New Plastics Can Speed Flexible Printed Electronics Development

Substrates play a huge role when designing any type of device, including printed and flexible electronics. From its compatibility with your printing process or with the inks and materials you’re using, to its thermal properties, the choice of substrate can have a significant impact on the effectiveness and manufacturability of your product.

However, substrate material capabilities tend to be overlooked when determining what should go into a product design. Sensor specifications, end-product functionality and other characteristics take precedence, while the substrate itself is rarely discussed. Yet, when wafers are purchased for R&D or manufacturing, some degree of customization is typically called for as bulk wafers must be tweaked to accommodate the process being implemented. This is particularly true for emerging technologies such as flexible and printed electronics.

Plastic materials for substrate films and coatings have existed for some time, but the options have been limited. The two basic types of polymeric materials currently in use are thermoplastics and curable plastics. While thermoplastics offer good mechanical strength, elongation and toughness when bonded or applied as a coating to another material, they have problems with adhesion due to their high viscosity. They are also susceptible to damage from organic processing solvents, as well as from elevated temperatures.

Curable plastics, which are widely used for semiconductor packaging, are cured by heating at 200°C or higher temperatures. Unlike thermoplastics, curable plastics adhere well to other materials, but due to their molecular structure, they are stiff and rigid; while tough, they can be brittle. This makes them subject to cracking during thermal processing.

Promising innovations in plastic
Developments in flexible electronics have created a need for new plastic packaging materials with properties better suited to their requirements. Two polymer-based materials have been developed that offer best-of-both-worlds alternatives to thermoplastics and curable plastics, and could prove invaluable for manufacturing flexible and printed electronics. Called photo-curable thermoplastics (PCT), these new materials can be used for thick, structural films and solvent-based coatings.

The first material, polycyanocinnamate, behaves like thermoplastic, but can be cured at high temperatures (175°C to 200°C) or by ultraviolet (UV) exposure, to form flexible thick films with excellent toughness and elongation. The second material, polyacylhydrazone, offers high adhesion to silicon, glass, copper and polyimide; is resistant to solvents (minimizing damage during processing); and can form strong, tough, flexible thick films with good elongation.

One particularly exciting aspect of these new plastic materials is that they can be laser etched quickly and cleanly using available laser ablation tools (see Figure 1). The process is much slower with other materials, such as polyimide, and leaves more residue.

Figure 1. In this example, a 308nm excimer laser ablation tool was used to conduct patterning on a polyacylhydrazone material. As these figures show, not only were very high etch rates achieved (above), but also the results proved to be virtually residue-free (below). Patterning/results courtesy of SUSS MicroTec.

These materials were specifically designed to be easily laser etched—i.e., their polymer chemistries were chosen because they allow the materials to interact well with the laser and provide high UV absorption, resulting in a smooth etch. Moreover, a very-low-power UV laser (<1 mW) can be used precisely because the materials absorb laser light so effectively.

In addition to the benefits described above, PCT materials offer thermal stability at high temperatures, as well as at room temperature over time (so they can be easily stored), and they don’t release volatile byproducts when baked. These new substrate materials hold great promise for flexible electronic applications, and it will be exciting to see how manufacturers choose to license and implement them going forward.