Solder mask ink for aerosol jet printing and methods for aerosol jet printing

Abstract

Solder mask ink for aerosol jet printing, comprising a resin or UV-curing monomer, a metal oxide and 20 to 50 wt%, based on the total weight of the ink, of a propylene glycol-based solvent, wherein the solvent is a 1:1 combination of propylene glycol monomethyl ether acetate and dipropylene glycol monomethyl ether acetate, wherein the solder mask ink has a viscosity of 0.05 Pa·s to 1 Pa·s (50 cps to 1,000 cps) at a shear rate of 10 1 / s at 25 °C and a structural viscosity index of 1.0 to 2.0.

Description

[0001] The embodiments disclosed herein relate to solder masks, such as those used in the manufacture of printed circuit boards. In particular, the embodiments disclosed herein relate to solder mask inks with a suitable viscosity for aerosol jet printing applications.

[0002] Printed circuit boards (PCBs or PCBs, hereinafter PCBs) are platforms that connect and interface electronic components with other electronic components and with other elements in computers, communication devices, consumer electronics, automated manufacturing, and testing equipment. PCBs can be manufactured from a base substrate, typically an insulating material, onto which a thin layer of copper is laminated or plated. Chemical etching is then used to remove areas of the copper to create electrically conductive paths or traces. These traces enable electrical connections between the components attached to the PCB.

[0003] Then, an insulating material called a solder mask is applied over the copper traces. Solder masks protect the traces on the PCB from being coated with solder during the soldering process, leaving only the conductor plates that need to be contacted with molten solder uncoated. The solder mask layer on simple PCBs can then be applied using screen printing or rotational molding techniques. However, more densely printed PCBs use lithographic techniques to create a patterned solder mask on the copper layer.

[0004] The lithographic techniques used to produce solder masks involve multi-step sequences that are material- and energy-intensive. The process typically includes film deposition, lithography, wet setting, and hardening, as shown in the flowchart of the Fig.Figure 1 shows an example. In such a process, solder masks are often epoxy-based materials that are rotationally coated or applied in a similar manner, followed by subtractive etching. The process tends to degrade the chemical and physical resistance of the final, cured solder mask. Because the first photolithography step is non-selective, passes in the PCB are often partially or completely filled with solder mask. Removing the solder mask from high stretch ratio passes is a very difficult and often impossible task. Finally, such processes can be expensive and generate a lot of waste.

[0005] Although digital methods for applying solder masks are desirable, attempts, e.g., with inkjet printing, have been limited due to the requirement for very high viscosity (less than approximately 20 cps). In contrast, commercial solder masks typically have very high viscosities (more than approximately 10,000 cps), and therefore inkjet printer solder mask systems are difficult to implement.

[0006] Alternatively, screen printing was developed for applying solder masks. While this can solve the problem of blocked channels, screen printing often results in solder masks with low resolution and poor coverage. Additionally, screen printing requires a flat surface. PCBs with textured surfaces are not suitable for screen-printed solder masks. Similarly, solder masks generally cannot be printed on curved surfaces or 3D electronics.

[0007] In some embodiments, the present embodiments provide solder mask inks for aerosol jet printing comprising a metal oxide and a propylene glycol-based solvent, wherein the solder mask ink has a viscosity of approximately 50 cps to approximately 1,000 cps at a shear rate of 10 1 / s at 25 °C and a structural viscosity index of approximately 1.0 to approximately 2.0.

[0008] In some embodiments, the present embodiments provide solder mask inks comprising: i) a resin or a UV-curing monomer; ii) an inorganic pigment; and iii) approximately 20 to approximately 50% by weight of the ink of a propylene glycol-based ether or ester solvent, wherein the solder mask ink formulation has a viscosity of approximately 50 cps to approximately 800 cps at a shear rate of 10 1 / s at 25 °C and a structural viscosity index of approximately 1.0 to approximately 1.5.

[0009] A method comprising aerosol jet printing of a solder mask ink in a pattern on a substrate, wherein the solder mask comprises: i) a resin or UV-curing monomer; ii) an inorganic pigment; and iii) approximately 20 to approximately 50 wt.% of a propylene glycol-based ether or ester solvent, wherein the solder mask ink has a viscosity of approximately 50 cps to approximately 800 cps at a shear rate of 10 1 / s at 25 °C and a structural viscosity index of approximately 1 to approximately 1.5, as well as hardening of the sprayed solder.

[0010] Various embodiments of the present disclosure are described below with reference to the figures, wherein:

[0011] Fig. Figure 1 shows a conventional solder mask application process.

[0012] Fig.Figure 2 shows a double application of the viscosity (at 25°C) and a structural viscosity index of an exemplary solder mask according to the present embodiments as a function of solvent addition. The structural viscosity index is the ratio of the viscosity at low shear rate of 10 1 / s to the viscosity at high shear rate of 484 1 / s.

[0013] Fig. Figure 3 shows a scratch resistance and adhesion test of an exemplary solder mask according to the present embodiments, formulated with a combination of propylene glycol monomethyl ether acetate and dipropylene glycol monomethyl ether acetate (PGMEA / DPGMEA). No material was transferred to the tape, demonstrating the excellent adhesion of 5B. The pencil hardness was 6H, the highest rating for a solder mask.

[0014] Fig.Figure 4A shows an image of a hardened exemplary solder mask according to present embodiments printed on polyethylene terephthalate (PET) substrate at different speeds (5.0 mm / s, 10 mm / s and 20 mm / s).

[0015] Fig. 4B an optical image of a line made of Fig. 4A shows the line printed at 10 mm / s; the line shows smooth edges.

[0016] The present embodiments provide solder mask inks suitable for aerosol printing. The solder mask inks generally have viscosities of less than approximately 1,000 cps at a shear rate of approximately 10⁻¹⁰ s at 25 °C and a structural viscosity index of less than approximately 2.0. In embodiments, the solder mask inks may comprise: i) resin or UV-curing monomers; ii) an inorganic pigment; and iii) at least approximately 20 wt.% of the total weight of the ink and up to 50 wt.% of a propylene glycol-based ether or ester solvent, wherein the solder mask inks have a viscosity of less than approximately 800 cps at shear rates of approximately 10⁻¹⁰ s at 25 °C and a structural viscosity index of less than approximately 1.5.It has been shown that such solder mask inks possess good printability in aerosol jet printers, and the cured masks exhibited comparable adhesion, pencil hardness, scratch resistance, and chemical resistance to commercial solder masks. The use of aerosol printing with these solder mask inks advantageously allows for the application of solder mask patterns to irregularly shaped surfaces, in stark contrast to conventional solder mask formulations.

[0017] As used here, the "shear-thinning index" (STI) is a dimensionless measure proportional to the ratio of the viscosities of a fluid measured at low and high rates. In embodiments, the STI is defined as the ratio of the viscosity at a shear rate of 10 1 / s to the viscosity at a shear rate of 484 1 / s at 25 °C. Thus, the shear-thinning index is a rate-dependent viscosity ratio. Those skilled in the art will recognize that the STI is occasionally also referred to as the thixotropy index, but this should not be confused with thixotropy.

[0018] Without being bound by theory, the advantages of the disclosed solder mask inks can be realized due to their high solids content and special rheology and structure-viscosity behavior, which enable application by an aerosol jet. In particular, the choice of propylene glycol-based solvents not only reduces the viscosity but also the structure-viscosity index of the solder mask inks, so that the viscosity is effectively reduced at relatively low concentrations. In fact, simply diluting the solder mask base materials to aerosol-sprayable viscosities is not sufficient, and alternative solvents that provide the required viscosity range can cause the application of very thin, pinhole-like layers, which prove unsuitable for solder masks.The specific solvents disclosed here therefore provide not only a desired viscosity, but also suitable structure viscosity properties to avoid difficult thin films with pinholes.

[0019] Aerosol jet printing with available solder mask inks offers several additional advantages: (1) It is a digital process that significantly simplifies or reduces process steps, thereby lowering manufacturing costs; (2) Solder masks are digitally applied to the desired area, reducing material waste and preventing clogging of passages; (3) Aerosol printing has been demonstrated for high-resolution prints (e.g., approximately 10 µm), making it suitable for producing high-density solder masks; (4) Aerosol printing can be used with much higher ink viscosities (up to approximately 1,000 cps) compared to inkjet printing; (5) Aerosol printing has proven to be a suitable method for printing on 3D surfaces or surfaces with 3D topographic relief structures. These are all desirable properties for PCB manufacturing as well as for 3D electronics printing.

[0020] In embodiments, the solder mask inks can employ metal oxide-based or other inorganic pigments in conjunction with the specified named solvents to provide solder mask inks that do not require the use of surfactants. This is particularly advantageous for achieving good performance characteristics of the cured masks, where surfactants can cause a performance degradation of the resulting mask. In embodiments, white solder mask base materials comprising a metal oxide, such as titanium dioxide, can be particularly suitable for PCBs used in LED applications.

[0021] In embodiments, solder mask inks for aerosol jet printing are provided, comprising a metal oxide and a propylene glycol-based solvent, wherein the solder mask ink has a viscosity of approximately 50 cps to approximately 1,000 cps at a shear rate of 10 1 / s at 25 °C and a structural viscosity index of approximately 1.0 to approximately 2.0.

[0022] As used here, a "solder mask ink" refers to compositions that are sufficiently fluid to be applied by aerosol jet printing and are used in the formation of solder mask patterns. The solder mask ink can be applied to copper traces of a printed circuit board (PCB) to protect against oxidation and prevent bridging between closely spaced solder pads. Solder masks are particularly useful in high-throughput PCB manufacturing. The solder mask inks can be based on epoxy liquids of suitable viscosity and structure viscosity index for application via aerosol printing processes. Solder mask inks typically employ either a thermally or UV-curing resin.

[0023] As used here, "aerosol jet printing" refers to a process that typically involves atomizing solder mask ink, which may optionally be heated, to form droplets on the order of one to two micrometers in diameter. The atomized droplets are typically conveyed in a gas stream and delivered to a printhead. At the printhead, an annular gas stream is introduced around the aerosol stream to focus the droplets into a tightly focused jet. The combined gas streams exit the printhead through a converging nozzle, which compresses the aerosol stream to a small diameter of approximately 1 to 10 micrometers. The jet exits the printhead and is deposited onto a substrate. The resulting patterns can have features ranging from approximately 5 µm to approximately 3,000 µm in width with a layer thickness of a few tens of nm to approximately 25 µm, encompassing approximately 1 µm to approximately 20 µm.

[0024] As used here, "structure viscosity index" refers to a velocity-dependent viscosity ratio. It can be obtained by determining the ratio of a viscosity measured at a low velocity of 10⁻⁶ 1 / s to a viscosity measured at a high velocity of 484 1 / s. In an ideal Newtonian fluid, the structure viscosity index is approximately 1.0.

[0025] In embodiments, solder mask inks comprise a resin or a UV-curing monomer. Such resins or UV-curing monomers may, but are not limited to, a bisphenol A epoxy, a novolac epoxy, an acrylic acid-modified epoxy, a cycloaliphatic or heterocyclic epoxy, and combinations thereof, wherein the resin is crosslinkable with a phenol, amine, or anhydride. In embodiments, the resin or UV-curing monomer may be provided in a solder mask base material, such as in commercially available solder mask pastes and the like, such as those distributed by Taiyo, America, including those under the PSR-4000 product line.

[0026] In embodiments, solder mask inks are provided comprising: i) a resin or UV-curing monomer; ii) an inorganic pigment; and iii) approximately 20 to approximately 50% by weight of the ink of a propylene glycol-based ether or ester solvent; wherein the solder mask ink formulation has a viscosity of approximately 50 cps to approximately 800 cps at a shear rate of 10 1 / s at 25 °C and a structural viscosity index of approximately 1.0 to approximately 1.5.

[0027] The solder mask inks presented here can advantageously be formulated without surfactants. In some embodiments, the solder mask is therefore free of surfactants. In other embodiments, the solder mask inks can contain a sufficiently low amount of surfactant to minimize its impact on the physical properties of the cured solder mask. If surfactants are used, they can be non-ionic. Examples of non-ionic surfactants include polysorbates such as polysorbate 20 (polyoxyethylene(20)-sorbitan monolaurate), polysorbate 40 (polyoxyethylene(20)-sorbitan monopalmitate), polysorbate 60 (polyoxyethylene(20)-sorbitan monostearate), and polysorbate 80 (polyoxyethylene(20)-sorbitan monooleate). Polyglycerol polyricinoleate, octadecanoic acid [2-[(2R,3S,4R)-3,4-dihydroxy-2-tetrahydrofuranyl]-2-hydroxyethyl] ester, octadecanoic acid [(2R,3S,4R)-2-[1,2-bis-(1-oxooctadecoxy)ethyl]-4-hydroxy-3-tetrahydrofuranyl] ester; C8- to C 221-chain alcohols, such as 1-octadecanol, cetylstearyl alcohol, hexadecan-1-ol and cis-9-octadecen-1-ol; substituted or unsubstituted octylphenol, in which the substituents may include a polyethoxyethanol group (e.g. to form octylphenoxypolyethoxyethanol) or any other substituent forming a non-ionic surfactant with octylphenol; polyethylene glycol monoisohexadecyl ether; 2,3-dihydroxypropyl dodecanoic acid ester; glucosides, such as lauryl glucoside, octyl glucoside and decyl glucoside; fatty acid amides, such as cocamide diethanolamine and cocamide monoethanolamine; and non-ionic surfactants with a hydrophilic polyethylene oxide chain and an aromatic lipophilic or hydrophilic hydrocarbon group, such as nonoxynol-9 and Triton X-100.

[0028] In one embodiment, the non-ionic surfactant is a polyalkylene glycol. The non-ionic surfactant can be, for example, a block copolymer comprising at least one polyethylene glycol block and at least one polypropylene glycol block, such as polyethylene glycol block-polypropylene glycol block-polyethylene glycol, or a triblock copolymer composed of a central hydrophobic chain of polyoxypropylene-(poly-(propylene oxide)) flanked by two hydrophilic chains of polyoxyethylene-(poly-(ethylene oxide)). An example of a commercially available non-ionic surfactant is SYNPERONIC F108, available from Aldrich.

[0029] The non-ionic surfactant can be used in any suitable quantity; for example, the amount of non-ionic surfactant can be approximately 0.01% by weight of the total weight of the solder mask ink composition. In other examples, the amount of non-ionic surfactant can range from approximately 0.05% to approximately 5%, such as approximately 0.5% to approximately 3% by weight, based on the total weight of the ink composition.

[0030] In embodiments, the solder mask inks employ a solvent that is a propylene glycol-based ether and / or ester solvent. In some such embodiments, the solvent is propylene glycol monomethyl ether acetate, dipropylene glycol monomethyl ether acetate, or combinations thereof. When used in combination, the solvent may be a 1:1 combination of propylene glycol monomethyl ether acetate and dipropylene glycol monomethyl ether acetate. In embodiments, the solvent comprises approximately 10 to approximately 50 wt.% of the solder mask ink, comprising approximately 10 to approximately 35 wt.% of the solder mask ink. In embodiments, the ratio may be approximately 9:1 to approximately 1:9, comprising approximately 7:3 to approximately 3:7.

[0031] In embodiments, the solder mask inks can comprise a metal oxide that is an inorganic pigment. In embodiments, the metal oxide is an oxide of titanium. In embodiments, the solder mask ink can comprise any inorganic pigment, including any inorganic metal-based pigments. Aluminum pigments can include, without restriction, ultramarine (PB29), a complex, naturally occurring pigment of sulfur-containing sodium silicate (Na₂O₃). 8-10 Al6Si6O 24 S 2-4 Ultramarine violet (PV15), a sulfur-containing silicate of sodium and aluminum, may be included. Copper pigments may, without restriction, include: Scheele's Green: copper arsenite CuHAsO3, Cu(C2H3O2)2·3Cu(AsO2)2; Paris Green: copper(II) acetoarsenite; Egyptian Blue, a synthetic pigment of calcium copper silicate (CaCuSi4O). 10 ), Han Blue: BaCuSi4O 10, Han Purple: BaCuSi2O6. Cobalt pigments may include, without restriction, the following: Aureolin (also known as Cobalt Yellow) (PY40): potassium cobalt nitrite (Na3Co(NO2)6), Cobalt Blue (PB28) and Cerulean Blue (PB35): cobalt(II) stannate, Cobalt Violet: (PV14) cobalt-containing orthophosphate. Manganese pigments may include, without restriction, the following: Manganese Violet: NH4MnP2O7 (PV16) manganese ammonium pyrophosphate; Iron pigments may include, without restriction, the following: Iron Black (PBk11) (CI No. 77499): Fe3O4, Yellow Ochre (PY43): a naturally occurring clay of hydrated iron oxide (Fe2O3·H2O), Prussian Blue (PB27): a synthetic pigment of iron hexacyanoferrate (Fe7(CN)3). 18Venetian Red, Oxide Red (PR102), Red Ochre (PR102): anhydrous Fe2O3, Burnt Sienna (PBr7): a pigment produced by heating raw sienna, clay pigments (naturally formed iron oxides), Raw Umber (PBr7): a natural clay pigment consisting of iron oxide, manganese oxide, and aluminum oxide: Fe2O3 + MnO2 + nH2O + Si + AlO3. When calcined (heated), it is referred to as Burnt Umber. Raw Sienna (PBr7): a naturally occurring yellow-brown pigment of limonite clay. Cadmium pigments can, without restriction, include the following: Cadmium Orange (PO20): an intermediate between Cadmium Red and Cadmium Yellow: cadmium sulfoselenide, Cadmium Yellow (PY37): cadmium sulfide (CdS), Cadmium Red (PR108): cadmium selenide (CdSe), Cadmium Green: a light green pigment comprising a mixture of Cadmium Yellow (CdS) and Viridian (Cr2O3).Chromium pigments may include, without restriction, the following: Chrome Orange: a naturally occurring pigment mixture composed of lead(II) chromate and lead(II) oxide (PbCrO4 + PbO); Chrome Yellow (PY34): a natural pigment of lead(II) chromate (PbCrO4); Chrome Green (PG17): chromium oxide (Cr2O3); Viridian (PG18): a dark green pigment of hydrogenated chromium(III) oxide (Cr2O3). Arsenic pigments may include, without restriction, the following: orpiment, natural monoclinic arsenic sulfide (As2S3). Lead pigments may include, without restriction, the following: Cremnitz White (PW1): basic lead carbonate ((PbCO3)2·Pb(OH)2); Red Lead: lead troxide, Pb3O4; Naples Yellow (PY41). Titanium pigments can include, without restriction, the following: Titanium White (PW6): titanium oxide (TiO2), Titanium Black, Titanium Yellow (PY53). Tin pigments can include, without restriction, the following: Mosaic Gold: tin sulfide (SnS2).Mercury pigments may include, without restriction, the following: Vermilion (PR106), mercury sulfide (HgS). Antimony pigments may include, without restriction, the following: Antimony White: antimony oxide (Sb₂O₃). Barium pigments may include, without restriction, the following: barium sulfate (PW5). Zinc pigments may include, without restriction, the following: Zinc White (PW4): zinc oxide (ZnO).

[0032] In embodiments, the inorganic pigment is selected from the group consisting of a metal oxide, metal sulfate, metal sulfide, metal borate, azurite, aureolin, cobalt blue, chrome yellow, cerulean, calcium chromate, bice cobalt green, Egyptian blue, Han purple, Han blue, lithopone, manganese violet, Prussian blue, ultramarine, Venetian ceruse, verdigris, vermilion, and viridian. In certain embodiments, the inorganic pigment may be a metal oxide. In some such embodiments, the metal oxide may be an oxide of titanium, such as titanium dioxide. Such titanium-based pigments are commonly used in white solder mask base materials. In embodiments, white solder mask base materials perform unexpectedly better than green solder mask base materials, although green solder mask base materials are also usable. In embodiments, the metal oxide is an oxide of titanium, antimony trioxide, chromium oxide, manganese dioxide, or lead oxide, or combinations thereof.

[0033] In particular, metal oxide-based pigments exhibit unexpectedly good performance in cured solder masks. While not bound by theory, this may be due to the interactions between the metal and the propylene glycol-based solvents. Combinations of such pigments and solvents appear to be at least partially responsible for the observed structural viscosity index. Thus, white pigment based on titanium dioxide provides a low structural viscosity index, as shown in the examples below. No comparable degree of performance improvement is observed for typical green solder substrate, even when the pasty carrier material for the pigment is the same. The carrier material itself, therefore, does not appear to be the determining factor of the resulting structural viscosity index.

[0034] In embodiments, methods are provided that include aerosol jet printing of a solder mask ink into a pattern onto a substrate, wherein the solder mask ink comprises: i) a resin or a UV-curing monomer; ii) an inorganic pigment; and iii) approximately 20 to approximately 50% by weight of the ink of a propylene glycol-based ether or ester solvent; wherein the solder mask ink has a viscosity of approximately 50 cps to approximately 800 cps at a shear rate of 10 1 / s at 25 °C and a structural viscosity index of approximately 1.0 to approximately 1.5; and curing of the sprayed solder. In embodiments, the curing step is a thermal or UV-curing step.

[0035] In some embodiments, the solder mask inks presented here can exhibit film integrity after curing, as measured by the industry standard set forth in IPC-SM-840C and its amendments. Some non-limiting, exemplary properties of the cured solder masks include pencil hardness, dimensional stability, adhesion, chemical resistance, flammability, and solderability. pencil hardness

[0036] This test is suitable for determining the hardness of the solder mask surface and its resistance to abrasion. The test is performed on three IPC-B-25A plates coated and cured with solder mask according to the manufacturer's instructions regarding application and curing. The plate is placed on a fixed horizontal surface. The hardest pencil (Eagle Turquoise brand, ranging from 6H to 4B) is selected and held firmly against the solder mask at a 45° angle. The pencil is then pushed away with uniform downward and forward pressure in a 1 / 4-inch stroke. If the solder mask is cut or hollowed out, the next softer pencil is used until one is found that does not cut into the mask. The pencil hardness that did not cut or hollow out the solder mask is then recorded. In embodiments, the solder masks shown exhibit a scratch resistance of 4–5B and a pencil hardness of 5–6H.

[0037] Embodiments disclosed herein also partially provide printed circuit boards comprising conductive patterns arranged on an insulating substrate and a hardened solder mask arranged on at least a part of the conductive pattern, wherein the hardened solder mask is formed from the solder mask inks described above.

[0038] Printed circuit boards can be manufactured using conventional techniques and can include glass as an insulating substrate over which a copper laminate sheet is arranged. In some embodiments, the insulating substrate can comprise a rigid or a flexible structure. In some embodiments, the insulating substrate is selected from a glass or a plastic resin.

[0039] In some embodiments, the conductive pattern can be formed directly on the insulating substrate. In other embodiments, a solder mask can be arranged on the copper laminate sheet, and the conductive pattern is formed on top of the solder mask. In other embodiments, a conductive pattern can be arranged on one or both sides of an insulating substrate, and in both cases, a solder mask can be arranged on one or both sides of the insulating substrate.

[0040] In some embodiments, the conductive pattern of a printed circuit board can itself be provided by a conductive ink. In such embodiments, the conductive ink can be placed directly on the insulating substrate, eliminating the need to etch a conductive copper-based pattern from a copper laminate sheet. When conductive inks are used, the conductive ink can be applied to the substrate using an inkjet printer, and subsequently, the solder mask can be printed over the conductive ink. Conductive inks generally comprise conductive particles dispersed in a carrier fluid, e.g., silver nanoparticles or other organically stabilized metal nanoparticles, as disclosed in U.S. Patent Application No. 2011 / 0305821.

[0041] PCBs for demanding environments may also include a leveling layer applied by dipping or spraying after the components are soldered. In some embodiments, such coatings can prevent corrosion and leakage currents or short circuits due to condensation. In some embodiments, the leveling layer comprises at least a wax, silicone rubber, polyurethane, acrylic resin, or epoxy resin. PCBs may also be designed with antistatic agents.

[0042] Embodiments disclosed herein also partially provide methods for printing solder masks, comprising the incorporation of a solder mask ink into an aerosol jet printing apparatus, which causes droplets of molten solder mask ink to be ejected onto a printed circuit board to form a patterned solder mask on the printed circuit board; and hardening of the patterned solder mask.

[0043] In some embodiments, the curing step can be catalyzed by one or more photostarters present in the solder mask ink. After printing the solder mask ink, the patterned mask can thus be cured by exposure to light, such as UV light. In some embodiments, the light curing can be performed across a broad spectrum of light, including UV, IR, near-IR, and visible light. In other embodiments, thermal curing is used instead of UV curing.

[0044] In some embodiments, the printing process may include printing a conductive pattern with conductive ink prior to printing the solder mask. In some embodiments, the printing of the solder mask can be performed essentially simultaneously with the printing of the conductive pattern. This can be achieved, for example, with a tandem printhead with multiple ink reservoirs that alternate between printing the insulator and the conductor onto the same substrate.

[0045] The following examples are submitted to illustrate embodiments of the present disclosure. These examples are intended for illustrative purposes only and are not to be understood as limiting the scope of the present disclosure. All parts and percentages are by weight unless otherwise stated. As used herein, "room temperature" refers to a temperature of approximately 20°C to approximately 25°C. Example 1

[0046] This example describes the presentation and testing of solder mask inks suitable for aerosol jet printing applications.

[0047] Commercial white solder mask (Taiyo PSR-4000 LEW1) was acquired from Taiyo America Inc. This solder mask paste is UV-curing, but thermally curing materials can also be used. Control Example A

[0048] First, the commercially available base solder mask paste was characterized. It exhibited a viscosity of 35,018 cps at a low shear rate of 10⁻⁵ 1 / s and 7,490 cps at a high shear rate of 484 1 / s at 25°C. The structure viscosity index, the ratio of the viscosity at low shear rate to the viscosity at high shear rate, was calculated to be 4.68. This viscosity is too high for aerosol jet printing. According to the present embodiments, this commercial solder mask was modified to achieve a target viscosity of less than 1,000 cps at a low shear rate of 10⁻⁵ 1 / s at 25°C using the solvents listed herein.Since the commercial paste has a high structural viscosity index, it was desirable to identify solvents that would not only reduce the viscosity of the original solder mask formulation but also lower the structural viscosity index, so that the solvents would effectively reduce the viscosity to below 1,000 cps at low shear rates.

[0049] The commercial solder mask paste was applied in a thin film. After UV curing under recommended conditions, a scratch resistance of 4–5B and a pencil hardness of 6H were measured, which is the highest rating for solder masks. When formulating the commercial paste into a composition for aerosol jet printing, the final ink ideally has no side effects from the final properties after curing, such as scratch resistance, pencil hardness, and the like. Furthermore, the inks should facilitate the formation of aerosols for printing by, for example, ideally possessing a relatively high boiling point and a low vapor pressure. Control Example B

[0050] Following procedures commonly used for formulating green solder masks, the white solder mask paste described above was diluted with the alcohol-solvent butylcarbitol in combination with a small amount of non-ionic surfactant (Synperonic F 108). Upon addition of 25 wt% butylcarbitol, the viscosity decreased dramatically to 436 and 555 cps at shear rates of 484 and 10⁻⁶ s, respectively. The composition was first applied to a copper substrate to test adhesion and hardness. After UV curing, scratch strengths of 3–4B and pencil hardnesses of 2–3H were observed, significantly worse than control example 1. Working example 1

[0051] Since solvents can be essentially completely evaporated, it was postulated that the addition of the non-ionic surfactant Synperonic F108 in control example B led to a deterioration in the pencil hardness of the final, cured solder mask. In the course of the search for aerosol-compatible solder mask inks, a solvent-only formulation was sought. Unexpectedly, it was found that propylene glycol-based ether and ester solvents provide excellent performance in aerosol jet-compatible solder mask inks. Fig.Figure 2 shows the viscosity (at 25 °C) and the structure viscosity index of the new white solder mask ink as a function of the amount of propylene glycol monoethyl ether acetate (PGMEA) and dipropylene glycol monoethyl ether acetate (DPGMEA) solvent mixture (PGMEA and DPGMEA in a 1:1 ratio). This solvent mixture very effectively reduced the viscosity. At 25 wt% solvent, the viscosity decreased dramatically to 179 and 214 cps at shear rates of 484 and 10 1 / s, respectively, significantly lower than in control example B. Even more significantly, the structure viscosity index was dramatically reduced from 4.68 for the commercial solder mask to 1.1–1.2. This helped to rapidly reduce the viscosity at low shear rates.

[0052] After adding the solvent, the low viscosity formulation was first applied to a copper-plated FR-4 substrate to test the film-forming properties. Fig.Figure 3 shows the applied film with a formulation containing 30 wt% PGMEA / DPGMEA. A smooth film with excellent wetting properties was observed on the copper surface. After UV curing, a scratch resistance of 5B and a pencil hardness of 6H were observed, results similar to those of the undiluted commercial paste (control example A). The results demonstrate that the use of the PGMEA / DPGMEA solvent mixture has no adverse effect on the properties of the final cured solder mask. Table 1 below summarizes the differences between the various formulations, including the scratch resistance and pencil hardness of the cured white solder mask coating for each formulation. Table 1 Samples solvent addition Scratch resistance pencil hardness Control A No 4–5B 6H Control B Butylcarbitol 3–4B 2–3H Working example 1 PGMEA / DPGMEA 5B 6H Working example 2

[0053] The formulation in Working Example 1 was printed using an aerosol printer with a pneumatic atomizer at approximately 50 °C. The atomizing gas was set to 1,000 to 1,300 standard cubic centimeters per minute (SCCM), the output to 900 to 1,200 SCCM, and the envelope gas to 200 to 600 SCCM. Aerosol was generated under these pressure conditions via a fog-generating probe refined by a virtual impactor. The fog stream was then focused using a nitrogen gas stream. The ink was printed on both PET and copper-clad FR-4 (a typical substrate for PCBs) substrates. Fig. Figure 4 shows solder mask lines printed at various speeds from 5.0 mm / s to 20 mm / s. Uniform lines with well-defined edges were observed. The printed lines exhibited excellent adhesion, scratch resistance, and chemical resistance, as observed above.

[0054] It should be noted that aerosol-sprayable solder mask inks can be formulated directly from epoxy resins, pigments, and solvents, with essentially optional surfactants, adhesion promoters, and / or other additives. Therefore, the present work examples are not limited to the use of commercial solder masks as starting materials, although these provide a very convenient starting point.

Claims

[1] Solder mask ink for aerosol jet printing comprising a metal oxide and a propylene glycol-based solvent, wherein the solder mask ink has a viscosity of approximately 50 cps to approximately 1,000 cps at a shear rate of 10 1 / s at 25 °C and a structural viscosity index of approximately 1.0 to approximately 2.

0. [2] Solder mask ink according to claim 1, further comprising a resin or UV-curing monomer. [3] Solder mask ink according to claim 2, wherein the resin or UV-curing monomer is selected from the group consisting of bisphenol A epoxy, novolac epoxy, acrylic acid-modified epoxy, cycloaliphatic or heterocyclic based epoxy and combinations thereof, wherein the resin is crosslinkable with phenol, amine or anhydride. [4] Solder mask ink according to claim 1, wherein the metal oxide is an inorganic pigment. [5] Solder mask ink according to claim 1, wherein the metal oxide is an oxide of titanium, antimony trioxide, chromium oxide, manganese dioxide or lead oxide. [6] Solder mask ink according to claim 1, wherein the solvent is a propylene glycol-based ether and / or ester solvent. [7] Solder mask ink according to claim 6, wherein the solvent is propylene glycol monomethyl ether acetate, dipropylene glycol monomethyl ether acetate or a combination thereof. [8] Solder mask ink, comprising: i) a resin or UV-curing monomer; ii) an inorganic pigment; and iii) approximately 20 to approximately 50 wt% of the ink of a propylene glycol-based ether or ester solvent; the solder mask ink formulation has a viscosity of approximately 50 cps to approximately 800 cps at a shear rate of 10 1 / s at 25 °C and a structural viscosity index of approximately 1.0 to approximately 1.

5. [9] Solder mask ink according to claim 8, wherein the inorganic pigment is selected from the group consisting of a metal oxide, metal sulfate, metal sulfide, metal borate, azurite, aureolin, cobalt blue, chrome yellow, cerulean, calcium chromate, bicine cobalt green, Egyptian blue, Han purple, Han blue, lithopone, manganese violet, Prussian blue, ultramarine, Venetian ceruse, verdigris, vermillion and viridian. [10] Procedures, including: Aerosol jet printing of a solder mask ink to a pattern on a substrate, wherein the solder mask ink comprises the following: i) a resin or UV-curing monomer; ii) an inorganic pigment; and iii) approximately 20 to approximately 50 wt% of a propylene glycol-based ether or ester solvent; wherein the solder mask ink has a viscosity of approximately 50 cps to approximately 800 cps at a shear rate of 10 1 / s at 25 °C and a structural viscosity index of approximately 1 to approximately 1.5; and Hardening of the sprayed solder.

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