Next-Gen Devices Call for Flexible, Lightweight Circuitry

Cutting-edge devices and electronics are pushing the envelope when it comes to manufacturing technologies. While printed circuit boards (PCBs) have dominated designs for decades and copper flexible printed circuits (FPCs) have provided much needed 3-dimensional (3D) capabilities, they present significant limitations as well. In response, technological advances in conductive inks have enabled designers to use silver printing on polyester as an alternative.

Shaping the Future of Electronics Designs

The manufacturing industry has seen some exciting new technologies in the past decade. From precision laser cutting to the recent advances in 3D printing, engineers can call upon a vast array of techniques and materials to design the next generation of consumer devices. The ability to easily create complex shapes, even in small quantities, has changed the design of electronic devices. These advanced techniques allow the manufacture of customer-driven designs in fields such as mechanical controls, wearable devices and medical equipment. However, these complex ergonomic shapes present challenges for the electronics designer. 

The primary solution for the electronics designer is the PCB. The backbone of the electronics industry for half a century, the PCB became popular in the 1970s at the same time the microprocessor was entering mainstream use. The reduction in size that the PCB and microprocessor delivered to the electronics industry was revolutionary. Almost overnight, computers that had previously needed a building to house them became small enough to fit on a desk.

The PCB: Dominating the Industry for 50+ Years

The Dominance (and Limits) of the PCB

The PCB is easy and cheap to manufacture in huge quantities, which has been crucial to its dominance in every sector of society. With the advent of surface mount technology (SMT)components, electronic equipment could grow smaller still. As a result, PCBs can now be found in everything from kitchen appliances and children’s toys to advanced medical equipment and satellites.

The PCB is an ideal solution for today’s advanced electronic devices, and yet it is limited by its 2-dimensional shape. The construction of the PCB, with layers of copper, woven glass and epoxy, makes it rigid and brittle. It is simply not possible to create a 3-dimensional shape with traditional materials.

In the search for a solution to creating a PCB in 3 dimensions, engineers turned to different materials to provide a flexible dielectric substrate. Replacing the conventional rigid material with a flexible alternative allows the circuit to be bent and folded to create complex shapes. The most popular choice for flexible circuits is polyimide foil. 

Flexible Circuit Construction

The construction process for polyimide-based circuits is similar to the one used for conventional PCBs. A copper layer is bonded to the substrate to provide conductivity. This layer is then masked and chemically etched, removing the copper from the unmasked areas to leave only the final circuit. Components can then be placed and soldered using conventional methods.

This is a subtractive process in which much of the copper is removed. This inefficient use of material, combined with the relatively high cost of the polyimide foil, makes copper FPCs an expensive solution. This is especially true for oddly shaped circuits that are frequently required in FPC applications, resulting in poor material utilization.

In the search for alternatives to polyimide, engineers have experimented with everything from woven and nonwoven fabrics to polymers and even paper. In many cases, their utility was limited by the ability to apply the conductive layers that form the circuit.

Conductive Ink Developments

However, recent advances in conductive inks have allowed designers to use silver printing on polyester as a lightweight, competitive alternative to copper FPCs. The construction of circuits with conductive inks is far more efficient than traditional etched copper techniques. The development of these inks now permits manufacturers to create traces as narrow as 0.127mm (0.005″) with spacing of a similar size, enabling these printed circuits to compete with traditional high-density PCBs.

Polyester as a Flexible Substrate

Designers can even turn to alternative substrate materials, depending on their application. For example, while a handheld device might use polyester to provide long-term stability at a reasonable cost, a single-use medical device could use paper to reduce the cost further and lessen any environmental impact when the item is discarded.

New advances in conductive inks have gone hand in hand with improved bonding technology. One of the greatest problems associated with flexible circuits was the bonding of components onto the substrate. Established techniques used conductive epoxies to anchor components such as LEDs, capacitors and resistors to the circuit. This imposed limitations on the attachment of   fine-pitch components onto the substrate. 

These limitations have been reduced with new bonding materials that allow the attachment of complex devices to flexible circuits. Microprocessors and other semiconductor components with pitches as fine as 0.50mm (0.02″) can now form part of the flexible circuit.

LEDs, Resistors, Capacitors and an Integrated Circuit Bonded to a Silver Flexible Printed Circuit

The manufacturing process is even flexible enough to allow multi-layered circuits to be created. Conductive inks can be printed onto either flat sheets or continuous rolls of substrate, as the solvent-based inks can cure in just a few minutes at 200 to 300°F. This is then followed by a dielectric layer to provide electrical isolation and environmental protection. Using this method, it is possible to add more ayers to create a more complex circuit.

Exciting New Applications

Silver FPCs have potential uses throughout electronics. Their benefits have led to their adoption both as an alternative to copper FPCs and as an innovative solution for new applications. 

Silver FPCs, with their low cost and light weight, have found uses in wearable devices. The technology is ideal for disposable sensor devices in the medical market, fitness and lifestyle applications, and even the world of fashion. Other disposable uses include data loggers and RFID tags for the food and beverage industry. 

These same features have led to new and interesting uses for printed silver circuits as frangible security seals for tamperproof bottles and other packaging. Printed silver circuits are also a cost-effective solution for the competitive home appliance market, including white goods, smart home devices and environmental controls. 

Whether as a solution for new and exciting applications or as a cost-effective alternative to existing technologies, silver FPCs offer a range of advantages. Their low cost, light weight and design flexibility make them ideal for applications across the electronics industry and beyond. They are the perfect companions to the latest production technologies, allowing engineers to make the most of the flexibility that the smart factory provides.

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