TWI907034B - Low dielectric constant solder resist resin composition - Google Patents

Abstract

This invention provides a solder resist resin composition with excellent resolution, high heat resistance and high elongation, and low dielectric constant, with a dielectric constant Dk≦3.10 (40GHz) and a loss factor Df≦0.015 (40GHz). The photocurable resin composition of this invention is characterized by comprising (A) a photopolymerizable, alkali-soluble prepolymer (oligomer) such as A1 (a) or A2 (b), having a carboxyl group (-COOH) and a photocurable unsaturated functional group: 20-50% by weight. It is prepared by any combination of (a1) 2,2-dimethylbutyric acid or (a2) 2,2-dimethylpropionic acid or (a1) and (a2), and (b) 3-isocyanate-methylene-3,5,5-trimethylcyclohexyl isocyanate IPD. The reaction proceeds from (I) to (c) a monocarboxylic acid containing one vinyl group; (B) styrene-maleic anhydride (SMA) resin: 10-20 wt%; (C) photopolymerizable vinyl monomer: 5-20 wt%; (D) epoxy resin: 5-20 wt%; (E) petroleum resin: 2-10 wt%; (F) photopolymerization initiator: 3-10 wt%; (G) hollow inorganic filler: 10-50 wt%; (H) accelerator: 0.5-2.0 wt%; (I) organic solvent: 15-40 wt%.

Description

Low dielectric constant solder resist ink resin composition

This invention relates to a novel low-dielectric-constant solder resist resin composition. This low-dielectric solder resist resin composition coating exhibits excellent resolution, high heat resistance, and high elongation. Its dielectric constant Dk ≦ 3.10 (40GHz) and loss factor Df ≦ 0.015 (40GHz) make it suitable for high-performance, high-frequency printed circuit boards.

To complement the high wiring density of printed circuit boards, liquid solder resist inks with excellent image quality and high dimensional accuracy are generally used. These inks are composed of phenolic polyfunctional epoxy acrylate resins with added acid anhydrides to form a UV-curable photopolymerizable prepolymer (oligomer), which is then combined with epoxy resins to form the solder resist ink composition. This composition can be cured under UV light and then further cured by heat, resulting in a solder resist ink hardened film with excellent physical properties.

Solder resist ink compositions based on photopolymerizable prepolymers formed from phenolic polyfunctional epoxy acrylates and commonly used epoxy resins have a cured coating dielectric constant Dk of approximately 3.6 (40GHz) and a loss factor Df of approximately 0.025 (40GHz), which cannot meet the transmission characteristics of next-generation 5G or 6G electronic signals. As electronic products develop towards higher frequencies and speeds, and the required transmission speeds increase, raw materials on printed circuit boards (PCBs) need to have lower dielectric constants Dk and loss factors Df. Solder resist ink is a crucial raw material for protecting circuit lines; therefore, developing solder resist inks with low dielectric constants, low loss factors, and that meet the physical property requirements of PCBs is an important project.

As electronic products demand increasingly higher frequencies and faster signal transmission speeds, there is a growing need for upstream raw materials for printed circuit boards (PCBs) to possess lower dielectric constants (Dk) and lower loss factors (Df). Solder resist inks, as crucial materials for solder resisting and protecting circuitry on PCBs, require advanced technology to develop inks with low dielectric constants (Dk) and low loss factors (Df).

To address the aforementioned challenges, this invention provides a novel solder resist resin composition with low dielectric constant and low loss factor, exhibiting excellent developing properties and photocuring capability. It provides high ductility, excellent adhesion, chemical resistance, heat resistance, and electrical properties for printed circuit board solder resist coatings. Its dielectric constant Dk ≦3.10 (40GHz) and loss factor Df ≦0.015 (40GHz) make it suitable for high-performance, high-frequency printed circuit boards.

This novel solder resist ink resin composition with low dielectric constant and low loss factor comprises the following components: (A) a photopolymerizable, alkaline-soluble prepolymer (oligomer) such as A1 (a) or A2 (b), having carboxyl (-COOH) and photocurable unsaturated functional groups: 20-50% by weight. It is prepared by any combination of (a1) 2,2-dimethylbutyric acid or (a2) 2,2-dimethylpropionic acid or (a1) and (a2), and (b) 3-isocyanate-methylene-3,5,5-trimethylcyclohexyl isocyanate IPD. The reaction proceeds from (I) to (c) a monocarboxylic acid containing one vinyl group, yielding formula A1 (I) or formula A2 (II); (B) styrene-maleic anhydride (SMA) resin: 10-20 wt%; (C) photopolymerizable vinyl monomer: 5-20 wt%; (D) epoxy resin: 5-20 wt%; (E) petroleum resin: 2-10 wt%; (F) photopolymerization initiator: 3-10 wt%; (G) hollow inorganic filler: 10-50 wt%; (H) accelerator: 0.5-2.0 wt%; (I) organic solvent: 15-40 wt%.

The novel solder resist ink resin composition of this invention is characterized by the proposed synthesis of a photopolymerizable prepolymer (oligomer, as shown in formula A1 (a) or A2 (b) above) with low dielectric constant and low loss factor. This is achieved by using a highly symmetric chemical structure of 2,2-dimethylbutyric acid or (a1)2,2-dimethylpropionic acid (a2) and (a3) b) Reaction of 3-isocyanate-methylene-3,5,5-trimethylcyclohexyl isocyanate (IPDI) with (c) a monocarboxylic acid containing one ethylene atom to synthesize a photopolymerizable prepolymer (oligomer) with high elongation, low dielectric constant, and low loss factor. This prepolymer is a mixture of styrene-maleic anhydride (SMA) resin, petroleum resin, and a thermosetting epoxy resin. The main formulation of solder resist ink consists of epoxy resins with low polarity and high symmetry, such as dicyclopentadiene-phenol polyfunctional epoxy resins or phenol-benzaldehyde polyfunctional epoxy resins and crystalline tetramethylbiphenol epoxy resins, to achieve low dielectric constant and low loss factor. The hardened coating of the solder resist ink is composed of photopolymerizable prepolymers (acid value 80~160 mg KO). A polymer coating is formed by bridging photopolymer prepolymer (H/g), styrene-maleic anhydride (SMA) resin (acid value 100~200 mg KOH/g), and epoxy resin through a UV-curing followed by heat curing reaction. Typically, the acid equivalent ratio of the photopolymer to the styrene-maleic anhydride resin is 1:0.8~2.0. The epoxy resin provides excellent heat resistance, adhesion, and chemical resistance.

The following details the composition of the solder resist ink resin with low dielectric constant and low loss factor of this invention: Component (A) is preferably used in an amount of 20 to 50% by weight. Its synthesis involves reacting (a1) 2,2-dimethylbutyric acid or (a2) 2,2-dimethylpropionic acid or any combination of (a1) and (a2) with (b) 3-isocyanate-methylene-3,5,5-trimethylcyclohexyl isocyanate IPDI, followed by reaction with (c) a monocarboxylic acid containing one vinyl group. The reaction conditions within the following range yield the best results. 2,2-Dimethylbutyric acid or 2,2-dimethylpropionic acid reacts with (b) 3-isocyanate methylene-3,5,5-trimethylcyclohexyl isocyanate IPDI, preferably using 1.3 to 1.7 moles per equivalent of hydrocarbon. (c) For the reaction of a monocarboxylic acid containing one vinyl group, preferably using 1.8 to 2.2 moles per equivalent of hydrocarbon. In this reaction, dilution is performed using an organic solvent, such as petroleum-based aromatic solvents, ethylene glycol monobutyl ether, propylene carbonate, ethyl 2-butoxyacetate, diethylene glycol butyl ether acetate, cyclohexanone, propylene glycol monomethyl ether, methyl carbitol acetate, and dipropylene glycol diethyl ether, etc. The optimal reaction temperature is 80 to 120°C, and the reaction time is 6 to 24 hours. The resulting photopolymerizable prepolymers A1 and A2 have an acid value (mgKOH/g) in the range of 80 to 180.

The solder resist ink resin composition of this invention with low dielectric constant and low loss factor comprises (B) styrene maleic anhydride (SMA) resin amount: preferably 10 to 20% by weight. The acid value (mgKOH/g) used in the composition of this invention is in the range of 100 to 200.

The solder resist ink resin composition of this invention with low dielectric constant and low loss factor includes (C) a photopolymerizable vinyl monomer content of 5 to 20% by weight. This monomer is selected from a variety including melamine acrylate, 2-hydroxyethyl acrylate, 2-hydroxybutyl acrylate, ethylene glycol monoacrylate, ethylene glycol diacrylate, propylene glycol monoacrylate, propylene glycol diacrylate, dimethylaminoethyl methacrylate, pentaerythritol tetraacrylate, polydipentaerythritol hexaacrylate, and glycidyl methacrylate. Melamine acrylate or polydipentaerythritol hexaacrylate are preferred.

The low dielectric constant and low loss factor solder resist ink resin composition of this invention comprises (D) epoxy resin compound in an amount of 5 to 20% by weight. It is selected from dicyclopentadiene-phenol polyfunctional epoxy resins or phenol-benzaldehyde polyfunctional epoxy resins, or any combination of the two.

Epoxy resin compound (D) is used as a thermosetting agent for crosslinking with photopolymerizable prepolymer (A) and styrene-maleic anhydride resin (B). The amount of epoxy resin compound used is based on the acid equivalent of the aforementioned photopolymerizable prepolymer and styrene-maleic anhydride resin, with 0.8 to 2.0 moles of epoxy equivalent per equivalent of carboxylic acid group.

The composition of the solder resist ink resin composition of the present invention with low dielectric constant and low loss factor: (E) Petroleum resin content: 2 to 10% by weight. The main component used in the composition of the present invention is C5 hydrogenated petroleum resin.

The low dielectric constant and low loss factor solder resist ink resin composition of this invention comprises (F) a photopolymerization initiator in an amount of 3-10% by weight. Known photoinitiators are selected such as benzoin methyl ether, benzoin isopropyl ether, 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexylphenyl ketone, 2-methyl-1-[4-methylthiophenyl]-2-morpholine, 2-isopropylthioxanthraphenone, and 2,2-dimethoxy-phenylacetophenone. These compounds can be used alone or in combination. The optimal combination is 2-methyl-1-[4-methylthiophenyl]-2-morpholine and 2-isopropylthioxanthraphenone.

The low dielectric constant and low loss factor solder resist ink resin composition of this invention comprises (G) a hollow inorganic filler of 10-50% by weight. The main filler used in the composition of this invention is a mixture of hollow inorganic fillers, such as silicon dioxide, glass powder, and calcium carbonate powder.

The low dielectric constant and low loss factor solder resist resin composition of this invention contains (H) an accelerator at a dosage of 0.5~2.0% by weight. Imidazole derivatives can be selected as the curing accelerator, with dimethylimidazole, diphenylimidazole, and dihydroxymethyldiphenylimidazole being commonly used.

The low dielectric constant and low loss factor solder resist ink resin composition of this invention comprises (I) an organic solvent in an amount of 15 to 40% by weight. The organic solvent is selected from, among others, propylene carbonate, butoxyethanol, ethyl butoxyacetate, toluene, xylene, butyl carbitol acetate, cyclohexanone, propylene glycol monomethyl ether, dipropylene glycol diethyl ether, and methyl carbitol acetate. Preferred examples are propylene carbonate and methyl carbitol acetate.

The technical solution of this invention has at least the following beneficial effects: Utilizing the excellent low dielectric constant Dk and low loss factor Df of 2,2-dimethylbutyric acid or (a2) 2,2-dimethylpropionic acid and (b) 3-isocyanate-methylene-3,5,5-trimethylcyclohexyl isocyanate IPDI, and then reacting with (c) a monocarboxylic acid containing one vinyl group, photopolymerizable prepolymers A1 and A2 with symmetrical structures, low dielectric constant Dk, and low loss factor Df are synthesized. These prepolymers are then mixed with styrene-maleic anhydride resin and petroleum resin and substituted into the ink. The formulation, combined with epoxy resin compounds of low dielectric constant (Dk) and low loss factor (Df) (such as dicyclopentadiene-phenol epoxy resin or phenol-benzaldehyde polyfunctional epoxy resin and crystalline tetramethylbiphenol epoxy resin), and hollow inorganic fillers, is first UV-cured and then heat-baked to form a solder resist ink coating. This coating exhibits excellent low dielectric constant and low loss factor properties, high resolution development, high elongation, and good adhesion, chemical resistance, electrical properties, electroplating resistance, and heat-resistant solder properties, meeting the requirements of high-frequency, high-speed, and high-end printed circuit boards.

To provide a clearer understanding of this invention, preferred specific embodiments will be further elaborated, as detailed in the following embodiments.

Synthesis Example 1: Photopolymerizable prepolymer (oligomer)-A1

1000g of 2,2-dimethylbutyric acid was added to 400g of methyl carbitol acetate solvent, heated to 80℃, and stirred until dissolved. Then, 1000g of IPDI was added dropwise to the solution for 100 minutes at 80℃. After the addition was complete, the temperature was maintained at 80℃ for a aging reaction for 4 hours. 1.0g of hydroquinone (HQ) was added, followed by 324g of acrylic acid (AA) to the solution for 60 minutes at 80℃. After the addition was complete, the temperature was raised to 120℃ for a aging reaction for 5 hours, yielding photopolymerizable prepolymer A1. The acid value was 145mgKOH/g, and the weight-average molecular weight (Mw) was 980.

Synthesis Example 2: Photopolymerizable prepolymer (oligomer)-A2

1000g of 2,2-dimethylpropionic acid was added to 400g of methyl carbitol acetate solvent. The mixture was heated to 80°C and stirred until dissolved. Then, 1100g of IPDI was added dropwise to the solution for 100 minutes at 80°C. After the addition was complete, the temperature was maintained at 80°C for a aging reaction for 4 hours. Next, 1.0g of hydroquinone (HQ) was added, followed by 358g of acrylic acid (AA) to the solution. The addition was carried out over 60 minutes at 80°C. After the addition was complete, the temperature was raised to 120°C for a aging reaction for 5 hours, yielding photopolymerizable prepolymer A2. The acid value was 155mgKOH/g, and the weight-average molecular weight (Mw) was 928.

Synthetic Comparison: Photopolymerizable Prepolymer (oligomer)-A3

1000g of o-cresol phenolic polyfunctional epoxy resin was mixed with 392g of methyl carbitol acetate solvent. The mixture was heated to 100℃, and then 6.0g of triphenylphosphine and 1.2g of hydroquinone HQ were added. After stirring and dissolving, 342g of acrylic acid (AA) was added dropwise to the solution over 90 minutes at a temperature of 95℃. After the addition was complete, the mixture was allowed to mature for 12 hours. Then, 384g of tetrahydrophthalic anhydride (THPA) and 392g of methyl carbitol acetate solvent were added, and the reaction was carried out at 110℃ for 4 hours. Finally, 256g of methyl carbitol acetate solvent was added to obtain photopolymerizable prepolymer A3. The acid value was 51mgKOH/g, and the weight average molecular weight (Mw) was 6000.

Implementation Examples 1-3

The low dielectric constant and low loss factor photopolymerizable prepolymers A1 and A2 of this invention are substituted into the solder resist ink formulation, the formulation of which is detailed in Table 1. After thorough grinding using a three-roll mill, the ink composition is obtained. The ink composition thus obtained is then screen-printed onto a patterned printed circuit board to obtain a coating thickness of 20 to 30 μm. The coating is then dried in a hot air dryer at 75°C for 40 minutes. A patterned photoresist negative film is then tightly adhered to the coating, and the film is irradiated with ultraviolet light at a dose of 25 mW/cm² using an ORC HMW-680GW UV exposure machine. Next, a 1% sodium carbonate aqueous solution is sprayed at a pressure of 2.0 kg/cm² for 60 seconds to develop the coating, dissolving and removing any unexposed areas. Finally, the coating is baked at 100-200°C for 0.5-1 hour to obtain a fully hardened film.

Comparative example 1

Comparative Example 1 is a solder resist ink formulation composed of commonly used photopolymerizable prepolymer A3, the composition of which is detailed in Table 1.

Table 1 lists the component information as follows: Hollow silica, type Suk-500; styrene; maleic anhydride resin, type SMA EF60; petroleum resin, type Eastotca H-142R; photopolymerization initiators: 2-methyl-1-[4-methylthiophenyl]-2-morpholine and 2-isopropylthioxanthraquinone; accelerator: 2-methylimidazole.

Weight-average molecular weight test (Mw)

The weight-average molecular weight was determined by GPC (gel permeation chromatography, standard material: polystyrene conversion).

Developability

The test board was brushed clean (linear speed: 2 m/min, pickling: 3% sulfuric acid, drying: 90℃). The ink was applied to the circuit board using a 36T screen and baked at 75℃ for 40 minutes. After removing from the oven, it was allowed to cool naturally to room temperature.

Imaging conditions:

Developer concentration: 1.0 ± 0.2% Na₂CO₃ aqueous solution

Spray pressure: 2.5±0.5Kg/cm2

Development temperature: 30~33℃

◎: Complete Imaging

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Adhesion

The hardened film has a grid pattern, as specified in JIS D 0202. After a peel test using Cyrrolidone tape, the peeling condition was visually assessed.

100/100, meaning that none of the 100 parts were peeled off.

From 50/100 to 90/100, that is, the portion from 50 to 90 out of 100 remains unpeeled.

From 0/100 to 50/100, that is, the portion from 0 to 50 out of 100 remains unpeeled.

Pencil Hardness

Similar to the adhesion method, after preparing the test piece, use a Mitsubishi pencil with a hardness of 2B to 9H, with the tip flattened (at a right angle), and then scrape the test piece at a 45-degree angle until the coating is scratched.

Heat resistance of tin plating

The test board was brushed clean (linear speed: 2 m/min, pickling: 3% sulfuric acid, drying: 90℃). Ink was applied to the test board using a 36T screen, with a wet film thickness of 36-42 μm. It was pre-baked at 75℃ for 40 min, cured with UV exposure energy of 500 mJ/cm² (under film), and then baked at 150℃ for 60 min. Flux was applied to the cured circuit board, and it was immersed in tin at 270℃ for 10 seconds each time. Each test required the board to cool completely before proceeding to the next. Testing was stopped if no peeling occurred after three tests.

Dielectric constant test:

The testing method involves inserting a 5cm x 5cm square ink-cured coating with a thickness of 100µm into a dielectric constant meter, measuring data at three points, and then averaging the results.

Dissipation factor test:

The testing method involves inserting a 5cm x 5cm square ink-cured coating with a thickness of 100µm into a dielectric constant meter, measuring data at three points, and then averaging the results.

Comparing the embodiments and comparative examples in Table 1 above, Embodiment 1 yields a photopolymerizable prepolymer by reacting 2,2-dimethylbutyric acid with IPDI and then with a monocarboxylic acid containing a vinyl group; Embodiment 2 yields a photopolymerizable prepolymer by reacting 2,2-dimethylpropionic acid with IPDI and then with a monocarboxylic acid containing a vinyl group. The photopolymerizable prepolymers synthesized in Embodiments 1 and 2 were mixed with styrene maleic anhydride resin, petroleum resin, and hollow filler to create a solder resist ink. Compared with the comparative example, the dielectric constant was reduced from 3.6 (40GHz) to below 3.1 (40GHz), and the loss factor was reduced from 0.025 (40GHz) to below 0.015 (40GHz), making it suitable for solder resist coatings on printed circuit boards used in high-frequency, high-speed signal transmission.

Claims (8)

A low dielectric constant solder resist ink resin composition includes: an acid-modified photopolymerizable oligomer A1 or A2 with low dielectric constant and low loss factor, having a carboxyl group (-COOH) and a photocurable unsaturated functional group; a styrene maleic anhydride resin having an anhydride group; a photopolymerizable monomer having two or more photocurable unsaturated functional groups; a thermosetting binder having two or more thermosetting functional groups; a petroleum resin; a hollow silica; and a photoinitiator; the low dielectric constant... The cured product of the constant solder resist ink resin composition includes: a structure in which the carboxyl groups of the acid-modified oligomer and the thermosetting functional groups are cross-linked; and a structure in which the unsaturated functional groups of the acid-modified oligomer and the photopolymerizable monomers are cross-linked. It is used for preparing a solder resist coating for a printed circuit board; its dielectric constant Dk≦3.10 (40GHz) and loss factor Df≦0.015 (40GHz) are suitable for high-performance, high-frequency printed circuit boards. The Al resin is as shown in structural formula (I). The A2 resin is shown in structural formula (II): As described in claim 1, the low dielectric constant solder resist resin composition, wherein the acid-modified oligomer A1 or A2 comprises a copolymer of a polymerizable monomer having a carboxyl group and a monomer comprising an acrylate compound, the content of the acid-modified oligomer is 20 to 50% by weight, and the acid value is 80 to 180 mg KOH/g. The low dielectric constant solder resist resin composition as described in claim 1, wherein the styrene-maleic anhydride resin content is 10 to 20% by weight, and the acid value is 100 to 200 mg KOH/g. The low dielectric constant solder resist ink resin composition as described in claim 1, wherein the content of the photopolymerizable monomer is 5 to 20% by weight, comprising one or more polyfunctional (meth)acrylate compounds: a polyfunctional acrylate compound having a hydroxyl group; an acrylate compound with an propylene oxide additive of a polyfunctional alcohol or polyphenol; a polyfunctional or monofunctional polyurethane acrylate compound; an epoxy acrylate compound; and a photosensitive (meth)acrylate compound. The low dielectric constant solder resist resin composition as described in claim 1, wherein the petroleum resin content is 2 to 10% by weight. As described in claim 1, the low dielectric constant solder resist resin composition, wherein the photoinitiator content is 3 to 10% by weight, comprising one or more compounds selected from the group consisting of benzoin compounds, acetophenone compounds, anthraquinone compounds, thiotonone compounds, acetal compounds, diphenyl ketone compounds, α-amine acetophenone compounds, phosphine oxide compounds, oxime ester compounds, diimidazole compounds, and triazine compounds. As described in claim 1, the low dielectric constant solder resist ink resin composition, wherein the thermosetting binder is selected from a dicyclopentadiene-phenol polyfunctional epoxy resin or a phenol-benzaldehyde polyfunctional epoxy resin or any combination of the two, and the content of the thermosetting binder is 0.8 to 2.0 equivalents per equivalent of carboxyl groups in the acid-modified oligomer. The low dielectric constant solder resist resin composition as described in claim 1, wherein the hollow silicon dioxide content is 10-50% by weight.