Conductive lacquer contact surface

In order to produce keyboards, two pairs of diagonally connected squares with rounded corners are used as fixed contacts. A direct diagonal connection electrically connects two non-adjacent squares. The squares are conductive lacquer (carbon lacquer) and cover copper pads on a printed circuit board.

BACKGROUND OF THE INVENTION 
The invention relates to an arrangement of contact surfaces for switches, 
pushbutton switches and keyboards. 
Switches, pushbutton switches or keypads, in particular entry keyboards for 
data processing systems, are often constructed on carrier boards. In this 
case, it has proved to be advantageous to use a printed circuit board as 
the carrier board and to implement fixed key contacts as specially formed 
conductor tracks, so-called contact surfaces. When a key is pressed down, 
a contact bridge, which is often made of conductive rubber, is pressed 
against the contact surfaces by the movable part of the key. 
An arrangement of two adjacent semicircles as in German reference DE A 37 
14 382 is already used for the contact surfaces. In this case, the contact 
surfaces composed of copper are coated on the surface, because otherwise 
any possible corrosion would prevent reliable contact-making. A gold layer 
which is applied by electroplating is used as the surface coating. In 
order to avoid the high production costs of gold plating, use is also made 
of a conductive lacquer which is applied, for example, by screen printing. 
However, the contact-making of this arrangement is not reliable enough for 
keyboards which have stringent requirements placed on the contact 
reliability. Therefore, use is also made of a form as in U.S. Pat. No. 
4,818,828, in the case of which the contact surfaces engage into one 
another in the form of fingers. However, in this form it is possible to 
carry out the surface coating by means of conductive lacquers using screen 
printing only when there are few fingers or large key spacings, since the 
meandering interspace between the fingers would have to be so narrow that 
the tolerances of the printing method for the conductive lacquer would not 
permit reliable production. 
SUMMARY OF THE INVENTION 
The object of the invention is to implement contact surfaces in such a 
manner that conductive lacquers can be used as the surface coating and 
better contact-making is achieved in comparison with the form of 
semicircles. 
The preferred embodiment of the invention uses a contact zone of four 
contact surfaces at the corners of an imaginary square, in each case two 
diagonally opposite surfaces being electrically connected to one another. 
In this way, respectively adjacent contact surfaces are connected to 
different poles of the switch. Therefore, contact is always made when the 
contact bridge touches two adjacent contact surfaces. Therefore, as 
illustrated in FIG. 4, a connection is already established even in the 
event of incomplete key actuation, when only that part of the contact 
bridge which is drawn in bold is effective, whereas this is not the case, 
under the same conditions, in the case of the known semicircle arrangement 
according to FIG. 3. As a result, the contact-making reliability is 
considerably improved because the number of possible contact-making 
surfaces is doubled. In the case of the diagonal form according to the 
invention, the contact surfaces can be dimensioned in such a way that they 
can be produced, as in the case of the known semicircle arrangement, from 
an electrically conductive lacquer, preferably a carbon lacquer, because 
the spacings are large enough to tolerate inaccuracies of a simple 
printing method. They are applied over, and so as to overlap, copper pads, 
from which the connection is carried out by means of a suitable printed 
circuit board track configuration. 
An advantageous development is to connect diagonally to one another a pair 
of contact surfaces within the contact zone. This connection contributes 
to the contact-making; in addition, only one electrical connection routed 
around the contact zone is now necessary. 
The contact surfaces preferably have a square configuration, but they can 
also be implemented as segments of a circle. Likewise, more than four, 
therefore six, for example, contact surfaces are possible. 
Since the contact surfaces can be applied using devices which are present 
in any case for every printed circuit board production process, for 
example for population printing, high quality keyboards can now be 
produced in a shorter time and with a lower outlay.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with FIG. 1, the contact zone of a key is constructed on an 
insulating carrier board 11, preferably a printed circuit board. Copper 
surfaces 12 are initially applied to the latter in accordance with a 
method which is known from printed circuit board production. These copper 
surfaces 12 are circular and have a diameter which is equal to 
approximately half the diameter of a circular contact bridge 22 which is 
arranged above them. The bridge 22 can be lowered in a known manner by key 
actuation. The bridge 22 is also illustrated in the section II--II in FIG. 
2 and the maximum possible effective area 16 of which is illustrated as a 
circle. The center points of the copper surfaces are situated slightly 
outside the circle which can be described by the contact bridge. As shown 
in FIG. 2, each of the contact surfaces has a convex configuration with 
beveled corners. The copper surfaces are electrically connected to an 
evaluation circuit by means of supply lines (not illustrated). Contact 
surfaces 13 are applied to the copper surfaces 12, for example by means of 
screen printing using conductive lacquers, preferably carbon lacquer. 
These contact surfaces 13 overlap the copper surfaces 12 on all sides and 
thus adhere additionally to the carrier board 11. The supply lines are 
protected in a conventional manner by a solder resist which covers the 
printed circuit board outside the contact surfaces and is likewise not 
illustrated in the figures. 
In order to illustrate the difference between the arrangement according to 
the invention and the prior art, the known design with semicircular 
surfaces is illustrated in FIG. 3, the maximum possible effective area 16 
of the contact bridge 22 being indicated as a circle. As an example of 
incomplete actuation, the contact bridge may be effective only partially 
and at the edge, as is illustrated by the part drawn in bold in FIGS. 3 to 
6. This is a frequent occurrence in entry keyboards when the key heads are 
pressed only at the edge. Whereas there is no connection of the contact 
surfaces in this case of the arrangement according to FIG. 3, a connection 
is achieved in FIG. 4 in the case of the diagonal arrangement which is 
illustrated in a simplified manner and has square contact surfaces. 
The same applies to the alternative embodiments with segments of a circle 
according to FIG. 5 and with six contact surfaces in the form of segments 
according to FIG. 6. 
As illustrated in FIGS. 1, 5 and 6, in a development of the invention, 
every second contact surface is connected to one another via the center of 
the contact zone on the surface of the contact zone, and the connection is 
configured like a contact surface. In FIG. 1, the connection is 
constituted by a copper track 14, which is covered by a conductive lacquer 
track 15 in the same way that the copper surfaces 12 are covered by the 
contact surfaces 13. The copper tracks are not illustrated in the 
arrangements according to FIGS. 5 and 6; they are implemented in the same 
way as in FIG. 1. 
After thorough testing, the following dimensions have proved to be 
particularly favorable for the contact zone of a keyboard corresponding to 
the standard for typewriters: the contact bridge 22 is circular with a 
diameter of 3.5 mm. The copper surfaces 12 are circular with a diameter of 
1.575 mm and their center points are situated on a square with an edge 
length of 2.7 mm. The contact surfaces 13 are square with an edge length 
of 2.044 mm and have rounded corners. Two diagonally opposite copper 
surfaces are connected by a copper web having a width of 0.3 mm. The 
contact surfaces are correspondingly connected by a web of carbon lacquer 
having a width of 0.6 mm. The two other contact surfaces are not rounded 
on the side facing this web, but rather are beveled with the result that 
their spacing from the web is 0.512 mm. The spacing between the contact 
surfaces is 0.685 mm along one side. Hence, the edges of the contact 
surfaces have a spacing of at least 0.5 mm and the surfaces themselves are 
at least 0.5 mm wide, with the result that known devices which are present 
for the solder resist mask and the population printing can be used for the 
application of the conductive lacquer. The bare part of the solder resist 
has an edge length of 4.496 mm and rounded corners. 
The invention is not limited to the particular details of the apparatus 
depicted and other modifications and applications are contemplated. 
Certain other changes may be made in the above described apparatus without 
departing from the true spirit and scope of the invention herein involved. 
It is intended, therefore, that the subject matter in the above depiction 
shall be interpreted as illustrative and not in a limiting sense.