Copper foil is the primary conductor material used in traditional flexible circuits. These thin metallic foils laminated on heat-resistant plastic films are called copper-clad laminates. Photosensitive chemicals are coated on the copper foil, and circuit patterns are printed. Next, a process called photolithography or subtractive is used, and the result is the production of electronic circuits (once unnecessary material is removed).
The photolithography process is now popular in the printed circuit industry, and circuit manufacturers have an etching line process in their basic manufacturing. The chemical etching is not environmentally friendly. Its by-products are considered chemical waste and require supplemental recycling; this adds cost to printed circuit products.
Thick-film circuits are another option to generate flexible circuits. They are created using a printing process that is very simple. Conductive ink (such as silver and graphite conductive powders) is printed on plastic films from a traditional screen-printing process, dried, and baked in a thermal oven. Copper-clad laminates are not required, so material expenses are minimized. Almost any material can be used (papers, textiles, or rubber sheets) with the flexibility from printing.
Wearable and medical devices require abnormal performances such as stretchability, transparency, moisture permeability, and more. These new products and applications require the use of flexible circuits and are dependent on non-traditional electronic materials. Conductive inks are very compatible with these non-traditional materials, and thick-film printing is very suitable. The screen-printing process can increase up to 1000 mm x 1500 mm, and an even larger circuit sizes are possible as long as the screen is available.
Another advantage of thick-film technology is the availability of functional materials. Various devices—such as flexible sensors, flexible ELs, photovoltaic cells, and switches—are built with a simple process using appropriate inks. The thick-film process does not generate any chemical waste; thus, the manufacturing is very cost-effective.
Thick-film circuits have conductivity and silver atom migration issues. Advancements have improved over the last few years, but the conductivity from a silver ink circuit is at least two orders smaller as compared to copper foil circuits. Applications may be limited.
With thick-film flexible circuits, advantages outweigh the disadvantages. Applications continue to expand, and a printable process is an excellent option for flexible electronics. Technological collaborations will create more opportunities for flexible circuits.
This column originally appeared in the November 2020 issue of Design007 Magazine.