Uncover savings and efficiencies with printed silver flexible circuits

Author : Wladimir Punt, Molex

20 July 2017

A massive and rapidly expanding range of products, from sensors and monitors to wearable and handheld devices, require flexible circuits. The bar is set high for these electronic devices and equipment.

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Consumers, healthcare providers, system integrators and other end-users expect high performance, preferably with sophisticated designs, intuitive smart controls and a compact footprint – all at a competitive price point. Without losing sight of the project budget, OEM engineers must identify the right materials and circuit technologies to achieve all of the above.

Etched copper circuits are foundational technologies in electronics, whether in rigid or flexible form. Printed circuit boards (PCBs) offer well-known mechanical integrity, electrical conductivity and reliability, but are also limited by their rigid construction. Their flat nature limits designers to two dimensions, which severely restricts design flexibility, especially as electronic devices reduce in size. Flexible printed circuits (FPCs) are able to bend and take advantage of three-dimensional space, while also accommodating soldered components. While FPCs improve the ability to package electronics, they typically come at a premium cost, compared with PCBs.

Long used in the appliance and membrane panel industry, printed silver circuits on polyester substrates are now increasingly being used in traditional electronic devices, where etched copper circuits have historically been used. Utilising printing methods to selectively apply conductive inks on flexible polyester (PET) substrates, printed technologies perform well in many applications at significantly lower cost.

Creating depth and space

Historically, a design engineer’s only recourse as electronic packaging became denser was to incorporate flexible PCBs to route traces around three-dimensional structures. Innovations in printing and component attach, however, mean that designers can now use light, flexible printed silver circuitry in applications where traditional etched copper circuits have been prohibitively expensive.

Advancements in silver ink printing now allow manufacturers to print traces as narrow as 0.127mm, with spacing as small as 0.127mm. The development of low-resistance inks makes a particularly effective circuit for a wide variety of low-power and low-speed signal applications. Utilising a high-volume roll-to-roll printing process gives manufacturers a path to scale as well.

Improvements in component bonding have been developed to overcome restrictions on attaching fine-pitch components on polyester. Membrane panel manufacturers have been using conductive epoxies for years to bond LEDs, capacitors and resistors onto polyester-based circuits. The development of enhanced bonding materials now allows the attachment of microprocessor and other semiconductor components with a pitch as tight as 0.50mm.

Economical replacement technology

A key advantage that printed silver circuits have over flexible etched copper circuits is cost. Not only are the base materials more cost-effective, but a far simpler manufacturing process is used.

Standard pick-and-place and reflow equipment is then used to bond surface mount components to the printed silver circuits. In the final step of the attachment process, a UV-encapsulant is dispensed which, when cured, makes the circuit more resistant to vibration and mechanical shock. Printed silver circuits are unsuitable, however, for through-hole or ball-grid array (BGA) components.

Conversely, traditional rigid and flexible copper circuits are fabricated via a subtractive method. The process starts with a sheet of polyimide or FR dielectric bonded to a sheet  of copper. The desired conductive paths are masked before a wet chemical process etches away all unwanted copper, leaving only the desired circuit patterns. 

In most cases, significantly more copper is removed than remains on the circuit, leading to inefficient material usage. Adding more than two signal layers also requires press, drill and plating processes. A multilayer circuit can have over 40 process steps. Additionally, due to the many wet chemical processes, etched copper fabrication facilities are required to have waste water treatment systems.

Open door to innovation

Printed silver circuits open the door to greater product innovation in virtually any application, from high-end, sophisticated electronics to frangible, disposable and other lower cost devices. Inexpensive polyester provides a flexible substrate alternative to a rigid PCB and a much more cost-effective alternative to traditional FPCs.

Screen-printed silver and roll-to-roll printed flexible circuits are proving to be a viable replacement technology for myriad applications, including:

•  Low-cost disposable sensor products, including adhesive tags, RFID and data logging devices
•  Economic alternative to traditional FPCs in portable and mobile devices, such as wearables, personal diagnostic sensor devices, fitness equipment and fashionables
•  Tamper-proof bottles and other frangible applications requiring a breakable circuit
•  Medical sensor equipment, such as lightweight circuits for EKG leads and oxygen sensing
•  Home appliances and controls, including white goods, kitchen equipment and heating/air-conditioning

OEM design engineers should not overlook economical, high-performing silver flexible circuits with finely spaced silver traces and complex components on a polyester substrate. Depending on the application, printed silver circuits on polyester can save over 25% on the substrate cost compared with an equivalent copper circuit. Engage with an experienced and qualified supplier early in the product design phase to maximize efficiencies and cost-benefits of screen-printed silver flexible circuitry.

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