Electronic connectors are normally designed with one primary objective: maintain a reliable electrical connection throughout the expected operating conditions of a product.
But in security-sensitive electronics, there may be another important requirement:
What happens to the interconnect if someone attempts to open, modify, or reassemble the device?
For products where physical tampering is a concern, an elastomeric connector can potentially become part of the product’s tamper-resistant mechanical design.
Rather than designing the connector to survive every possible mechanical condition, the interconnect can be engineered around a defined mechanical operating envelope. Under normal assembly conditions, the connector provides reliable electrical contact. If the device is manipulated in a way that subjects the connector to excessive compression or other conditions outside that envelope, the interconnect may become permanently deformed or damaged.
This concept can be useful in security-sensitive products such as ithardware wallets, authentication devices, payment terminals, encrypted storage devices, access-control systems, and other electronics where unauthorized physical access is a concern.
Controlled Compression of Elastomeric Connectors
Z-Axis wire-based elastomeric connectors use vertically oriented conductive elements embedded within silicone rubber. Electrical contact is established by compressing the elastomer between two mating contact surfaces.
Under normal conditions, the connector is designed to operate within a specified compression range. The mechanical assembly controls the connector height and compression so that consistent electrical contact is maintained.
For a security-sensitive application, those mechanical limits can also become part of the product’s tamper-resistant architecture.
A connector and enclosure can potentially be designed so that:
- Normal assembly produces the intended connector compression and reliable electrical contact.
- Opening, removing, or manipulating components changes the mechanical relationship of the assembly.
- Unauthorized reassembly can expose the connector to excessive compression or altered compression conditions.
- Compression beyond the qualified range can permanently deform or damage the interconnect, preventing the device from simply being returned to its original condition.
Using the Interconnect as Part of the Security Architecture
The key is establishing a clear distinction between normal operating compression and abnormal or destructive compression.
During normal operation, the connector remains within its qualified compression range and provides a reliable electrical connection.
If tampering causes the assembly to exceed those mechanical limits, the connector may experience irreversible deformation or damage.
This creates the possibility of an interconnect serving two functions:
During normal operation: provide a reliable, compact electrical connection.
During a tamper event: act as a mechanical element that makes unauthorized disassembly and reassembly more difficult without affecting the interconnect.
The exact behavior must be characterized through testing, particularly when connector deformation or electrical failure is intentionally incorporated into a security strategy.
Applications for Tamper-Resistant Interconnects
This approach can potentially be applied anywhere physical access to sensitive electronics must be controlled.
Examples include hardware wallets used to protect cryptographic keys, payment and point-of-sale devices, secure authentication products, encrypted storage systems, access-control electronics, security modules, and other devices containing sensitive information or functionality.
In these applications, the electrical connector can be considered as more than simply a means of transferring signals between two surfaces.
It can become part of the mechanical security architecture of the product itself.
Example: Secure Access-Control Module
Consider a compact access-control device containing a main PCB and a separate authentication or sensor PCB connected through an elastomeric connector.
The enclosure can be designed so that mechanical stops establish a controlled PCB-to-PCB spacing. When the device is properly assembled, the elastomeric connector is compressed between the two contact surfaces and maintains electrical continuity.
If the enclosure is opened or the PCB assembly is disturbed, the distance between the mating surfaces increases. As the connector expands toward its free height, compression decreases. Once sufficient compression is lost, reliable electrical contact is no longer maintained.
The electronics can monitor this connection and detect the resulting open circuit as a potential tamper event.
Calculating Connector Compression
Connector compression can be calculated as:
Compression (%) = (Free Height − Installed Height) / Free Height × 100
Assume an elastomeric connector has a free height of 2.00 mm.
In the properly assembled device, the mechanical design establishes a connector height of 1.70 mm:
Compression = (2.00 − 1.70) / 2.00 × 100
Compression = 15%
The connector therefore operates at 15% compression during normal operation.
Now consider what happens when the enclosure begins to open.
If tampering increases the PCB separation by 0.20 mm, the installed connector height increases from 1.70 mm to 1.90 mm:
Compression = (2.00 − 1.90) / 2.00 × 100
Compression = 5%
The connector has now lost two-thirds of its original compression.
If the separation increases another 0.10 mm, the connector reaches its free height of 2.00 mm:
Compression = (2.00 − 2.00) / 2.00 × 100
Compression = 0%
At this point, there is no remaining mechanical compression holding the connector against both contact surfaces. Additional separation creates a physical gap and therefore an open circuit.
Example Tamper Sequence
| Installed Gap | Connector Compression | Change From Normal Assembly | Example Condition |
|---|---|---|---|
| 1.70 mm | 15% | — | Normal assembly |
| 1.75 mm | 12.5% | +0.05 mm | Enclosure movement |
| 1.80 mm | 10% | +0.10 mm | Reduced compression |
| 1.90 mm | 5% | +0.20 mm | Near loss of contact |
| 2.00 mm | 0% | +0.30 mm | Free connector height |
| >2.00 mm | 0% + physical gap | >+0.30 mm | Open circuit / tamper condition |
Values shown are illustrative. The minimum compression required for reliable electrical contact must be established for the specific connector and application.
This means that in the example assembly, only 0.30 mm of additional separation is required to move the connector from its normal 15% compressed condition to its completely uncompressed free height.
The mechanical sensitivity can also be expressed relative to the original compression distance.
At normal assembly:
Compression distance = 2.00 mm − 1.70 mm = 0.30 mm
Therefore, an increase in PCB separation of approximately 0.30 mm consumes the entire original compression distance.
This provides a straightforward method for incorporating the interconnect into a tamper-detection system.
Using the Open Circuit as a Tamper Signal
The connector does not necessarily need to be permanently damaged during a tamper event.
Instead, one or more conductive paths through the elastomeric connector can be monitored by the electronics.
During normal operation:
Connector compressed → electrical continuity → normal condition
During enclosure opening:
Gap increases → compression decreases → contact is lost → open circuit
The system can then interpret the loss of continuity as a tamper event.
For example, a dedicated tamper-monitoring circuit could continuously check a conductive path routed through the elastomeric interconnect. If opening the enclosure increases the PCB separation beyond the designed mechanical threshold, the monitored connection opens.
The device could then respond according to the requirements of the application, such as recording a tamper event, disabling sensitive functionality, generating an alert, or initiating another predefined security response.
Designing the Tamper Threshold
The required enclosure movement can also be designed mathematically.
If:
H = connector free height
C = desired compression as a decimal
then the installed connector height is:
Installed Height = H × (1 − C)
For a 2.00 mm connector designed for 15% nominal compression:
Installed Height = 2.00 × (1 − 0.15)
Installed Height = 1.70 mm
The available mechanical travel before reaching the connector’s free height is therefore:
Tamper Travel = 2.00 − 1.70
Tamper Travel = 0.30 mm
This relationship allows connector height and nominal compression to become design variables in the tamper-detection system.
A taller connector or greater initial compression provides more mechanical travel before contact is lost. A smaller initial compression provides less travel and potentially greater sensitivity to enclosure movement.
The actual electrical opening point may occur before the connector reaches exactly 0% compression and should be determined experimentally.
Characterizing the Actual Detection Point
For a production design, Z-Axis can evaluate the connector at progressively decreasing compression levels while monitoring electrical resistance.
For example:
| Compression | Installed Height for 2.00 mm Connector | Test Objective |
|---|---|---|
| 20% | 1.60 mm | Electrical performance |
| 15% | 1.70 mm | Nominal operating condition |
| 10% | 1.80 mm | Verify contact margin |
| 7.5% | 1.85 mm | Monitor resistance |
| 5% | 1.90 mm | Determine low-compression behavior |
| 2.5% | 1.95 mm | Determine contact threshold |
| 0% | 2.00 mm | Uncompressed condition |
Resistance measurements across this range can establish two useful design points:
Normal operating range: the compression range over which reliable electrical contact is maintained.
Tamper detection threshold: the mechanical separation at which the monitored electrical path reliably transitions to an open circuit.
The difference between these values provides a measurable mechanical margin between normal product operation and tamper detection.
Rather than simply connecting two circuit boards, the elastomeric connector can therefore function as both an electrical interconnect and a mechanically actuated tamper-detection element.
Developing a Custom Elastomeric Interconnect
Z-Axis Connector Company develops custom wire-based elastomeric connectors for applications requiring precise mechanical and electrical performance.
Connector dimensions, conductive element pitch, compression, contact geometry, and mechanical integration can be tailored to the application.
For applications involving unusual compression requirements or tamper-resistant designs, connector behavior can also be evaluated through prototype testing to establish the appropriate operating and failure conditions.
If you are developing a security-sensitive electronic product and need an interconnect with a defined mechanical operating range, contact Z-Axis Connector Company to discuss your application