Conductive threads and tape function as a composite system to maintain connectivity across different layers of a flexible circuit. The conductive thread passes through vias (holes) in the substrate, physically bridging the top sensing pads to the bottom wiring layer. The conductive tape is then applied to pin these threads against copper foil pads, creating a secure, conductive seal without the need for heat or melting.
By replacing brittle solder joints with a flexible, low-profile mechanical bond, this method ensures electrical continuity withstands the intense compression and bending forces typical of wearable applications.
The Anatomy of the Connection
Bridging Vertical Layers
In this wiring architecture, the conductive thread acts as the primary interconnect.
It is threaded through vias in the sensor substrate, creating a physical link between the sensing elements on the top layer and the wiring paths on the bottom layer.
Locking the Contact
While the thread provides the path, the conductive tape provides the anchor.
The tape is applied directly over the thread ends, pressing them firmly against the copper foil pads. This ensures contact integrity and prevents the thread from shifting or losing connection with the circuit during movement.
Why This Replaces Solder
Eliminating Brittle Points
Standard electronics typically rely on solder, which creates a rigid, metal-fused joint.
However, in flexible applications like the sole of a foot, these rigid points are prone to cracking and failure. The repeated mechanical stress fractures the solder, severing the electrical connection.
Maintaining a Low Profile
The combination of thread and tape creates an exceptionally thin connection point.
This low-profile design is critical for wearables, as it minimizes bulk and prevents the user from feeling hard components underfoot during compression.
Understanding the Trade-offs
Mechanical Flex vs. Rigid Permanence
The primary trade-off in this approach is sacrificing the permanent fusion of a solder joint for the mechanical compliance of a taped thread.
While solder offers a chemically fused bond, it cannot survive dynamic environments. The thread-and-tape method relies on mechanical pressure and friction, which is superior for flexibility but requires precise application to ensure the tape holds secure over time.
Making the Right Choice for Your Goal
When designing interconnects for sensor systems, consider the environmental stresses your device will endure.
- If your primary focus is dynamic durability: Utilize the conductive thread and tape method to allow for bending, twisting, and compression without risking connection fractures.
- If your primary focus is standard structural rigidity: Stick to traditional soldering methods, as they provide a fused bond but are unsuitable for areas subject to high mechanical flex.
Reliable wearable data depends not just on the sensor, but on the ability of the wiring to move with the user.
Summary Table:
| Feature | Conductive Thread & Tape Method | Traditional Solder Joint |
|---|---|---|
| Flexibility | High (Mechanical compliance) | Low (Brittle/Rigid) |
| Profile | Ultra-thin / Low-profile | Bulky / Protruding |
| Durability | Resists compression & bending | Prone to stress fractures |
| Application | Dynamic wearables (e.g., shoe soles) | Rigid PCBs & stationary devices |
| Bond Type | Mechanical pressure & friction | Chemically fused metal bond |
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References
- Sarah De Guzman, Gautam Anand. The Development of a Built-In Shoe Plantar Pressure Measurement System for Children. DOI: 10.3390/s22218327
This article is also based on technical information from 3515 Knowledge Base .