PSI - Issue 80

Marilyne Philibert et al. / Procedia Structural Integrity 80 (2026) 65–76 Author name / Structural Integrity Procedia 00 (2019) 000–000

68 4

2. Methodology 2.1. Design and manufacturing

In this study, the printed strain gauges are patterned into a simple measuring grid, similar to commercially available strain gauges. For electrical properties assessment, lines or serpentine designs are also printed. Direct-write printing is achieved using Voltera V-One printer for directly printing on PCB substrates during preliminary tests and NOVA printer for directly printing on flexible substrates thanks to its vacuum table. The width of the wire or line pattern is more than 0.1 mm and different widths are tested for optimisation. Thin and long traces are aimed for higher final resistance, which would allow accurate measurement of resistance changes. The measuring grip pattern (zig-zag pattern) allows printing traces with length up to 1.5 m in this study. Even though the pattern allows printing long trace in a compact design, increasing the length implies larger design prone to defects during printing or handling. For printing resistive strain sensors, a conductive ink is needed. Silver conductive inks are widely available as it is less expensive than gold and with higher electrical conductivity and stability than copper. Carbon resistive inks are also good candidate for printing piezoresistive strain gauges due to its low price, high resistance and flexibility. However, piezoresistive strain gauges are often less consistent in performances and with lower long-term stability, as previously discussed in the introduction. Therefore, this study investigates printing conductive silver ink on flexible substrates in a measuring grid pattern for manufacturing resistive strain gauges. Silver conductive inks used in this study are from Voltera or ACI Materials with characteristics presented in Table 1. Flexible inks allow flex and crease ability. The sheet resistance, ( , is proportional to the resistance, , the width, , and length, , of the trace and defined as: ( = ) ! (3) Table 1. Silver conductive inks characteristics. Silver ink Sheet resistance (mΩ/sq) Density (g/mL) Solvent Curing Substrates

Isopropyl Alcohol Isopropyl Alcohol Acetone, MEK

30 min @ 210°C 15 min @ 170°C 5-15 min @ 150°C

Voltera Conductor 2 Voltera Conductor 3

2.05 @ 50 µm

3.35

Rigid

Rigid; Flexible Rigid; Flexible

2.4 @ 15 µm

3.72

2 @ 25.4 µm

3.72

ACI FS0142

The ink is cured on a hot plate. Substrates used in this study are rigid PCB from Voltera, and flexible films such as polyethylene terephthalate (PET) films, Kapton (polyimide) films or thermoplastic films. Conductive inks are stored in cartridges and different dispensing nozzle tips can be used, such as 230 μm disposable polypropylene (PP) or 150 μm metal tips. Preheating the ink during printing allows reduced viscosity and stable flow rate for more consistent printing. The printing calibration is tuned by adjusting print speed, print height, and pressure. The designed trace width may differ from measured width and toolpaths (printing path) may need to be manually improved for each design. In strain gauges design, only one toolpath is performed for the sensitive trace for achieving traces as thin as possible. The printing parameters are also optimised for each ink to minimise cross-section area of the resulting trace while ensuring consistency and repeatability. The print height was kept at 60 µm, while the print speed is maximised and the dispense pressure is minimised. The printing parameters optimisation for the ACI FS0142 silver ink is shown in Fig. 1.