Machine readable memory card with capacitive interconnect

An interconnection system for transferring either electrical energy in the form of power and data signals or both is disclosed. Capacitive coupling devices are shown connected to a memory device such as an erasable programmable read only memory chip, in a form suitable for use in a smart data entry card. The capacitive coupling devices employ a dielectric medium having a relatively high dielectric constant due to the orientation of the crystals of the dielectric medium. Barium Titinate having a dielectric constant of 18,000 in the direction of the crystal axis is used. Only small bearing pressures sufficient to wipe contaminants from the exposed contact pads are required.

FIELD OF THE INVENTION 
The present invention relates to an electrical connector and method for 
interconnecting circuit paths to transmit digital and other signals 
through a low pressure bearing engagement between two circuits, such as a 
circuit in memory card and another circuit in a reader, in which signal 
information is transmitted by a fundamental change in capacitive coupling 
technique. 
BACKGROUND OF THE INVENTION 
Interconnections for electric assemblies typically include low reactance 
characteristics based on metal-to-metal contact; permanent 
interconnections being on the order of micro-ohms and disconnects being on 
the order of milliohms. Desirably, such interconnections have 
characteristics of stability in the presence of heat and time, of 
reproduceability from one interconnection to the next within a given 
process. The so-called permanent connections are achieved through 
processes such as soldering, welding, brazing, crimping, and wire bonding. 
Interconnections achieved through disconnect structures utilize relatively 
high pressure, spring brased interfaces frequently involving surface 
finishes employing noble metal plating thereon. 
Interconnections of the disconnect type typically require normal forces on 
the order of 80 to 150 grams and frequently employ wiping action to 
eliminate surface oxides and/or dielectric debris. Forces of this 
magnitude, if applied in the form of wiping action, degrade the contact 
interface and limit the life or number of interconnects that can be made. 
The alternative is to utilize relatively thick coatings of precious metal, 
which practice is costly. 
Interconnection of intelligence channels for intelligence transfer through 
digital or other signals can be accomplished magnetically as through the 
scanning of a magnetic medium such as a tape by magnetic head pickup. 
Optical techniques are also available wherein intelligence is transmitted 
as between phototransmitters and photoreceptors. Finally, as part of the 
background, a variety of techniques are employed utilizing radio frequency 
transmission, the so-called RF techniques widely employed through 
frequency modulation, audio amplitude modulation, and pulse code width 
modulation. 
All of the foregoing have their advantages and applications, but all have 
certain shortcomings. Interconnections which require a physical 
metal-to-metal engagement under relatively high spring pressures and/or 
utilizing noble metals have quite limited lives in terms of the number of 
cycles of engagement possible and as well are vulnerable to the effects of 
moisture, industrial gases, and/or corrosive fluids and gases. Magnetic 
and optical techniques likewise are subject to environmental constraints. 
The optical techniques require a medium through which light can pass or at 
least a medium transparent to particular frequencies. The magnetic 
techniques require close proximity and are not readily sealable. RF 
devices are notably subject to interference such as static and 
field-caused noise. 
SUMMARY OF THE INVENTION 
The present invention utilizes a memory card containing capacitive input 
devices employing a material having a relatively large dielectric 
constant. In the preferred illustrative embodiment, the devices are formed 
of laminated structures having metal electrodes on each side of a barium 
titanate material, the electrodes being interconnected to signal channels 
capable of developing analog or digital signals in the range from 0 to 6 
volts, both unipolar or bipolar, if desired. The relatively high 
dielectric constant of the barium titanate dielectric medium permits a 
reliable low force interconnection to be made because only a single point 
contact with a card reader is necessary for each output pad.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, the element numbered 10 represents a plastic card 
of the type frequently referred to as a credit card or a data card or a 
"smart card," the latter term being employed particularly with respect to 
cards which contain integrated circuits therewithin. The card may contain 
on its face symbols, including names and numbers and sources of the card, 
or in certain instances, may be preferably be blank. FIG. 1 shows a series 
of interconnection pads or zones 20, six of which are in card 10 for 
illustrative purposes. Cards of this type are fairly well standardized to 
a dimension on the order of 3.365 inches by 2.120 inches and on the order 
of 0.030 inches in thickness with some substantial variation in thickness 
dependent upon the use and type of embossment and/or inclusion of 
integrated circuits within the usual plastic laminations of the card. Such 
cards may have additional intelligence in the form of magnetic stripes, 
bar codes for optical reading, and as mentioned, embossments to 
effectively render imprinting upon sales slips, records, and the like. In 
general, the amount of intelligence as measured in bits effected through 
embossments, magnetic stripes, and optical markings is quite limited. 
This had led to the development of the so-called "smart card" which 
contains integrated circuits including various logic circuits and memory 
devices, including ROM, EROM, and EPROM, the latter standing for Erasable 
Programmable Read Only Memory. Through the use of such circuits, the 
various memories may be accessed, read into, read out of, erased, changed, 
and otherwise manipulated, depending upon the circuits chosen. In this 
way, individual files, including medical histories, personal data, 
historical data, financial records, and considerable information may be 
stored to become portable and to travel with an individual for a wide 
variety of uses. In accordance with prior art techniques, a number of ways 
have been employed to effect interconnections to the logic and memory 
integrated circuits within the card, including gold plated contact fingers 
disposed on the edge of a card which are read by card reader fingers 
contained in a card reader and to which the card is inserted. The use of 
precious metals to enhance the life of the fingers on a card is almost 
necessitated by the presence of oxide-causing moisture and various 
chemicals resulting from the handling of the card. Wear and tear upon the 
card fingers can cause false readings as well as the presence of 
dielectric debris. 
FIG. 2 shows the logic and memory integrated circuits as element 14. Power 
for such circuits is provided through an inductive coupling with a coil 16 
proximate the upper surface of the card just beneath an outer membrane 18 
which covers the power coupling 16 and the interconnection paths leading 
to contact pads 20. The power windings of 16 may typically be formed of 
etched copper in a suitable pattern to be inductively coupled by coil 
mounted in a card reader head generating an alternating current field 
which induces a voltage in the coil of 16 which in turn effects current 
flow through leads connected to 
Referring now to FIGS. 2 and 3, the interconnection paths or leads 28, 30 
may be seen to reside at the upper surface of card 10 just beneath the 
membrane 18. The upper contact surfaces or electrodes of contact pads 20, 
however, are exposed on the upper surface of the cards. The contact pads 
20 each comprise upper and lower electrodes 22 and 24 sandwiched and 
bonded to a dielectric medium 26 in lamina fashion The electrodes 22 and 
24 are connected to the integrated circuit 14 through a pair of leads 
shown as 28 and 30 in FIGS. 2 and 3. 
The capacitive contact pads 20 can be formed by using devices purchased 
from Piezo Electric Products, Metuchen, N.J. These commercially available 
devices are comprised of a lamination of thin copper or nickel foils 
having therebetween a dielectric material such as barium titanate 
(Ba.sub.2 TiO.sub.3) of the oriented variety. This barium titanate has a 
dielectric constant on the order of 10,000 in the direction of the crystal 
axis. FIG. 8 shows a barium titanate crystal, the structure of which is 
well known as evidenced by Huheey, J, Inorganic Chemistry, Harper & Row 
1983. The direction of the dipole movement is shown by the arrows in FIG. 
8. It is along this direction or axis that the dielectric constant of 
barium titanate is on the order 10,000. The dielectric constant of barium 
titanate in the direction perpendicular to the crystal axis is less than 
ten times that of air. The capacitors employed in this invention are 
formed with the crystal axis of the barium titanate dielectric medium 
perpendicular to the electrodes 22 and 26 so that the effective dielectric 
constant is on the order of 10,000. 
It would be practical to employ devices 20 energized by voltages ranging 
between 1 and 6 volts in a frequency from roughly 20 hertz to 10 
megahertz. In practice, the devices 20 may be bonded to a carrier shown as 
29 affixed in a premoulded package in the body of the plastic card 25. 
Wiping forces were on the order of 10 grams. 
FIG. 4 shows that contact can be established with the upper electrode 22 by 
the reader contact finger 21. Unlike conventional ohmic contacts, this 
contact need only comprise a single point contact, since the primary 
interconnection mechanism is capacitive. With conventional ohmic contact, 
a number of points or asperities must be in contact so that sufficient 
cross-sectional area is provided to carry a detectable signal current. 
With this invention, only a single point contact is necessary since only 
the voltage on the upper electrode 22 will be sensed by the finger when 
the card is read or altered when information is read into the memory of 
the card. Large signal transport currents are not required. Therefore the 
high forces otherwise necessary for data signal transmission are not 
needed. Significantly these same high forces employed with conventional 
card readers would be felt by both the contact pads 20 on the card and the 
reader finger 21. Some wiping action would be necessary between the finger 
21 and the upper electrode of the contact pads to remove contaminants, but 
sufficient wiping action can be provided with a contact of much less 
force. The relatively small changes in voltage on the electrodes can be 
capacitively sensed because of the relatively high dielectric constant of 
the dielectric medium 26. FIG. 4 shows that a similar output waveform 
would be generated using this input/output mechanism. 
These capacitive input devices need not employ precious metal plating, 
although precious metal platings would increase the useful life of both 
the cards and the contact elements needed for a card reader. Typical smart 
cards, and especially card readers, must experience many connections and 
disconnections over their lifetime. Therefore, the contact force 
reductions which can be achieved using these capacitive input devices, 
with or without precious metal platings, will greatly prolong the useful 
life of such devices. 
The instant invention differs from conventional capacitive coupling in 
which an input capacitance on an electrode on an input device is sensed by 
another electrode in the output device. In the instant invention a point 
contact is made between a first contact member such as a card reader 
finger 21 and the outer electrode 22 of the capacitive input pad 20. By 
virtue of this point contact, there is no potential difference between 
contact finger 21 and electrode 22. Little transport current occurs 
between contact finger 21 and electrode 22. Therefore, only a small 
contact area, essentially point contact need be established. However, due 
to the relatively high dielectric constant of the material between 
electrodes, a small change in voltage on one electrode will produce a 
relatively large detectable displacement current. 
The only force necessary for the inventive interconnection is the force 
necessary to wipe away contaminants. FIG. 6 illustrates the condition if 
point contact is not established for example by the failure to remove 
contaminates or the presence of air between exterior conductive surfaces. 
As shown in FIG. 6, the input voltage on element 121 (corresponding to 
card reader finger 21) is not sensed by a capacitive input pad 120 
(corresponding to page 20) even though a relatively high dielectric 
material 126 is employed. FIG. 6 shows that the first circuit 130 is 
separated from the second circuit 132 by two dielectrics having dielectric 
constants .epsilon..sub.1 and .epsilon..sub.2 where .epsilon..sub.2 
&gt;&gt;.epsilon..sub.1. The electric field strength across each dielectric is 
then given by: 
##EQU1## 
The potential difference .gamma. is given by: 
##EQU2## 
Since .epsilon..sub.2 &gt;&gt;.epsilon..sub.1 
##EQU3## 
In other words, virtually the entire voltage drop would occur across the 
dielectric having the lowest dielectric constant. Therefore, such a change 
in voltage at 130 would not be sensed at 132 if air or some other low 
dielectric constant material such as dirt, grease or some other 
contaminant is present on the surface of the outer electrode 122 as shown 
in FIG. 6. The capacitance would be given by: 
##EQU4## 
In other words, the capacitance is not a function of any parameters 
representative of the input capacitive element 120, especially the large 
dielectric constant of dielectric material 126. 
The low dielectric material could be eliminated by establishing only a 
point contact between element 130 and the outer electrode 122. The 
impedance between 130 and 132 would then be a function of the capacitance 
between electrodes 122 and 124 which is a function of the dielectric 
constant .epsilon..sub.2. 
##EQU5## 
For barium titanate .epsilon..sub.2 .apprxeq.10.sup.4. To calculate the 
impedance for an input device of the type represented by this invention: 
EQU .epsilon..sub.0 =8.854.times.10.sup.-12 
EQU .epsilon..sub.2 .beta.10.sup.4 
For a practical capacitive input device: 
##EQU6## 
A small voltage change on one side of the high dielectric material will 
cause a large current to flow if there is point contact with one 
electrode. 
##EQU7## 
The preferred embodiment of this invention shows a smart card in which 
capacitive input devices 20 are incorporated into the card. It should be 
understood, however, that these capacitive input devices could be 
incorporated into the card reader in the manner shown in FIG. 7. In this 
embodiment only traces 50 comprising a single electrode would be required. 
Card reader finger 21' would comprise an extension of the outer electrode 
22' on the capacitive input element 20' on the card reader 42'.