Applying additive manufacturing techniques for semiconductor and electronics packaging.
By Gity Samadi and Paul Semenza
The FLEX Conference, held again this year in conjunction with SEMICON West 2023, provided numerous examples of continued developments in flexible, printed, and flexible hybrid electronics technologies applied to sensing, robotics, communications, and other applications. At the same time, there is growing focus on applying various additive manufacturing equipment, materials, and processes to solving the challenges of heterogenous integration and other forms of advanced packaging adopted by the semiconductor industry. Following are key takeaways from the conference.
A key theme at the event was the integration of electronics functionality into processes for additive manufacturing of 3D structures. New developments point the way toward customized fabrication of components for communications, power, display, and other components and subsystems used in medical, industrial, aerospace, and other applications.
Jared Wittkopf, Research Manager at HP, highlighted the benefits of multi-jet fusion (MJF), which uses conductive agents such as silver nanoparticle ink to create conductive properties, enabling printed conductive traces, vias, and pads anywhere within or on any part’s 3D geometry. Similarly, resistive agents such as carbon black, and insulating agents, such as barium titanate, can be used to create other circuit elements.
HP Labs has demonstrated pick and place assembly of packaged components onto the 3D printed electronic structures, and aerosol jet post-processing to create features smaller than 100 mm (the MJF process currently has design rules of 250 mm). Working with Binghamton University’s Center for Advanced Microelectronics Manufacturing (CAMM), HP Labs has constructed demonstrator devices with RF and WiFi capabilities, an OLED display with touch control, and battery power management circuits.
Binghamton University’s CAMM presented an update to its ongoing research on using aerosol jet printing to replace wirebonding in high-performance applications including power and RF. These approaches enable packages with novel form factors and excellent electrical performance and reliability. Mark Poliks, SUNY Distinguished Professor & Director, presented work with GE Research on creating silicon carbide power modules using direct write processes for insulators and conductors, enabling compact, conformable packages, with lower inductance, a combination of characteristics that neither wire-bonded nor power overlay packaging can achieve. Stephen Gonya, a Research Scientist at CAMM, presented the center’s ongoing research on using aerosol jet printing to deposit impedance-controlled interconnections to silicon die bond pads in place of wirebonding in RF circuits.
Kexin Hu, a Ph.D. student at Georgia Tech, presented new developments in additively manufactured flexible system-on-package antenna arrays for 5G/mmWave bands. Georgia Tech researchers identified polypropylene as an attractive 3D-printable dielectric material with a very low loss tangent of 0.001, and used ink-jet printed silver nanoparticle material for conductive traces. The team also fabricated an inkjet-printed reconfigurable phased array with an integrated microfluidic cooling channel on 3D printed substrate.
David Keicher, VP at IDS, discussed recent results from aerosol printed gold bumps using high-speed shuttering. IDS has demonstrated that a wide range of gold bumps can be aerosol printed with low to high aspect ratios and small diameter features.
In an alternative approach to bumping, John Yundt, VP of Sales and Business Development at SunRay Scientific, described ZTACH pressure-free anisotropic conductive epoxy for die-to-die bonding. ZTACH uses ferromagnetic particles within an epoxy; the particles within Z-axis columns are fixed in place during curing using a magnetic field (magnetic pallet) without pressure. The epoxy can be deposited using stencil printing or syringe dispensing as no patterning is required. As opposed to traditional die bumping, which requires underfill around the bumps for mechanical stability, the ZTACH assembly process involves only a single adhesive application. The recent work presented entailed reducing the interconnect pitch to 30 mm in order to maintain reliability.
A variety of printing techniques have been developed for or adapted to the creation of electronics functionality (conductors, resistors, capacitors, antennas, etc.) on flexible substrates or integrated onto 3D structures. Two new approaches presented at FLEX 2023 demonstrated the potential for high-volume manufacturing.
Doug Schadt, Director of Product Management at Komori America, described the company’s new approach to improving productivity and yield in traditional screen printing, depicted in figure 1 (left). Komori has developed a gapless roll-to-roll screen printing system, also shown in figure 1 (right).
Fig. 1: Comparision of traditional sheet-based and gapless roll-to-roll screen printing. Source: Komori
NanoPrintek, founded by Auburn University Professor Masoud Mahjouri-Samani, is developing an inkless multi-material printing technology, in which pure nanoparticles of various materials are generated in situ and on demand, and then directed through a printer nozzle, and laser-sintered in real time (figure 2). This system promises multimaterial printing of hybrid and tunable nanocomposite materials and structures. The use of dry printing avoids the need for complex ink formulations, surfactants/contaminants, limited printing inks, and high-temperature post-processing to sinter the particles and remove surfactants.
Fig. 2: Dry multimaterial printing system. Source: NanoPrintek
Substrates, conductive inks, and other materials are critical to economic fabrication of durable flexible and hybrid electronic systems. Several presentations added to the growing library of materials that developers may be able to choose from.
Electroninks has developed particle-free metal complex inks using metal organic decomposition in which solvents are combined with metal precursors to form complexes that are free of particles and can be cured at low temperatures. As described by Technical Project Manager Sima Hannani, the company has used the process to produce silver, gold, and platinum inks, with nickel and copper formulations under development. Such inks are intended for use in EMI shielding, while other applications are addressable via a variety of deposition techniques (spray, screen, aerosol jet, inkjet). One application uses spray coating for backside metallization of wafers.
UES described its ELMNT liquid metal printable, stretchable, and conductive ink based on technology developed at Air Force Research Laboratory (AFRL). UES Program Manager Jack Ly and Division Director Lucas Beagle showed laboratory data indicating high conductivity at a high strain of 700%. The company deposited the ink using blade coating and screen printing, and development with aerosol and ink jet print deposition is underway.
Fereshteh Kouchi, a graduate student at Boise State University, presented work on titanium carbide (Ti3C2Tx), a MXene nanomaterial ink for additive electronic manufacturing techniques. MXenes facilitate rapid electron transport through electrodes in energy storage devices, enabling fast charge storage of supercapacitors and batteries. In the Boise State work, the ink was deposited using aerosol jet on gold-coated Kapton, and then annealed.
Professor Pradeep Lall of Auburn University reported on initial characterization of water-based inks for printed conductors. These materials have the beneficial property of not containing the volatile solvents that typical solution-based inks utilize, but the impact on performance has not been evaluated rigorously. Lall’s team deposited test traces via direct-write, aerosol-jet, and ink-jet systems, before attaching components the traces with ECAs and low-temperature solvents with good results.
Franz Selbmann, Research Scientist at Fraunhofer ENAS, proposed Parylene as a new substrate material. A thermoplastic polymer, Parylene has a variety of unique properties such as optical transparency, biostability, biocompatibility, thermal stability, and low permeability to gases and water – all useful for a variety of applications. Fraunhofer has used Parylene as a substrate, a dielectric between the metallic redistribution layers (RDL), as well as for encapsulation. The Parylene is formed through a CVD process on a sacrificial silicon wafer, metallized, patterned, and separated from the wafer, forming a printed circuit board only 20 mm thick, hundreds of times thinner than rigid PCBs. Screen and aerosol jet printing have been used to deposit metal traces for interconnect, as well as soldering and wire bonding.
One application to which flexible and hybrid electronics manufacturing is uniquely suited is the creation of devices that are able to locomote, grasp, and carry out other mechanical motions. By being flexible – or soft – these devices can access interiors of complex industrial systems and interact with difficult to access components.
Nancy Stoffel, Principal Scientist at GE Research, discussed the need for and challenges of developing bio-inspired soft robots for inspection and repair of high-value industrial assets. GE is developing a soft, electronic skin-innervated robotic worm (Sensiworm) for rapid maintenance of strategic assets. The work enables robust, three-degree-of-freedom locomotion in unstructured environments, including against gravity.
The worm also sports highly stretchable conformable multiparameter sensors capable of simultaneously measuring temperature, humidity, material characteristics (cracks, corrosion, coating thickness) and can transmit power and video signals.
Professor Greg Whiting of the University of Colorado-Boulder described his team’s research of soft robots using hydraulically amplified self-healing electrostatic actuators (HASELs, which were introduced by Artimus Robotics at FLEX 2022). Whiting is focused on developing modular robots based on high-voltage soft actuators with FHE control electronics and thin-film drive electronics (figure 3).
Fig. 3: Example modular untethered robotic system. Source: Greg Whiting, UC Boulder
The industry’s investigation of flexible and hybrid electronic devices and systems to interact with the human body to provide sensing, feedback, or assistive functionality is ongoing. Applications of interest include healthcare, complex system operators, human-machine interaction, and other areas.
Professor Pratap Rao of WPI presented results of a project to develop a glove and sleeve human-machine interface system for cases for workers who need to teleoperate or train robotic systems to perform dexterous manipulation tasks, or in which people need to interact seamlessly with augmented or virtual reality environments. Printed stretchable sensors on each glove finger track the flexion of the wearer’s fingers and are connected via printed stretchable traces to a digital converter chip on a flexible circuit board mounted on the glove hand. Inertial measurement unit sensors mounted on flex carrier boards track the motions of the hand, lower arm, and upper arm. Printed stretchable traces running the length of the sleeve carry power and data for the sensors (figure 4). The WPI team fabricated thin pneumatic haptic muscles using a fiber braiding and silicone coating process and integrated them into the glove fingers to provide force feedback.
Fig. 4: Printed stretchable electronics and sensors for human-machine interface. Source: Pratap Rao, WPI
Shizuo Tokito, Distinguished Research Professor at Yamagata University, presented work on screen-printed flexible organic sensors that exploit the resistance change of conductive composite layers of carbon-based and polymer-based host materials. These sensors show high sensitivity and reliability and have been demonstrated in applications including pulse wave and respiration detection, as well as gripper control for robotics.
Professor Pradeep Lall of Auburn University described a research project seeking to evaluate automotive in-mold electronics with integrated biosensors capable of measuring electrodermal activity (EDA), which can be related to driver stress levels. Rather than using fixed electrodes adhered to the skin, these EDA sensors are fabricated through direct-write printing and thermoformed onto the steering wheel.
Mike Hopkins, VP of R&D at Liquid Wire, described a new application of the company’s metal gel materials to finger tracking gloves for extended reality. An alloy of gallium, indium, and tin, the metal gel can be printed directly onto fabrics and other substrates.
As printed and flexible sensors make ECG, EMG, and other human monitoring available in real time, issues such as coupling of sensors to the body, fidelity of sensing, data collection and analysis are attracting increased attention. Presentations from AFRL, ITRI, and SMD Solutions addressed different aspects of human monitoring in medical and military applications.
Paul Semenza is an advisor to SEMI on special projects. He was previously with NextFlex, the Flexible Hybrid Electronics Manufacturing Innovation Institute.
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