding 10. Superimposed on the first antenna layer is 
a second crossed slot antenna layer formed of a second dielectric sheet 13 
carrying a second set of four patches 14a-14d, aligned with the etched 
first set of patches. The patches 14a-14d are also formed by etched copper 
cladding on the upper face and each have an effective length of 
.lambda..sub.o.sup.(2) /4. where .lambda..sub.o.sup.(1) is greater than 
.lambda..sub.o.sup.(2). Each superimposed pair of patches e.g. 11a, 14a 
are shorted to the ground plane 10 by a common set of shorting links e.g. 
15a. The patches of the first set are fed by respective coaxial feeds 
16a-16d, the outer conductors of which are connected to the ground plane. 
The patches of the second set are fed by respective coaxial feeds 17a-17d, 
the outer conductors of which pass through the ground plane and are 
connected to both the ground plane and the patches of the first set. It is 
to be noted that the patches 11a-11d are larger than patches 14a-14d so 
that the radiating edges of each stacked pair of patches e.g. 11a and 14a 
are arranged so that the top patch 14a does not obstruct the radiation 
from the bottom patch 11a. The antenna structure support sheets 12 and 13 
are assembled onto a conducting base plate 5 as shown in FIG. 3. In 
between the ground plane 10 and the base plate 5 are two dielectric 
substrates 3 and 4 which carry interconnections between a coaxial signal 
output connector 22 and the coaxial feeds 16a-16d and 17a-17d. 
Substrate 3 has copper cladding 3a on its lower side which has been etched 
to provide interconnecting tracks between the male pin 20 of connector 22 
and the longer male centre conductor 38 formed of swollen tubular braid of 
coaxial feed-throughs 16a-16d (see FIG. 5). 
The sheets 12 and 13 are made of a dielectric material having a low 
dielectric constant and the thickness is about 3.2 mm. The material also 
has a high temperature coefficient of expansion in the direction of its 
thickness and conventional through hole connections for the shorting links 
etcetera are not satisfactory as explained in the preamble. 
FIG. 4 shows an embodiment of a through hole connection suitable for the 
antenna described, particularly the shorting links 15a to 15d. 
As boards 12, 13 are assembled they are held spaced apart by a temporary 
spacer 21. 
Through holes 13' and 12' are aligned: 
A mandrel 22 having a tubular flexible conductive link 23 formed of a 
conductive braid supported thereon is inserted through the holes 13', 12' 
from above board 13, as shown in FIG. 4A. 
The mandrel 22 is inserted further as shown in FIG. 4B and enters a stop 
member 24 held by a jig against the underside of board 12. The stop member 
has a hole 24a which is a sliding fit around mandrel 23. Mandrel 23 has a 
shoulder 22a and the length of the flexible tubular link 23 is such that 
when fully inserted a predetermined amount, the shoulder and the stop 
member co-operate to compress link 23 along its longitudinal axis, causing 
an expanded region "B" of the link at its mid-portion intermediate its 
ends where the space "S" between the boards caused by the spacer defines 
where this bellying is allowed to occur. 
The spacer 21 is removed and the boards 12 and 13 compressed together thus 
flattening the bellied portion into a radial flange-like portion and 
trapping it between the boards and connecting the expanded region B to 
intermediate conductive layer 11a-11d, as well as to ground plane 10 and 
batches 14a-14d. 
The mandrel 23 and the stop 24 are removed and the projecting end portions 
23a and 23b are belled over prior to fusing the boards together. 
This provides a shorting link such as 15a in FIGS. 1 to 3. A set of such 
links is required for each pair of superimposed patches such as 15a for 
patches 11a and 14a, 15b for patches 11b, 14b, and so on. 
Alternatively the braids can be preformed. 
Another embodiment of the invention is shown in FIG. 5. Here a tubular 
flexible conductor 30 made also of copper braid has been inserted in a 
through hole 31 in the copper clad board 12 having the cladding 11a 
forming one of the patches and the ground plane 10. It has been belled 
over onto the claddings at 35 and 36 and soldered to the claddings. 
The board 12 is then assembled with boards 13 having the cladding 14a (one 
of the patches) on its upper side only, and substrates 3 and 4, as 
described earlier, with a rigid insulating sleeve 37 inserted in the 
conductor 30. 
A swollen-diameter flexible tubular conductive braid 38 is soldered to 
patch 14a at 38a and to distribution conductor 3a at 38B 
This arrangement forms a coaxial feed between patch 14a and the 
distribution conductor 3a to connect with the connector 22. 
Then the antenna is completed by bonding the four boards 12, 13, 3, 4, to 
the baseplate 5 fitting connector 20, 22, and securing cover 15 with 
rivets 16 to conductive base 5. 
In order to prevent leakage or crosstalk, a screening arrangement is 
provided. FIG. 6 shows boards 3 and 4 in plan view. Several through 
channels have been formed around the various feed-throughs forming 
E-shaped and question-mark shaped channels. In these channels are located 
elongated beryllium copper springs. When, the four boards are bonded to 
the baseplate these springs become compressed transversely of their 
longitudinal axes to form conductive screens around the feed-throughs. 
Also a circular surrounding screen 41 is formed by eight channels 
centering similar springs 40. The springs provide a short circuit path 
between the cladding 10 under board 12 and the conductive base plate 5. 
In both the embodiments described we have found that the flexible tubular 
conductors 31 in FIG. 5 and 23 in FIG. 4 in conjuction with the flexible 
central tubular conductor 38 of the coaxial feed of FIG. 5, accommodates 
expansion and contraction of the boards 12 and 13, and substrate 3, 
without discontinuities forming. Braids forming inner co-axial connections 
16a-16d also withstand expansion and contraction of substrates 12 and 13. 
Furthermore these links are easy to make and show low impedances e.g. 3 
m.OMEGA. which are regularly reproducible. 
In the example illustrated, the difference between frequency f.sub.1 and 
f.sub.2 is 30% approximately, where f.sub.2 is higher than f.sub.1. 
In this embodiment sheets 12 and 13 have the same dielectric constants 
although sheets of different dielectric constant could be used to alter 
the relative patch lengths involved approximately equal to: 
##EQU1## 
The lateral dimensions of the antenna are governed by 
.lambda..sub.o.sup.(1), the larger wavelength, and .epsilon..sub.r, the 
dielectric constant of sheets 12 and 13. 
Whilst the particular embodiment described utilises a cross slot structure 
it will be appreciated that other multiple patch antenna structures can 
also be constructed in a superimposed arrangement utilising the links 
described in FIGS. 4 and 5.