Method for fabrication of a soft-matter printed circuit board

Abstract

A fabrication process for soft-matter printed circuit boards is disclosed in which traces of liquid-phase Ga—In eutectic (eGaIn) are patterned with UV laser micromachining (UVLM). The terminals of the elastomer-sealed LM circuit connect to the surface mounted chips through vertically-aligned columns of eGaIn-coated ferromagnetic microspheres that are embedded within an interfacial elastomer layer.

Claims

1. A method for fabricating a soft-matter printed circuit board comprising: providing a layer of an elastomeric substance; patterning one or more openings in the elastomeric substance; placing one or more solid state devices in the one or more openings; coating the layer of the elastomeric substance with a layer of an anisotropically conductive substance; coating the layer of the anisotropically conductive substance with a layer of liquid metal; etching a one or more circuit paths in the layer of liquid metal; and coating the layer of liquid metal with a sealing layer; wherein the anisotropically conductive substance comprises a plurality of ferromagnetic microspheres forming vertically aligned columns in a cured elastomer.

2. The method of claim 1 wherein the layer of the elastomeric substance is cured polydimethylsiloxane.

3. The method of claim 1 wherein the layer of the anisotropically conductive substance is formed by: coating the ferromagnetic microspheres with a liquid metal; suspending the liquid metal-coated ferromagnetic microspheres in a liquid elastomer; coating the layer of the elastomeric substance with the liquid elastomer; and curing the liquid elastomer in the presence of a magnetic field such that the lines of the magnetic field extend through the thickness of the layer.

4. The method of claim 1 wherein the layer of liquid metal is eGaIn.

5. The method of claim 1 wherein etching the circuit in the layer of liquid metal comprises etching circuit paths such that the circuit paths align with appropriate contacts on the solid state devices and wherein the circuit paths and contacts are separated by the layer of an anisotropically conductive substance.

6. The method of claim 1 wherein the layer of sealing material is PDMS.

7. The method of claim 1 wherein the one or more circuit paths are etched in the layer of liquid metal using UV laser micromachining.

8. The method of claim 1 wherein the vertically-aligned columns of ferromagnetic microspheres creates vias between the circuit paths.

9. The method of claim 2 wherein the layer of elastomeric substance is disposed on a glass plate.

10. The method of claim 3 wherein the ferromagnetic microspheres are coated with silver.

11. The method of claim 3 wherein the ferromagnetic microspheres are coated with liquid metal.

12. The method of claim 9 wherein the one or more solid state devices are placed having a board-side of the one or more solid state devices facing the glass plate.

13. The method of claim 12 further comprising: coating the layer of elastomeric substance opposite the glass plate with a second sealing layer comprising an elastomeric substance.

14. The method of claim 12 further comprising: removing the layer of the elastomeric substance from the glass plate to expose the board-side of the one or more solid state devices.

15. The method of claim 14 wherein coating the layer of elastomeric substance with the layer of the anisotropically conductive substance comprises covering the board-side of the one or more solid state devices with the anisotropically conductive substance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A shows an exemplary soft-matter printed circuit board an exploded form fabricated in accordance with the invention.

(2) FIG. 1B shows vertically aligned columns of Ag—Fe.sub.2O.sub.3 microparticles coated in eGaIn forming z-axis vias between LM trace terminals and surface-mounted components of the PCB of FIG. 1A.

(3) FIG. 2 is an illustration of eGaIn being patterned using the UVLM process. The inset shows a Ga.sub.2O.sub.3 passivation layer which is necessary to hold the shape of the LM circuit.

(4) FIGS. 3(A-F) show the steps of the fabrication process of the soft-matter PCB in accordance with the invention.

(5) FIGS. 4(A-D) show examples of representative demonstration circuits used to validate the fabrication method of the present invention.

DETAILED DESCRIPTION

(6) The above challenges are addressed via a unique layup for LM-based soft-matter electronics using innovations in materials selection and processing.

(7) The term liquid metal, as used herein, refers to any metal or metal alloy that is in the liquid state, including, but not limited to, Ga-based alloys such as eutectic Ga—In (eGaIn) and Ga—In—Sn (Galinstan).

(8) The term zPDMS, as used herein, refers to liquid metal-coated microspheres suspended in a cured elastomer, preferably PDMS, produced as described below, wherein the microspheres comprise ferromagnetic microspheres having a liquid metal, for example, Ag-coated FE.sub.2O.sub.3 particles having an eGaIn coating. The microspheres are arranged in vertically-oriented columns, such that a layer of zPDMS is anisotropically conductive only in the direction of the z-axis.

(9) As shown in FIG. 1A, a UV laser micromachining (UVLM) system is used to pattern a coating of LM on a polymer substrate 108 to form circuit paths 102. In preferred embodiments, the coating of LM may be approximately 20 μm in thickness. The UVLM-patterned eGaIn circuits 102 can interface with surface-mounted SSTs 104 using an anisotropically conductive “z-axis film” 106 composed of vertically-aligned columns 110 of eGaIn-coated Ag—Fe.sub.2O.sub.3 microspheres 112 embedded in a PDMS matrix, as shown in FIG. 1B, which serve as vias between SSTs 104 and the UVLM-patterned eGaIn circuit 102. In contrast to conventional z-axis films with “dry” ferromagnetic microparticles, the LM-coating 114 allows for electrical connectivity even as the rigid microspheres 112 separate under mechanical deformations induced by circuit bending, compression, stretching, or twisting.

(10) As shown in FIG. 2, a Nd:YAG laser 202 with 355 nm wavelength, 200 kHz pulse rate, and 15 μm diameter beam that is capable of direct photophysical ablation of metal 208 may be used to pattern the eGaIn film 204 to form circuit paths 102. Using a 1 W beam power, the liquid metal features can be reliably patterned with planar dimensions of 50 μm. In contrast, prior art attempts at eGaIn patterning with a CO.sub.2 laser required indirect material expulsion through vapor recoil force that limits the minimum feature size to ˜250 μm.

(11) As with eGaIn stencil lithography and additive manufacturing, the ability to laser pattern eGaIn on an elastomeric substrate depends on the formation of a self-passivating, nanometer-thin Ga.sub.2O.sub.3 “skin” 206, shown in the inset of FIG. 2. The oxide skin allows the patterned liquid metal 102 to hold its shape after laser ablation and during deposition of an elastomer seal (not shown in FIG. 2). This feature of liquid moldability is critical for extending conventional UVLM PCB prototyping to Ga-based LM alloys. Combining UVLM processing with liquid metal and z-axis connectivity allows for a versatile method to produce elastically deformable electronics that are mechanically robust and compatible with natural human tissue.

(12) FIGS. 3(A-F) show the steps in the fabrication process of a soft-matter PCB with embedded LM wiring in accordance with this invention.

(13) As shown in FIG. 3A, a layer of PDMS 302 (for example, 10:1 base-to-curing agent ratio) is applied on a glass substrate by spin coating (KW4A, SPI) at 1500 RPM for 10s, or by other means, and then cured. Preferably, the PDMS 302 will be cured at 100° C. for 20 min.

(14) Next, a polymer film 304 is placed on top of the PDMS layer 302 and the film is patterned using the UVLM to create placement openings 306 for the rigid integrated circuits (ICs) 104, as well as fiducials 308 for alignment within the UVLM. In an alternative embodiment, the PDMS layer 302 may be patterned directly, without the polymer film 304.

(15) As shown in FIG. 3B, surface mount SSTs 104 are then placed into the openings 306. The board side of the components may be dipped in polyvinyl acetate (PVA), to adhere the SSTs 104 to the glass substrate. PVA may then be cured, preferably by heating at 70° C. for 5 min on a hotplate. The SSTs 104 are then sealed with a second layer of PDMS 310 (10:1). Multiple sealing layers of PDMS 310 may be applied until the surface mount components are covered. Preferably, the sealing layers are spin-casted (600 RPM, 5 s) and cured (100° C., 20 min).

(16) The SST-embedded elastomer is then peeled from the substrate and placed on the glass disc with the board side of the components exposed, as shown in FIG. 3D. The exposed components are rinsed with deionized water to remove the PVA.

(17) After drying, as shown in FIG. 3D, a layer of zPDMS 312 is deposited on top of the exposed SSTs 104. Preferably, the zPDMS 312 is deposited by spin coating at 1500 RPM for 10s; KW-4A, SPI) and cured on top of a flat magnet (for example, ˜1448 Gauss, 2″×2″×¼″ NdFeB). Preferably, the zPDMS is cured at 100° C. for 20 min. Curing the zPDMS 312 in the presence of the magnetic field causes the eGaIn-coated 114 ferromagnetic microspheres 112 to arrange in vertically-aligned columns 110, which will become vias between the LM etched circuit 102 and the contacts of the SSTs 104. It should be noted that the zPDMS layer 312 is only electrically conductive in the areas where the vertically aligned columns 110 of microspheres 112 extend through the thickness of the layer. Coating the microspheres 112 with eGaIn 114 prior to mixing with the PDMS allows the zPDMS 312 to physically deform without the vertically-aligned columns 110 of microspheres 112 breaking apart and losing their ability to conduct in the z-axis direction.

(18) As shown in FIG. 3E, a film of eGaIn 314 is then applied on the zPDMS 312, preferably using a PDMS roller 316, and then patterned with a UVLM system, as shown in FIG. 3F. Lastly, the LM circuit 102 is sealed in PDMS and released from the glass substrate. Multiple sealing layers of PDMS may be applied by spin-casting at 600 RPM for 5s, and then cured at 100° C. for 20 min to match the thickness of the SST-embedded elastomer layer.

(19) In a preferred embodiment, the zPDMS 312 may be prepared by mixing 70 wt % of 40 μm diameter Ag-coated Fe.sub.2O.sub.3 particles (20% Ag by wt.) with 30 wt % of eGaIn (75 wt % Ga and 25 wt % In) using a mortar and pestle or other means. The microsphere/eGaIn mixture is then mixed at 50 wt % with uncured PDMS (10:1). In alternate embodiments, the microspheres 112 may be composed of any ferromagnetic material, for example, nickel. In alternate embodiments, the microspheres 112 may optionally be coated with silver or other highly conductive materials prior to mixing with the eGaIn, to improve conductivity.

(20) With the fabrication process shown in FIG. 3, packaged SST chips 104 can be embedded in elastomer and wired together with eGaIn interconnects that share dimensions similar to those of conventional copper PCB traces (50 μm width, 100 μm spacing). The UV laser-patterned eGaIn traces exhibit properties that are consistent with classical predictions for electrical resistance and capacitance.

(21) The layup and fabrication process shown in FIGS. 3(A-F) was used to produce several representative demonstration circuits that integrate UVLM-patterned traces of eGaIn with packaged SSTs, as shown in FIGS. 4(A-D). The demonstration circuits are intended to validate the fabrication method and compatibility of materials.

(22) FIG. 4A shows a demonstration circuit consisting of an array of surface mounted LED 402 and 331Ω, resistor chips 404.

(23) FIG. 4B shows a soft-matter bioelectronic circuit for non-invasive measurements of heart rate (HR) and arterial blood oxygenation (SpO.sub.2). The circuit contains an integrated pulse oximetry unit 406 that houses a red and IR LED, photodetector and optics, and a low-noise analog signal processor for reflective PPG waveforms. A flex PCB with Cu traces is used as an interface board 408 that connects the circuit to a battery-powered Bluetooth module that transmit signals to a host computer for signal processing. The device is configured to collect the PPG waveforms 410 at 200 Hz using LED pulse widths of 400 μs and a current of 20.8, as shown in FIG. 4C. With appropriate filtering and calibration, the waveforms can be used to obtain HR from spacing of Systolic peaks 412 and SpO.sub.2 by comparing the amplitude (i.e. optical reflection) of IR and red light, which are absorbed differently by oxygenated or deoxygenated hemoglobin within arterial blood. Because the circuit board is intrinsically soft and deformable, the pulse oximetry band 405 can wrap around the wrist 414 and form intimate contact with skin with limited interfacial pressure, as shown in FIG. 4D.

(24) The fabrication technique disclosed herein can be used to produce soft and deformable circuits with liquid metal and UV laser micromachining. To match the mechanical properties of soft natural tissue, the relatively stiff materials typically used in existing PCBs (metal wiring, soldered connections, and glassy polymer substrate) are replaced with liquid-phase metal alloy and elastomer. This biomechanically compatible “soft-matter” PCB can be rapidly produced using the same commercial UVLM system used for conventional electronics prototyping. Because the circuit is composed entirely of soft and deformable material, fabrication does not depend on a limited selection of geometric patterns or the specialized microfabrication techniques required for thin-film metal circuits with deterministic architectures. In this respect, the method represents a relatively inexpensive, scalable, and user-accessible alternative that complements previous achievements in stretchable and thin-film electronics based on cleanroom lithography.

(25) The intrinsic compliance of the soft-matter PCB is of particular importance in wearable bioelectronics and computing. For these applications, mechanical impedance mismatch can constrain natural body motion or cause irritation, discomfort, or tissue damage due to interfacial stress concentrations. Impedance matching is especially critical in optical or electrode-based bioelectronics applications, such as pulse oximetry, that depend on intimate contact with the skin for accurate physiological measurements. Incorporating soft materials, SSTs, and processing steps into a single UVLM-based fabrication method enables the rapid production of customizable wearables. Such systems could be user/patient-specific and capable of physiological sensing for activity, fitness, and health monitoring.

(26) The method of the present invention has been explained in terms of examples utilizing specific equipment and fabrication parameters. It should be realized by one of skill in the art that alternatives in the equipment or variations in the fabrication parameters may be used to produce results that are intended to be within the scope of the invention. Additionally, examples of specific circuits have been provided as a validation of the fabrication method. As further realized by one of skill in the art, the fabrication method is not meant to be limited to these specific applications but may be used to produce circuits intended for any purpose.