Mechanical and Ka-Band Electrical Reliability Testing of Interconnects in 5G Wearable System-on-Package Designs Under Bending

Additive printing is being increasingly used for the fabrication of flexible electronics. Low-loss interconnects are required to achieve heterogeneous integration for 5G/mm-wave electronics packaging. In addition, the packaged systems need to have good mechanical and electrical reliability under bending conditions and survive from a large number of cyclic bending to be applied under flexible conditions. A microstrip line is designed and inkjet printed on a 3D printed polypropylene (PP) substrate. Three different metallization thicknesses of printed ink structures, 3 layers (6.13 mu m), 6 layers (9.52 mu m), and 9 layers (14.82 mu m), are fabricated for characterization. Nine-layer microstrip line samples have the lowest insertion loss (less than 0.08 dB/mm) over 20-40 GHz. The maximum loss measured at bending condition is 0.17 dB/mm when the 3-layer samples are bent over the smallest mandrel with radius of 1 in. In the cyclic bending test over a mandrel with a radius of 1 in., 6-layer samples are able to maintain < 0.10 dB/mm loss before 10000 cycles. Scanning electron microscopy (SEM) images show that thicker printed ink tends to have more micro defects formed during fabrication, and cracks grow longer as more numbers of bending cycles are performed. As the loading is similar among samples of different thicknesses, based on mechanical finite-element analysis (FEA) simulations, the original defects from fabrication dominate the reliability of microstrip lines. Ramp interconnects are designed and fabricated, and each interconnect features 0.1-0.5 dB insertion loss over 20-32 GHz. Preliminary bending tests with a bending radius of 1 in. show that the ramp interconnect has no physical changes up to 10000 bending cycles.