PROTECTION OF WIRE-BOND BALL GRID ARRAY PACKAGED INTEGRATED CIRCUIT CHIPS

Abstract

A chip includes a substrate having a first surface and a second surface opposite the first surface, and an integrated circuit mounted on a landing zone on the first surface of the substrate. The chip also includes contacts provided about the first surface in the peripheral region, and wire-bonds providing electrical connections between the integrated circuit and the contacts. The chip further includes solder ball connections provided in the peripheral region on the second surface, and connections provided in the substrate for connecting the electrical contacts on the first surface with the solder ball connections on the second surface. The substrate includes at least one conductive track routed through the landing zone region of the substrate, and the chip is configured such that an alteration in the at least one conductive track prevents operation of the integrated circuit.

Claims

1. A chip comprising: a substrate having a first surface and a second surface opposite the first surface; an integrated circuit mounted on a landing zone on the first surface of the substrate, the landing zone defining a landing zone region of the substrate being surrounded by a peripheral region of the substrate; a plurality of contacts provided about the first surface in the peripheral region; a plurality of wire-bonds providing electrical connections between the integrated circuit and the plurality of contacts; a plurality of solder ball connections provided in the peripheral region on the second surface; a plurality of connections provided in the substrate for connecting the electrical contacts on the first surface with the solder ball connections on the second surface; wherein the substrate comprises at least one conductive track routed through the landing zone region of the substrate, wherein the chip is configured such that an alteration in the at least one conductive track prevents operation of the integrated circuit.

2. The chip according to claim 1, wherein a ground plane is provided as a layer of conductive material in the region of the substrate and wherein the at least one track follows an isolated path partitioning the ground plane.

3. The chip according to claim 1, wherein a power plane is provided as a layer of conductive material in the landing zone region of the substrate and the at least one track follows an isolated path partitioning the power plane.

4. The chip according to claim 1, wherein the at least one conductive track is electrically connected to at least one solder ball, wherein the at least one solder ball is positioned in the landing zone region of the substrate.

5. The chip according to claim 1, wherein the at least one conductive track provides an electrical connection to the integrated circuit which is required for the normal operation of the integrated circuit.

6. The chip according to claim 1, wherein the at least one conductive track is a security track forming part of a security mechanism, wherein the security mechanism is configured to detect an interference with the integrity of the security track and in response to enable at least one counter measure.

7. The chip according to claim 6, wherein the security mechanism comprises: a signal transmitter on the integrated circuit for providing a signal to a first end of the security track; and a detection circuit connected to a second end of the security track for detecting an alteration to the signal.

8. The chip according to claim 7, wherein the security mechanism further comprises a response circuit responsive to the detection circuit for causing the performance of the at least one counter measure and wherein the at least one countermeasure comprises resetting or disabling all or part of functionalities of the chip.

9. A method for protecting a wire-bond packaged ball grid array chip against alterations, comprising: forming a substrate having a first surface and a second surface opposite the first surface having a landing zone for an integrated circuit, the landing zone defining a landing zone region of the substrate, the forming of the substrate including providing a plurality of contacts about the first surface in a peripheral region, the peripheral region surrounding the landing zone region; mounting the integrated circuit on the landing zone; connecting a plurality of wire-bonds between the integrated circuit and the plurality of contacts; providing a plurality of solder ball connections on a peripheral region of the second surface; wherein the step of forming the substrate include providing a plurality of connections in the substrate for connecting the electrical contacts on the first surface with the solder ball connections on the second surface; wherein the forming of the substrate comprises routing at least one conductive track through the landing zone region of the substrate, wherein the chip is configured such that an alteration in the at least one conductive track prevents operation of the integrated circuit.

10. The method according to claim 9, wherein the method comprises the step of forming a layer of conductive material in the landing zone region of the substrate to act as a ground plane wherein the at least one track follows an isolated path partitioning the ground plane.

11. The method according to claim 10, further comprising providing a layer of conductive material in the landing zone region of the substrate to act as a power plane and the at least one track follows an isolated path partitioning the power plane.

12. The method according to claim 10, further comprising the step of forming a conductive path using the at least one conductive track between one of the contacts of the first surface and at least one solder ball, wherein the at least one solder ball is positioned in the landing zone region of the substrate.

13. The method according to claim 9, wherein the at least one track provides an electrical connection to the integrated circuit which is required for the subsequent correct operation of the integrated circuit.

14. The method according to claim 9, wherein the at least one conductive track is a security track, the method further comprising providing a security mechanism including the security track, wherein the security mechanism is configured to detect an interference with the integrity of the security track and in response to enable at least one counter measure.

15. The method according to claim 14, further comprising providing by the security mechanism a signal to a first end of the security track; and detecting an alteration to the signal at a second end of the security track and in response to detecting an alteration causing the performance of the at least one counter measure and wherein the at least one countermeasure comprises resetting or disabling all or part of the functions of the chip.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The present application will now be described with reference to the accompanying drawings in which:

[0052] FIG. 1 is a cross sectional view of a wire-bond BGA package known in the art;

[0053] FIG. 2 is a bottom view of the package of the type generally shown in FIG. 1;

[0054] FIG. 3 is an illustration of a potential route of attack to the substrate of the package of FIG. 1;

[0055] FIG. 4 is an illustration highlighting a region in which a protective measure may be provided in a wire-bond BGA package to reduce the opportunity for attack by the means shown in FIG. 3;

[0056] FIG. 5 is an exemplary arrangement illustrating how a conductive plane may be partitioned;

[0057] FIG. 6 is a further exemplary arrangement illustrating partitioning of the conductive plane;

[0058] FIG. 7 is a further exemplary arrangement in which a solder ball connection to a critical signal is provided to reduce the opportunity for attack by the means shown in FIG. 3; and

[0059] FIG. 8 is a security mechanism which may be employed with the arrangements of FIGS. 4 to 7.

DETAILED DESCRIPTION OF THE DRAWINGS

[0060] As discussed above a vulnerability to attack has been identified in wire-bond BGA packages and the present application provides an inexpensive method to frustrate such attempts.

[0061] As shown in FIG. 4, the present application provides a wire-bond packaged ball grid array, BGA. chip 40 in which protective measures are employed in the region 42 of the substrate 8 underneath the integrated circuit die. For the avoidance of repetition, like features with the prior art of FIG. 1, employ the same reference numerals.

[0062] It will be appreciated that the protective measures are applied during the creation of the BGA package for the package and more particularly during the formation of the substrate as well as in the design of the chip.

[0063] The protective measures are based upon forcing physical destruction of one or more protective tracks during the opening, by milling or otherwise of the chip's package from the solder ball side to gain access to the die.

[0064] Conventionally opening up of large sections of the landing zone region under the die is possible as there is significant redundancy provided for power and ground connections and thus a large portion of the region under the die can be removed with limited effect on the operation of the integrated circuit thus opening the die to attack.

[0065] More specifically, BGA wire-bonded packages are typically routed to provide the most efficient pin-outs, with power and ground balls arranged about and within the landing zone region of the package (underneath the chip). This also allows heat dissipation into the large GND plane. Typically, these power and ground planes are re-connected at multiple points between PCB layers. It is the large area and the multiple connections that allows large areas to be opened beneath the silicon of the die without any effect on functionality.

[0066] The present application places protective connections (protective tracks) through the vulnerable landing zone region 42. The substrate will then comprise connecting tracks for the normal operation of the chip and connecting a solder ball to the electrical contacts on the first surface as well as protective tracks, not used to convey a signal from or to the chip from the solder ball but to detect an intrusion into the landing zone. For that purpose, at least a part of the protective track is situated within the landing zone.

[0067] Connecting tracks (also known as traces) would be familiar to those in the art. Generally, the length of a track is an order of magnitude, for example at least ten, times their width.

[0068] In this context, the protective tracks suitably start at a point outside the landing zone region. It will be appreciated that protective tracks run parallel to the surfaces of the substrate. This is in contrast to connecting tracks which run substantively perpendicular to the surfaces such as vias which are employed conventionally to connect the ground plane to solder balls below. The protective tracks may be provided in one or more layers of the substrate. In certain configurations, the protective track will end at a point outside the landing zone region, for example where it is a security track and a connection is made to a detector on the integrated circuit. In other configurations, the protective track will terminate at a point within the landing zone region and an external electrical connection will be made to the second via a solder ball connection (as will be described below).

[0069] The physical destruction or interference with of one or more of the protective tracks can either disable connectivity and thus the functionality of the chip or where the track is a security track allow detection and action by a security mechanism of the IC.

[0070] The security mechanism may employ active or passive shield techniques (referenced above and described below) which would be known to those skilled in the art.

[0071] By routing one or critical signals directly underneath the silicon substrate through the power and/or ground planes, this limits the area which can be opened without having to reconnect these signals and the ground and power planes.

[0072] It will be appreciated that the more critical signals that are routed through the landing zone region, the more unfeasible it is to reconnect multiple cut signals during an attack.

[0073] The signals routed through the landing zone region are selected to be suitably critical to the operation of the chip.

[0074] In a first approach, the critical nature may be directly, i.e. that the signal routed along a conductive track in the landing zone region is required for the normal functioning (correct operation) of the integrated circuit. As an example, in may be an input or output signal connection from the integrated circuit. It will be appreciated that a signal connection is distinct from a power connection, i.e. a ground or a supply voltage.

[0075] In a second approach, the critical nature is indirect. In this approach, the conductive track is a security track (or protective track) which forms part of a security mechanism. The security mechanism is not required for normal operation of the integrated circuit and does not generally interfere in the operation of the integrated circuit unless and until it detects an attack on the integrity of the security track.

[0076] It will be appreciated that the two approaches are not mutually exclusive. Both may be employed at the same time using different tracks.

[0077] At the same time, by providing tracks in the same layer as the ground plane or a power plane, the GND and power domains are suitably less interconnected as the contiguous area of the planes may be segmented thus creating more opportunity to isolate critical circuitry/detector circuits. Where the tracks are provided in a different layer to the GND or power domains, the tracks still reduce the contiguous area available for attack by limiting the space available to be opened.

[0078] As shown in FIG. 4, an integrated circuit die 2 is positioned and mounted generally on top of a substrate 8. The outline or perimeter of the die defines an area, referred to as a landing zone, which in tune defines a landing zone region of the substrate 8 which extends between the two surfaces and is defined by the area of the landing zone. Wire-bonds 4 connect the top of the die to contacts 16 on a first surface of the substrate. These contacts 16 are arranged in a peripheral region of the substrate surrounding the landing zone region 22. Solder ball connections 10 are provided on the second side of the substrate.

[0079] Through connections, for example via 15, are provided to electrically connect individual solder ball connections to associated contacts on the top surface.

[0080] To hinder attack through the landing zone region of the die at least one conductive track is routed through the landing zone region 42 of the substrate. The at least one track does not form part of a ground or power plane. The conductive track may be co-planar with one or other of a ground or power plane. The chip is configured such that an alteration in the at least one conductive track prevents operation of the circuit.

[0081] Several potential approaches to routing critical paths through the landing zone region, which may be used in isolation or combined together, will now be described with reference to FIGS. 5 to 7.

[0082] In FIG. 5, a ground plane 52 which is provided in the landing zone region of the substrate is shown. A plurality of thermal vias 14 are provided which provide both thermal and electrical connections to the ground plane from solder balls (not shown) on the underside of the substrate. For convenience, regions of the plane extending beyond the landing zone region defined by the position of the die are not shown. In practise, connections will extend from the landing zone region to contact pads onto which wire bonds will make connections to the die. Generally, there will be multiple different ground and power connections arranged to the die.

[0083] Thus, in a conventional approach, ground connections may be made to conductive tracks along the exterior sides of the landing zone region with power connections in the regions adjacent to and exterior the corners of the landing zone region.

[0084] A conductive track 56 which is insulated from but co-planar with the ground plane follows an isolated path through the ground plane. This isolated path separates the ground plane into two separate contiguous areas 54a and 54b. The conductive track is connected through a contact pad and wire-bond on one side. In a first approach, the conductive track is employed as part of an active or passive shield and is connected by a second contact pad and wire-bond to the integrated circuit and forms part of a detection circuit which is configured to monitor for any alteration in the protective track.

[0085] In a second approach, the conductive track provides a connection to a required signal (a critical connection) on the integrated circuit and the second end of the conductive track is connected by means of a via to a solder ball connection at the bottom of the substrate. Thus a critical signal connection is routed through the landing zone region whose interruption prevents the correct operation of the integrated circuit.

[0086] At the same time, it will be appreciated that the separation of the ground plane into two separate ground plane areas 54a and 54b again limits the scope of removing material to access the substrate of the die.

[0087] It will be appreciated that the number of conductive tracks may be increased to increase the complexity and the necessity to bypass and re-establish connections. Thus, in FIG. 6, the ground plane 62 is partitioned into four separate contiguous areas 64a, 64b, 64c, 64d by two separate conductive tracks 66, 67 each of which may be one or other of a critical signal for the operation of the circuit or connected to a detection circuit. It will be appreciated that the approach is not restricted to two conductive tracks.

[0088] Although, FIGS. 5 and 6 are referenced in the context of a ground plane, it will be appreciated that it equally may be applied to a power plane. In the case of a power plane, it is likely that the thermal vias would be isolated from the plane as thermal vias are conventionally ground connections.

[0089] A further approach to protecting the vulnerable region is shown in FIG. 7, in which a wire-bond connection 74 is made to a conductive pad 70 on the top surface of the substrate. As is conventional, the conductive pad 70 is located in the peripheral region surrounding the landing zone region of the die. The conductive pad is for a signal connection rather than a ground or power connection to the integrated circuit.

[0090] The conductive pad is connected by means of a conductive track 72 into the landing zone region of the die where a via 78 provides a connection to the underside of the substrate and where in turn an electrical connection is made to a solder ball 80. The solder ball 80 is provided in the landing zone region of the substrate directly under the die. It will be appreciated that the conductive track may be positioned in a layer coplanar with a ground plane, power plane or be in an intermediate layer. Equally, the conductive path between the contact pad and the solder ball may include additional vias or tracks in different layers of the substrate.

[0091] By positioning one or more solder ball connections centrally under the die for critical signals other than power or ground connections, the vulnerability of the chip to attack through the identified vulnerable region is significantly reduced. It will be appreciated that this approach runs entirely counter intuitive to the design of wire-bond BGA packages where the approach is to have all signal connections arranged about the peripheral region.

[0092] It will be appreciated that the technique of FIG. 7 may readily be combined with that of FIGS. 5 and 6.

[0093] The nature of where the conductive track is a security track will now be described in more detail. In this approach, the critical nature of the conductive track is indirect.

[0094] The interference with a security track in itself does not disrupt the normal operation of the integrated circuit.

[0095] Instead, as shown in FIG. 7, the security track 86 forms part of a security mechanism 82 which reacts to a disruption of the security track. The security mechanism comprises the security track and connections between the security track and the integrated circuit 2 and circuitry 87, 88, 89 within the integrated circuit which monitors for an attack on the integrity of the security track. The security mechanism 82 does not generally interfere in the operation of the integrated circuit 2 unless and until it detects an attack on the integrity of the security track 86.

[0096] In the FIG. 8, the security mechanism 82 employs a transmitter 87 to direct a signal along the security track 86. This signal may be a voltage, a current or s dynamic signal such as a clock or digital stream of bits. Equally, it may be a DC or an AC voltage or current. A detection circuit 88 monitors for any change to the signal at the opposite end of the security track to the transmitter. The detection of a change to the signal by the detection circuit would result in counter measures by a response circuit 89 preventing the normal operation of the chip. The countermeasures may comprise resetting or disabling all or part of the functionalities of the chip. These counter measures may include disabling the functioning of the integrated circuit or erasing the contents of memory on the integrated circuit or both.

[0097] A security track may be considered as a conductive track which is electrically connected, for example using a combination of tracks, vias and wire-bonds, to at least two terminals of the integrated circuit. This allows a first connection at one end of the security track to a first terminal and in turn to the transmitter of the security mechanism. The connection to a second terminal is provided at the opposite end of the security track. The second terminal in turn provides a connection to the detector of the security mechanism.

[0098] The security mechanism may be a passive shield or an active shield. In both instances, a signal is provided by a signal transmitter to a first end of the conductive track and the detection circuit is employed to monitor the opposite end of conductive track.

[0099] In the case of a passive shield, the transmitter and detection circuit may be configured to allow a detection in the change of the impedance of the conductive track. This change may be measured, for example, as a capacitance or as a resistance. Passive shields typically employ analog circuitry for the transmitter and detector.

[0100] In active shielding, sequences of bits (typically random) are injected at the start of the conductive track and subsequently tested at the other end by the detection circuit to check whether the sequence of bits arrive unaltered after their journey along the security track.

[0101] It will be appreciated that a variety of different techniques are available which may be selected to advantage to implement the security mechanism as a passive or active shield.

[0102] The words comprises/comprising when used in this specification are to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.