KNOWLEDGE BASE

Articles

Stretchable Copper Board, SCB, printed circuit technology

Edward Snelleman, Boy van Veghel

In September 2016, FineLine Global acquired the UK-based Exception VAR Ltd, a successful company with annual sales of over USD 12million and an enviable customer base. Subsequently, joining forces with the already established FineLine Global UK saw the formation of FineLine VAR Ltd.

Some history about the development of stretch PCB

The definition of the original development project, Stretchable Electronics Technology for Large Area Applications (STELLA) did among others include the integration of electronic circuitry, stretch up to 50% and a circuitry which could withstand machine washing. During the development program a large number of possible solutions were evaluated, some of them were already in the early development phase stopped, several were developed up to prototype circuitry.

The project with an additive Cu process on a breathing layer as the substrate was stopped as the substrate layer was heavily affected by the chemistry. Another technology which was explored was the creation of conductive polymers on non-woven substrates.

The advantage is the breathing of the substrate, however this technology turned out to be very unreliable.

The stretchable copper board technology, which makes use of a polyurethane type basis, is not breathable though turned out to have some big advantages. The productions processes are rather similar to the traditional production technology 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 contact with the skin, the disadvantage of the non-breathing of the material can be overcome by a patron of small holes in the poly urethane substrate.

Development of SCB technology

The development of new stretchable and flexible substrates carrying stretchable conductors does include a few additional challenges. Electronics cannot stretch and also the interconnection of the components with the substrate should not bend and/or stretched. Therefore several new interconnection technologies are developed to assure reliable circuitry.

  • Interposer technology. The active electronics are placed on a interposer to assure that no stress is put on the electronics and the interconnection of the components
  • Stretch-rigid. Similar to the technology of Flex-rigid a circuitry is built with a stretchable interconnection part and a rigid part on which the electronics are mounted
  • In case of a limited amount of electronics also a stiffener like solution can be an option. The effect of stretching is hardly noticeable on these reinforced islands.
chip
  ➝

Stiffener construction to support the mounted parts

base material

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. The best results are obtained with foil of 100 micron (4mils) nominal thickness and copper film of 18micron (0.5Oz). Depending on the design a coverlayer of polyurethan can be used to protect the copper wires. The adhesion of the copper onto the polyurethan is very strong, and is a factor that has a positive impact on the reliablity of the wiring in case stretched.

chip

Horse shoe like patron, gradually change to lesser flexibility at the interconnection

chip

Actual processed patron

Typical build

The wiring is copper, as copper can be stretched due to its ductility the typical wiring of the track, also called horse shoe like assures that the PCB can be stretched without harming the copper wiring.

The technology can be made in a 1 or 2 layer construction, whereas the conductors should be above each other to assure even stretch of the complete circuitry, unlike the flex circuitry construction just avoiding that the tracks are above each other. Under development is even a more layer construction.

Due to the 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 an alternative. Also alternative builds are under evaluation which would make it even possible to solder with Sn.

Cross section build of a 2 layer construction

Finish Im. Ni. Au

Im. Ag

OSP

cross section

Cover layer PU Cooper traces Base material PU Cooper traces Cover layer PU

Options: Stretch - Bending – Twisting

Unlike other interconnection techniques the SCB technology allows many degrees of freedom, besides stretch and bending is also twisting an option. The polyurethane base material is unlike the polyimide base material not sensitive for tearing or cracking. This make that specially incase unexpected movements can be expected the SCB technology is a real option.

base material

Possible applications and cases

Functional clothing

base material

An important application area for stretchable printed circuit boards is in functional clothing. Adding sensors to clothing the functionality of the clothing can increased. This can be a simple signage or integration of remote sensing and control, including signaling by means of vibration. This way additional information can be gathered but also information can 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 and still allowing free movement without destroying the electronics.

Complicated positioning of eg. sensors

Measuring and monitoring of vital signs, such as heart rate and breathing require 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 make that easy wearable clothing can be designed.

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

Smart Sensor Glove for Arthritis Rehabilitation

Diabetics causes among others the loss of sensing pain in the feet of the 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 not only that the part can easily bend but also the shoe size can be “adjusted” by more or less stretching of the SCB. The unit includes a memory unit to store the data for later analysis. In this case a connector interface with a stiffener construction is developed to connect to the memory unit using a ZIF connector. Realized: Multiple circuits.

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

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.

Based on the biomechanical model the degrees of freedom are identified and the sensors and. their positioning are determined. The construction of the glove includes bend sensors, pressure sensor and 3-axis accelerometer. Realized: assembled stretch PCBs for evaluation

Mechanical decoupling

The low E-modulus of the base material in combination with the horse shoe like shape of the copper wires makes the SCB technology very suitable for application were decoupling of mechanical vibration is crucial. The decoupling can realized with a far less need for space compared with traditional polyimide circuitry.

compared to Polyimide 3.5GPa, which is used in flex or flex rigid constructions).

base material
base material

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

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.

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.

Articles