Stretchable PCB Technology: RUSH PCB UK LTD Innovates for Flexible Applications
Traditional Printed Circuit Boards (PCBs) are rigid, meaning they are not meant to be bent during use. A different type of circuit board is available for use in applications that need the board to flex or bend repeatedly—flexible circuit boards. Both these are not very useful if the application demands the circuit board be stretched. For this, RUSH PCB UK LTD recommends using stretchable PCB technology. 
Stretchable PCB Construction
Although stretchable PCB technology uses classical processes for the production and assembly of such PCBs, the laminate is either Polyurethane or Polyimide. This has the advantage of realizing stretchable PCBs with relatively low investments. For ease of assembly of components on the substrate, manufacturers use one of two methods as follows.
Manufacturers reinforce the laminate locally using an interposer or a special coating. The alternate method is to use Stretch-Rigid technology. Rather than connect two rigid boards with a flexible PCB as in Rigid-Flex construction, Stretch-Rigid technology connects multiple rigid boards using stretchable substrates with embedded copper interconnection traces. The electronic components are soldered on the rigid parts. 
Properties of Stretchable PCBs
PCBs with stretchable substrates are useful for applications that require the PCB to stretch, twist, bend, or any combination thereof. The stretchable substrate is ductile enough to decouple mechanical resonances, which reduces the effort necessary for compensating mechanical tolerances.
Stretchable PCBs come in single or double layers, with Polyurethane being the usual stretchable substrate. Typical base material thickness varies between 90 and 100 µm or 3.5 and 3.9 mil, while the copper weight is usually 0.5 Oz or 17.5 µm.
As the substrate must stretch, manufacturers take special care to give the copper a high peel strength of about 5 N/mm or 456 Oz/in, and a tensile strength of 6 MPa or 870 psi at 50% strain.
The above features of the substrate allow the stretchable PCB a maximum stretchability of 30% of its original length and 10% stretchability for repeated elongations. This, however, depends on the structure of the copper pattern on the stretchable substrate. As the maximum allowed temperature for soldering on the substrate is about 150°C, the assembly process uses SnBi solder and FR4 interposers.
This allows a usable operational temperature range of 0 to 100°C for stretchable PCBs. Where the application requires a stretchable substrate of short length and low volume, manufacturers prefer to use Polyurethane as the substrate material. If the application demands a long and high-volume substrate link between the rigid parts, Polyimide is preferable. 
Advantages of Stretchable PCBs
Stretchable PCBs are very useful in the industry where two parts of a machine move relative to each other and must be interconnected electrically. For instance, a sensor executing complex movements on a stationary machine is best interconnected using a stretchable PCB as it allows the sensor to move in multiple degrees of freedom, including linear and rotational. Apart from being able to twist and bend, the stretchable interconnect can also allow the sensor to move linearly away from the machine (stretch) when needed, with a maximum elongation of 30% of its original length.
Therefore, two or more rigid boards connected by stretchable substrates can change their positions very easily, can change their positional angles relative to each other, and move apart or come close to each other, while remaining electrically tethered to each other all the time. However, for repeated stretching and contractions, RUSH PCB recommends limiting the elongation of stretchable PCBs to 10% of the original length. 
Mechanism of Stretchable PCBs
Although the thermoplastic Polyurethane that manufacturers use as a substrate for stretchable PCBs can stretch inherently, copper traces in straight lines on the substrate prevent it from doing so, as copper is not ductile enough for the purpose. Manufacturers use special press and confidential lay-up techniques for bonding the standard ED or RA copper foil on the Polyurethane substrate. Once this is done, they use regular subtractive wet-etching PCB processing steps such as drilling, metalizing, imaging, plating, and etching for fabricating stretchable circuits.
As adding multiple layers of adhesive and Polyurethane substrates reduces the stretchability of the product, stretchable PCBs are mostly double-sided and have four layers at the most. To maintain a homogeneous elastomeric construction, manufacturers apply a Polyurethane solder mask or overlay on the finished PCB. 
Assembly of a stretchable PCB uses the standard off-the-shelf surface mounting components soldered on its copper tracks. As these components are rigid, the areas where the components are positioned cannot stretch. Therefore, the concept of the stretchable circuit is small islands of a rigid nature holding a few SMD components interconnected with conductive copper foil on stretchable substrates. For a mechanically reliable PCB, the manufacturing technique follows a gradual transition from the rigid area to the flexible area and ultimately to the stretchable region.
To allow the copper traces on the substrate to flex without damage, the designer gives the traces a horseshoe shape rather than allowing them to travel in straight lines. The designer then places the horseshoe shapes alternately facing 180°, allowing them to meander along the path the straight trace would have normally taken. When stretched, the horseshoe tracks will uncurl without much stress. Other shapes such as triangular and sinusoidal interconnect traces can also stretch, but exhibit higher stresses, leading to lower reliability. This has led manufacturers to standardize the horseshoe shape. 
Designers must note that stretching copper traces lead to a change in their resistance. For instance, tests conducted on copper traces with a thickness of 15 µm, width of 1 mm, and length of 80 mm showed an original resistance of 7.4 Ω, which increased to 13.5 Ω when the trace was stretched by 10%, to 23.8 Ω when stretched by 20%, and to 37.6 Ω when stretched by 30%. However, lab tests have verified that the trace maintained its conductivity even after a 300% stretching. 
Uses of Stretchable PCB
Applications that demand the PCB be placed on a non-flat surface are the major users of stretchable PCBs. A conventional rigid PCB cannot be comfortably integrated on a non-flat surface such as that in wearable and implantable devices. Devices such as those used in smart textiles, safety, sports and leisure, and biomedical applications often follow irregular shapes, and the printed circuit must follow the shape for proper integration.
Although it is possible to form a flexible circuit in the shape of a cone or a cylinder, only a stretchable circuit can be deformed onto any type of surface, as it has stretchable interconnects. 
For instance, a stretchable PCB placed in the sole of a shoe can measure pressure with embedded sensors, collecting data with free movement of the user. Placed inside bandages, the pressure sensors on a stretchable circuit can measure the tightness of the applied bandage. 
A completely new range of electronic devices can make use of stretchable PCBs providing comfortability as their unique characteristic. Apart from the few uses listed above, stretchable PCBs are already being used in applications involving artificial skins, randomly shaped biomedical implants, and conformable light sources.