Patent Application
20220306887
Conductive inks are generally formulated with metallic (conductive) micro/nanoparticles (e.g. silver, copper, and gold) or carbon particles (e.g. graphene and carbon nanotubes) in an insulate or weak conductive retention matrix. The retention matrix includes organic additives (e.g. dispersants and stabilizers) to prevent agglomeration caused by the high surface energy of the micro/nanoparticles. Such additives encapsulate the particles.
The conductive inks can be deposited onto substrates according to designed circuit patterns by a number of printing techniques such as screen printing, syringe-needle type dispenser, inkjet, and 3-D printers. After printing and solvent evaporation, the metal micro/nanoparticles usually do not become adequately conductive due to the organic additives added to the conductive ink. For example, movement of electrons between metal particles can be prevented by an organic layer as thin as a few nanometers. Therefore particle-ligand bonds must be broken to allow for removal of polymeric ligands between contacting particle surfaces to establish metal-to-metal atomic diffusion among particles.
Thus in order to create a contact path with the conductive particles, it is necessary that the volume of the matrix be reduced either with a curing or an evaporation process, also called the sintering process. Sintering decomposes organic additives, allowing the conductive particles to interact.
There are many sintering techniques, such as thermal, chemical, electric, and laser sintering, for example, thermal sintering to over 100 degrees Celsius or extended
ultraviolet or infrared development. Advanced sintering techniques involve microwave, or laser radiation, xenon flash light, electrical or chemical sintering, and plasma. However, these processes result in extra costs and time after the printing process, and often involve high-cost equipment, require high energy, and/or require complex pre- or post-treatments.
Sintering the printed metal micro/nanoparticle ink at an elevated temperature (e.g., in a thermal oven) makes it electrically conductive, while thermal decomposition (known as pyrolysis) and vaporization might be used to remove the polymeric ligands. Simultaneously, the contacting metal micro/nanoparticles are fused together via interfacial atomic diffusion and neck formation.
However, sintering silver nanoparticle inks in a thermal oven, for instance, although conceptually straightforward, poses practical limitations for printed electronics on common low-cost polymer substrates (e.g., polycarbonate and polyethylene terephthalate) with glass transition temperature well below 200° C. To avoid thermal damage to the plastic substrate, the printed parts should not be sintered above the heat deflection temperature of substrate. On the other hand, it is often desired to obtain lowest possible resistivity of the sintered ink material for the desired electronic performance, thus it is important to remove the polymer ligands via the sintering process. When producing functional electronic devices, it is also important to have adequate adhesion and cohesion of the sintered nanoparticle material on a substrate, which may also require increased sintering temperature.
The method of thermal oven sintering is still widely used in printed electronics production. Some modern silver nanoparticle inks can now be sintered at temperatures as low as about 100° C. However, this comes with the undesired result of an impractical long duration (e.g., days) just to achieve marginal values of electrical conductivity.
The following presents a simplified summary in order to provide a basic understanding of the disclosure. The summary is not an extensive overview of the disclosure. It is neither intended to identify key or critical elements nor to delineate the scope of the disclosure. The following summary merely presents some concepts in a simplified form as a prelude to the more detailed description below.
Described herein are examples of self-sintering conductive inks that can be printed and self-sintered with a simple and low-cost process mechanized by exothermic alkali metal and water reaction, with enhanced electrical and thermal performance by liquid metal fusion. Such self-sintering conductive inks may include a gallium-alkali metal component and a water absorbing gel component.
After patterning, the self-sintering inks, on reaching a designed trigger temperature (including room temperature), may metallize through a two-step process. A first step may comprise raising the temperature to the trigger temperature so as to cause the gallium-alkali metal component to activate and react with water released from the water absorbing gel component. The exothermic reaction between the water and the alkali element creates an intense and highly localized heating effect, which liquefies all metallic components in the ink and, on cooling, creates a solid metal trace or interconnect. Post cooling, the metal trace or interconnect cannot be reflowed without a significant temperature increase (well above 100′s of degrees Celsius) or other energetic input.
Pre-existing conductive inks that enable formation of conductive patterns require a step of thermal sintering to over 100° C., extended ultraviolet or infrared development, or other external energetic stimulus. Certain advanced sintering processes such as microwave, or laser radiation, xenon flash light, electrical or chemical sintering, and plasma involve high-cost equipment and require high energy or complex pre- or post-treatments.
Certain electronic applications require low-temperature substrates (e.g., paper, polyethylene terephthalate (PET), polyether ether ketone (PEEK), bioelectronics, organic membranes, polyethylene films) and/or processing including die-bonding and 2.5/3D electronic packaging. The step of thermal sintering to over 100° C., extended ultraviolet or infrared development, or other external energetic stimulus may adversely affect such substrates. Despite years of intensified research efforts, the method of thermal oven sintering is still widely used in printed electronics production.
Mitsubishi Paper Mill and The Hebrew University of Jerusalem developed so called “built-in” or halide-triggered silver inks sintered at or near room temperature. However, the halide used may cause environmental concerns with advent of “green” electronics and environmental-friendly industries. Further, due to corrosion issues, silver is not a good stand-alone conductor (especially in bioelectronic applications).
Aspects of the disclosure allow for low temperature self-sintering without the environmental concerns of “built-in” or halide-triggered silver inks. Self-sintering conductive inks are mechanized by exothermic alkali metal and water reaction and enhanced electrical and thermal performance are obtained with liquid metal fusion.
The self-sintering conductive inks can be processed and applied at low or near room temperatures, and used as solders or electrically conductive adhesives, thermal interface materials, and/or conductive traces/interconnects for both printed rigid and flexible electronics on the substrates of metals, ceramics, and polymers. Also or alternatively, they can be formulated and tuned to function as electrical and/or thermal conductors, semiconductors, dielectrics, or insulators. By modifying the ink to change its temperature set point, new desired circuits can be added to a system on reaching different temperatures. Pre-application surface treatments can also be used to improve adhesion or reduce wetting so the ink stays localized. The materials and processes disclosed herein are cost-effective for many applications.
Aspects of the present disclosure relate to formulation, process, and application of self-sintering conductive inks mechanized by exothermic reaction between metal and water or other media and performance enhanced by liquid metal fusion.
The self-sintering conductive inks may be formulated based on micro- and/or nano-sized conductive fillers, gallium-alkali metal low melting point alloys, and a water-absorbing gel prepared from water and/or hydrogen peroxide and a water-soluble resin.
The micro-/nano-sized conductive fillers may provide electrical and/or thermal conductivities. These conductive fillers may include, for example, nanoparticles, nanowires/whiskers, and/or micron size particles that are highly conductive. The conductive fillers may have electrical and thermal conductivities comparable or the same to the relevant highly conductive bulk metals that are typically used in forming interconnections and the like on, for example, printed circuit boards. Suitable conductive fillers include, but are not limited to, silver, gold, copper, zinc, aluminum, nickel, or carbon (carbon black, carbon nanotube and graphene); silver or gold coated copper, zinc, aluminum, nickel, or carbon; other metals and alloys; and a combination of different fillers.
In general, particle sizes may range from 20 nm to 2000 p.m. The particle size is usually from 100 nm to 100 p.m due to the combined considerations of material processing cost, performance, and easy operation. The minimum particle size may be as low as 4 nanometers or even lower, but at least 20 nm may be more common for commercial applications, mainly due to the consideration of material processing cost and operation difficulty.
The gallium-alkali metal alloy has a low melting point and provides non-oxidized metal for exothermic water reaction to trigger self-sintering and liquid metal fusion. The exothermic reaction provides the heat to allow the conductive fillers to melt and form liquid metal. The liquid metal can flow to fill voids and modify connectivity of the conductive network to enhance electrical and thermal conductivities. The resulting concentration and distribution of the liquid metal can be tuned to improve flexibility and stretchability of the printed patterns or structures. The gallium-alkali alloy may also improve adhesion performance or bonding strength between the substrate and the printed patterns or structures.
The melting point range of the gallium-alkali metal may be −15° C. to 300° C. Commercially, the melting point of low melting alloys is usually below 150° C. In aspects described herein, the melting point can be below 85° C., or around room temperatures (e.g. 23 to 35° C.), especially for some polymer substrates with low glass transition temperatures
Gallium-alkali metals typically contain 1-50 wt.% alkali metal and additional elements. In one aspect the gallium-alkali metal contains 1-30 wt.% alkali metal and additional elements. Examples suitable gallium-alkali metal alloys may include Ga-Al, Ga-AlTiC, Ga-Al-Ti-B, Ga-Mg, Ga-Zn, Ga-Fe, Ga-Li, Ga-K, Ga-Ba, Ga-Ca, and Ga-Na with or without a combination of other elements, such as In, Sn, Ti, B, C, Ag, Cu, Fe, Si, Pb, Zn, Ni, Cr, Bi, and rare earth elements etc. At least one aspect utilizes a gallium-aluminum (Ga-Al) alloy, with or without a combination of other elements. The gallium-alkali metal may be a liquid or solid when it reacts with water.
The water absorbing gel functions as a binder and suspending agent for the conductive fillers and gallium-alkali metal alloys during printing and also provides water for the exothermic reaction during self-sintering. The water absorbing gel may be formed with a water-soluble resin and water or hydrogen peroxide or mixtures thereof. Water-soluble resins may comprise nonionic polymers such as nonionic poly(ethylene oxide) polymer. Suitable commercial water-soluble resins include, for example, DowDuPont's POLYOX™WSR N750 or POLYOX™WSR 301.
The water absorbing gel may include other water-soluble additives such as phosphorus oxoacid compound, halogen compound, gelatin, polyvinyl alcohol, polyvinyl acetal, polyvinylpyrrolidone, carrageenan, carboxylmethylcellulose, or hydroxylpropyl cellulose. Also or alternatively, the water absorbing gel may include additives such as KOH, KCl, NaCl, HCl, Ba2Cl2, BiOCl, NaBH4, NaMgH3, Al(OH)3 to enhance the exothermic reaction (changing the reaction strength and temperature) and promote the self-sintering at relatively low temperatures. In addition, polymeric binders, such as acrylic, silicone, styrene, fluoroelastomers, or urethane backbones, may be added in suitable amounts to aid homogeneous dispersion of the fillers and the gallium-alkali alloys (both liquid and solid), to hold the ink components together upon solvent evaporation, and/or to help bind the printed trace onto the substrate.
Other additives may be used in suitable amounts to adjust to the desired rheological, wetting, healing, stretching, and/or other properties of the inks for different printing technologies, such as screen printing, syringe-needle type dispenser, inkjet, 3-D printing, etc. Additives in the form of surfactants, adhesion improvers, humectants, penetration promoters, and stabilizers may be used to tailor the ink properties for specific applications.
Ink solvents other than, or in addition to, water may be used to provide enhanced solubility to the water-soluble polymer or other polymeric binder and impart favorable viscosity, surface tension, and homogeneity.
The ratio (weight percentage) of the water-soluble resin in water or hydrogen peroxide solution is typically from 1-20 wt.% for different printing processes and applications. The higher the percentage of the resin, e.g., 10 wt.% to 20 wt.%, or 15 wt.% to 20 wt.%, the higher the viscosity of the formed gel. The lower the percentage of the resin, e.g., 1 wt.% to 10 wt.%, or 1 wt.% to 5 wt.%, the lower the viscosity of the formed gel. The ratio between the water or hydrogen peroxide and water-soluble resin in the solution gel, and the gel's concentration in the ink, can be tuned or manipulated to make the exothermic reaction provide high enough temperature for the self-sintering, meanwhile the formation of oxides, metal oxyhydrides, and H2 bubbles can be manipulated to minimize the volume of formed voids and remaining reactants after the self-sintering.
The gel's concentration in the ink (weight percentage) may be 20 to 40 wt.%. The ink formulations may be tuned by adjusting the ratio of the water-soluble resin over Ga-alkali metal. For example, (1) when the gel percentage gets closer to 40 wt.%, e.g., 30 wt.% to 40 wt.%, the Al or Mg in the Ga liquid metal tends to be consumed completely as the self-sintering is completed within the designed time; (2) when the gel percentage gets closer to 20 wt.%, e.g., 20 wt.% to 30 wt.%, the alkali metal tends to remain available for the exothermic reaction to continue even after self-sintering, which may be used, for example, for sensing and anti-temper or other applications.
In at least one aspect, water absorbing poly(ethylene oxide) gel is generally a mixture of water (H2O) and/or hydrogen peroxide (H2O2) and 1-15 wt.% water-soluble poly(ethylene oxide) polymer. In one particular, non-limiting example, the water-soluble poly(ethylene oxide) polymer may have, for example, a general composition of 95% to 100% poly(ethylene oxide), up to 3% fumed silica, and up to 1% calcium as mixed salts.
The above constituents can be stably mixed, stored, and printed below the gallium-alkali metal eutectic reaction temperature (e.g. about 26.8° C. for Ga-Al eutectic reaction) to avoid the exothermic reaction. The self-sintering conductive inks can be processed and applied at low or near room temperatures, or at higher temperatures. The self-sintering conductive inks can be self-sintered when the environmental temperature is above the gallium-alkali metal eutectic reaction temperature to trigger the exothermic reaction and liquid metal fusion.
A relatively higher temperature (about 100° C. for example) also can be used for self-sintering to reduce the sintering time from about 2 hours (self-sintering at about 30° C. for Ga-Al eutectic reaction) to several minutes or less. This higher temperature self-sintering is useful for shortening the time on substrates that can withstand the higher temperature. Limited optical, thermal or sonic excitation could also be used to accelerate the self-sintering process.
The substrates include, for example, metals or alloys, (e.g., copper, aluminum, stainless steel), polymers (e.g., polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polyethersulfone (PES), polyimide (PI) and polyarylate (PAR), and polydimethylsiloxane (PDMS) as stretchable substrate), glasses, ceramics, papers, and/or textiles.
The self-sintering conductive inks enable metal traces and interconnects to be formed in electronic circuits without any external heating or other forms of energetic input (microwave, laser, sonic, electroplating etc.) Thus this disclosure enables technologies that cannot endure extreme temperatures during manufacture, but still require traces and interconnects with conductivity on par with bulk metal. The self-sintering conductive inks can be formulated for different applications with multiple functions. Representative applications include, without limitation, flexible/wearable electronics, bioelectronics (including implants), batteries, die-bonding, 2.5/3D electronic systems-on-a-chip, and multi-step electronics packaging where multiple solder reflows would destroy precursor assembly steps and hybrid chemical/electronic systems.
In a first step (110), water or hydrogen peroxide is mixed with a water-soluble resin to form a water absorbing gel. In a second step (120), the water absorbing gel is mixed with conductive fillers and suitable solvents and/or additives. In a third step (130), the water absorbing gel is mixed with gallium-alkali metal alloy low melting alloy below the eutectic reaction temperature. Step (110) may be combined with steps (120) and/or (130). For example, water or hydrogen peroxide may be mixed with a water-soluble resin and the conductive fillers and/or gallium-alkali metal alloy. During all of these steps the mixtures are kept below the eutectic reaction temperature of the gallium-alkali metal alloy.
In a fourth step (140), the resulting conductive ink is printed and/or otherwise applied onto the desired substrate at a temperature below the eutectic reaction temperature of the gallium-alkali metal alloy. Optionally, prior to printing and/or other application, the substrate may be pretreated by, for example, plasma clean or atomic layer deposition of thin film to inhibit wetting or promote adhesion.
In a fifth step (150), the temperature is raised above the eutectic reaction temperature whereby the alkali metal reacts with water in an exothermic reaction causing an increase in temperature whereby the printed material self-sinters. The increase in temperature caused by the exothermic reaction is sufficient for the conductive fillers to melt. In a sixth step (160), liquid metal resulting from the exothermic reaction fills voids and then fuses into the desired interconnect, for example. Optionally, sonic, thermal, or optical excitation may be used to accelerate the self-sintering process or reduce voids.
The self-sintering inks can be used as solders or electrically conductive adhesives, thermal interface materials, and/or conductive traces/interconnects for both printed rigid and flexible electronics on the substrates of metals, ceramics, and polymers. They also can be formulated and tuned to function as electrical and thermal conductors, semiconductors, dielectrics, or insulators.
Compared with other printable inks and thermal interface materials:
The sintering temperatures (for example, about room temperature 23-35° C.) of one or more of the self-sintered inks described herein may be tuned to values comparable with liquid metal inks (such as EGaIn and Bi35In48.6Sn16Zn0.4), and/or may be much lower than other printable inks such as nano-silver inks (typically ≥150° C.), and solder based interface materials (320-340° C. for eutectic AuSn solder as an example).
The electrical thermal conductivity of the self-sintered ink is able to be tuned better than, or comparable to, liquid metal ink (EGaIn—3.4×106 S/m, Bi35In48.6Sn16Zn0.4—7.3 ×106 S/m) and nano-silver ink (Ag-DDA—3.45×107 S/m, Ag-PVP—6.25×106 S/m), and much better than carbon conductive ink (CNT(carbon nanotube)—5.0×103 S/m).
The thermal conductivity of one or more of the self-sintered inks described herein may be comparable with solder based thermal interface materials such as indium (56 W/mK) and eutectic Au-Sn (57W/mK).
The bonding strength, with a metal substrate, of one or more of the self-sintered inks described herein may be comparable with commercial solders such as to SnPb or SnBi solders (about 40-70 MPa), and lower than eutectic Au-Sn (about 270MPa).
Mechanical flexibility of one or more of the self-sintered inks described herein may be much better than solders such as Au-Sn to match with different substrates, which may facilitate improved handling of thermal stress, and which can be tuned to be soft enough for some applications where the heated materials expand at remarkably different rates.
The thermal interface material (TIM) used to attach a die to a heat spread/component substrate (more common) or directly to a heat sink is usually named TIM1. The TIM used between a heat spreader and a heat sink is named TIM2. A solder like Au-Sn is generally used as TIM1. A TIM consisting of or comprising one or more of the self-sintering inks described herein may be used for both TIM1 and TIM2.
Example 1
An ink formulation was prepared with a water-soluble gel (10 wt.% DowDuPont
POLYOX™WSR N750, water solution), 43 wt.% liquid metal (Ga+2wt.%Al5Ti0.15C), and 30 wt.% Ag powder having and average size of 5μm.
(a) N750 powder was mixed with water to form a uniform gel (used vacuum furnace and ultrasonic to remove the gas bubbles in the gel) at room temperature (about 30° C.).
(b) The gel was mixed with Ag powder at room temperature, and then mixed with the Ga-AlTiC liquid metal under 0 to 20° C.
(c) A syringe dispenser was used to print the mixed ink on Cu and Al foil and made Cu-Cu joints under 0 to 20° C. for electrical resistance and adhesion strength testing.
(d) The printed samples were brought back to room temperature (about 30° C.) for self-sintering for about 2 hours until the inks completely dried.
The pull strength was 19 lbs. for a Cu-Cu joint with a jointing area of 1 cm2 (about 201bs for the same Sn60Zn40 soldering joint).
Example 2
The following is a general formulation for different applications.
Comparative Example
A pre-existing silver nanoparticle ink was heated to 70° C., the particles appeared to touch each other as the organic molecules started moving away from particle surface; but the electrical resistivity reached minimum value only at a temperature >200° C. and then increased with further heating to temperature beyond 250° C. due to growing pore size with reduced connectivity among coalesced large metal domains. To achieve a high conductivity, further sintering is required to transform the initially very small contact areas into thicker necks and, eventually, to a dense layer.
In the initial stage of sintering, the driving forces are mainly surface energy reduction due to the silver particle's large surface-to-volume ratio, a process known as Ostwald ripening. Ostwald ripening triggers surface and grain boundary diffusion within the coalesced Ag nanoparticles. Grain boundary diffusion allows for neck formation and neck radii increase, which is diminished by the energy required for grain boundary creation. As the sintering develops into a deep level, the relative density of the printed Ag particle based conductive trace or film increase and the electric conductivity increases.
At present, silver nanoparticle-based inks are the most commonly used inks because silver possesses the highest electrical conductivity that is not retarded by its oxidation. Copper nanoparticle-based ink is another promising material for printed electronics, which can potentially replace silver nanoparticle ink. Copper's conductivity is only 6 percent lower than that of silver, while its cost is less than a few percent of the price of silver. However, spontaneous formation of copper oxides (both Cu2O and CuO) on the surface of the particles at ambient conditions can cause substantial decrease in electrical conductivity and an increase in the sintering temperature.
Methods used to overcome the oxidation issue include performing the synthesis in organic solvents (such as polyols, octyl ether, octylamine, toluene, heptanes) often under inert atmosphere (Ar, N2), as well as minimizing the exposure of copper nanoparticles to oxygen by forming a protective layer that surrounds the particles. For example, performing synthesis in water in the presence of CTAB (cetyl trimethylammonium bromide) and CTAB-PVP (PVP stands for polyvinylpyrrolidone) mixture.
Stable copper nanoparticles may also be obtained by the formation of a copper formate shell, induced by a reaction of an interfacial copper oxide with formic acid. Such shell can be transformed into copper at a low temperature, resulting in highly conductive metallic films. Due to the susceptibility of Cu to oxidation, the usual post-deposition treatment methods include expensive and instrumentally elaborate flash lamp and laser sintering approaches in the pre-existing copper nanoparticle-based ink.
An easy to scale process is reported by using formic acid to sinter structures that are inkjet-printed using an industrial scale Cu nanoparticle ink at temperatures at or above 130 ° C. Up to 16% bulk Cu electrical conductivity is obtained when sintering is carried out at 130 ° C. and more than 25% bulk Cu conductivity is observed above 150 ° C. It is suggested that when the sintering temperature exceeds the melting point of nano-copper (>350° C.), a stable conductivity channel is formed on the substrate surface.
In contrast, the self-sintering conductive inks such as those described herein can be processed and applied at low or near room temperatures and do not require expensive or complicated equipment or extra process steps to avoid oxidation. Ga-alkali alloys having low melting points can supercool far below their melting point making it possible to process and apply the ink at much lower temperatures. The self-sintering conductive inks can be used as solders or electrically conductive adhesives, thermal interface materials, and/or conductive traces/interconnects for both printed rigid and flexible/stretchable electronics and soft robots. They also can be formulated and tuned to function as electrical and thermal conductors, semiconductors, dielectrics or insulators, and/or transition from insulator to conductor by thermal or mechanical stimulations. The used materials and processes are cost-effective for many appropriate applications.
Depending on difference in conductivity between the preset and set inks, for example, the printing process described herein may be useful for anti-tamper and emergency temperature response such as bringing an auxiliary cooling unit online. By modifying the ink to change its temperature set point, new desired circuits can be added anytime to a system on reaching different temperatures.
The foregoing has been presented for purposes of example. The foregoing is not intended to be exhaustive or to limit features to the precise form disclosed. The examples discussed herein were chosen and described in order to explain principles and the nature of various examples and their practical application to enable one skilled in the art to use these and other implementations with various modifications as are suited to the particular use contemplated. The scope of this disclosure encompasses, but is not limited to, any and all combinations, subcombinations, and permutations of structure, operations, and/or other features described herein and in the accompanying drawing figures.
This invention was made with Government support under Contract No. N00178-04-D-4119-FC2846 awarded by the United States Navy. The Government has certain rights in this invention.
