EP1883087A1

EP1883087A1 - Secure switchboard

Info

Publication number: EP1883087A1
Application number: EP06118033A
Authority: EP
Inventors: Wim Bervoets; Guy De Cremer; Michel Leonard
Original assignee: Intellect International NV-SA
Current assignee: Intellect International NV-SA
Priority date: 2006-07-28
Filing date: 2006-07-28
Publication date: 2008-01-30
Also published as: AU2007203384A1

Abstract

The present invention is related to a switchboard comprising a PCB (10), a membrane (20) and at least one switch contact (11, 21) of the force-sensitive resistor type interposed between the PCB and the membrane and characterised in that on top of said at least one switch contact at least one mesh (31) of electrically conductive tracks is provided for protection against tampering. The conductive mesh is printed on the membrane, and is interposed between membrane and switch contact. The membrane is glued onto the PCB.

Description

Field of the Invention

The present invention is related to a switchboard in which the switch contacts are secured against unwanted probing or monitoring. The present invention is equally related to an apparatus incorporating such switchboard. Additionally, the present invention is related to a method of securing a switch on a switchboard.

State of the Art

Point of sale (POS) or point of interchange (POI) terminals require a high level of PIN protection. Classical POS keyboards based on ON/OFF switches with e.g. matrix scanning are quite easy to monitor even with fake key-scanning being active. In most cases it is always difficult and costly to add an extra protection layer to protect the electrical contact or "pole" of the keyboard switches from unwanted probing or monitoring. The same rule applies to security switches avoiding mechanical opening of a POS or POI terminals. For a robust (tentative) fraud detection, the appropriate security countermeasures should be taken. Hence, the present invention concerns the issue of providing highly secure keyboards, preventing probing of the key switches in order to remotely gain user PIN data inserted on the keyboard.
The state of the art does not solve the problem of ensuring, each time an extra protection layer is added (e.g. to protect the switches), that this protection layer is easily bypassed and then removed.
Document US 2003/0025617 discloses a foil keyboard built up out of two layers that are glued one on top of the other. The upper layer is a membrane comprising key contact pads. The key pads on both layers are surrounded by dashed conductive lines, such that when the two layers are joined, one shorted conductive track is formed around the keys. Any attempt to lift up the upper layer will result in the conductive track opening. The key contacts themselves are however still reachable through the membrane and are thus not secured.
Document WO 2005/086546 discloses a card reader, such as for a POS terminal, having a layered printed circuit board (PCB) with layers containing conductive serpentine tracks for tamper detection. The key contacts are provided on top of the PCB and are aligned with protrusions on a keyboard membrane. There are two sets of contact pairs: one for the keys (default open), a second set (default short) for tamper detection. An attempt to open the cover (and the keypad) is detected by an interruption in current flowing through the second set of contact pairs. Hence the terminal disclosed in the abovementioned document provides tamper detection for the cover, but not for the keys. In case that the tamper evidence switches can be bypassed, the key contacts can be easily probed.
Documents EP 1432031 and US 6438825 disclose PCBs with conductive serpentine tracks, either embedded into the PCB or wrapped around. Any attempt to reach to the enclosed components necessarily damages one of the conductive tracks.

Aims of the Invention

The present invention aims at providing a secure switchboard in which the switch contacts are protected against probing and which overcomes the drawbacks of the prior art.

Summary of the Invention

The present invention is related to secure switchboards, as set out in the appended claims, in which a high level of protection against unwanted probing and monitoring of the switches is obtained by providing a mesh comprising conductive tracks on top of the switch contacts. The present invention is equally related to devices, such as point of sales terminals, incorporating the switchboards of the present invention. The present invention is also related to a method of securing a switchboard.
According to a main aspect of the invention, there is provided a switchboard comprising a carrier (e.g. a PCB), said switchboard further comprising a membrane on top of said carrier and at least one switch contact interposed between the carrier and the membrane. On top of the switch contact at least one layer comprising a mesh of electrically conductive tracks is provided for protection against tampering. Said mesh of conductive tracks covers the switch contact.
Preferably, the layer comprising the mesh of conductive tracks is interposed between the membrane and the switch contact. An insulation layer is interposed between said switch contact and said mesh. The conductive tracks of the mesh are preferably connected to circuitry arranged to detect a change in electrical resistance of said tracks. Preferably, more than one mesh of conductive tracks is interposed between the switch contact and the membrane. More preferably, an insulation layer is interposed between two meshes.
The switch contact is preferably a normal-closed contact. The switchboard preferably comprises at least one switch contact which is a normal-open contact. More preferably, the switchboard comprises both normal-open switch contacts and normal-closed switch contacts. The membrane, the carrier and the switch contacts may form part of a keyboard.
Preferably, the switch contact is of the force-sensitive resistor (FSR) type comprising a pad of ink whose electrical resistance is pressure-sensitive. The ink pad makes contact with a second pad provided on the carrier thereby forming a switch contact pair. The second pad is an electrically conductive pad and preferably comprises at least two electrically conductive tracks. In a preferred embodiment, the switch contact is connected to circuitry arranged for detecting a change in pressure force acting on the force-sensitive resistor pad.
According to a preferred embodiment, the switch contact comprises two contact pairs connected in parallel. A light emitting diode (LED) is preferably interposed in between said two contact pairs. More preferably, the LED is connected to the carrier, and the membrane covers the LED. Even more preferably, the membrane comprises for each LED a bump accepting the LED.
According to another aspect of the invention, there is provided an apparatus, such as a point of sale terminal, comprising a switchboard of the invention.
The invention equally provides a method of securing a switch contact provided on top of a carrier, comprising the steps of: providing conductive pads on a top surface of the carrier; providing a membrane; printing at least one electrically conductive mesh at one side of the membrane; printing at said one side of the membrane an insulating layer on top of the mesh; printing at said one side of the membrane, on top of the insulating layer, a force sensitive resistor pad; and applying the membrane onto the top surface of the carrier with said one side facing the carrier, whereby said resistor pad on the membrane corresponds to a conductive pad on the carrier, and whereby the mesh is coupled to a conductive pad providing connection to a tampering detection circuit. The method preferably further comprises the steps of: providing at said one side of the membrane and/or onto the top surface of the carrier glue in between said pads. More preferably, the method further comprises the step of embossing the membrane so as to form a bump in the membrane.

Brief Description of the Drawings

Figure 1 represents the top surface of a PCB, showing conductive pads.
Figure 2 represents a membrane with pressure-sensitive pads printed on it.
Figure 3 represents a membrane printed with a mesh of conductive tracks.
Figure 4 represents an expanded cross section of the switchboard, with the different layers visualised.

Detailed Description of the Invention

In the following preferred embodiment, the secure switchboard of the invention is applied to a keyboard, such as for a point-of-sale terminal. The keyboard is of the membrane type. This keyboard comprises switch contacts, as shown in figures 1 and 2, based on force sensitive resistor (FSR) technology. A switch contact comprises at least one contact pair. Each contact pair consists of two pads, as is explained in the following. Figure 1 depicts the top surface of a printed circuit board (PCB) 10 with contact pads 11 and 12. Figure 2 depicts a foil (membrane) 20 with pads 21 and 22 printed on it. Pads 21 and 22 consist of an ink printed on the membrane 20, whose electrical resistance changes with varying pressure (force) applied on the pads. The position of ink pads 21, 22 on the membrane 20 corresponds to the position of pads 11, 12 on the PCB. The membrane 20 is applied onto the PCB 10 with the ink pads 21, 22 faced towards the PCB. Hence, ink pads 21, 22 make contact with contact pads 11, 12 respectively. Contact pairs 11, 21 and 12, 22 may constitute a switch contact. Pads 11 and 12 may be connected to a resistor sensing circuit (not shown).
Pads 11 and 21 constitute together a switch contact pair. As can be seen in figure 1, pad 11 comprises two separate tracks 111 and 112 that are bridged by ink pad 21. Upon pressing pad 21, the ink is pressed and its electrical resistance changes, which can be measured by a circuitry (not represented on the figures) connected to tracks 111 and 112. The same principle applies to the switch contact pair 12 and 22. The FSR technology is not new and is disclosed e.g. in Belgian patent document BE 906081 the contents of which are incorporated herein by reference.
A FSR-type switch contact does not move like a classical switch and hence does not suffer from the drawback of having a blind travel such as is the case for e.g. a classical detection switch. In contrast, a FSR-type switch gives an indication of the pressure exerted on the switch, giving the possibility to be very sensitive in the detection of attempts to tamper with e.g. the cover of a keyboard.
Switch contacts constituted by pads 11, 21 and 12, 22 may be employed both for normal-open and normal-short switches, the configuration depending on the circuitry detecting the resistor change. In the present embodiment, let the switch contact pair 11 and 21 be configured to be a normal-open switch. Such a switch may be employed for a key contact of the keyboard. The resistance change of pad 21 is sensed whenever a key is pressed.
The same type of contact pair can be configured to obtain a normal-closed switch. In the present embodiment, let contact pair 12 and 22 be configured to be a normal-closed switch. The latter type of switch can be provided on the keyboard to detect mechanical tampering (e.g. opening attempt) of the terminal frame in which the keyboard is mounted. The terminal frame in this case is equipped with a stud exerting a pressure (force) on the ink pad 22. Any attempt of tampering with the frame (e.g. trying to open it) will cause a pressure variation, modifying the electrical resistance of ink pad 22, which is detected by the circuitry connected to pad 12.
In the configuration as depicted in figures 1 and 2, the switch contacts can be easily probed, either through the membrane 20, or by lifting the membrane from the PCB 10. The present invention adds a number of security measures to prevent the switch contacts from being probed.
The invention provides the following method to secure the switch contacts. Before printing the ink pads 21 and 22 onto membrane 20, a mesh comprising electrically conductive tracks running in serpentine lines, loops, etc. is printed at the same side of the membrane. A possible mesh 31 is shown in figure 3. Figure 4 depicts an expanded cross-sectional view of the assembly. After printing mesh 31 on membrane 20, an insulating layer 41 is printed on top of mesh 31, to insulate mesh 31 from pads 21 and 22. On top of this insulating layer, pads 21 and 22 are printed. As is clear from figure 3, the mesh 31 runs underneath the ink pads 21 and 22.
The membrane 20 may be applied onto the PCB without adhering means, or may be glued. Glue is applied around the pads on the membrane (layer 42) and/or the top surface of the PCB (glue applied on the PCB is represented by numeral 44 in figure 4). Thereafter, the membrane is applied on the PCB, with the printed side facing the PCB. The glue may be a single type of glue. Alternatively, multiple types of glue (e.g. a strong type and weak type) may be employed simultaneously. Use of two different glues allows to develop a membrane where any trial to lift up the membrane from the PCB will ensure partial damage of the mesh allowing tampering detection.
More than one mesh 31 can be applied on the membrane in order to increase the efficiency of protection. An insulating layer 41 can be interposed in between two meshes 31, as represented in figure 4. The membrane, printed with one or more meshes and insulating layers and with the ink pads on top is subsequently assembled onto the PCB, e.g. by glueing.
The method described above results in switch contact pairs 11, 21 and 12, 22, covered with one or more meshes of electrically conductive tracks and one or more insulating layers. The meshes are interposed between the switch contact pairs and the membrane. The electrically conductive tracks of each of the meshes are connected to a detection circuit through contact pads 15 on PCB 10. Each mesh may have its own contact pads and detection circuit. Hence, as the tracks of the mesh have to be in electrical connection with contact pads 15, no insulating material is printed on the membrane where the mesh is to make contact with pads 15. Additionally, the printed layers applied on the membrane, between the mesh and the PCB may comprise corresponding pads 23 (see figure 2) for ensuring electrical connection between the tracks of the mesh and contact pads 15. This is shown in figure 2, where the final printed layer with the ink pads 21 and 22 of the switch contacts comprises also pads 23 ensuring electrical contact between mesh 31 and contact pads 15.
The ink used for printing the mesh and the resistive pads is preferably a resistive-type ink. Such a type of ink may be easily removed. Partial damage to the mesh, e.g. when attempting to tamper with, bridge or open the contact, is immediately detected by sensing a change in one of the mesh's tracks' electrical characteristics. Shorts between consecutive meshes can also be detected.
The mesh 31 provides tamper detection when the membrane is drilled through and when peeling off of the membrane from the PCB is attempted. Attempts to peel off the membrane will result in the glue destroying part of the mesh (e.g. the ink comes off from the membrane by the glue), allowing detection.
In a further preferred embodiment, the keys (the normal-open switches) are equipped with a LED 43 for background lighting the key (see figure 4). The LED 43 is provided on the PCB 10, and connected to pins 14. As the LED is placed centrally with reference to the key, the key contact in this case can not be placed centrally anymore, but is shifted. Different configurations are possible. Firstly, the ink and contact pads can be shaped e.g. circularly around the LED. Alternatively, the switch contact may be comprised of two or more contact pairs placed around the LED. The latter configuration is depicted in figures 1 and 2, wherein two identical contact pads 11 and 13 are shown. The pads 11 and 13 are connected in parallel (not shown) to form one key switch. Figure 2 shows the corresponding ink pads 21.
As the light emitted by the LED has to be visible by a user of the keyboard, the membrane 20 is not printed on the spots corresponding to the LEDs. This can be seen in figures 3 and 4, where the mesh 31 and insulating layer 41 are made to circumvent the LED at spot 32, while still protecting the switch pads 21 and 22. In the embodiment shown in the drawings, the LED 43 is mounted on top of the PCB 10 and protrudes out of the PCB surface. In order for the membrane 20 to be applied firmly onto the PCB (by glue 42 and 44), the membrane comprises bumps 45 for housing the LEDs. The bumps are embossed in the membrane 20 after the printing process. The membrane encloses the LEDs, which provides an additional security protection. The LEDs are thus not provided on top of the membrane, but interposed between the PCB 10 and the membrane 20.
In a further preferred embodiment of the invention, the contact pads 15 for electrical connection of mesh 31, and the contact pad 12 of the frame tamper detection switch may be merged.
The different layers printed on the membrane (e.g. ink pads and mesh) can be made invisible by use of transparent ink and can be visually protected by an extra colour background (additional printed layer). The membrane is generally a polyethylene terephtalate (PET) foil. The foil thickness is about 100 µm.
The hereabove described preferred embodiments provide a keyboard in which both tamper detection switch contacts 12, 22 and key switch contacts 11, 21 are secured by means of mesh 31. The present invention is equally applicable to e.g. a tamper detection switch contact pair that is secured by means of mesh 31.

Claims

A switchboard comprising a carrier (10), such as a printed circuit board, said switchboard further comprising a membrane (20) on top of said carrier and at least one switch contact (11, 21, 12, 22) interposed between the carrier (10) and the membrane (20), characterised in that on top of said at least one switch contact at least one mesh (31) of electrically conductive tracks is provided for protection against tampering.
The switchboard according to claim 1, wherein said at least one mesh (31) of conductive tracks is interposed between the membrane (20) and the switch contact (11, 21) and wherein an insulation layer (41) is interposed between said switch contact and said mesh.
The switchboard according to claim 1 or 2, wherein the conductive tracks of the mesh are connected to circuitry arranged to detect a change in electrical resistance of said tracks.
The switchboard according to any one of the preceding claims, wherein at least one switch contact is a normal-closed contact (12, 22).
The switchboard according to any one of the preceding claims, wherein at least one switch contact is a normal-open contact (11, 21).
The switchboard according to any one of the preceding claims, wherein said switch contact comprises a force-sensitive resistor pad (22) and an electrically conductive pad (12), said resistor pad and said conductive pad forming a contact pair.
The switchboard according to claim 6, wherein said switch contact is connected to circuitry arranged for detecting a change in pressure force acting on the force-sensitive resistor pad (22).
The switchboard according to claim 6 or 7, wherein said switch contact comprises at least two contact pairs (11, 13) connected in parallel.
The switchboard according to claim 8, wherein a LED (43) is interposed in between two contact pairs.
The switchboard according to claim 9, wherein the LED is connected to the carrier, and the membrane (20) covers the LED.
The switchboard according to claim 9 or 10, wherein the membrane (20) comprises a bump (45) housing the LED.
An apparatus, such as a point of sale terminal, comprising a switchboard according to any one of the preceding claims.
A method of securing a switch contact provided on top of a carrier (10), characterised in that the method comprises the steps of:
- providing conductive pads (11, 12, 15) on a top surface of the carrier;

- providing a membrane (20);

- printing at least one electrically conductive mesh (31) at one side of the membrane;

- printing at said one side of the membrane an insulating layer (41) on top of said mesh;

- printing at said one side, on top of the insulating layer, a force-sensitive resistor pad (21, 22); and

- applying the membrane onto the top surface of the carrier with said one side facing the carrier, whereby said resistor pad (21, 22) on the membrane corresponds to a conductive pad (11, 12) on the carrier, and whereby the mesh is coupled to a conductive pad (15) providing connection to a tampering detection circuit.
The method according to claim 13, further comprising the step of providing at said one side of the membrane (20) and/or on the top surface of the carrier (10) glue (42, 44) in between said pads.
The method according to claim 13 or 14, further comprising the step of embossing the membrane so as to form a bump (45) in the membrane.