Multiphysics Testing and Numerical Modeling of Fully 3-D/Inkjet-Printed Microstrip Line Under 12500 Cycles of Bending for 5G/mm-Wave Flexible Hybrid Electronics Packaging

Additive manufacturing is increasingly used in the fabrication of flexible electronics. A critical component of flexible electronics packages for 5G/mm-wave applications, the interconnect needs to demonstrate low loss and adequate mechanical reliability while surviving a large number of bending cycles to fulfill its intended functionality under flexible conditions. In this study, a proof-of-concept inkjet-printed microstrip line on a 3-D-printed polypropylene (PP) substrate is designed and fabricated, with metallization thicknesses of three, six, and nine layers. Initial tests reveal excellent insertion loss values at flat conditions, with thick samples showing lower insertion losses. In monotonic mandrel bending tests, the insertion loss is observed to increase with the strain but remain below 0.130 dB/mm even for the smallest bending radius of 1 in. Electrical numerical models developed in CST Studio Suite are used to analyze the impact of various factors on the observed insertion loss, with results showing minimal contribution from the shape change and more significant contribution from changes in conductivity and surface roughness, which effectively explains discrepancies between simulations and empirical observations. Mechanical-loading-induced geometry modifications and property changes in the printed silver nanoparticle (AgNP) layer are also quantified using mechanical finite-element analysis (FEA) simulations, scanning electron microscopy (SEM), profilometer surface scan, and four-point probe resistance measurements, which provide further mechanistic insights that help improve electrical models to capture empirical observations. Cyclic mandrel bending tests up to 12500 cycles are conducted and results show that six-layer samples offer the best balance between electrical performance and mechanical reliability for 5G and mm-wave applications. Numerical models in CST Studio Suite suggest that while local cracks and microstructure changes due to cyclic bending could not be fully represented by the decrease in conductivity alone, the increase in macroscopic surface roughness serves as a representative equivalent. This study contributes to the understanding of additively fabricated flexible electronics components by providing important empirical data under bending conditions and valuable mechanistic insights that help improve predictive models.