Flexible inks and interconnect for high-performance wearables
- khashayar Ghaffarzadeh
- Sep 22
- 5 min read
Authors: Andrew Stemmerman, John Yundt, Kathy Ritter | SunRay Scientific Inc., Eatontown, NJ USA | andrew@sunrayscientific.com johny@sunrayscientific.com, kathy@sunrayscientific.com
SunRay Scientific of Wall Township, NJ, USA has developed a versatile interconnection technology for flexible and stretchable electronics on Thermoplastic Polyurethane (TPU) substrates for wearables.
This article will outline the developments of this suite of conductive and dielectric stretchable inks with strong adhesion to mechanically flexible substrates, paired with a high-performance magnetically aligned Anisotropic Conductive Epoxy (ACE). The ACE interconnect material, with ferromagnetic conductive particles aligned along the z-axis, forms vertical conductive pathways to connect multiple component styles to the stretchable circuits on flexible TPU. The ACE additionally provides the strong mechanical bond for the components, with the entire assembly capable of surviving wearables’ wash cycles.
Introduction
Thermoplastic urethane (TPU) substrates, with flexibility, wear resistance and skin-compatibility, are ideal for wearable electronics applications. Wearables require stretchable circuitry with high density for integration and miniaturization. Printable materials enable low-cost electronic circuit fabrication processes.
SunRay Scientific has developed a complementary set of additively printed circuit materials; a conductive silver ink called StretchS and a stretchable dielectric. StretchS ink is a low-resistance polymer thick film silver ink designed for compatibility with SunRay’s ZTACH® ACE, an anisotropic (Z-axis) conductive epoxy. This ink is designed to have reduced silver migration and is intended for applications where mechanical performance, environmental stability, low resistance, and cost effectiveness are needed.
SunRay Scientific’s low-temperature thermal-cured anisotropic conductive epoxy offers a way to integrate and miniaturize electronic assemblies without the thermal and mechanical penalties of traditional interconnects. Temperature-sensitive components and low-temperature substrates often force compromises in design and process, while conventional anisotropic conductive adhesives and films depend on controlled pressure and heat, lengthening cycle time and risking damage to components. Thermode bonding adds keep-out constraints and can threaten neighboring features. There is an industry demand for an interconnect that supports fine pitch, cures rapidly at low temperature, protects delicate materials, and reduces total manufacturing cost—without sacrificing reliability.
ZTACH® ACE meets that need with a pressure-less, low temperature, 80°C-160°C cure, which consolidates interconnect and underfill into a single operation. An illustration of the novel approach is shown below in Figure 1. The formulation is a curable resin system loaded with ferromagnetic particles bearing a highly conductive coating. During cure, the magnetic field generated from SunRay’s patented ZMAG® Magnetic Pallet causes these particles to align into vertical, z-axis columns across the thin bond-line. These columns create low-resistivity pathways between component terminations and substrate pads while maintaining lateral insulation between adjacent pads, delivering anisotropic conduction.
Because conduction arises from column formation in the epoxy matrix, ZTACH® ACE can be stencil-printed or dispensed over all the entire circuit’s component footprints, reducing reliance on intricate, precision-tooled deposits. Alignment tolerances are more forgiving than patterned solder, ECA, and ACA/ACF processes. Figure 2 is an example of a component area, full footprint stencil deposit of ZTACH® ACE, clearly illustrating the simplicity of application. This view is prior to a 24-pin Quad Flat No-lead (QFN) package placement and magnetic cure of the epoxy. The result is a scalable, high-throughput process that protects heat-sensitive parts and flexible substrates, enables lower-profile attachments, and miniaturization. The entire assembly process and materials set is compatible with surface mount technology (SMT) manufacturing lines.
Figure 3 is a view of a bonded QFN, with the vertical columns of ferromagnetic particles seen from a side angle.
Figure 4 is a photo of a TPU test vehicle for stretch testing and 85/85%RH testing showcasing the full material set of StretchS conductive ink, stretchable dielectric, and ZTACH® ACE bonded components.
A test demonstrator, seen in Figure 4, was created utilizing printed StretchS stretchable silver ink for the circuitry, with resistors, voltage regulators and LEDs mounted with ZTACH® ACE onto TPU for a wearable test product that needed to perform in the field and meet exceptional performance requirements like multiple washing machine cycles.
The level of robustness can be clearly seen in the Figure 5 video link below:
An experimental study to understand ZTACH® ACE’s adhesion and performance under active stretching was done. The design involved two TPU circuits laminated into fabric swatches, with ZTACH® ACE as the bond between the TPU circuits. In the research illustrated in Figure 6, 11 samples and controls were tested. The ZTACH® ACE electrical and mechanical bond remained intact through 100 cycles of fatigue cycling at 30% strain without losing conductivity. Electrical resistance of the bond increased with strain but recovered, returning to value upon release.
In another wearable product development, a 3.5” x 6” TPU substrate was used, with 2” x 4.5” of the area dedicated for circuitry and devices. StretchS was stencil printed for the first conductor layer and thermally cured at 125°C for 15 minutes. Next, the dielectric was selectively printed at locations defined for second conductor layer crossovers, where insulation was required, and cured at the same conditions. StretchS was used for the second conductor-crossover layer and cured.
ZTACH® ACE was stencil printed over the circuitry on the TPU substrate, using a 0.004” thick stencil. The ACE was applied in a single print, for bonding all 27 of the components of various package styles. Devices were SMT resistors, capacitors, diodes, a sensor, and two QFN packages. One was a thin plastic QFN with 24 terminals at 0.5mm pitch; the other was a transistor. After the components were positioned by a pick-and-place machine, the entire TPU circuit was put over the magnetic pallet and ZTACH® ACE thermally cured for 125°C for 15 minutes. The populated TPU circuit was then laminated onto the wearable demonstrator. Eight out of eight assemblies had 100% electrical yield. The assemblies were connected to power, NFC and read temperature accurately.
Another example was a pulse oximeter product demonstration shown in Figures 7 and 8. SunRay’s StretchS silver ink conductor was printed on the TPU flexible substrate, and two components were interconnected to the circuit traces with thermally cured ZTACH® ACE.
The final example in this article illustrates the flexibility of a large area TPU circuit with 36 LEDs mounted with ZTACH® ACE onto conductive StretchS traces. The flexible circuit can be folded, unfolded, rolled and remain fully functional. The video in Figure 9 captures the flexibility of a TPU electronic device with these high performance, compatible inks and ACE interconnect.
Conclusion
A suite of complementary materials developed for flexible electronics has been demonstrated in high performance wearable applications. The stretchable conductive ink and stretchable dielectric provide strong adhesion to low surface energy substrates, allowing for multilayer conductors with crossovers. Multiple device package styles, from 0201 to 0603 SMT passives to QFNs, can be bonded at the same time with the 80°C-160°C low temperature cure ACE. Strong adhesion, low contact resistance, high x–y isolation and high yield were achieved. The inks and interconnect material set are compatible with standard SMT manufacturing lines.
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