Low Current Voltage Supply Watchdog

Abstract

The voltage source watchdog comprises a passive device placed in series between a voltage source and a load. The passive device includes an electromigration (EM) junction of a known material that generates an electromigration void after a specified amount of current passes through the EM junction. After the known amount of current has passed, the void is generated and no voltage is sensed, thus providing a guaranteed safe mode situation. When the voltage source is a battery, selectively enabling the voltage measurement operation of the proposed watchdog can extend battery life.

Description

[Technical Field]

[0001] The present invention relates generally to the field of power management, and more particularly to monitoring the power of a voltage source. [Background technology]

[0002] Encrypted cards require security protection while on battery power, in accordance with PCI (Payment Card Industry) data security standards.

[0003] MEMS (Micro-Electro-Mechanical Systems) and related micromechatronics and microsystems constitute a technology for microdevices, especially those with moving parts. MEMS are miniature machines that contain both mechanical and electronic components. MEMS are embedded in silicon and are capable of performing specific mechanical tasks based on an electrical input. Summary of the Invention

[0004] In one aspect of the invention, an apparatus includes: (i) a component substrate having a wire bond site configured for electrical connection between a voltage source and an electrical device; (ii) a wire bond having a wire including two terminations, the wire having a fusion site including a first length and two end portions, each end portion including one of the two terminations, the fusion site including a smaller diameter than the two end portions, the two terminations attached to the component substrate at the wire bond site; and (iii) a first electromigration (EM) bond of the first termination, the EM bond formed at a ball bond connecting the first termination of the wire to a metal pad at the wire bond site.

[0005] In another aspect of the present invention, a circuit for battery backup of an encryption card includes: (i) a cryptographic circuit board; (ii) a battery; (iii) a processor; and (iv) an electromigration (EM) junction assembly including a first wire bond having a first length of fused wire, the EM junction assembly electrically connected in series with the battery and the processor. [Brief explanation of the drawings]

[0006] [Figure 1] 1 is a schematic diagram of a circuit according to the present invention;

[0007] [Figure 2] FIG. 2 is a perspective view of a wire bond according to the present invention.

[0008] [Figure 3] 1 is a perspective view of an electromigration timer according to the present invention;

[0009] [Figure 4] FIG. 2 is a perspective view of an exemplary structure after a first layer of material has been deposited on a substrate.

[0010] [Figure 5] 1 is a perspective view of an exemplary structure of an electromigration timer in accordance with the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0011] Some embodiments of the present invention are directed to a voltage source watchdog comprising a passive device placed in series between a voltage source and a load, including an electromigration (EM) junction of a known material, that creates an electromigration void after a specified amount of current passes through the EM junction. After the known amount of current has passed, the void is created and no voltage is sensed, thus providing a guaranteed safe mode situation. When the voltage source is a battery, selectively enabling the voltage measurement operation of the proposed watchdog can extend battery life. Applications for a battery watchdog include battery-backed processor memory.

[0012] Low-voltage applications often require extended battery life. For example, encryption cards require security protection while under battery power in accordance with the Payment Card Industry (PCI) data security standard. Batteries used in these applications may prematurely deplete if continuous monitoring is performed to ensure sufficient battery power is available. This specification presents a low-current battery watchdog for periodically ensuring battery power over an extended period of time, which may be, for example, one to five years. Other areas where a low-current voltage source watchdog may be applied include the Internet of Things (IoT), automotive applications, and computing servers. IoT devices may be improved by monitoring the health of low-power circuits using the disclosed watchdog, which uses an EM junction to monitor voltage source power. With regard to automotive applications, any circuit with a fuse may be improved by using the disclosed watchdog, which uses an EM junction to monitor voltage source power. The use of an EM junction eliminates the need for additional circuitry to monitor overcurrent conditions, since the EM junction opens when exposed to an overcurrent condition.

[0013] When a low battery power condition occurs, preventative measures such as shutting down the system or discarding stored data are often taken for security reasons. A low battery condition may also be a false positive event. Managing battery life over a five-year lifespan is achieved by inserting a low-current voltage source watchdog, also referred to herein as a battery watchdog, into existing circuitry that periodically checks battery power and operates to identify a failed battery before the system performs a preventative low-battery action. The voltage source watchdog device utilizes an EM-controlled or EM-regulated sensor.

[0014] By targeting wirebond to pad interface failure, a failure can be designed into the circuit, creating a predictable time period until failure occurs. Establishing a predictable time period allows the voltage source to produce the expected failure at the target time, knowing the voltage source is still functional. Multiple electromigration-based sensors are configured to fail at various times, establishing periodic voltage checks upon failure or undervoltage. Voltage measurement circuitry can continuously or periodically monitor the voltage across the load or DUT.

[0015] Some embodiments of the present invention recognize the following facts, potential problems, and / or improvements with respect to the current state of the art: (i) encryption hardware is subject to a battery function requirement of 3-5 years without service; (ii) if encryption hardware detects low voltage in the battery area, the hardware automatically enters safe mode and discards all stored data; (iii) achieving a 5-year battery life requires careful consideration of battery life conservation; (iv) with sufficient current flow, metals can move via electromigration, causing voids and / or shorts in microelectromechanical systems (MEMS); (v) electromigration is dependent on current flow, material, cross-sectional area, and metallurgy; (vi) accurate electromigration design helps determine the time at which the desired Kirkendall voids occur; (vii) some encryption hardware maintains current consumption below 100 μA (approximately 30-60 μA); and (viii) monitoring battery life tends to drain battery power.

[0016] In one aspect of the invention, an apparatus includes: (i) a component substrate having a wire bond site configured for electrical connection between a voltage source and an electrical device; (ii) a wire bond having a wire including two terminations, the wire having a fusion site including a first length and two end portions, each end portion including one of the two terminations, the fusion site including a smaller diameter than the two end portions, the two terminations attached to the component substrate at the wire bond site; and (iii) a first electromigration (EM) bond of the first termination, the EM bond formed at a ball bond connecting the first termination of the wire to a metal pad at the wire bond site. The EM bond advantageously provides a sensor for battery condition in that a period during which the EM bond conducts current indicates that the battery is in good condition.

[0017] In another aspect of the invention, the diameter of the fusion bond is defined as a function of a specified current passing through the wire bond. By setting the diameter to a specified current associated with the application, the EM joint of the wire bond can be designed to fail after a specified number of years of operation.

[0018] In yet another aspect of the invention, the voltage source is a battery and the electrical device is an encryption card. Some embodiments of the invention advantageously support battery-backed memory in encryption cards.

[0019] In yet another aspect of the invention, the diameter and first length of the fused joint are sized to carry a specified current for a specified number of years before electromigration at the EM joint causes an open circuit, and in this manner, electromigration failure of the EM joint is designed such that as long as the EM joint carries current, the measured voltage indicates the condition of the battery, and if failure occurs by the specified number of years, the battery is stressed and therefore indicates a potentially bad battery.

[0020] In yet another aspect of the invention, the specified current is less than 100 μA. Some embodiments of the invention are directed to relatively low current applications, less than 100 μA, and in some applications as low as 30 μA.

[0021] In one aspect of the present invention, a circuit for battery backup of an encryption card includes: (i) a cryptographic circuit board; (ii) a battery; (iii) a processor; and (iv) an electromigration (EM) junction assembly including a first wire bond having a first length of fused wire, the EM junction assembly electrically connected in series with the battery and the processor. The EM junction assembly advantageously provides a sensor for battery status in that while the EM junction assembly is conducting current, it indicates that the battery is in good condition.

[0022] In another aspect of the present invention, a circuit for a battery backup of an encryption card includes a voltage measurement circuit electrically connected in parallel with the processor for reading the voltage across the processor, which can advantageously monitor battery power while verifying current flow through the EM junction assembly.

[0023] In yet another aspect of the present invention, an EM junction assembly of a circuit for a battery backup of an encryption card has a second wire bond including a second length of fused wire, the first and second wire bonds being electrically connected in parallel, and the second length being longer than the first length. One or more pairs of wire bonds in the EM junction assembly allow for intermediate verification of proper function of the EM junction assembly while providing more controlled assurance of continued proper battery function.

[0024] In yet another aspect of the present invention, the first length of fused wire is an intermediate portion of a wire including two end portions, the first length of fused wire being between the two end portions, each end portion including one of two terminations, the fused portion including a smaller diameter than the two end portions, and the two terminations being attached to a component substrate of an EM junction assembly. The EM junction advantageously provides a sensor for battery condition in that while the EM junction is conducting current, it indicates that the battery is in good condition.

[0025] In yet another aspect of the present invention, an EM junction assembly for a battery backup circuit for an encryption card includes an EM junction disposed at a first end of a wire, the EM junction formed at a ball bond connecting the first end of the wire to a metal pad at a wire bond site on a component substrate. The EM junction advantageously provides a sensor for battery status in that while the EM junction is conducting current, it indicates that the battery is in good condition.

[0026] The Kirkendall effect and Kirkendall voids are the result of thermally or electrically (electromigration) driven atomic diffusion. The Kirkendall effect refers to the movement of an interface between two metals resulting from differences in the diffusion rates of metal atoms. When one atomic species diffuses faster across an interface from one element, phase, or compound compared to the diffusion rate of another element, phase, or compound, the resulting interfacial movement and the accumulation of a concentration of atomic vacancies at the interface result in the formation of voids. As diffusion progresses, atomic vacancies aggregate into microscopic voids at the interface. The accumulation of a large number of voids at an interface can compromise the mechanical and electrical integrity of the interface, leading to electrical failure. Kirkendall voids have been observed to occur in various electronic interconnects, including (i) solder joints; (ii) wire bonds; and (iii) sintered co-fired metals and / or ceramics.

[0027] The mean time to failure (MTTF) due to electromigration can be calculated using Black's formula.

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[0028] Some embodiments of the present invention are directed to identifying and / or monitoring the operational status of an encryption card's battery backup, often for periods exceeding seven years. By reliably identifying the operational status of the battery backup, failures due to incorrect battery function may be avoided, thus reducing unnecessary repairs and / or replacements. Some embodiments of the present invention employ a series of EM fuses or EM / thermal fuses, allowing for monitoring of battery function with very low current, typically less than 100 uA.

[0029] According to some embodiments of the present invention, a passive device is placed in series between a voltage source and a load. The passive device is constructed and / or inserted to include an electromigration junction. The electromigration junction is made of a material known to create an electromigration void after a specified current density j flows through the junction for a predetermined period of time T. Thus, in some embodiments of the present invention, exposure to a specified current density for a specified time or period creates a void and no voltage is sensed, thus providing a guaranteed safe mode condition. Alternatively, the circuit is evaluated for a specified period of time T to determine whether the passive device has an open circuit. If an open circuit occurs for the specified period of time, the voltage source power is confirmed to be at an operational level. If no open circuit occurs, the voltage source power is flagged as being below the operational level. The voltage source power is suspect or flagged because the absence of an open circuit suggests that the passive device is being exposed to a current density lower than that required by the operating voltage source or battery cell. If the evaluation reveals that the open condition occurred before the specified time period T, it can be predicted that the voltage source may fail prematurely due to excessive current density through the passive device.

[0030] 1 is a circuit diagram showing a battery and load with an electromigration (EM) junction that monitors the operating condition of the battery. The circuit 100 includes a voltage source 104; an EM junction 102; a load or device under test (DUT) 106; and a voltage measurement circuit 108. In this example, the EM junction includes a battery watchdog packaged in the EM junction. The EM junction is connected to a load from T0 to T f is designed for electromigration to cause an open circuit at the EM junction for a predetermined time period up to T, where T is the initial time, and T f is the time to failure or open circuit. Upon failure, the EM junction interrupts the flow of current through the junction, causing an open circuit condition.

[0031] According to some embodiments of the present invention, measurements of the voltage across the load / DUT 106 are made by the voltage measurement circuit 108 during normal operation to establish a baseline for the duration of the EM junction. If the voltage source 104 is a battery, the voltage may be monitored periodically, for example, at 1 Hz, 0.1 Hz, once per minute, or once per hour. If battery life is critical, the voltage measurement circuit is disabled except when a measurement operation is required. By disabling or limiting the frequency of voltage measurements, battery life is longer than if voltage measurements were made continuously. The load / DUT resistance may be reduced for a variety of reasons, including: (i) during tampering with the encryption card; and (ii) during operations that are out of specification and may damage the powered circuitry.

[0032] Some embodiments of the present invention are directed to multiple electromigration (EM) sensors in a single system with different specified time periods to EM failure. For example, a first electromigration sensor can be designed to have a time period to EM failure of T1, and a second electromigration sensor can be designed to have a time period to EM failure of T2, where T2 is greater than T1. If the two EM sensors are placed in a parallel circuit with a battery generating a known current, the first EM sensor will fail after the first time period, sooner than the second EM sensor. Intermediate battery condition determination is achieved by multiple electromigration (EM) sensors in a single system with different specified time periods to EM failure.

[0033] Some embodiments of the present invention operate in applications with current demands in the range of 100 μA to 300 mA. Some embodiments of the present invention are deployed with on-chip metallization capabilities of less than 100 μA. The on-chip metallization may be implemented within new semiconductor chips, within an ASIC (Application-Specific Integrated Circuit), or as a small standalone chip operating as part of a regulator feedback loop. Furthermore, with respect to wirebond embodiments of the present invention, the addition of pre-stress burns consisting of high continuous currents or high temperature thermal aging exposures, used independently or simultaneously, can promote a significant amount of interface degradation on burned-in hardware, potentially resulting in electromigration joint failure within a specified functional time window at relatively low levels of current and low temperatures during operation.

[0034] 2 illustrates a wire bond according to an embodiment of the present invention. Wire bond 200 includes a bond wire 202, a Rayleigh filament 204, a ball bond 206, and a wedge bond 208. The bond wire of its original diameter is processed to form a Rayleigh filament of a specified length and diameter, resulting in interfacial failure due to electromigration. Interfacial failure can be engineered to occur at the ball bond or the wedge bond. Diffusion occurs at the bond weld.

[0035] Fusion wire bonds can be first prepared and then attached using a fusion process. The initial preparation may require the bond wire to be subjected to an external heat treatment. After attachment, the wire bond to pad interface may degrade, ultimately causing electromigration failure of the target. The fusion process can be applied to bond wires made of noble metals such as gold, which form filaments for sensor purposes.

[0036] According to some embodiments of the present invention, past failures of specific wire bonds in the field provide the data necessary to predictably create electromigration fuses to periodically check battery life.

[0037] According to some embodiments of the present invention, the time period, or period of time, during which wire bonds are subject to failure may be from one month to up to six years.

[0038] Some embodiments of the present invention are directed to proposing a sensor packaging solution as a stand-alone electronic package consisting of one or more wirebonds attached to a substrate with the wirebond connections terminating to peripheral leads or array pads.

[0039] One example use of electronic packaging is as a battery watchdog in a cryptographic card application. Electromigration fuses are made extremely small to induce significant Joule heating in the interconnects of the fuse itself, ensuring that electromigration diffusion can cause failure when used in that application. This small size is achieved by making the wire bonds small enough to create a wire bond package so that a low current can be passed through them to significantly heat the bond interface, creating an electromigration fuse or series of fuses.

[0040] 3 is a packaged set of wirebond sensors for attachment to a circuit such as circuit 100 (FIG. 1). Wirebond sensor package 300 includes gold-plated pads 302, wirebond jumper pads 304, a component substrate 306, and a set of wirebond timers or sensors 310a, 310b, 310c.

[0041] The current carrying capacity limit of a wire bond of a given material and wire diameter before the onset of fusion (melting) in air is usually approximated by Preece's equation.

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[0042] For variable length wire bonds, a modified Preece equation is used and is highlighted in the MIL specification standard MIL-M-38510, which is based on the Joule heating model and simplifies to a simplified form where the maximum current to fusion, I (in amperes), is proportional to the following equation:

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[0043] According to some embodiments of the present invention, a post-wire bond processing method is provided that includes adding short bursts of pulsed current to melt and / or fuse the individual bonded wires. This is one way to make wire bonds small enough to operate as cryptographic battery watchdogs. With the proposed method, the individual bond wires melt approximately at their centerlines, and the molten metal is drawn back by capillary action and surface tension control, creating very thin filaments.

[0044] The very thin filaments described herein are also referred to as Rayleigh filaments. The formation and ultimate stability of the filaments was studied and determined by Lord Rayleigh, where the maximum fusion gap or filament length is equal to 4.51*d, where d is the starting diameter of the wire. The specific current amount and duration of the current pulse can be tailored to the specific bond wire and wire bond length, allowing for variable length fusion filaments to be created on variable length wire bonds, using filaments with lengths up to the Rayleigh instability limit of 4.51*d. By creating wire bond packages using Rayleigh filaments, very high operating temperatures of the wire bonds can be achieved at very low current levels.

[0045] Using a sensor package with bond wires with variable fused lengths allows for the duration of time T to be adjusted when battery operation is confirmed. The ability to create sensors that fail over for various durations can be used in various watchdog applications, including cryptographic cards. Paralleling multiple wire bonds results in current sharing, and the presence of different length wire bonds with different Rayleigh filament lengths allows for proper battery function detection over time, as these wires may fail at different times over the product's lifetime. According to some embodiments of the present invention, if one wire in parallel fails, the remaining wires share more current, but are also designed to carry more current, similar to part of a "timer circuit." For example, any remaining wire bonds can be made using relatively short lengths of wire and / or larger diameter filaments, ensuring that the longest wires / filaments have the shortest time to failure. When a wire fails, a change in voltage and / or resistance is detected. This process continues as parallel wires in the circuit fail over time.

[0046] For example, using a starting bond wire with a diameter of 0.8 mil (0.02 mm) (20 μA), the fusion process can create a Rayleigh filament of approximately 0.08 mil (0.002 mm) (2 μA). According to Preece's equation, the maximum current that can be carried through a 0.08 mil (0.002 mm) wire is 7.3 mA. Using the modified Preece's equation, the maximum current for a Rayleigh filament length of 125 microns (125 micrometers) with gold material properties at a temperature of 400°C is approximately 3.1 mA.

[0047] According to another embodiment of the present invention, chip metallization establishes electromigration junctions through lithography of semiconductor structures. As described with respect to Figures 4 and 5, lithography techniques are used to create electromigration junctions with specific properties designed to cause electromigration failure when exposed to a specified current density for a defined period of time.

[0048] 4 and 5, an exemplary structure for forming an electromigration timer or sensor device includes a semiconductor substrate 402. A trench is formed in the semiconductor substrate 402 to a thickness less than the thickness of the substrate. Material layers 406 and 408 are deposited on the surfaces of the trench using thin film methods known in the art to form the substructure 400 of FIG. 4. Material layer 406 can be one of the following metals: Cu, Al, Chrome, Ag, Au, and Ni. Material layer 408 is a dielectric material to electrically isolate material layer 406.

[0049] The electromigration (EM) timer 500 is formed by depositing a layer of substrate 402 and forming a trench through the deposited layer to the top surface of structure 400. A second layer of material is deposited over material layers 406 and 408, the second layer including metal layer 504. The second layer is positioned such that metal layers 504 and 406 overlap at distance 506, creating an electromigration interface 502. Metal layers 504 and 406 may be made of various metals, including Cu, Al, Chrome, Ag, Au, and Ni. Material layer 408 is a dielectric material that electrically isolates metal layers 504 and 406. A dielectric 510 of the second layer of material electrically isolates metal layers 504 and 406, except for the electrical connection at electromigration interface 502.

[0050] The electromigration interface 502 can be scheduled for a specific period of time before electrical connection is lost due to electromigration-induced voids. Dimensions 508 and 512 are adjustable characteristics of the thin current-carrying member 514, limiting current flow to a specified amount. If current flow exceeds the specified amount, EM-induced voids can occur at the EM interface 502 sooner than the specified period. Alternatively, if current flow exceeds the specified amount, the thin member 514 fails, creating an open condition across the thin member. Dimension 506 is an adjustable characteristic of the EM interface. The surface area of ​​the EM interface combined with the current flow limitation establishes a target time-to-failure before electrical connection is lost due to electromigration-induced voids. Figure 5 shows a single EM timer. Multiple EM timers with different times to failure allow for periodic battery integrity checks based on the actual time-to-failure of each EM timer.

[0051] The electromigration interface may be based on metal layer EM pairs (404 / 406 pairs) of Cu to Al, Al to Chrome, Cu to Chrome, Cu to Ag, Cu to Au, and Al to Au. Other EM pair metals may also be used according to embodiments of the present invention.

[0052] According to some embodiments of the present invention, the structure and process flow for creating a watchdog sensor package with an electromigration (EM) fuse includes: (i) building a watchdog component substrate; (ii) creating a wire bond assembly; (iii) performing a high current pretreatment on the wire bond assembly to create Rayleigh filaments on the individual bond wires; (iv) applying a protective cover to the assembly; and (v) assembling the wires to a target circuit, such as a cryptographic board in a security card.

[0053] The watchdog component substrate 306 can be made from, but is not limited to, the following: (i) circuit board laminate, (ii) ceramic, and (iii) lead frame (FIG. 3). For example, the component substrate 306 includes gold-plated pads 302 suitable for wire-bonding aluminum slugs that can be attached to a device or substrate to which the aluminum pads are attached. The component substrate can be made using nickel-plated and hard gold or doped gold-plated wire bond jumper pads 304. The component substrate can include a single wire bond site 310a for creating a single electromigration timing connection. The component substrate can include multiple wire bond sites 310a, 310b, 310c for series or parallel connections to create multiple timing watchdogs. The wire bond sites can be within the component substrate or can be wired to the substrate at the application assembly level.

[0054] The wire bond assembly may be made by ultrasonic thermal assembly, or by ultrasonic assembly or other assembly techniques now known or to become known in the future. The individual bond wires may be made of materials including, but not limited to, (i) aluminum; (ii) copper; (iii) gold; (iv) silver; and (v) palladium.

[0055] High temperature pre-conditioning of the assembly wire bonds can be achieved by varying the current / pulse to create filaments of desired length up to the Rayleigh instability limit of the material, 4.51*d.

[0056] Assembly of the packaged watchdog component to the target circuit can be achieved using several techniques, including, but not limited to, (i) soldering; and (ii) separable connectors. The assembly can be a peripheral lead or array package for SMT soldering or a connector socket. The assembly can consist of single or multiple wirebond wires, and the presence of parallel circuitry to the wirebonds can provide additional options for monitoring electromigration failure over time, especially if the assembly package is made with wirebonds of varying lengths or different bondwire materials and / or diameters.

[0057] Some embodiments of the present invention may include one or more of the following features, characteristics, and / or advantages: (i) a safe mode condition is detected by physical evidence of voids forming at strategically defined electromigration junctions; and (ii) monitoring for periodic faults according to an EM timer reduces battery wear that would be detected by continuous monitoring of the battery itself.

[0058] A wide variety of tuning parameters are available to the skilled engineer that will allow for a range of times to failure over the life of the product when using multiple individual watchdog packages with a single wirebond, or when using watchdog packages with multiple wirebonds that can be wired in parallel in the application.

[0059] The time to failure period of a heat spreading / electromigration fuse package made with wirebonds can be adjusted by varying several structural and / or watchdog process parameters, including: (i) wirebond or chip / thin film wire length; (ii) substrate pad metallurgy (Al, Au, and Ag) for wirebond or chip metallization; (iii) pad plating dopant addition (e.g., making wirebond or chip pads with hard gold plating, including palladium, iron, silver, platinum, and silicon); (iv) pad metallurgy thickness; (v) wirebond or chip wire material, such as Au, Pd, Ag, Pt, Cu, Al; (vi) starting wire diameter or chip trace width; (vii) fusing current and time pulse (wirebond only); (viii) thermal pretreatment (burn-in) after wirebond or chip trace; and (ix) electrical pretreatment (burn-in) after wirebond or chip trace.

[0060] Here are some helpful definitions:

[0061] The present invention: The subject matter described by the term "the present invention" should not be taken as an absolute indication that it is covered by either the claims at the time of filing or any claims that may eventually issue after patent prosecution; the term "the present invention" is used to help the reader get a general sense that the disclosures herein are believed to be potentially new, but as indicated by the use of the term "the present invention," this understanding is hypothetical and provisional, and is subject to change during the course of patent prosecution as relevant information develops, as the claims are potentially amended.

[0062] Embodiments: See definition of "present invention" above. A similar caution applies to the term "embodiments."

[0063] and / or: Inclusive or; for example, A, B "and / or" C means that at least one of A or B or C is true and applicable.

Claims

1. a component substrate having wire bond sites configured for electrical connection between a voltage source and an electrical device; a wire bond having a wire including two terminations, the wire having a fusion portion including a first length and two end portions, each end portion including one of the two terminations, the fusion portion having a smaller diameter than the two end portions, and the two terminations attached to the component substrate at the wire bond site; and a first electromigration (EM) bond at the first termination, the EM bond formed at a ball bond connecting the first termination of the wire to a metal pad at the wire bond site; An apparatus comprising:

2. The apparatus of claim 1 , wherein the diameter of the fusion bond is defined as a function of a specified current passed through the wire bond.

3. The apparatus of claim 1 , wherein the voltage source is a battery.

4. The apparatus of claim 1 , wherein the electrical device is an encryption card.

5. 10. The apparatus of claim 1, wherein the diameter and the first length of the fused joint are sized to carry a specified current for a specified number of years before an open circuit forms due to electromigration at the EM joint.

6. 6. The apparatus of claim 5, wherein the specified current is less than 100 μA.

7. 1. A circuit for battery backup of an encryption card, comprising: Cryptographic circuit board; Battery; a processor; and an electromigration (EM) joint assembly including a first wire bond having a first length of fusion wire, the EM joint assembly being electrically connected in series with the battery and the processor; A circuit comprising:

8. 8. The circuit of claim 7, further comprising a voltage measurement circuit electrically connected in parallel with the processor for reading a voltage across the processor.

9. 9. The circuit of claim 8, wherein the voltage measurement circuit is normally open and voltage is measured when the circuit is closed, thereby allowing periodic voltage measurements.

10. the EM junction assembly comprising:

8. The circuit of claim 7, further comprising a second wirebond comprising a second length of fused wire, the first wirebond and the second wirebond being electrically connected in parallel, the second length being longer than the first length.

11. 8. The circuit of claim 7, wherein the first length of fusion wire is an intermediate portion of a wire comprising two end portions, the first length of fusion wire being between the two end portions, each end portion comprising one of the two terminations, the fusion wire comprising a smaller diameter than the two end portions, and the two terminations being attached to a component substrate of the EM junction assembly.

12. the EM junction assembly comprising:

12. The circuit of claim 11, further comprising an EM bond disposed at a first termination, the EM bond formed at a ball bond connecting the first termination of the wire to a metal pad of a wire bond site on the component substrate.

13. 1. A semiconductor structure for an electromigration timer, the semiconductor structure comprising: a semiconductor substrate; and an electromigration (EM) timer embedded in the semiconductor structure and including a first contact and a second contact, the first contact having adjustable characteristics for defining a duration of electrical connection, the duration of electrical connection terminating when an EM-induced void forms at the EM interface, creating an open circuit condition; where: the first contact is in electrical communication with the second contact at the EM interface where the first contact overlaps the second contact; and The adjustable characteristics of the first contact include a surface area of ​​the EM interface and a thin current-carrying member that restricts current flow through the EM interface by a specified amount. A semiconductor structure comprising:

14. 14. The semiconductor structure of claim 13, wherein the tunable properties at the electromigration interface include a contact width, overlap length, and contact thickness of each of the first contact and the second contact, and a contact width and thickness of the thin current-carrying member of the first contact.

15. 14. The semiconductor structure of claim 13, further comprising a dielectric material separating a portion of the first contact from a portion of the second contact to define an overlap length at the electromigration interface.

16. 14. The semiconductor structure of claim 13, wherein the first contact and the second contact are composite metals that comprise a dissimilar metal electromigration pair.

17. 17. The semiconductor structure of claim 16, wherein the first contact is copper (Cu) and the second contact is aluminum (Al).

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