Testing and Reliability Characterization Methods for Flexible Hybrid Electronics
Pradeep ball, Jinesh Narangaparambil, and Kartik Goyal
Flexible electronics in wearable application may be subjected to twisting, folding, or flexing depending on the form factor and the location of use. The trend toward weight reduction and miniaturization of electronics is driving the emergence of flexible electronics. A number of additive printed electronics methods are available including aerosol-jet printing (AJP), inkjet printing, screen printing, and gravure printing. Reduction in size and weight are especially important for applications focusing on sports, leisure, healthcare, military and security apparel, fashion, and wearable consumer electronics [Tao 2017]. Mechanical stresses are induced on flexible electronics when subjected to bending, which can lead to delamination, cracking, or shearing of interconnects and additively printed layers [Dai 2015].
An understanding of the shear strength of the deposited layer is important in addition to the effect of process conditions on the reliability and survivability of the printed structures.
The AJP is a popular method in flexible electronics because of its high-resolution in comparison with inkjet, screen printing. It has also gained much popularity as a low-volume, custom print electronics manufacturing technique. The vulnerability of the conductors for mechanical stresses needs to be investigated thoroughly as they are an essential block in flexible electronics [Happonen 2015]. From a bendability point of view, high mechanical strength and stability are key parameters required from the substrate material. A common choice for the substrate is plastic film, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or polyimide (PI) [Yakimets 2009; Lacerda 2013]. The relationship between microstructural evolution, such as particle growth, porosity, and electrical conductivity during post heat treatment has been widely studied. Reducing the porosity and forming a dense microstructure can improve the electrical conductivity, which can be attained by controlling annealing temperature [Lee 2005; Park 2006; Jeong 2006], annealing time [Greer 2007], heat treatment method [Kim 2011], and ambient atmosphere [Yia 2010]. According to the end application, the process parameters can be varied in order to get a lower resistance but, at the same time, enough shear strength to suffice the need. Flexing is one of the common body joint motions. Wearable application in flexible electronics being the major area, it is important to analyze and improve reliability.
Flexure Reliability of Flexible Hybrid Electronics (FHE)
Figure 13.1 shows the flexure setup developed at Auburn University's CAVE3 Research Center. The setup includes a data acquisition unit connected to the test vehicle to measure the resistance across each individual trace, which records the data into the computer. A stepper motor, power supply source, stepper motor driver, microcontroller, lead screw actuator, and hinge setup are used to produce flexure.
The acquisition unit measures real-time data and records it into the software, which helps in analyzing the fatigue failure point more conveniently. Figure 13.2 shows the
Experimental setup (flexure).
Motion of the test board (flexure).
motion of setup. The test setup replicates the v-bend encountered in body motions like folding hands at elbow or folding of legs. These bends allow the testing of local features in the line allowing for applying stress at a region or over a region of line and not on the entire test board [ball 2018]. The wires are connected to the traces on the substrate using silver conductive epoxy. Figure 13.2 shows how the actuator moves in the horizontal direction imparting flexure to the test-specimen. The samples are fixed with the help of two metal sheets of width 4 mm at 10 mm from the center of the hinge. Figure 13.3 shows the experimental setup with the flexible electronics test vehicle.
Figure 13.4 shows the experimental stepwise flow. Each end of the traces is connected to the data-acquisition system wire using silver conductive epoxy. The wire connection is routed to the multiplexer (34901A) which is connected to the Agilent Data Acquisition Unit (34970A). The acquisition unit is connected to a computer through general purpose interface bus-to-universal serial bus (GPIB-USB) cable. Cycle time for the setup is 2 sec (0.5 Hz). The acquisition unit takes readings at an interval of 1 sec such that it takes reading at the start of the cycle and at the bent position similarly in twist state. The data recorded by the software is saved with '.csv', an extension, which can be imported to the excel for analysis.
Experimental setup with a test vehicle (flexure).
Block diagram of the experimental setup.
Figure 13.5 shows the actual graph plot using the high-speed camera for its position versus time graph to measure frequency and velocity.