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High-Density PTF-Inlaid Traces on Flexible Substrates
Keywords: high-density flexible circuits, printed electronics, PTF conductor
The etching of copper foil is the most common subtractive technology for generating circuitry on flexible substrates. Although the most advanced subtractive processes can generate 30 m-pitch traces, the traditional equipment and materials struggle with pitches smaller than 150 m. In recent years, electronic devices have increasingly employed printed circuits made from polymer thick film (PTF) pastes applied to the substrate by screen printing. This additive method is environmentally less harmful and much more cost-effective than the chemical etching but its resolution is limited to about 100 m. A number of sequential direct-write additive methods have been developed to address the mesoscale (1 m to 100 m) range; however, they are characterized with a low throughput and high production cost. Described here is an inlaid method for generating PTF conductive traces on flexible substrates. First, the substrate surface is treated for hydrophobicity. In the next step, a pattern of trenches with the desired width and depth is formed. Finally, the trenches are filled with conductive paste using a squeegee. Reported are results from experiments in which high-precision conductive traces with linewidths of less than 20 m and pitches as low as 6 m were fabricated. In some experiments, vibration was added to promote the filling of trenches. By way of comparison, 100-m wide and 35-m deep PTF-inlaid traces and similar direct-write and screen-printed traces were fabricated and subjected to cyclic thermal and bending mechanical stresses. The PTF-inlaid traces demonstrated better adhesion and higher reliability compared to the other methods. The conclusion is that the PTF-inlaid method holds a promise as a low cost solution for the volume production of high density flexible circuits.
Valery Marinov, Professor
North Dakota State University
Fargo, ND

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