Presentations at this year’s FLEX Conference illustrated the ongoing development of manufacturing tools and processes, materials, and test and reliability evaluation techniques for the growing field of hybrid electronics, which includes printed electronics and flexible hybrid electronics (FHE). Additionally, the field includes the use of additive manufacturing processes for electronics packaging and system assembly, from die attach to flexible printed circuits.Hosted by FlexTech, a SEMI Strategic Technology Community, the conference provides an opportunity for the device making supply chain to connect to R D, design and manufacturing innovations. A review of some of the key developments highlighted in FLEX presentations follows.Innovations in Flexible Printed CircuitsTokyo-based Elephantech has been focused on using advanced inkjet systems to produce flexible printed circuits. Using additive methods instead of subtractive to produce PCBs can enable reductions in carbon footprint, copper usage and water consumption. In order to achieve these benefits, Elephantech has developed processes for combining inkjet printing of metals and electroless plating. The company synthesizes copper nano particles, which it uses to formulate metal ink. It has implemented artificial intelligence to increase print accuracy, showing the capability of average drop position error of less than 2μm, and depositing 20μm droplets into 40μm grooves and wells (Fig 1).Fig. 1. Elephantech inkjet results showing ~2μm precision and prototypes with 50μm line widthExamples of Elephantech’s use of flexible printed circuit technology include a set of switches for a curved monitor and a pressure sensor with reduced footprint and component count. The company intends to directly compete with larger, rigid PCBs, and is developing a mass-production system with 57,840 nozzles that can process sheet sizes of 500 x 830 mm.Traditional processes for component attach on PCBs include mass reflow ovens, thermal compression bonding, and spot laser reflow. Laserssel has developed laser selective reflow, which promises warpage- and damage-free bonding at increased processing speeds. In addition to improving the productivity of rigid PCB production, the laser selective reflow could also enable in-line processing of roll-to-roll flexible printed circuits, replacing the use of trays for bonding to flexible printed circuits.Scrona, which spun out from ETH Zurich, has developed MEMS-based printheads to improve electrohydrodynamic (EHD) printing. By using an electric field to pull droplets out of the print nozzle, EHD can enable much higher print resolution (sub-micron, compared to tens of microns), and enable the use of higher viscosity inks than would be possible with traditional inkjet heads. While EHD has been under development for some time, its application has been limited by crosstalk, in which the electric fields of adjacent nozzles interact with each other, and the requirement for the nozzle to be within tens of microns from the substrate to enable high print accuracy.Scrona’s MEMS-based nozzles address these EHD problems by shielding adjacent nozzles to prevent crosstalk and by creating a uniform electric acceleration field, which increases print distance to the order of a millimeter. The company has used its system to print a variety of inks on different substrates, as well as conformal printing on 3D surfaces (Fig. 2).Fig. 2. Example of printing silver wires across a polished glass edge; line pitch 25μm, glass thickness 1mmThe Rochester Institute of Technology (RIT) has been developing an additive technique called liquid metal droplet jetting, which can deposit metal traces functionally equivalent to solid wires. The process uses metal wire as a feedstock, which is a fraction of the cost of nanoparticle metals. While tin, zinc, and aluminum have been used, silver and copper are still under development. The wire is melted in a micro-crucible, which feeds a nozzle; metal droplets are then jetted on demand in an argon environment to prevent oxidation (Fig. 3, l). Upon hitting the substrate, the drops solidify into metal traces equivalent to solid wire, quickly enough to avoid melting flexible films, and without curing or drying.Several methods have been explored to eject the jets from the nozzle, including magnetohydrodynamic using electromagnetic pulses, piezo-actuated pistons, and pneumatic jetting using compressed gas (Fig. 3, r). These techniques range from high-jetting-frequency and high-cost to simple and low-cost but low-frequency. Higher frequency enables overlap of droplets, increasing conductivity, and reduced processing time.Fig. 3. Concept of liquid metal droplet jetting (l); pneumatic droplet ejection approach (r)In addition to ongoing development of deposition tools and processes, the material set for additively printed electronics continues to expand. Iris Light Technologies, which spun out of Argonne National Lab and Northwestern University, is developing photonic inks for wafer-scale production of active devices including photodetectors, LEDs, and lasers. The semiconductor-based ink can be deposited via aerosol jet onto silicon wafers. Iris Light is focused on 2D semiconductors, specifically black phosphorous, which has a wider spectral coverage than graphene, is tunable in emission and absorption, and has high mobility.An example of the broadening of the additive manufacturing supply chain, Kraetonics has developed software for creating slices to be used in designing 3D-printed structures and elements. The software enables manufacturing 3D volumetric circuits with reduced size, weight, and power compared to 2D PCBs. The process involves 3D printing of hybrid mechanical-electrical assemblies such as circuits and antennas.Innovations in Test and ReliabilityAn area of active interest in the hybrid electronics community is that of test and reliability. American Semiconductor, a developer of flexible circuitry, and Bayflex, a value-added partner of Japanese equipment company Yuasa, are conducting a project on dynamic harsh environmental FHE reliability testing. The goal is to identify root causes of FHE material and system failures.The companies are developing extended temperature and humidity tests to determine FHE system lifetimes and identify causes of failures from physically deforming FHE materials and systems in harsh temperature and humidity environments. Materials under consideration for testing include:Copper on polyimide substrate with a small outline package IC and surface-mounted componentsNobleflexTM, a multilayer substrate with gold on polyimide in development for medical devicesSilver on PET substrate, with small outline package IC.The team is soliciting other test devices and is planning to coordinate with ongoing development of FHE test standards coordinated by SEMI.Henkel reported on an investigation of accelerated temperature cycling test methods, in which the company applied different combinations of temperature range, stress, and frequency of mechanical force in an effort to reduce cycle time for testing component attach reliability. The study was able to achieve similar failure modes using an accelerated test method in the case of a bonding position shift in which cracking of the die attach film was the failure mode (Fig. 4, approach 4). The study found the greatest acceleration in the case of reduced thermal shock cycles (Fig. 4, approach 1).Fig. 4. Approaches evaluated for accelerated testing of component attach.Engineering consulting firm Exponent presented the results of a study on mechanical testing for characterizing fatigue performance of flexible electronics, conducted with continuous monitoring of fatigue for 6-pin flexible flat cables from seven different vendors. Exponent found that continuous monitoring during bending fatigue testing provided greater resolution in test results including detection of intermittent failure in each sample. The study also found that strain amplitude was a critical factor for determining fatigue life, and that flat flexible cables with larger pitches showed improved fatigue performance.About SEMI FlexTechFlexTech, a SEMI Strategic Technology Community, promotes the growth, profitability and success of the flexible hybrid electronics industry by developing educational forums, directing research, and promoting technology innovation.SEMI FlexTech members benefit from speaking and business networking opportunities, introductions to key industry players, research reports, technical funding, access to end users and industry advocacy at FLEX Conferences.Gity Samadi is Director of SEMI research and development funding programs and SEMI FlexTech and SEMI Nano-Bio Materials Consortium (NBMC). Paul Semenza is an advisor to SEMI on special projects. He was previously with NextFlex, the Flexible Hybrid Electronics Manufacturing Innovation Institute.
