Additive Techniques For Flexible Hybrid Electronics Packaging And Integration With Human Body

By Gity Samadi and Paul Semenza

Flexible hybrid electronics (FHE) has spawned the development of novel packaging techniques to overcome the application limitations of the rigid boards, high-temperature solders, bulky component packages, and insertion processes used in traditional printed circuit boards. Thin, flexible substrates, bare die, and combinations of printed and small-format packaged components require the use of additive processes for circuitization, mechanical attach, and encapsulation.

Additive techniques such as printing with a master (e.g., screen, gravure) or direct write (e.g., ink jet, aerosol jet, extrusion) deposit conductors, insulators, adhesives, and other materials precisely to attach electronic components to flexible substrates, and the materials can be cured (e.g., thermal, infrared, ultraviolet, photonic) without the need for extended exposure to high temperatures.

FHE manufacturers have been working to use these novel interconnect and attach techniques in advanced packaging to enable more compact, lightweight, and higher-performance microelectronic systems. While these techniques are not yet commonly used, presentations at FLEX 2022 in July pointed to intriguing possibilities.

Wafer-level printing for packaging

One example is applying gravure offset printing to ball-grid array packaging (figure 1). Komori, a provider of precision printing equipment, demonstrated a process to deposit 30mm pillars of flux paste at 60mm spacing directly on a wafer using gravure offset; 30mm solder balls are then placed on top of the pillars in a separate process.

Fig. 1: Gravure offset printing used to create pillars directly on wafer (l); solder balls placed on pillars. Source: Komori

Gravure offset printing has also been used to deposit fan-out conductor lines for connecting them to the die pads on flip-chip bare die assembly on flexible substrates (figure 2). NextFlex, the industry consortium whose mission is to advance U.S. manufacturing of printed flexible electronics, has demonstrated a process that combines gravure offset to print fine (30mm) fan-out lines to the die pad, screen printing for the trace lines, and gravure offset to place bumps on the die pads.

Fig. 2: Gravure offset printing can be used to create fine lines for fan-out (l) and bumps for die pads (r) and combined with screen printing to create traces (c). Source: Komori

Direct-write techniques for power and RF modules

While FHE is not normally associated with high-power, high-temperature applications, groups working on high-performance applications such as power and RF modules are exploring combining additive circuit materials and unpackaged components on substrates. In addition to the potential for compact, low-profile, and conformable packages, additively manufactured modules promise low inductance, critical to these high-performance modules.

General Electric, working with Binghamton University, has explored direct-write techniques for constructing silicon carbide power module packages. The hypothesis is that such techniques can replace wirebonded and planar packages, which are bulky and limited in performance (figure 3). Prototypes developed with selected combinations of insulating and conducting materials and deposition techniques are being evaluated for meeting smaller package size and lower inductance requirements.

Fig. 3: In order to overcome the performance limitations of wirebonded power modules (l), planar modules have been developed, which require semiconductor packaging and PCB techniques (c); using direct-write techniques to deposit insulators and conductors (r) offers the potential for conformal/low profile packages. Source: GE

Binghamton University reported on using an Optomec aerosol jet system for selective deposition and patterning of both dielectric and conductive materials in a multi-layer structure to create high-resolution 3D circuit elements for millimeter-wave electronics.

Researchers used direct-write patterning in conjunction with conventional low-k substrates and etched copper foil circuitry to enable a simpler, more compact package. These printed connections showed similar RF and electrical performance as wire bonds (figure 4). In addition, the aerosol jet deposition enabled Binghamton to fabricate 3D antenna structures.

Fig. 4: Metal-insulator semiconductor (MIS) capacitor connected via gold wire bonds (l) and aerosol jet ribbon (r). Source: Binghamton University

Photonic curing as an alternative to solder reflow

Photonic curing has been used to apply conductors, adhesives, and solders to flexible substrates that are not able to withstand high temperatures. Photonics curing delivers intense bursts of energy in optical wavelengths to cure and sinter inks and pastes, as well as reflow solder – an approach that has not typically been used in traditional (rigid) electronics, where high-temperature solder reflow ovens are used.

PulseForge, a provider of 3D printing and design services, has developed a digital thermal process that uses a precise, sub-millisecond intense light pulse from a flashlamp to generate surface temperatures hundreds of degrees higher than those inside the substrate material, enabling flowing of solder without impacting substrate or component materials. The process enables the use of standard solders with low-temperature substrates, sensitive coatings, as well as other components such as batteries and optical devices and can be used with a variety of connectors. While the process is not geared toward rigid board use, it enables intermetallic thickness to be controlled by selecting process parameters (figure 5).

Compared to reflow ovens, photonic curing enables processing times over 10 times faster in a third of the footprint and consumes only 15% of the power.

Fig. 5: Digital Thermal Processing used with a variety of connection types (l). Control of process parameters enables control of intermetallic thickness on rigid boards (r). Source: PulseForge

Convergence of FHE techniques and advanced packaging?

Nextflex pointed out ways FHE can eliminate distinctions between package, printed circuit board (PCB) and system (figure 6). For example, while hybrid electronics approaches enable additive printing of circuits and multichip modules, structural electronics enable circuits on and within parts, non-planar surfaces, and 3D structures.

Fig. 6: FHE in electronics manufacturing. Source: NextFlex

Hybrid packaging combines additive deposition techniques, chips, and materials for substrates and boards. NextFlex presented application examples for these novel packaging approaches, including wearables and textiles, conformal monitors, and active shipping labels. Future component technologies used in hybrid packaging could potentially include embedded fluidics, MEMS integration, and photonics and optoelectronics.

Commenting on the potential for integrating 3D printing and additive electronics in packaging, Matthew Dyson of market research firm IDTechEx noted that commercialization of partially additive semiconductor packaging using techniques including laser induced forward transfer (LIFT), laser direct structuring (LDS), aerosol jet, extruded conductive paste, two-shot molding, and print then plate is likely in the next few years, with fully additive semiconductor packaging a possibility within five years. Looking out over the next decade, he predicted that fully additive electronics components will be integrated with surface-mount devices (SMD), additive electronics will be used in full Heterogeneous Integration.

Integrating FHE with the body

Bio-inspired technologies are key to enabling flexible mechanical systems that can sense and actuate in concert with the human body. FLEX 2022 presentations by Shane Mitchell of Artimus Robotics, Bryan Starbuck and colleagues from Georgia Tech, and Sheng Xu of UC San Diego pointed to FHE advances that are enabling thin, conformal devices that can sense a variety of conditions as well as provide actuation.

Artimus Robotics is developing a muscle-like HASEL (hydraulically amplified self-healing electrostatic) actuator technology that can be designed to expand, contract, or rotate like the human body. The technology uses electro-hydraulic actuation, in which electrodes coat both sides of a flexible polymer pouch encapsulating a dielectric liquid. By placing a charge across the electrodes, the pouch is compressed, enabling controlled increases in pressure. The manufacturing uses mature and widely available materials and processes. The pouch can be made with inexpensive materials such as polypropylene (PP), polyethylene terephthalate (PET), thermoplastic polyurethane (TPU), or polyvinylidene fluoride (PVDF), and the dielectric fluid can likewise use common materials such as vegetable or silicone oil.

Manufacturing HASEL devices involves using heat to seal and shape films, screen printing or other bulk deposition for the electrodes and filling the pouches with dielectric fluid and dip coating to encapsulate the system. Artimus Robotics has also designed a high-voltage power supply that independently addresses 10 channels up to 10 kV each, powered by two 3.7 V, 2500 mAh lithium polymer batteries; the power supply weighs 250g and measures 8.4cm × 13.3cm × 2cm. An integrated microcontroller enables a feedback loop that allows capacitive sensing across the electrodes to gauge the spacing of the electrodes and the degree of movement.

Fig. 7: Human-machine interfaces (top) and industrial automation (bottom) are among the applications Artimus Robotics envisions. Source: Artimus Robotics

Focusing on applications and technologies that help assist movement in people with neuromotor disorders and aging muscle, Georgia Tech is developing a flexible system that uses sensory feedback through electromyography (EMG) to assess muscle and motor neuron health and augments strength through pneumatically actuated artificial muscle. The electromyography system uses nanomembrane electrodes, fabricated through E-beam evaporation that conform to the skin and measure voltage differences in muscle tissue.

The George Tech team developed a flexible circuit board with A/D converter and Bluetooth low-energy transceiver chips. The flexible board is encapsulated on top of the electrodes and features a rechargeable battery. The group is converting the EMG data to images used to train a convolutional neural network and predict motions using deep learning. The predictions can then be used to direct a pneumatic system to augment human muscle.

Existing wearable devices using soft electronics can detect vital signs on or near the skin using electrical signals such as electrocardiogram (ECG or EKG), oxygen level (pulse oximetry), temperature, some motion data, and chemical data via sweat. Sheng Xu’s group at UCSD has embedded electrodes and ultrasonic transducers in silicone to fabricate soft ultrasonic transducer arrays that conform to the skin and can fold, stretch, and twist. In addition, the team developed wearable phased arrays that can penetrate deep into tissue at high resolution and with active steering, as well as apply Doppler imaging to cardiac tissue.

Paul Semenza is advisor to SEMI on special projects. He was previously with NextFlex, the Flexible Hybrid Electronics Manufacturing Innovation Institute.