Silver nanoparticle-based inks

Abstract

The present invention relates to ink formulations based on silver nanoparticles which can be advantageously used for electromagnetic interference ("EMI") shielding coatings of semiconductor chips, in particular epoxy molding compound ("EMC") chips; in particular, the present invention relates to such silver nanoparticle-based ink formulations which are stable, have improved electrical conductivity and are particularly suitable for application by spraying.

Description

[0001] The present invention relates to an ink formulation based on silver nanoparticles, which can be advantageously used in electromagnetic interference (EMI) shielding coatings for semiconductor chips, especially epoxy molding compounds (EMC) chips; specifically, the present invention relates to such an ink formulation based on silver nanoparticles, which is stable, has improved conductivity and is particularly suitable for application by spraying.

[0002] The present invention also relates to the use of the ink in the field of electromagnetic interference (EMI) shielding coatings by spraying the ink on a semiconductor chip, especially an epoxy molding compound (EMC) chip.

[0003] The applicant has described similar inks in its patent applications WO20160184979A1 published on November 24, 2016 and WO2020120252A1 published on June 18, 2020, which are particularly suitable for application by screen printing and inkjet, respectively.

[0004] The development of conductive nanoparticles has made it possible to provide new uses and envision many new applications. Nanoparticles have a very large surface area / volume ratio, and the replacement of their surface by surfactants causes certain property changes and brings the possibility of dispersing them.

[0005] Their small size can lead to quantum confinement effects in some cases. The term "nanoparticle" is used when at least one dimension of the particle is less than or equal to 250 nm. Nanoparticles can be spherical (1 to 250 nm), rod-shaped (length L less than 200 to 300 nm), wire-shaped (hundreds of nanometers or even several micrometers), disk-shaped, star-shaped, pyramid-shaped, tetrapod-shaped, cubic or crystalline when there is no predefined shape.

[0006] A variety of methods for synthesizing conductive nanoparticles have been developed. Among them, the following methods can be non-exhaustively listed:

[0007] - Physical methods: Chemical vapor deposition (CVD) method, which occurs when a substrate is exposed to volatile chemical precursors that react or decompose on its surface, and this method usually results in the formation of nanoparticles, whose morphology depends on the conditions used; thermal evaporation method; molecular beam epitaxy method, which occurs when atoms constituting the nanoparticles are bombarded onto the substrate at high speed in the form of a gas stream (and combine on the substrate);

[0008] - Chemical or physico - chemical methods: microemulsion method; laser pulse method in solution, which occurs when a laser beam irradiates a solution containing precursors, and nanoparticles form along the laser beam in the solution; microwave irradiation synthesis method; surfactant - assisted directed synthesis method; ultrasound - mediated synthesis method; electrochemical synthesis method; organometallic synthesis method; synthesis method in alcohol medium.

[0009] Physical synthesis consumes more starting materials and has significant losses. It is usually time - consuming and requires high temperatures, which makes it unsuitable for transfer to industrial - scale production. Thus, it is not suitable for certain substrates, such as flexible substrates. Additionally, the synthesis is carried out directly on substrates with small - sized structures. These production methods prove to be relatively rigid and do not allow production on large - sized substrates; however, they may be perfectly suitable for the production of silver nanoparticles used in the ink formulations according to the present invention.

[0010] Chemical synthesis, by itself, has many advantages. The first is that it is carried out in solution: the resulting conductive nanoparticles are already dispersed in the solvent, which facilitates their storage and use. In most cases, the nanoparticles do not adhere to the substrate at the end of the synthesis, thus providing greater freedom in their use. This opens the way for using substrates of different sizes and different properties. These methods also allow better control of the starting materials used and limit losses. The correct adjustment of the synthesis parameters enables good control of the synthesis and growth kinetics of the conductive nanoparticles. This makes it possible to ensure good reproducibility between batches and good control of the final morphology of the nanoparticles. It is possible to produce nanoparticles chemically quickly and in large quantities, while ensuring a certain flexibility of the product, making it possible to envisage industrial - scale production. The production of dispersed conductive nanoparticles opens up numerous prospects for their customization. Thus, the nature of the stabilizer present on the surface of the nanoparticles can be adjusted according to the expected application. Specifically, there are various methods of wet deposition. In each case, special attention must be paid to the physical properties of the ink, such as surface tension or viscosity. The additives used in the formulation of nanoparticle - based inks will make it comply with the requirements of the deposition method. However, the surface ligands also affect these parameters, and their choice proves to be decisive. Therefore, it is important to have a comprehensive understanding of the ink in order to combine all the elements involved (nanoparticles, solvents, ligands, and additives) and obtain a product compatible with the target application.

[0011] The object of the present invention is to overcome one or more drawbacks of the prior art by providing an ink that is suitable for non - contact spraying and improved uniform coating for use as an electromagnetic interference (“EMI”) shielding coating. The ink is characterized by a series of improved properties, among which the following properties may be cited for illustrative purposes:

[0012] - Higher stability, such as better stability over time than existing inks, and / or

[0013] - non-toxicity of the solvent and nanoparticles, and / or

[0014] - Improved surface tension, and / or

[0015] - retain the intrinsic properties of the nanoparticles after application by spraying, and / or

[0016] - better annealing properties (homogeneity of the deposit), and / or

[0017] - improved adhesion to the substrate, and / or

[0018] - does not produce bubbles / foam after spraying, and / or

[0019] - Excellent electrical conductivity for annealing temperatures typically between 120°C and 220°C.

[0020] Although the inks described in WO20160184979A1 and WO2020120252A1 already meet a large number of the requirements set out above, the applicant has sought to develop new ink compositions that make it possible to further improve their properties and their sprayability; in particular for use in the field of electromagnetic interference ("EMI") shielding coatings by spraying said inks onto semiconductor chips, in particular onto epoxy molding compound ("EMC") chips.

[0021] ink

[0022] Therefore, the present invention relates to an ink composition comprising at least:

[0023] - 25wt% silver nanoparticles,

[0024] - 15 wt% of aliphatic monohydric alcohol,

[0025] - 10 wt% of polyols and / or polyol derivatives, and

[0026] -5wt% of terpenoid alcohols,

[0027] Less than 0.5 wt% water, and

[0028] The sum of the silver nanoparticles, aliphatic monohydric alcohols, polyhydric alcohols and polyhydric alcohol derivatives, terpenoid alcohols and water constitutes at least 90 wt % of the ink composition.

[0029] The present invention also relates to the synthesis of the ink composition.

[0030] The present invention also relates to the use of the ink composition in the field of electromagnetic interference ("EMI") shielding coating by spraying the ink composition onto a semiconductor chip, particularly an epoxy molding compound ("EMC") chip.

[0031] The present invention also relates to a method for spraying the ink composition onto a semiconductor chip, in particular an epoxy molding compound ("EMC") chip, as an electromagnetic interference ("EMI") shield. Description of the Drawings

[0032] Figure 1 Figure 1 Shows the visual difference in the uniformity of the spray deposits on the EMC-chip between the sprayed formulation S-CS91544 (comparative example - left) and S-CS91547 (present invention - right).

[0033] Figure 2 Figure 2 and

[0034] Figure 3 Figure 3 Shows the microscopic difference in the uniformity of the spray deposits on the EMC-chip between the sprayed formulation S-CS91544 (comparative example - Figure 2 ) and S-CS91547 (present invention - Figure 3 ).

[0035] The viscosity of the ink composition is an important property for EMI, and for reasons of improving the spraying ability and fluidity, a high degree of uniformity and homogeneity of the electromagnetic wave shielding film on all shielding surfaces of the semiconductor chip must be ensured. The viscosity of the ink according to the present invention, measured at 20 °C, is typically 5 to 100 mPa·s, for example 7.5 to 50 mPa·s, for example 9 to 25 mPa·s.

[0036] The applicant has found that the combination of the silver nanoparticle-based ink composition according to the present invention with the claimed compounds and their respective concentrations enables the obtaining of an ink having improved properties, in particular improved stability, excellent spraying ability, and conductivity within a viscosity range particularly suitable for use in the field of non-contact spraying for EMI shielding; in particular, the silver nanoparticle-based ink formulation according to the present invention has a series of improved properties, among which the following properties may be listed for illustrative purposes:

[0037] - Higher stability, for example better stability over time than existing inks, and / or

[0038] - Non-toxicity of the solvent and nanoparticles, and / or

[0039] - Improved surface tension, and / or

[0040] - Retaining the inherent properties of the nanoparticles after application by spraying, and / or

[0041] ​​​​​​- Better annealing performance (uniformity of the deposit after spraying), and / or

[0042] - Improved adhesion to the substrate, and / or

[0043] - No formation of bubbles / foam after spraying, and / or

[0044] - Excellent electrical conductivity for annealing temperatures typically between 120 °C and 220 °C, with a significant improvement between 150 °C and 180 °C.

[0045] The conductive nanoparticle-based ink according to the invention can advantageously be sprayed on all types of carriers. Examples to be mentioned include the following carriers: paper, polymers and polymer derivatives, composite materials, organic materials and / or inorganic materials; in particular, the claimed ink is particularly suitable for EMI shielding of integrated circuit ("IC") packages, such as EMI shielding of epoxy molding compounds ("EMC"). In fact, most of these packages are molded from epoxy plastics, which provide proper protection for semiconductor devices and provide mechanical strength to support the leads and handling of the IC package. Exemplary examples of such plastics are cresol-novolaks, silicone polyimides, parylenes, silicones, polyepoxides and / or benzocyclobutene materials.

[0046] In one embodiment of the invention, a semiconductor package is provided that includes an electromagnetic wave shielding layer based on the claimed ink for protecting a semiconductor chip in the semiconductor package from external factors and shielding electromagnetic waves.

[0047] With the expansion of the electronic product market, the demand for more functions and smaller portable devices has grown rapidly, and thus, technology has promoted the development of electronic components in electronic products towards miniaturization and light weight. In this regard, not only the sizes of various electronic components are reduced, but also multiple discrete semiconductor chips can be placed in one semiconductor package.

[0048] Although this does not constitute a preferred embodiment according to the invention, the semiconductor package may further include an insulating layer between the semiconductor chip and the shielding layer, which contains a thixotropic material or a hot melt material. The thixotropic material may include at least one of the following: complex fine silica, bentonite, surface-treated calcium carbonate fine particles, hydrogenated castor oil, metal soaps, aluminum stearate, polyamide wax, polyethylene oxide and linseed polymerized oil. The hot melt material may include at least one of the following: polyurethane, polyurea, polyvinyl chloride, polystyrene, acrylonitrile butadiene styrene, polyamide, acrylic acid and polybutylene terephthalate. The thixotropic material or the hot melt material can be cured by ultraviolet curing or thermal curing.

[0049] In one embodiment of the present invention, the semiconductor chip package to be shielded is a three-dimensional object having a top surface and side surfaces, which surfaces can be advantageously treated with the claimed ink; since the three-dimensional object can be made of layers of different materials, it is crucial that the shielding layer provides proper adhesion and uniformity of the sprayed layer on all the surfaces to be shielded. The materials include not only the epoxy-based compounds cited above, but also, as exemplary examples, copper, polyimide (such as Kapton), silicon, silicon-based compounds, silica, etc.

[0050] The present invention also relates to a method of spraying the claimed ink composition onto at least two different materials of a semiconductor chip package to serve as an electromagnetic interference (“EMI”) shielding layer, wherein the package substantially comprises an epoxy molding compound (“EMC”), and further comprises copper, polyimide (such as Kapton), silicon, silicon-based compounds or silica.

[0051] The claimed silver nanoparticle-based ink can be advantageously sprayed on electronic components to protect the device from electromagnetic interference. This technique allows for the direct formation of an effective EMI shielding layer on the chip through a one-step spraying process, which one-step spraying is preferably carried out before pick-and-placing the component onto an electronic flexible circuit board. A very thin sprayed ink layer can be used to achieve high shielding efficiency, which allows for the reduction of the weight and size of the final electronic device. In the prior art for such shielding, one can cite the direct addition of a metal casing to the chip that needs to be shielded, but this solution has several drawbacks as it requires a large production line and has a serious impact on the environment. It also results in an increase in the system weight and takes up space on a printed circuit board (“PCB”). Another prior solution aims at metallization through physical vapor deposition (PVD). This method is lighter for the final device, but due to its complex process, it requires a large investment in the production line.

[0052] On the other hand, the claimed solution is compatible with high-volume industrial production equipment, such as ultrasonic spraying equipment. The claimed ink has been optimized and tuned to exhibit strong adhesion on EMC, copper, PCB, and silicon wafers, and to have a homogeneous and ultra-thin shielding layer on the top and side walls of the external package. Compared with copper foil, the ink shows better electrical properties, thus allowing for better EMI shielding on top of the chip. In addition, the claimed coating solution is sufficiently resistant to pass electronic post-treatment standards, such as solvent cleaning baths, tolerance / reliability tests under environmental conditions, etc. The claimed solution is environmentally friendly as it uses non-toxic and non-carcinogenic, mutagenic, and reprotoxic (CMR) solvents for its formulation. And it only requires a low investment to adapt the existing production line.

[0053] Silver nanoparticles

[0054] The silver nanoparticles constitute at least 25 wt% of the ink composition according to the invention. In particular, the silver nanoparticle content in the ink composition according to the invention is at least 30 wt%, such as at least 35 wt%; it is preferably less than 60 wt%, such as less than 50 wt% (weight of silver nanoparticles divided by the weight of the ink composition).

[0055] According to a variant of an embodiment of the invention, the object of the invention can be particularly achieved when the compound "a" consists of silver nanoparticles with a size between 1 and 250 nm. The size of the nanoparticles is defined as the average diameter of the silver-containing particles (excluding the stabilizer), for example, determined by transmission electron microscopy.

[0056] According to a variant of an embodiment of the invention, the silver nanoparticles have a quasi-spherical and / or spherical shape. In the present invention and the following claims, the term "quasi-spherical" refers to a shape similar to a sphere but not completely circular ("sub-spherical"), such as the shape of an ellipsoid. The shape of the nanoparticles is usually identified by images taken with a microscope. Therefore, according to this variant of the embodiment of the invention, the nanoparticles have a diameter between 1 and 250 nm.

[0057] According to a variant of an embodiment of the invention, the silver nanoparticles can be spherical (1 to 250 nm), rod-shaped (length L less than 200 to 300 nm), wire-shaped (hundreds of nanometers or even several micrometers), cubic, flaky or crystalline when there is no predefined shape.

[0058] According to a specific embodiment of the invention, the silver nanoparticles have been pre-synthesized by physical synthesis or chemical synthesis methods. Any physical or chemical synthesis method can be used in the context of the present invention. In a specific embodiment according to the invention, the silver nanoparticles are obtained by a chemical synthesis method using an organic or inorganic silver salt as a silver precursor. Non-limiting examples that can be cited include silver acetate, silver nitrate, silver carbonate, silver phosphate, silver trifluorate, silver chloride, silver perchlorate, used alone or as a mixture. According to a variant of the invention, the precursor is silver acetate.

[0059] Therefore, the nanoparticles preferably used in the present invention are characterized in that the D50 value is preferably between 1 and 250 nm, regardless of the synthesis method (physical or chemical); it is also preferably characterized by a monodisperse (homogeneous) distribution and no aggregates. For quasi-spherical silver nanoparticles, a D50 value between 10 and 150 nm is preferably adopted.

[0060] Aliphatic monohydric alcohol

[0061] The aliphatic monohydric alcohol constitutes at least 15 wt% of the ink composition according to the present invention. In particular, the content of the aliphatic monohydric alcohol in the ink composition according to the present invention is at least 20 wt%, for example at least 25 wt%; it is preferably less than 40 wt%, for example less than 30 wt% (the weight of the aliphatic monohydric alcohol divided by the weight of the ink composition).

[0062] The aliphatic monohydric alcohol is preferably selected from the group consisting of: ethanol, propanol, butanol, pentanol, and hexanol and their isomers (such as isopropanol, n-butanol, tert-butanol), and / or a mixture of two or more of the above aliphatic monohydric alcohols. Isopropanol is particularly suitable for EMI shielding spraying applications of the ink.

[0063] Polyhydric alcohol and / or polyhydric alcohol derivative

[0064] The polyhydric alcohol and polyhydric alcohol derivative constitute at least 10 wt% of the ink composition according to the present invention. In particular, the content of the polyhydric alcohol and polyhydric alcohol derivative in the ink composition according to the present invention is at least 15 wt%, for example at least 19 wt%; it is preferably less than 35 wt%, for example less than 27 wt% (the weight of the polyhydric alcohol and polyhydric alcohol derivative divided by the weight of the ink composition).

[0065] The polyhydric alcohol and / or polyhydric alcohol derivative is preferably characterized by a boiling point below 260 °C. Exemplary examples include: dihydric alcohols (such as ethylene glycol, propylene glycol, diethylene glycol, trimethylene glycol, 1,3-butanediol, 1,2-butanediol, 2,3-butanediol, pentanediol, hexanediol, etc.), and / or alcohol ether solvents (such as monomethyl or dimethyl ethers of alcohols, exemplary examples include ethylene glycol propyl ether, ethylene glycol butyl ether, ethylene glycol phenyl ether, propylene glycol phenyl ether, diethylene glycol methyl ether, diethylene glycol ethyl ether, diethylene glycol propyl ether, diethylene glycol butyl ether, propylene glycol methyl ether, propylene glycol butyl ether, propylene glycol propyl ether, dipropylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diglyme, diethylene glycol diethyl ether, dibutylene glycol diethyl ether, diglycol, ethyl diglycol, butyl diglycol), and / or alcohol ether acetates (such as ethyl 2-butoxyacetate, diethylene glycol monoethyl ether acetate, diethylene glycol butyl ether acetate, propylene glycol methyl ether acetate), and / or a mixture of two, three or more of the above solvents. Dipropylene glycol monomethyl ether (also known by the name Dowanol DPM), 2-(2-butoxyethoxy)ethanol (also known by the name butyl carbitol), and propylene glycol methyl ether acetate (also known by the abbreviation PGMEA) are solvents particularly suitable for the formulations according to the present invention; in a preferred embodiment according to the present invention, the ink contains a mixture of these three solvents.

[0066] Terpenoid alcohol

[0067] Terpenoid alcohols constitute at least 5 wt% of the ink composition according to the invention. In particular, the content of terpenoid alcohols in the ink composition according to the invention is at least 6 wt%, for example at least 8 wt%; it is preferably less than 18 wt%, for example less than 15 wt% (weight of terpenoid alcohol divided by weight of ink composition).

[0068] The terpenoid alcohol is preferably selected from menthol, nerol, eucalyptol, lavandulol, geraniol, terpineol (α-, β-, γ-terpineol and / or 4-terpineol; preferably α-terpineol), isoborneol, citronellol, linalool, borneol, geraniol, and / or a mixture of two or more of the above alcohols; a mixture of α-terpineol and γ-terpineol has been found to be particularly suitable for the formulation according to the invention.

[0069] Water

[0070] Water constitutes less than 0.5 wt% of the ink composition according to the invention (weight of water divided by weight of ink composition).

[0071] According to one embodiment of the invention, the ink does not incorporate water into its composition. However, since some components of the ink may tolerate trace amounts of water depending on their purity, it should be understood that the sum of these corresponding trace amounts of water is acceptable in the claimed ink. Thus, the water content in the final ink generally depends substantially on the water content in the solvents used in preparing the ink. According to a specific embodiment of the invention, the water content in the ink composition according to the invention is less than 0.25 wt%, for example less than 0.1 wt%, or even zero.

[0072] According to a preferred embodiment of the invention, no water is added during the formulation of the claimed ink composition, except for trace amounts of water that may be present in the compounds used in preparing / formulating the ink.

[0073] Adhesion promoter

[0074] According to a variant of an embodiment of the present invention, the ink composition further comprises at least one adhesion promoter, the concentration of which will preferably be greater than 0.1 wt% of the ink composition, preferably greater than 0.25 wt%, for example greater than 0.5 wt%; and preferably less than 2.5 wt% of the ink composition (weight of the adhesion promoter divided by the weight of the ink composition). Examples of adhesion promoters that can be cited include vinyl pyrrolidone / vinyl acetate copolymers, acrylic polymers, silanes, siloxanes and / or polysiloxanes, the purpose of which is to improve the tolerance to various mechanical stresses, such as adhesion to various substrates; linear random copolymers of N-vinyl-2-pyrrolidone (VP) and vinyl acetate (VA) with a monomer molar ratio of VP / VA from 30:70 to 70:30, such as a linear random copolymer of N-vinyl-2-pyrrolidone and vinyl acetate with a ratio of 60:40, are preferred adhesion promoters. According to a preferred embodiment of the present invention, the ink composition comprises at least 0.7 wt% of a vinyl pyrrolidone / vinyl acetate copolymer.

[0075] For illustrative purposes, a specific example of preparing an ink according to the present invention is described below:

[0076] The optional adhesion promoter is preferably first dissolved in one of the solvents in a first step. Then the silver nanoparticles are added to the mixture together with the remaining solvents, and stirring is continued for 3 hours. Then sonication is carried out for 15 minutes, followed by an end filtration step using a glass microfiber filter with a cut-off particle size of 1 μm.

[0077] Another advantage of the ink according to the present invention is that its preparation can be carried out under non-restrictive pressure and / or temperature conditions, for example under pressure and / or temperature conditions close to or the same as normal or ambient conditions. Preferably, it is maintained below 40% of normal or ambient pressure and / or temperature conditions. For example, the applicant has found that it is preferred to keep the pressure and / or temperature conditions fluctuating by no more than 30%, preferably no more than 15%, around the values of normal or ambient conditions during the preparation of the ink. Therefore, advantageously, the control of these pressure and / or temperature conditions can be incorporated into the ink preparation device to meet these conditions. This advantage related to the preparation of the ink under non-restrictive conditions is clearly also reflected in the convenience of using the ink.

[0078] According to an embodiment of the present invention, the ink composition comprises at least:

[0079] - 25 wt% of silver nanoparticles,

[0080] - 15 wt% of aliphatic monohydric alcohols,

[0081] - 10 wt% of polyols and / or polyol derivatives,

[0082] - 5 wt% of terpenoid alcohol, and

[0083] - 0.5 wt% of vinyl pyrrolidone / vinyl acetate copolymer

[0084] Less than 0.5 wt% of water,

[0085] The sum of all the compounds defined above constitutes at least 95 wt% of the ink composition. According to a specific variant of the present invention, the sum of all the compounds defined above will preferably constitute at least 97.5 wt%, at least 98 wt%, at least 99 wt% or even 100 wt% of the final ink.

[0086] According to one embodiment of the present invention, the silver ink may optionally further contain an antioxidant. Examples of antioxidants that can be cited include:

[0087] - Ascorbic acid or vitamin C (E300), sodium ascorbate (E301), calcium ascorbate (E302), 5,6 - diacetyl - L - ascorbic acid (E303), 6 - palmitoyl - L - ascorbic acid (E304);

[0088] - Citric acid (E330), sodium citrate (E331), potassium citrate (E332) and calcium citrate (E333);

[0089] - Tartaric acid (E334), sodium tartrate (E335), potassium tartrate (E336) and sodium potassium tartrate (E337);

[0090] - Butylated hydroxyanisole (E320) and butylated hydroxytoluene (E321);

[0091] - Octyl gallate (E311) or dodecyl gallate (E312);

[0092] - Sodium lactate (E325), potassium lactate (E326) or calcium lactate (E327);

[0093] - Lecithin (E322);

[0094] - Natural tocopherol (E306), synthetic α - tocopherol (E307), synthetic γ - tocopherol (E308) and synthetic δ - tocopherol (E309), and these tocopherols together constitute vitamin E;

[0095] - Eugenol, thymol and / or cinnamaldehyde;

[0096] - And mixtures of two or more of the above antioxidants.

[0097] According to an embodiment of the present invention, the silver ink may optionally further comprise a dispersant. For example, an organic dispersant containing at least one carbon atom. These organic dispersants may also contain one or more non-metallic heteroatoms, such as halogenated compounds, nitrogen, oxygen, sulfur or silicon. For illustrative purposes, thiols and their derivatives, amines and their derivatives (such as amino alcohols and amino alcohol ethers), carboxylic acids and their carboxylate derivatives and / or mixtures thereof may be cited.

[0098] Although this does not constitute a preferred embodiment according to the present invention, the ink composition may also comprise a rheology modifier, such as a urea-based rheology modifier (such as modified urea, polyurea and / or mixtures thereof) and / or a cellulose-based rheology modifier (such as alkyl cellulose (such as ethyl cellulose), nanocellulose, nitrocellulose and / or mixtures thereof). If a rheology modifier is included, the total concentration of the rheology modifier in the ink will preferably be less than 5 wt%, less than 1 wt%, less than 0.1 wt% or even less than 0.01 wt%.

[0099] Although this does not constitute a preferred embodiment according to the present invention, the ink composition may also allow for the presence of additional compounds, among which solvents may be cited for illustrative purposes. For example, hydrocarbons; alkanes containing 5 to 20 carbon atoms, among which pentane (C5H12), hexane (C6H14), heptane (C7H16), octane (C8H18), nonane (C9H20), decane (C10H22), undecane (C11H24), dodecane (C12H26), tridecane (C13H28), tetradecane (C14H30), pentadecane (C15H32), hexadecane (C16H34), heptadecane (C17H36), octadecane (C18H38), nonadecane (C19H40), eicosane (C20H42), cyclopentane (C5H10), cyclohexane (C6H12), methylcyclohexane (C7H14), cycloheptane (C7H14), cyclooctane (C8H16) (preferably when it is not used as compound "b"), cyclononane (C9H18), cyclodecane (C10H20) may be cited for illustrative purposes; aromatic hydrocarbons containing 7 to 18 carbon atoms, among which toluene, xylene, ethylbenzene, ethyltoluene may be cited for illustrative purposes; and mixtures thereof. If included, the total concentration of the solvent in the ink will preferably be less than 5 wt%, less than 1 wt%, less than 0.1 wt% or even less than 0.01 wt%.

[0100] According to an embodiment of the present invention, the ink can be advantageously used in the spraying process detailed below.

[0101] Spraying

[0102] With the growing demand for advanced computing systems and the improvement in the performance of personal portable devices, front-end manufacturers need to integrate more functions while reducing the physical size of chips. According to the present invention, ultrasonic spraying technology is a preferred technology for coating the claimed ink because it can spray a variety of materials and produce a uniform coating on a substrate. The ultrasonic spraying technology preferably uses a nozzle tip (or nozzle) vibrated by an ultrasonic transducer.

[0103] As an example, the nozzle head is preferably an integrated component, including an ultrasonic transducer and a nozzle tip, a liquid applicator, and an air director (by adjusting the air flow of the air director, the spraying rate can be determined).

[0104] The ultrasonic frequency at which the ultrasonic transducer vibrates can advantageously be between 16 kHz and 180 kHz; for example, between 20 kHz and 100 kHz; and again for example, between 20 kHz and 60 kHz.

[0105] Silver ink is delivered to the nozzle on the ultrasonic transducer through a liquid applicator. The silver ink is stored in a reservoir and supplied to the liquid applicator at a precisely controlled rate by a positive displacement pump. The ultrasonic vibration from the nozzle tip breaks the liquid into small droplets and sprays them out of the nozzle tip as a spray. The air director is used to generate an air flow to shape and accelerate the ultrasonic-generated spray. The spraying parameters are as follows, for example: pressure: 40 - 50 (PSI); nozzle head speed: 150 - 400 (mm / s); height Z = 40 mm; step distance = 10 mm; ink flow rate: 0.1 - 3 mL / min; nozzle head tilt angle = 30°; number of layers: 1 - 4; number of directions: 1 - 4.

[0106] In one embodiment of the present invention, the nozzle is capable of coating a layer of spraying material with a thickness of 0.5 to 1.5 microns. For electromagnetic interference shielding in semiconductor chip applications, the thickness of the final ink layer (after curing at 150 °C for 30 minutes) is preferably between 1 and 5 microns; for example, it means that the nozzle device sprays the chip one to five times.

[0107] Therefore, it is obvious to those skilled in the art that the present invention allows many other specific forms of embodiments without departing from the scope of the claimed invention application. Therefore, these embodiments should be considered illustrative, but can be modified within the scope defined by the appended claims.

[0108] The present invention and its advantages will now be illustrated by the formulations collated in the following table. The ink formulations are prepared according to the embodiments described above in this specification. The chemical compounds and properties used are shown in the table.

[0109] The stability of the ink according to the present invention can be measured according to any suitable method. As an example, the measurement of the ink stability follows the standard procedure of Guinness ink: during the stability test, the materials are tested monthly (stored between ambient conditions and 4°C - 5°C) for a total of 6 to 12 months. To verify the stability of the materials, all characterization methods will be carried out, such as viscosity, ink solid content, and conductivity of the coated samples, etc.

[0110] S-CS91547 is a formulation according to the present invention, while S-CS91544 and S-CS11520 are formulations given for comparison purposes.

[0111] Table 1

[0112]

[0113]

[0114] Silver nanoparticles (Ag NPs) are spherical-like and have a D50 of 50 nm.

[0115] Butyl carbitol is 2-(2-butoxyethoxy)ethanol (i.e., diethylene glycol butyl ether).

[0116] Dowanol DPM is dipropylene glycol monomethyl ether.

[0117] PGMEA is propylene glycol methyl ether acetate.

[0118] TEGME is triethylene glycol monomethyl ether.

[0119] PVP-VA is a vinylpyrrolidone / vinyl acetate copolymer; in particular, it is Ashland TM PVP / VA S-630 copolymer, i.e., a 60:40 linear random copolymer of N-vinyl-2-pyrrolidone and vinyl acetate prepared by free radical polymerization.

[0120] Table 2

[0121]

[0122] Spraying is carried out on the same EMC chip according to the following parameters [printer: Spray USI Prism 400; pressure: 50 PSI; nozzle speed: 300 mm / s; height Z: 40 mm; step distance: 10 mm; ink flow rate 0.8 mL / min; nozzle tilt angle: 30°C; number of layers: 4, 1 layer in each direction]. During the spraying process, the EMC sprayed sample is placed on a hot plate at 60°C and dried for 5 minutes, and then cured in a conventional oven at 150°C for 30 minutes.

[0123] The sheet resistance of the ink mentioned in the present invention can be measured according to any suitable method. As an example, corresponding to the measurements tabulated, it can be advantageously measured for a cured EMC spray sample according to the following method:

[0124] - Machine model: S302 resistance measurement bench of Microworld;

[0125] - Four-probe test head model: SP4-40045TFY (four equally spaced collinear probes); - Current source model: Agilent U8001A;

[0126] - Multimeter model: Agilent U3400;

[0127] - Measurement temperature: room temperature;

[0128] - Voltage / resistance conversion factor: 4.5324;

[0129] - Apply a DC current to the outer probes of the test head and raise the voltage obtained between the other two probes of the test head. Then, calculate the sheet resistance according to the following equation:

[0130]

[0131] - Here, Rs is the sheet resistance (in ohms per square), ΔV is the voltage change measured between the inner probes (in volts), and I is the current applied between the outer probes (in amperes). The coefficient 4.53236 is provided by the user manual of the S302 resistance measurement bench and corresponds to π / ln(2) = 4.53236.

[0132] The content of silver nanoparticles mentioned in the present invention can be measured according to any suitable method. As an example, corresponding to the measurements tabulated, it can be advantageously measured according to the following method:

[0133] - Thermogravimetric analysis;

[0134] - Instrument: TGAQ50 of TA Instruments;

[0135] - Crucible: alumina crucible;

[0136] - Method: ramp;

[0137] - Measurement range: from room temperature to 600 °C;

[0138] - Heating rate: 20 °C / min.

[0139] The ink viscosity mentioned in the present invention can be measured according to any suitable method. As an example, it can be advantageously measured according to the following method:

[0140] - Instrument: AR-G2 rheometer from TA Instrument;

[0141] - Conditioning time: Pre-shear for 1 minute / Equilibrate for 1 minute at 40 s -1 ;

[0142] - Test type: Shearing stage;

[0143] - Stages: 40 s -1 , 100 s -1 and 1000 s -1 ;

[0144] - Duration of each stage: 5 minutes;

[0145] - Mode: Linear;

[0146] - Measurement frequency: Once every 10 seconds;

[0147] - Temperature: 20 °C;

[0148] - Curve reprocessing method: Newtonian;

[0149] - Reprocessing region: The entire curve.

[0150] The surface tension mentioned in the present invention can be measured according to any suitable method. As an example, it can be advantageously measured according to the following pendant drop method:

[0151] This measurement is performed using a DataPhysics OCA 15Plus device equipped with a camera. The surface tension range that can be measured by this device given by the manufacturer is from 0.01 to 2000 mN / m, with an accuracy of ±0.05 mN / m. This method is the Pendant drop left.

[0152] During the measurement of surface tension, a droplet of the solution to be analyzed is formed in air through a needle connected to a syringe. The automatic system allows controlling the push of the syringe piston. Then, the shape of this droplet or a series of droplets is recorded by the camera. Next, the software calculates the surface tension based on these images.

[0153] Then, the surface tension γ (N.m -1 ) is given by the following formula (simplified Laplace–Young equation):

[0154] γ = Δρg d 2 / H

[0155] where:

[0156] Δρ (kg.m -3(): The density difference between the liquid of the droplet and its surrounding phase;

[0157] g (acceleration due to gravity at the Earth's surface): 9.81 m.s -2 ;

[0158] d (m): Equatorial diameter of the droplet;

[0159] H: Shape factor;

[0160] 1 / H is a function of S = ds / de, where ds is the diameter measured at a distance de from the bottom of the droplet.

[0161] - Instrument: OCA 15 from DataPhysics;

[0162] - Method: Left-side sessile drop method;

[0163] - Measured volume: 0.2 μL;

[0164] - Flow rate: 0.5 μl / s;

[0165] - Probe: 1.65 mm probe;

[0166] - Input ink density: 1.1675;

[0167] - Measurement: Ink volume: 1 mL ink / number of measurements = 4 times.

[0168] The adhesion test mentioned in the present invention can be measured according to any suitable method. As an example, it can be advantageously measured according to the following simplified version of the ASTM D3359 standard. Parallel and perpendicular scribes are made using a diamond-tipped pen and a metal ruler to form a grid pattern. The pen is held at an angle between 15° and 30° with respect to the plate. Sufficient force is used during scribing to scribe down to the substrate surface. The ruler should be positioned on the uncut portion of the surface. The tape model is 3M610 (12.7 mm; 4.7 N / cm). A section of tape approximately 75 mm long is cut, its center is attached to the grid, and then it is spread over the entire surface. A free area is left so that the tape can be easily removed. The tape is firmly pressed with a finger to adhere to the entire surface. Wait for 90 ± 30 seconds (using a stopwatch). The tape is quickly removed at an angle close to 180° with respect to the substrate without tearing. The results are recorded according to the grades of the standard from 0B to 5B: 0B means 100% of the coating is removed, and 5B means 0% of the coating is removed.

[0169] As reflected by the sheet resistance measurement, it has excellent conductivity at annealing temperatures typically between 120 °C and 220 °C.

[0170] The ink composition according to a variant of the present invention is characterized in that its sheet resistance attribute value is less than 350 mOhms / sq (annealing temperature: 150 °C) when the thickness is less than or equal to 500 nm.

[0171] The thickness mentioned in the present invention can be measured according to any suitable method. As an example, its measurement can be advantageously carried out using a one-dimensional mechanical profiler (model: Alpha Step IQ (KLA Tencor)). A straight line is drawn at the sheet resistance measurement position using a metal fixture. Measurement method: Measurement is carried out using the default settings (range: 20 μm, length: 500 μm, speed: 50 μm / s, frequency: 50 Hz, duration: 1 time, 2-layer deposition). Resistivity (μΩ·cm) = Rs (μΩ / sq) × thickness (cm).

[0172] The comparison of the spray coatings prepared with the above formula is a good example of visualizing the advantages provided by the present invention; specifically, a significant improvement in the uniformity of the spray formulation S-CS91547 is observed relative to the uniformity of S-CS91544.

[0173] The above visual observation is also confirmed by microscopic observation: The deposits are observed using a Dino-Lite Edge AM4000 digital microscope, which allows measurement in the range of x650 to x950 (1280 * 1024 pixels). This microscope has only one zoom, and the focus needs to be adjusted to obtain a clear image. The microscopic images of the S-CS91544 deposits show larger ink droplets that are not fully fused and have larger gaps between the droplets compared to the S-CS91547 deposits.

Claims

1. An ink composition, which at least comprises: - 25 wt% of silver nanoparticles, - 15 wt% of aliphatic monohydric alcohols, - 10 wt% of polyols and / or polyol derivatives, and - 5 wt% of terpene alcohols, less than 0.5 wt% of water, and the sum of silver nanoparticles, aliphatic monohydric alcohols, polyols and polyol derivatives, terpene alcohols and water constitutes at least 90 wt% of the ink composition.

2. An ink composition, which at least comprises: - 25 wt% of silver nanoparticles, - 15 wt% of aliphatic monohydric alcohols, - 10 wt% of polyols and / or polyol derivatives, - 5 wt% of terpene alcohols, and - 0.5 wt% of vinyl pyrrolidone / vinyl acetate copolymer, less than 0.5 wt% of water, and the sum of silver nanoparticles, aliphatic monohydric alcohols, polyols and polyol derivatives, terpene alcohols, vinyl pyrrolidone / vinyl acetate copolymer and water constitutes at least 95 wt% of the ink composition.

3. The ink composition according to any one of the preceding claims, wherein the viscosity of the ink measured at 20 °C is between 5 and 100 mPa·s.

4. The ink composition according to any one of the preceding claims, wherein the silver nanoparticles are spherical-like and have a D50 value between 10 and 150 nm.

5. The ink composition according to any one of the preceding claims, wherein the aliphatic monohydric alcohol is isopropyl alcohol.

6. The ink composition according to any one of the preceding claims, wherein the polyols and / or polyol derivatives consist of a mixture of alcohol ethers and alcohol ether acetates.

7. The ink composition according to any one of the preceding claims, wherein the polyols and / or polyol derivatives consist of a mixture of dipropylene glycol monomethyl ether, 2-(2-butoxyethoxy)ethanol and propylene glycol methyl ether acetate.

8. The ink composition according to any one of the preceding claims, wherein the terpene alcohol is terpineol.

9. The ink composition according to claim 2, wherein the vinyl pyrrolidone / vinyl acetate copolymer is a linear random copolymer with a monomer ratio of N-vinyl-2-pyrrolidone to vinyl acetate of 30:70 to 70:

30.

10. The ink composition according to any one of the preceding claims, wherein the water content in the ink composition according to the present invention is less than 0.25 wt%, for example less than 0.1 wt% or even zero.

11. A use of spraying the ink composition according to any one of the preceding claims on a semiconductor chip for electromagnetic interference shielding.

12. A use of spraying the ink composition according to any one of the preceding claims on an epoxy molding compound chip for electromagnetic interference shielding.

13. The use as electromagnetic interference shielding according to claim 11, which is sprayed on the semiconductor chip by using an ultrasonic nozzle.

14. The use of the ink composition according to the previous claim, wherein the ultrasonic transducer of the ultrasonic nozzle vibrates at an ultrasonic frequency between 20 kHz and 60 kHz.

15. An epoxy molding compound chip sprayed with an ink according to any one of the preceding claims, the chip having a final ink layer thickness between 1 micron and 5 microns after curing at 150 °C for 30 minutes.

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