What are Stretchable PCBs?

Stretchable PCBs are advanced printed circuit boards that are designed to keep functionality even when they are being subjected to stretching, bending, or other dynamic deformations. Stretchable PCBs are known for their elasticity, which means that these materials can stretch and return to their original shape. Similar to flexible PCBs, stretchable PCBs can remain dynamically conformable to complex shapes but with the added advantage of the ability to undergo great mechanical deformation without giving up functionality. They are suitable for wearable technology, medical devices, etc., applications.

Structure of Stretchable PCBs

The most common substrate material used to make stretchable PCBs is thermoplastic polyurethane (TPU). It can be elongated or stretched by up to 30% and can handle repeated elongations by up to 10%. This material can ensure that connectivity doesn't break when stretched due to components moving linearly away from one another. TPU is biocompatible (safe for skin contact in wearable applications), plasticizer-free, and resistant to UV light, moisture, and microbial activity. Other substrates used for stretchable PCBs include Silicone (e.g., PDMS) or other elastomers. 

Copper traces are laid in straight lines, which can limit flexibility due to the relatively low ductility of copper. Overcoming this requires special bonding and etching techniques like using a stretchable copper foil in serpentine shapes and creating traces in shapes such as horseshoes that flex & uncurl under strain for durability. Stretchable metals like liquid metals (e.g., gallium-indium alloys), metal traces in serpentine shapes, or conductive inks are also used for conductors in these PCBs.

A polyurethane solder mask or overlay is applied to add strength and uniformly retain an elastomeric structure. The assembly is to attach standard surface-mount components onto the copper tracks. These are then kept on rigid "islands" connected by stretchable interconnects.

Manufacturing Process:

• Stretchable Copper foil is laminated on the TPU by specialized techniques that ensure strong adhesion with flexibility. The copper thickness can range from 9 to 70 µm, depending on the application. The circuit design can be structured onto the laminated TPU substrate through traditional PCB manufacturing techniques like printing and etching. Design geometries are adjusted to accommodate stretching with special geometric patterns such as meanders, horseshoes, or waves. These patterns accommodate stretching and contraction with no deformation in electrical performance.
• A solder mask is applied to protect the circuit and make it more durable. Different surface finishes, like Electroless Nickel Immersion Gold or silver, are adopted to make the solderability much better while preventing oxidation. Some of the cases apply conductive adhesives, depending upon the assembly requirements.
• Components are attached to the circuit by using low-melting-point solder, for example, Indalloy 282, with a melting temperature around 140°C. The crimping method and conductive adhesives are used for attaching components. These are particularly applied where the integrity of the TPU substrate might be compromised if soldered.
• To protect the circuits, encapsulation techniques such as injection molding or glob-top methods are used. For applications in wearable electronics, the stretchable PCB is often laminated onto textiles or other carrier materials. This involves pressing the TPU circuit onto fabric or rigid carriers like polycarbonate under controlled temperature and pressure. Depending on the design, reinforcement layers may be added to areas such as connector regions or assembly zones to make them stronger.

Applications

Stretchable PCBs offer significant advantages in applications in which electrical interconnections must track dynamic movements. They are suitable for connecting parts in machinery that move relative to each other, such as attaching a sensor to a moving mechanism. Such PCBs enable the sensor to make complex movements, including twisting, bending, rotating, and stretching, while still making a reliable electrical connection.

Stretchable PCBs can be integrated on non-flat or irregular surfaces. They are suitable for wearable and implantable devices, smart textiles, and biomedical applications. Some examples of such applications are:

• Wearables: Pressure sensors in shoe soles that track the movement of users.
• Medical Applications: Sensors in bandages to measure tightness or pressure.
• Smart Textiles: Electronics integrated into fabrics for safety, sport, and recreation.