The market for additive printed circuit boards (PCBs) and flexible electronics is full of possibilities, with applications including IoT-connected skin patches, environmental sensing devices, flexible electronic displays, and much more. However, making such devices requires a carefully developed selection of materials (not all of which are electrically conductive). This article provides an overview of three key groups of materials required for additive PCBs and flexible electronics: conductive inks, substrates, and solder pastes. Throughout the article, we look at the most important materials within each group and see how additive PCB pioneer Voltera is pushing electronics material possibilities to the limit.

Conductive Inks in Additive Manufacturing: Making Prints Electronic

A conductive ink is a type of functional ink that can conduct electricity after it has been printed and processed. Printing involves depositing the ink in a computer-specified pattern with a dedicated machine, and processing typically involves applying heat to the printed pattern. Once processed, the ink can carry electrons from one place to another.

Conductive inks are made up of several components. The conductive element or filler must be a conductive material like silver, copper, or graphene particles. However, for the ink to flow freely and be accurately printable, it needs to contain many other components too, components collectively known as the vehicle. These ingredients may include binders to give the ink its polymeric backbone, solvents to dissolve the other components, and dispersants to help the ink flow and cure.

Types of Conductive Inks

• Silver: Perhaps the most important type of conductive ink for additive PCBs and flexible electronics is silver ink. This is partly due to the good conductive properties of silver and partly because of its ubiquity: silver inks are widely available and inexpensive, allowing for low-cost printed electronics like sensors and antennae.

• Copper: An alternative to silver ink is copper ink. Copper is also a conductive metal and is consequently used in traditional PCB production techniques like etching. However, because it oxidizes when cured in air, it requires extra processing which can increase the overall cost of printing.

• Carbon: Materials like carbon black, carbon nanotubes, and graphene — a carbon allotrope with a very high level of conductivity — can also be used as fillers in conductive inks, with almost limitless potential uses. Electronics applications include sensors, resistors, and heating elements.

• Conductive Polymers: Certain polymers such as polythiophene can be used as conductive printable inks. These polymers become conductive when they are oxidized due to the delocalization of their electrons.

Innovative Materials and Future Trends

Inks used in printed electronics are not just limited to conductivity. Other applications include the use of electroluminescent inks such as those developed by Saralon, which emit light when an electrical current passes through them. 

By printing with these inks, it is possible to create an Electroluminescent Displays (ELD) on a substrate. Another exciting area of printed electronics is the creation of bioelectronic medical devices. Researchers have had some success developing biocompatible inks that are both conductive and harmless to cells.

Popular Conductive Inks

Substrates: The Foundation of Flexible Electronics

Conductive inks open up a world of opportunities in additive PCBs and electronics. But electronics printing works differently compared to ordinary additive manufacturing: you cannot print an entire device out of conductive ink. Instead, the ink needs to be printed onto something. The durable material required here is known as the substrate, and it forms the foundation of flexible electronics and other devices.

As with conductive inks, there are several options to choose from when it comes to the substrate. Established types of rigid substrates include FR4, a glass-reinforced epoxy laminate, and FR1, a low-cost alternative made of paper and phenolic resin.

While rigid substrates are incredibly useful, one of the most exciting applications of printed electronics is the use of flexible substrates for devices like wearables. Flexible substrates can endure bending and various motions while maintaining functionality. They include films, foils, and even paper (electronic RFID tags are regularly printed on paper transit tickets, for example). Flexible substrates require specially developed conductive inks that will not crack or deform when the substrate bends.

Some flexible substrates can also be classified as stretchable substrates. Unlike papers and foils, these materials have good tensile strength and can have additional uses in printed electronics. However, it can be slightly harder to match stretchable substrates with a suitable ink; this is because the ink must stretch congruently with the substrate to avoid circuit breakage and separation from the underlying substrate or layer. When choosing a stretchable substrate like TPU, the ink must be carefully selected to accommodate its stretchability.

Types of Flexible Substrates

• PI (Kapton): Polyimide films possess excellent thermal stability, electrical insulation, and flexibility, making them a good choice for applications like flexible displays and flexible batteries. Though polyimide is its scientific name, the material is referred to as Kapton, a brand name owned by DuPont.

• PET: Polyethylene terephthalate, the most widely used material in the polyester family, is commonly used to make items like single-use water bottles. However, it also makes an excellent flexible substrate for flexible electronics like force-sensitive resistors. It is recyclable and more affordable than polyimide but less thermally stable.

• TPU: Thermoplastic polyurethane makes a very good substrate for flexible and stretchable electronics due to its elasticity and resistance to abrasion and oil. Its rubber-like consistency makes it suitable for wearable electronics like smart watches and even textile-based smart clothing.

Popular Substrates

Solder Paste: The Secret Sauce for Printed Electronics

We have so far looked at two very important groups of materials in PCBs and flexible electronics: the conductive inks that allow for the transfer of electrons and the substrates upon which these inks can be deposited in a specific pattern (and which can sometimes bend and stretch for added functionality). A third material, less glamorous but no less important, is solder paste. This critical substance allows PCB designers to add pre-existing electronic components onto their boards.

Solder is a metal alloy that can be fused to bond two pieces of metal. In many industries, a solder bar or wire is used in conjunction with a tool like a soldering iron or gun and a separate wetting agent called flux. In printed electronics, things are done a little differently. Here, solder paste — containing powdered solder and flux (which acts as a cleaning, flowing, and purifying agent) already mixed together — is deposited alongside conductive inks onto the substrate. After its application alongside conductive inks on the substrate, the solder paste undergoes a reflow process. During reflow, the assembly is heated, causing the solder paste to melt and flow — forming solid, electrically conductive connections upon cooling. The solder itself typically contains tin and other metals.

Voltera’s Approach to PCBs and Flexible Electronics

Additive electronics expert Voltera, based in Canada, develops printers that dispense conductive inks for PCBs and functional electronic devices. Its V-One PCB printer lets users prototype printed circuit boards in less than an hour, while its NOVA materials dispensing system can print a wide array of conductive materials onto a range of substrates, including flexible, stretchable, and porous materials. Both machines also deposit solder paste, allowing for the creation of functional electronics.

In terms of the specific inks, substrates, and solder pastes that the V-One and NOVA use to make PCBs and flexible electronics, perhaps the most important material of all is Conductor 3. Voltera’s own silver-based conductive ink formulation boasts a 30-minute curing time, a low soldering temperature of 180°C, and compatibility with rigid and flexible substrates. Importantly, however, NOVA is also compatible with other screen-printable inks.

Voltera also brings a new level of substrate flexibility to flexible substrates. NOVA’s vacuum table creates uniform suction which keeps flexible and soft substrates in place, enabling high-precision printing upon the chosen material, while a Smart Probe maps the height of the substrate. There is also a threaded mounting grid to secure custom rigid substrates of different shapes and sizes. 

Finally, both Voltera systems offer precision solder paste deposition, with the company offering tin-lead eutectic alloy and a tin-bismuth-silver alloy in T4 or T5 particle sizes, so users can solder components to their boards with precision.

Voltera enables the streamlining of research and development with direct-ink-write dispensing. Credit: Voltera

Conclusion

The PCB and flexible electronics industry moves at a fast pace. Material scientists are constantly seeking solutions to current challenges in order to evolve the technology and create electronics with greater functionality, longevity, and sustainability. And with interest rising in products like flexible displays, wearable technology, and flexible photovoltaics, the demand for printed electronics materials — inks, substrates, and pastes alike — will continue to grow.