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Stretchable Copper Board, (SCB), Printed Circuit Technology

Edward Snelleman Boy van Veghel

PCB quality control

The Stretchable Copper Board Technology has been developed on the basis of results obtained during a project defined within the Sixth Framework Programme of the European Commission to promote European research and technology development. Rigid Printed circuit boards are not bendable and not stretchable. Flexible circuit boards are only bendable. Printed circuit boards made with the Stretchable Copper Board (SCB) technology are both stretchable and bendable enabling totally new interconnection solutions.

PCB quality control

Some history about the development of SCB PCB

The definition of the original development project, Stretchable Electronics Technology for Large Area Applications (STELLA) did amongst others include the integration of electronic circuitry, stretch up to 50% and a circuitry which could withstand machine washing. During the development program a number of possible solutions were evaluated. Some which were already in the early development phase ceased, whilst several continued development through to prototype circuits.

One project which used an additive Cu process on a breathing layer as the substrate ceased as it became apparent that the substrate layer was heavily affected by the chemistry used. Another technology which was explored was the creation of conductive polymers on non-woven substrates.

The perceived advantage of this technology is the breathing of the substrate. However in reality, the technology turned out to be very unreliable. The stretchable copper board technology, which makes use of a polyurethane type substrate, whilst not breathable, was proven to have some big advantages. Firstly, productionprocesses are very similar to the traditional fabrication methods used in the manufacture of PCBs. Stretch can be up to 30% depending on the design chosen for the circuitry and a reliable interconnection can be created. The polyurethane base material can be in contact with the skin, the disadvantage of the non-breathing of the material can be overcome by a pattern of small holes in the poly urethane substrate.

Development of SCB technology

The development of new stretchable and flexible substrates carrying stretchable conductors does present a few additional challenges however. Electronics cannot stretch and additionally, theinterconnection of the components with the substrate should neither bend nor stretch. To overcome this, several new interconnection technologies were developed, in order to ensure reliable interconnection.

  • Intverposer technology. The active electronics are placed on an interposer to ensure that no stress is put on either theelectronics, or the interconnection of the components.
  • Stretch-rigid. Similar to the technology of Flex-rigid a circuit is fabricated with a stretchable interconnection part and a rigid part on which the electronics are mounted.
  • Where components require to be mounted, a stiffener can be added in same manner as currently adopted on flexible circuits. The effect of stretching is barely noticeable on these reinforced islands.

Stiffener constructions to support the mounted parts

The Base material used for SCB

The stretchable copper board technology is based on a polyurethane film plated with a copper foil. Both the polyurethane film and the copper can vary in thickness. Optimal results have been achieved with a polyurethane film of 100 micron (4mils) nominal thickness and copper foil of 18micron (0.5Oz). Design Dependant, a coverlayer of polyurethane can be used to protect the copper wires. The adhesion of the copper onto the polyurethane is very strong, and is a factor that has a positive impact on the reliability of the wiring when stretched.

Typical build

The wiring is copper, as copper can be stretched due to its high ductility. The wiring of the track, also called horse shoe on account of it’s shape and appearance, ensures that the PCB can be stretched without impacting reliability of the copper wiring.

The technology can be made as a single or 2 layer construction, but unlike traditional flex circuits, the conductors should be placed above one another to ensure even stretch of the complete circuit.

Traditional flex circuits are typically designed such that tracks on one layer are clear in same area of the other layer. Under development are constructions which support multiple layers. Due to the relatively low melting point of polyurethane, traditional Pb free (Sn only) soldering is not an option. Low melting solder materials like SnBi (Tin Bismuth) are an excellent alternative solution. Z-glue is also an alternative. Alternative builds are under evaluation which would make it even possible to solder with Sn.

Horse shoe like patron, gradually change to lesser flexibility at theinterconnections

Actual processed patron

Options: Stretch - Bend – Twist

Unlike other interconnection techniques the SCB technology offers almost unparalleled freedom of movement. Together with stretching and bending, twisting is also an option. Unlike polyimide base material, polyurethane is not sensitive to tearing or cracking meaning that unexpected movement can be accommodated with ease and total reliability maintained.

Possible applications and Uses

Functional clothing

An important application area for stretchable printed circuit boards is in functional clothing. Adding sensors to clothing can increase the functionality of the clothing. This could be a simple signage or a more complexintegration of remote sensing and control, including signaling by means of vibration.

This way additional information can be gathered but crucially, information can also be shared with people in a remote area or in an area with no sight.

The SRB interconnection technology allows sensors to be close to the body whilst still permitting freedom of movement, without damage to the electronics.

Complicated positioning eg. sensors

Measuring and monitoring of vital signs, such as heart rate and breathing dictate that the sensors used for this purpose are worn close to the body for accurate measurements. As such it makes sense to integrate them in fairly tight, form-fitting clothing. The stretch and the low bend and twist forces supports the design of easily wearable clothing. Realized: Sensor network with vibrators in clothing to people directions in the dark.

Medical applications

Many constructions can be designed with just simple linear movements or rotations and with flex PCB the inter- connection can be realized. In case of the human body this is totally different. The movement of the joint of the elbow or the knee par example is not a simple 2 dimensional movement. Not to speak of the fact that the human body is never the same.

Stretchable PCBs can be a solution in those cases. Particular as three dimensional stretch is possible, like a knee in a pair of trousers. It’s expected that there will be numerous possible applications in the medical sector. Stretchable substrates with chemical or temperature sensors could be integrated in bandages to detect infected wounds. By integrating a pressure sensor, the contact pressure of the bandage could be measured, so that bandages could be applied with the correct contact pressure, an important factor when dealing with open wounds.

project

Wound treatment, intelligent bandage

Wound treatment of the lower leg by pressure measuring. The build in pressure sensors assist in the applying the bandage with the correct pressure on the wound. With GSM communication the correct pressure of the bandage can be monitored on a distance.Disposable stretch part with 2 pressure sensors build in. Realized: prototypes for evaluation

wound-treatment-image

project

Smart Sensor Glove for Arthritis Rehabilitation

Rheumatoid arthritis is a chronic, inflammatory joint disease. Stiffness, swelling and deformity are the most common symptoms. Approximately 20% to 30% of people become work disabled within the first two to three years of the disease. Today clinical assessment techniques are based on manual assessment techniques, are time consuming and several measurements are difficult to measure empirically, like join stiffness.

sensor-glove-image

project

Foot sole pressure

Diabeties causes amongst other problems, the loss of sensation of pain in the feet of the affected person. Location of the pressure points, and known the rate will enable the orthopedic shoemaker to improve the design of the foot sole. The advantage of the SCB technology is that not only does the part bend easily but also the shoe size can be “adjusted” by more or less stretching of the SCB. The unit includes a memory device to store data for later analysis. In this case a connector interface with a stiffener construction was developed to connect to the memory unit using a ZIF connector. Realized: Multiple circuits.

foot-sole-pressure-image

To overcome tolerances in a construction

Diabeties causes amongst other problems, the loss of sensation of pain in the feet of the affected person. Location of the pressure points, and known the rate will enable the orthopedic shoemaker to improve the design of the foot sole. The advantage of the SCB technology is that not only does the part bend easily but also the shoe size can be “adjusted” by more or less stretching of the SCB. The unit includes a memory device to store data for later analysis. In this case a connector interface with a stiffener construction was developed to connect to the memory unit using a ZIF connector. Realized: Multiple circuits.

overcome-tolerances-first-image

To overcome tolerances in a construction

A combination of vibration of a construction with tide tolerances can be challenge for PCBs. Will a rigid PCB not allow tolerances in the construction, or variation caused by stress put on a construction, the flex construction can give some relieve, but therefore the flex material is rather sensitive for tearing. In all those cases the SCB technology could be a very interesting option.

A SCB print was develop to cope with tolerance variations of 3%. Therefore a relative simple patron was designed which could withstand the stretch. Realized: small scale production of tenth of circuits including assembly

overcome-tolerances-last-image

Three dimensional car interior parts

Stretchable substrates allow industrial designers greater design freedom. The SCB print could be used as part of the design of an interior, with the substrates stretched to form a three dimensional shape without the need for space for with traditional wiring, thus giving more freedom and a higher quality product. Particular as the integrated wiring will not issue any problems with vibration or noise.

Consumer clothing

Integration of electronics in clothing is already a couple of years a rather hot topic. Many examples has been shown from dresses up to simple T-shirts. Most applications are connected to illumination or signage. Cost is a rather big issue for consumer applications and is key to create a successful solution. The stretchable copper board could be a real option as it can very well withstand mechanical stress in case of clothing.

Concerning the cost, it depends largely on the design, the circuit area and production quantities. The SCB is well suited to be scaled-up to large volume production.