Simple, inexpensive, and reliable, high density interconnect technology for flexible electronics applications

The ultra-small form factor of the electronic components along with a reliable and inexpensive high-density interconnect technology are the key factors in bringing flexible electronics to the next level of miniaturization without compromising performance and substantially increasing production cost. The common thick-film methods used today to fabricate conductive trace patterns on flexible boards are ineffective for resolutions below 50 mum. Thin-film technologies can fabricate high resolution patterns; however, they are prohibitively expensive for the low-cost flexible electronics applications. A number of direct-write methods were developed to address the mesoscale (i.e., from 1 mum to 100 mum) range; however, these sequential material additive technologies are characterized with a low throughput and high production cost.This paper describes a modified mill and fill (M&F) interconnect technology, in which the surface of a flexible substrate is first treated, next trenches with the desired width, depth and configuration are formed and then filled with a conductive polymer thick film (PTF) to produce a high- resolution conductive trace pattern. In the final step, the PTF material in the trenches is cured. Reported are results from two series of experiments. In both the substrate was a 0.125- mm thick polyimide film that was chemically treated to modify its surface energy. In the first series of experiments, 100-mum wide and 35-mum deep trenches were micromachined using an excimer laser. The trenches were then filled with PTF material using a squeegee and cured. The M&F traces were subjected to cyclic thermal and bending mechanical stresses and compared to similar direct-write and screen-printed conductors. The conclusions were that the M&F conductors are characterized with better adhesion and higher reliability compared to the other methods. In another experiment, high- precision conductive traces with an excellent edge definition, linewidths of less - than 20 mum and a distance between lines as low as 6 mum were fabricated. The minimum linewidth was only limited by the laser micromachining capabilities and the size of the metal particulates in the PTF. The overall conclusion is that M&F is an inexpensive and reliable high-throughput technology suitable for high-density interconnect applications on flexible printed circuit boards.