Stretchable mould interconnect optimization : peeling automation and carrierless techniques

The primary bottleneck of the stretchable mold interconnect (SMI) technology is its reliance on carrier boards. These are necessary to handle the meandered circuit during production and to ensure dimensional stability of the flexible circuit board before encapsulation. However, for all the problems it solves, it also introduces a new major problem by requiring a peeling step – which is difficult to automate. This manuscript aims to present some of the work that went into eliminating this problem, discussing both unsuccessful and functioning methods to tackle this conundrum and some of the experimental work that went into verifying these techniques. First, alterations to the design to simplify peeling are considered, followed by adhesivebased peeling processes and mechanical pin-based systems. Next, masking and structuring of the carrier board adhesive are considered. Finally, two carrierless methods which circumvent the problems are discussed, a two-step process – which cuts temporary support structures after partial encapsulation – and a technique whereby the frame is designed to fail in a controlled manner during the first use of the circuit, creating a carrierless process feasible for high-volume production

One-time deformable thermoplastic devices based on flexible circuit board technology

This contribution describes an efficient process flow for production of one-time deformable electronic devices based on standard circuit board technology and demonstrates multiple devices fabricated using this technique. The described technology has the potential to streamline and simplify the production of complex user interfaces which typically require extensive mechanical design and many components. The employed technique allows for the production of complex 3D shapes without the need to modify existing circuit board manufacturing equipment or processes significantly. To achieve this the device is manufactured in a flat state, encapsulated in a thermoplastic polymer laminate and deformed afterwards. This allows the usage of standard electronic components in surface mount packages, which are assembled using lead-free high-temperature solder. The circuit is deformed using a high-volume cost-effective thermoforming approach, where the encapsulating polymer is heated above its glass transition temperature and forced against a mold where it is allowed to cool down again. To enable significant out-of-plane deformations stretchable meandering interconnects are used, which were traditionally developed for dynamically stretchable devices. Fabrication of the circuit starts using a standard flexible copper clad laminate which is processed using the default techniques, the resulting circuit is then attached to a carrier board coated with a reusable high-temperature pressure sensitive adhesive. The interconnect and circuit outline is then defined using laser routing or punching, cutting the flexible circuit without damaging the carrier. The residuals not part of the circuit are removed, in a process akin to protective film removal, and solder paste is stencil printed on the circuit. Afterwards components are placed using a pick-and-place machine and the boards are reflow soldered. After functional testing and repair (if necessary) the circuits are placed in a vacuum press with a thermoplastic laminate, consisting of a thermoplastic elastomer and a rigid thermoplastic sheet. During this lamination the components are protected by a highly conforming press pad. Because the adhesion between the elastomer and the circuit far exceeds that between the circuit and the carrier the circuit is released readily as the thermoplastic laminate is peeled away. The resulting laminate is built up further using thermoplastic films and sheets, and finally deformed using a vacuum forming machine. The resulting device, which is trimmed to remove the clamping edges, can then be mounted in the final assembly. The advantages of this approach are demonstrated using a series of test vehicles, demonstrating the integration of complex circuits, connectors, and power circuitry. Finally, a series of design considerations that became apparent after initial reliability testing are discussed, together with the resulting design rules

Plastic electronics based conformable electronic circuits

In this contribution, results on technology developments are presented aiming to realize conformable electronic systems based on plastic electronics technologies. Focus of the developments is on low cost with an acceptable reliability in function of the end-application. The feasibility of this technology is demonstrated by digging into the different process steps and their characteristics. A number of demonstrators (large-area conformable illumination tiles) have been realized and are discussed. The major part of this contribution is on the mechanical characterization of these plastic electronics based technologies

2.5D smart objects using thermoplastic stretchable interconnects

This contribution describes the technology used to produce thermoplastically deformable electronics, based on flexible circuit board technology, to achieve low-cost 2.5D free-form rigid smart objects. These one-time deformable circuits employ a modified version of the previously developed meander-based “polymer-last” technology for dynamically stretchable elastic circuits. This is readily achieved by substituting the dynamically stretchable elastomeric materials (e.g. silicone) with thermoplastic polymers (e.g. polycarbonate). Afterwards the circuit is given its final form using widely available thermoforming techniques, such as vacuum forming, where the material is heated above its glass transition temperature and drawn against a forming tool by a strong vacuum. After cooling down the thermoplastic retains its shape without inducing large internal stresses. The presented method allows for the production of these circuits on a flat substrate, using standard printed circuit board production equipment, with deformation only taking place afterwards; eliminating the need for large investments and reducing the cost of fabrication. Potential advantages over competitive methods are reductions in weight and material usage, decrease of mechanical complexity; lower tooling cost, increased resilience, and a higher degree of manufacturer independence due to adhering to standard industrial practices. This is realized by starting production from a flexible circuit board, manufactured by an industrial supplier using polyimide flexible copper clad laminate, which is attached to a temporary reusable carrier board through means of a silicone based high-temperature pressure sensitive adhesive. Through selective laser structuring the meander and island outlines of the flexible circuit are defined, without causing damage to the carrier board or pressure sensitive adhesive. After removing the residual material the circuit is assembled using high-temperature lead-free solder, made possible by the temporary carrier keeping the circuit in place at these elevated temperatures. The circuit is then transferred into a thermoplastic laminate, which is deformed into its final shape. After demonstrating the need for stretchable electronics for this application, this contribution describes the method used to design, fabricate, and test the first one-time deformable circuits manufactured using the presented technology. Using the initial set of observations a series of preliminary design rules is established, both for the circuit and choice of materials. The feasibility of this manufacturing method was then demonstrated through a small scale production run using lab scale equipment, where a large quantity of high power LEDs was integrated into a one-time deformable device made out of polystyrene and thermoplastic polyurethane. These devices were then tested by exposing them to real world conditions for several days

Arbitrarily shaped 2.5D circuits using stretchable interconnections and embedding in thermoplastic polymers

AbstractThis contribution describes considerations and very preliminary results in the technology development of thermoplastically deformable electronics and sensor circuits, with the intention to eventually achieve the low-cost fabrication of 2.5D free-form rigid smart objects. The technology is based on the one for elastic circuits, developed and characterized before, which is using soft elastic polymers as materials for the circuit carrier. For 1-time deformable circuits the elastic carrier needs to be substituted by a thermoplastic material. An additional step of thermoforming is necessary after the entire circuit is fabricated on a flat surface, which is the normal industrial practice for circuit fabrication and which thus is also pursued here. First tests have been executed and simple circuits fabricated, using meandered Cu tracks as 1-time stretchable interconnects, PET-G as the thermoplastic carrier and SMD LEDs and zero-ohm resistors as circuit components