Conformal Circuit Fabrication via Flash Light Sintering and Forming

This paper investigates sequential Thermoforming and Flash Light Sintering (FLS) of conductive silver nanowire interconnects printed on planar polymer sheets. The resulting interconnect-polymer assemblies are pre-shaped to a desired 3D geometry and can be robustly attached to the surface. This approach avoids interconnect delamination in manual conformation of planar flexible electronics, eliminates heating of the 3D object in direct conformal printing, and enables easy circuit replacement. The effect of the forming strain and FLS fluence on the resistance of the interconnect are studied. The interconnect resistance increases after thermoforming but is reduced significantly by subsequent FLS. The resistance depends nonlinearly on the forming strain, interconnect thickness, and FLS fluence. With the optimal parameters found here this process achieves interconnect resistance of < 10 Ω/cm within 90.8 seconds at 100% maximum strain over a 1 square-inch forming area. The application of this process for complex surfaces is demonstrated via a simple conformal LED-lighting circuit.

Scalable Forming and Flash Light Sintering of Polymer-Supported Interconnects for Surface-Conformal Electronics

Conformally integrating conductive circuits with rigid 3D surfaces is a key need for smart materials and structures. This paper investigates sequential thermoforming and flash light sintering (FLS) of conductive silver (Ag) nanowire (NW) interconnects printed on planar polymer sheets. The resulting interconnect–polymer assemblies are thus preshaped to the desired 3D geometry and can be robustly attached to the surface. This conformal circuit integration approach avoids interconnect delamination in manual conformation of planar flexible electronics, eliminates heating of the 3D object in direct conformal printing, and enables easy circuit replacement. The interconnect resistance increases after thermoforming, but critically, is reduced significantly by subsequent FLS. The resistance depends nonlinearly on the forming strain, interconnect thickness, and FLS fluence. The underlying physics behind these observations are uncovered by understanding interconnect morphology and temperature evolution during the process. With the optimal parameters found here, this process achieves interconnect resistance of <10 Ω/cm within 90.8 s at 100% maximum strain over a 1 square inch forming area. The application of this process for complex surfaces is demonstrated via a simple conformal LED-lighting circuit. The potential of this approach to enable surface size and material insensitivity, robust integration, and easy replaceability for conformal circuit fabrication is discussed.

Use of Soldered or Glued PCSB as Interconnection between PCB and Ceramic Package in Harsh Environment

Polymer Core Solder Balls (PCSB) have been used as interconnects between a 16 pin leadless chip carrier (LCC) ceramic package and a small FR4 board. A comparison was made between two different volumes of SnPb solder and an isotropic conductive adhesive (ICA) for the attachment of the PCSB to the board and to the package. Shear testing and electrical measurements were performed to characterize the interconnects as bonded and during thermal shock cycling (TSC) tests. No significant reductions of the measured fracture forces was observed for any of the sample groups. However, using a larger volume of solder or ICA resulted in less degradation of interconnect resistance during TSC, and the results for the solder were overall better than for ICA.