Thick-film Conformal Circuit Forming Method Based on Surface Partitioning and Flexible Frame Screen Printing

Abstract

Conformal circuits integrate mechanical structures and electrical performance, improving the design flexibility and equipment reliability. It enables the conformal circuit a promising alternative to the traditional printed circuit boards and independent electric wires in the fields of communication devices, automotive electronics, and medical equipment. Conformal circuits can be fabricated by several techniques such as 3D printing and 3D molding interconnect device (3D-MID) fabrication technique. However, the point-by-point reconstruction of the curved surfaces and the complex configuration of the equipment significantly reduce the efficiency and the cost effectiveness. The small thicknesses and sizes of the conductive layers fabricated by these techniques limit the application of the conformal circuits in the high current circuits. In this paper, a thick-film conformal circuit forming method based on the surface partitioning and the flexible frame screen printing is presented. The partitioning process divides the complex surface into finite number of subareas with generatrices of simple shapes and guide lines with single extension directions, limiting the mean curvature variations and the lengths of guide lines. In each subarea, flexible frames can be fixed along its guide lines and the screen can then be tensioned along its generatrix uniformly, ensuring the forming of circuits on the curved surfaces which cannot be achieved by the traditional screen printing with a rigid frame. The conformal circuits were fabricated on the external surface of an actual workpiece. The Gaussian curvature of the surface varied from - 2.2×10-3 to 1.1×10-3 mm-2. The maximum variation of the mean curvature among all subareas after the partitioning was 0~6×10-3 mm-1, two orders of magnitude lower than that of the entire surface. The uniformity and the deviation of the linewidths were around ±41 μm and ±43 μm, comparable to the printed circuits on the planar surfaces. The resistivity of the printed circuits was 7.1±0.2 μΩ•cm, 29% higher than the nominal resistivity of the silver paste used as the conductive materials in this work. The results demonstrate the effectiveness of the method presented in this paper and the potential to efficiently prepare complicated conformal circuits with a high aspect ratio on curved surfaces.