A heat-shrinkable polyimide adhesive film for reducing a thermal affected zone of laser drilling and a preparation process thereof

Abstract

This invention belongs to the field of polymer materials, specifically relating to a heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling and its preparation process. The film comprises a diamine derivative containing a benzoxazole group and a carboxylic acid compound containing a triazine ring. The preparation process first involves reacting a diamine monomer and a dianhydride monomer in a solvent under an inert atmosphere to generate a polyamic acid resin. Subsequently, the triazine carboxylic acid compound is added and mixed uniformly. The mixture is then coated onto copper foil and subjected to a multi-stage programmed temperature heat treatment. Both modified compounds have corresponding specific preparation methods. This invention, by introducing rigid benzoxazole side groups and a crosslinkable triazine structure, synergistically improves the dimensional stability and laser thermal shock resistance of the film, effectively suppressing resin thermal decomposition and shrinkage during laser drilling. This keeps protrusion and indentation defects in the hole walls to an extremely low range, meeting the precision machining requirements of high-density interconnect circuit boards for micro-vias.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling and its preparation process. Background Technology

[0002] As electronic information products rapidly evolve towards miniaturization, high integration, and high frequency and speed, the spacing between conductors and the diameter of vias on printed circuit boards are constantly shrinking, placing near-extreme precision demands on micro-hole processing technology. Traditional mechanical drilling techniques, when processing micro-holes with diameters less than 0.15mm, have gradually revealed a series of insurmountable defects, including insufficient processing precision, burrs and mechanical stress on the hole walls, easy drill wear, and low processing efficiency. Therefore, laser drilling technology, with its significant advantages of non-contact operation, high precision, high flexibility, and high efficiency, has become the mainstream technology for micro-hole processing of high-end circuit boards. However, the laser drilling process essentially utilizes a high-energy laser beam to instantaneously vaporize and remove material. Under this extreme thermal effect, the heterogeneous materials constituting the circuit board (mainly the polymer resin insulating layer and the copper foil) will exhibit different thermal responses due to their vastly different thermophysical properties (such as thermal conductivity, coefficient of thermal expansion, absorption coefficient, and ablation threshold). This results in excessive shrinkage and depression of the resin material at the hole wall interface due to thermal decomposition and carbonization, while the copper foil protrudes outward due to melting and resolidification, forming so-called "copper bumps" and "resin depressions." These defects severely affect the uniformity of subsequent electroless and electroplated copper plating, causing incomplete or voided plating layers on the hole walls, ultimately becoming a hidden danger to the reliability of signal transmission and restricting the improvement of product yield. Therefore, how to suppress the negative effects of laser thermal shock from the material itself has become a key technical bottleneck that the industry urgently needs to overcome.

[0003] Currently, industry improvements to laser drilling defects mainly focus on optimizing laser process parameters, such as using ultrashort pulse lasers, adjusting beam energy distribution and scanning strategies, or introducing auxiliary gases and cooling media. While these methods have some effect, they are often only temporary solutions, have demanding equipment requirements, narrow process windows, and are costly. On the other hand, improvements from a materials science perspective often focus on physical blending modifications of general-purpose polyimides, such as adding inorganic fillers to improve thermal conductivity or copolymerizing with other polymers to adjust mechanical properties. However, these conventional modification methods often fail to simultaneously address the material's dimensional stability, thermal decomposition behavior, and interfacial deformation capabilities with copper foil under the instantaneous ultra-high temperature of laser treatment. Simply adding inorganic fillers may introduce new problems such as dielectric degradation and decreased interfacial bonding; while ordinary copolymerization modification has little effect on suppressing the severe shrinkage of the resin under laser irradiation. Therefore, developing a special polyimide film material with inherent low thermal shrinkage properties that can actively adapt to and resist laser thermal shock is of great significance for achieving ultra-high precision through-hole processing with a pore size of less than 50 μm and controlling the hole wall deformation to a level below 15 micrometers.

[0004] Therefore, the purpose of this invention is to overcome the limitations of existing technologies. Instead of a passive approach relying on external process adjustments, it aims to create a novel low-heat-shrinkage polyimide film by starting from the source of molecular structure design. The core idea of ​​this invention is to synthesize two modified compounds with specific functional groups and molecular structures, and introduce them as key monomers or additives into the polyimide system. One compound, by introducing rigid and large side groups, aims to restrict chain segment movement at the molecular chain level, directly reducing the material's thermal shrinkage rate. The other compound, by constructing reactive crosslinking points, forms a stable three-dimensional network structure within the material, further enhancing its shape retention capability at high temperatures. The synergistic effect of these two compounds is expected to fundamentally strengthen the dimensional stability and structural integrity of the film under the instantaneous high temperature of laser treatment, effectively absorbing or dissipating heat stress, thereby minimizing the thermal decomposition shrinkage of the resin layer and the melting protrusion of the copper foil. This results in a laser drilling effect with smooth hole walls and extremely high geometric precision, providing crucial basic material support for next-generation ultra-high density interconnect technology. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling and its preparation process.

[0006] A first aspect of the present invention provides a process for preparing a heat-shrinkable polyimide film to reduce the heat-affected zone of laser drilling, comprising the following steps:

[0007] S1. By weight, in a dry, nitrogen-filled reactor, add 400-600 parts of N-methylpyrrolidone and stir at 15-25°C. Then, add 60-80 parts of 4,4'-diaminodiphenyl ether and 5-15 parts of N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether and stir. Add 30-50 parts of pyromellitic dianhydride and 20-40 parts of biphenyltetracarboxylic dianhydride and continue stirring at 25-30°C to obtain a polyamic acid resin solution.

[0008] S2. Add 3-10 parts of 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine to the polyamic acid resin solution, stir and mix, filter to remove air bubbles, and then coat it on the surface of electrolytic copper foil to obtain a wet film. Heat treat the wet film and let it cool naturally to room temperature.

[0009] In this invention, the preparation of the heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling is a complex process involving sequential polymer synthesis, physical blending modification, and thermochemical transformation. Its core mechanism lies in achieving in-situ construction of a three-dimensional network structure and increased polymer chain rigidity through molecular design and process control. The first stage is the synthesis of a polyamic acid resin solution, a typical step-growth polymerization reaction. In an inert atmosphere and solvent, the diamine monomer and the dianhydride monomer undergo a ring-opening addition reaction. The amino group of the diamine nucleophilically attacks the carbonyl carbon of the dianhydride, causing the anhydride ring to open, forming an amide bond and a free carboxyl group, generating polyamic acid. This reaction is carried out near room temperature to control the chain growth rate and avoid gelation, ultimately yielding a high-molecular-weight linear or rigid-side-group-containing polyamic acid prepolymer solution. This prepolymer already exhibits the chain motion restriction effect brought about by the rigid benzoxazole side groups. The second stage involves uniformly mixing the previously synthesized triazine tricarboxylic acid compound into the polyamic acid solution. At this stage, the triazine compound exists primarily in a physically dispersed state. Its multiple carboxyl groups may form secondary interactions such as hydrogen bonds with the amide or carboxyl groups on the polyamic acid chain, but strong covalent bonding has not yet occurred. The third stage, the programmed temperature heat treatment after coating, is the decisive step in determining the material's properties, involving several key chemical transformations. First, at lower temperatures, solvent evaporation and preliminary imidization mainly occur. As the temperature increases, the polyamic acid begins to undergo intramolecular dehydration and ring-closing reactions, i.e., imidization, generating the polyimide backbone and releasing water molecules. This process significantly improves the rigidity and heat resistance of the polymer chain. Second, at even higher temperatures, two key chemical reactions dominate the formation of the final properties: one is the continued deep imidization of the remaining polyamic acid portion, ensuring complete conversion; the other is the condensation reactions such as esterification and amidation between the active carboxyl groups in the triazine tricarboxylic acid compound and the residual amino groups at the ends of the polyimide chain, thereby establishing covalent chemical crosslinking points between the polyimide macromolecular chains. This cross-linked network, connected by a rigid triazine core, weaves the originally linear or side-chain polymer chains into a stable three-dimensional whole. In summary, the final molecular structure of the film is a super-network system with a rigid polyimide main chain, large benzoxazole rigid groups suspended on the side chains, and rigid triazine cross-linking bridges connecting the chains. This unique synergistic structure endows the film with exceptional dimensional stability: the rigid side groups and main chain greatly suppress chain segment movement and thermal expansion; the chemical cross-linking network locks in the overall topology, exhibiting extremely strong resistance to thermal stress. Therefore, when subjected to the instantaneous extreme heat of laser drilling, the film exhibits extremely low thermal shrinkage and resistance to thermal decomposition, effectively suppressing resin layer collapse and excessive copper foil protrusion, thereby controlling the heat-affected zone and hole wall geometric defects of laser drilling at the micron level.

[0010] According to a preferred embodiment of the present invention, in step S1, the stirring time is 12-18 hours.

[0011] According to a preferred embodiment of the present invention, the specific steps of heat treatment of the wet film in step S2 are as follows: heat treatment at 80-100°C; then heat treatment at 175-185°C; heat treatment at 245-255°C; and finally heat treatment at 300-320°C.

[0012] According to a preferred embodiment of the present invention, the method for preparing the N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether comprises:

[0013] A1. Under the protection of dry nitrogen, anhydrous N-methylpyrrolidone and 4,4'-diaminodiphenyl ether were added to a round-bottom flask and stirred at room temperature to obtain a reaction mixture; the reaction mixture was cooled to 0-5°C, and 2-chlorobenzoxazole was added while stirring continuously. The mixture was then heated to room temperature and stirred continuously to obtain a reaction solution.

[0014] A2. Heat the reaction solution to 85-90℃ and reflux to obtain a mixture. Cool the mixture to room temperature and pour it into ice water with stirring. Filter to collect the solid and wash it with deionized water and then with ethanol to obtain a crude product. Dry the crude product in a vacuum drying oven at 78-82℃ and recrystallize it with a mixed solvent of N,N-dimethylformamide and ethanol.

[0015] In this invention, the preparation of N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether is a classic aromatic nucleophilic substitution reaction process. Its core lies in the attack and substitution of the highly reactive amino group in the diamine monomer on the halogenated heterocyclic compound. Specifically, 4,4'-diaminodiphenyl ether is used as a bifunctional nucleophile. The nitrogen atoms of its two ends, being aromatic primary amino groups, contain lone pairs of electrons, exhibiting strong nucleophilicity. In polar aprotic solvents such as N-methylpyrrolidone, these amino groups can attack the carbon-chlorine bond attached to the heterocycle in the dichlorobenzoxazole molecule. Due to the strong electron-withdrawing effect of the benzoxazole ring, the carbon atom containing the attached chlorine atom exhibits significant positive charge, meaning the carbon-chlorine bond is easily polarized, thus becoming an excellent nucleophilic attack site for the aromatic amine. The initial reaction is carried out at low temperature to control the reaction rate and avoid side reactions caused by violent exothermic reactions, such as excessive reaction of the amino group with multiple halogenated compounds or polymerization. Under these conditions, nucleophilic substitution occurs gradually. The amino nitrogen atom attacks the chlorinated carbon, and the chloride ion, acting as a good leaving group, detaches to form a new carbon-nitrogen covalent bond, while simultaneously generating hydrogen chloride as a byproduct. Subsequently, the reaction system is heated to room temperature and continuously stirred to allow the remaining amino group on the monosubstituted intermediate generated in the first step to undergo the same nucleophilic substitution with the second molecule of di-chlorobenzoxazole, thus completing the bifunctionalization. The final reflux heating process is to drive the reaction to complete and ensure full conversion of all reaction sites. After the reaction, precipitation in ice water is used for preliminary separation and purification, taking advantage of the product's extremely low solubility in low-temperature water. Subsequent recrystallization using a mixed solvent is based on the difference in solubility between the product and impurities in specific solvent pairs. Through the physical processes of dissolution and crystallization, unreacted monomers, homologue byproducts, or salts are effectively removed, ultimately yielding a high-purity, structurally well-defined bisbenzoxazole diamine monomer. The molecular structure of this product combines the moderate flexibility of diphenyl ether, the extreme rigidity of the benzoxazole ring, and the reactivity of aromatic amines, laying a crucial molecular foundation for the subsequent construction of a high-performance polymer backbone.

[0016] According to a preferred embodiment of the present invention, in step A1, the stirring time is 2-4 hours.

[0017] According to a preferred embodiment of the present invention, in step A2, the reflux reaction time is 12-14 hours.

[0018] According to a preferred embodiment of the present invention, the method for preparing the 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine includes:

[0019] B1. Under dry nitrogen protection, anhydrous dimethyl sulfoxide and cyanuric chloride are added to a three-necked round-bottom flask and cooled to -5~0℃ to obtain an anhydrous dimethyl sulfoxide solution of cyanuric chloride; anhydrous potassium carbonate is dissolved in anhydrous dimethyl sulfoxide and methylparaben is added, and stirred to obtain a suspension; the suspension is added dropwise to the anhydrous dimethyl sulfoxide solution of cyanuric chloride to obtain a reaction mixture; the reaction mixture is heated to room temperature and stirred, then heated to 48-52℃ to continue the reaction to obtain a reaction solution;

[0020] B2. Filter the reaction solution to obtain a filtrate. Pour the filtrate into an aqueous hydrochloric acid solution while stirring, filter again, and wash the precipitate with water until neutral to obtain a wet filter cake. Transfer the wet filter cake to an aqueous sodium hydroxide solution, heat to 78-82℃ and stir to obtain a reaction solution. Cool the reaction solution to room temperature, adjust the pH to 1.8-2.2 while stirring, filter to obtain a crude product, wash the crude product with deionized water to obtain the product, and vacuum dry the product at 98-102℃.

[0021] In this invention, the synthesis of 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine is a stepwise, multi-step reaction, mainly involving two key steps: nucleophilic substitution and hydrolysis. The core of the first step is the nucleophilic substitution of the chlorine atom on the triazine ring by the phenolic ester. Cyanurium chloride serves as the core framework, with three highly reactive chlorine atoms attached to its symmetrical triazine ring. These chlorine atoms are extremely sensitive to nucleophiles due to the strong electron-withdrawing inductive effect of the adjacent nitrogen atom, which significantly reduces the electron cloud density of the carbon atoms they are attached to. The reaction occurs between the potassium salt of methylparaben and cyanurium chloride. The phenoxy anion is a strong nucleophile; under strictly controlled low-temperature conditions, it is slowly added dropwise to the cyanurium chloride solution, sequentially attacking the three chlorine atom sites. Low temperature is crucial because the three chlorine atoms in cyanurium chloride exhibit different reactivity. Low temperature facilitates stepwise, controllable nucleophilic substitution, avoiding side reactions such as cross-linking or decomposition caused by excessively vigorous reactions, thereby ensuring the regularity and uniformity of the target product structure. With the attack of the phenoxy anion, the chloride ion departs, forming a stable carbon-oxygen (aryl ether) bond, generating a trisubstituted ester intermediate. This step, after the dropwise addition, involves a sustained reaction at a moderate temperature to ensure complete substitution of all three chloride atoms, yielding a fully functionalized triester compound. The second step is the hydrolysis of the ester groups. The ester intermediate obtained in the previous step is heated in an alkaline aqueous solution, where its three methyl ester groups undergo saponification hydrolysis under strong alkali. Specifically, hydroxide ions attack the carbonyl carbon of the ester, forming a tetrahedral intermediate, followed by the departure of the methoxy group as a methanol salt, generating the corresponding carboxylate. This hydrolysis process is carried out under heating to accelerate the reaction rate and ensure complete conversion of all ester groups into water-soluble sodium carboxylate. Finally, acidification is achieved by adding a strong acid, causing the sodium carboxylate to protonate in the acidic environment, precipitating as a low-water-soluble free carboxylic acid. After filtration, washing, and drying, the target product, the tricarboxylic acid compound, is obtained. This molecule has a highly symmetrical and rigid triazine ring as its core, and is connected to a benzene ring with a carboxyl group at the end by three ether bonds. This forms a multifunctional molecule that has a rigid planar structure, high thermal stability and multiple reactive carboxyl functional groups, providing an ideal building block for introducing crosslinking points into polymers and enhancing the overall rigid network.

[0022] According to a preferred embodiment of the present invention, in step B1, the reaction continues for 8-10 hours.

[0023] According to a preferred embodiment of the present invention, in step B2, the time for heating to 78-82°C and stirring the reaction is 2-4 hours.

[0024] The present invention also provides a heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling, prepared according to the method for preparing the heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) This invention represents a fundamental breakthrough in materials science, significantly enhancing the dimensional stability and resistance to instantaneous thermal shock of polyimide films. By introducing a self-designed diamine monomer containing rigid benzoxazole side groups, this monomer acts like a stable rigid pillar anchored on the polymer backbone, effectively restricting the movement and relaxation of polymer chain segments under heat, and directly reducing the thermal expansion and contraction tendency of the material from the molecular source. At the same time, the addition of another self-designed triazine cyclic carboxylic acid compound can react with the polymer system during the final heat treatment process, constructing a stable and uniform moderately cross-linked network. These two modified compounds produce a significant synergistic effect: the rigid groups inhibit chain segment movement, and the cross-linked network locks the overall structure, so that the prepared film has a much better shape and size retention capability than conventional polyimide materials when subjected to the high temperature and heat generated by laser drilling, exhibiting an extremely low thermal shrinkage rate, which lays a solid material foundation for controlling the geometry of the pore walls.

[0027] (2) Based on the improved intrinsic properties of the material, the adhesive film of the present invention exhibits excellent processing adaptability and hole wall forming quality during laser drilling. Due to the greatly improved thermal stability of the material itself, the diffusion of heat to the surrounding area and the resulting thermal damage are effectively suppressed when laser energy acts on the adhesive film. The adhesive film no longer undergoes severe and uneven thermal decomposition and carbonization shrinkage, thereby avoiding hole wall indentation defects caused by excessive shrinkage of the resin layer. At the same time, the stable resin substrate also provides uniform support for the copper foil bonded to it, reducing copper foil melting protrusion caused by the mismatch of their thermal behaviors. Therefore, when laser drilling is performed using the adhesive film of the present invention, the resulting micropores exhibit a smoother and more uniform morphology, the width of the heat-affected zone is compressed to a very small range, and the resin indentation and copper foil protrusion of the hole wall are strictly controlled, providing a near-ideal high-quality substrate surface for subsequent hole metallization processes.

[0028] (3) The technical effects of this invention are ultimately reflected in a substantial improvement in the manufacturing capabilities and reliability of high-end electronic products. By controlling the hole wall deformation to an extremely low micron level, this invention supports the fabrication of micro-vias with smaller diameters on circuit boards, directly contributing to the further improvement of circuit wiring density and meeting the high integration requirements of next-generation electronic devices. The excellent hole wall quality ensures that chemical copper plating and electroplating can form a uniform, complete, and dense metal layer inside the hole, greatly improving the reliability of interlayer electrical interconnection and reducing signal transmission loss and short-circuit risk. In summary, this invention not only solves specific process problems in laser drilling, but also provides strong support for the precision manufacturing and technological progress of the entire high-density interconnect circuit board industry by providing a high-performance key basic material. Detailed Implementation

[0029] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0030] Example 1

[0031] This embodiment provides a method for preparing a heat-shrinkable polyimide film to reduce the heat-affected zone of laser drilling, the steps of which include:

[0032] Preparation of N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether: Under dry nitrogen protection, 500 mL of anhydrous N-methylpyrrolidone was added to a 1000 mL three-necked round-bottom flask equipped with a magnetic stirrer, thermometer, and reflux condenser. Stirring was started and maintained at 300 rpm. 27.8 g of 4,4'-diaminodiphenyl ether was added to the solvent, and the mixture was stirred at 25 °C until the solid was completely dissolved, yielding a clear solution. The reaction flask was then placed in an ice-salt bath, and the mixture was continuously stirred while cooling the internal temperature of the reaction mixture to 3 °C. While maintaining this temperature, 34.8 g of 2-chlorobenzoxazole was slowly added dropwise over 30 minutes using a constant-pressure feeding funnel. During the addition, the temperature of the reaction solution was kept below 10 °C by adjusting the ice bath. After the addition was complete, the ice-salt bath was removed, and the reaction system was allowed to warm naturally to 25 °C, where the mixture was stirred for another 3 hours. Next, the reaction apparatus was switched to an oil bath heating system, and the temperature of the reaction solution was raised to 88°C. The mixture was then refluxed and stirred at this temperature for 13 hours. After the reaction was complete, the heating was turned off, and the reaction mixture was allowed to cool naturally to 25°C. In another 5000mL plastic beaker, 3000g of crushed ice and an appropriate amount of water were added beforehand. While stirring vigorously, the cooled reaction solution was slowly poured into the ice water. A pale yellow solid immediately precipitated. After standing for 10 minutes, the solid was collected by vacuum filtration using a Buchner funnel and quantitative filter paper. The filter cake was washed three times with 1000mL of deionized water, and then twice with 500mL of ethanol. The resulting wet filter cake was transferred to a watch glass and placed in a vacuum drying oven at 80°C, where it was dried for 12 hours under a vacuum of -0.095MPa. The dried crude product was transferred to a 1000mL Erlenmeyer flask, and 200mL of N,N-dimethylformamide was added. The mixture was stirred in a 60°C water bath until completely dissolved. Subsequently, 200 mL of ethanol was slowly added to the clear solution while stirring, and the solution gradually became cloudy. Stirring was stopped, the conical flask was sealed, and the solution was allowed to crystallize overnight at room temperature. The next day, a large number of needle-like crystals precipitated. The crystals were filtered again using a Buchner funnel, collected, and washed twice with 100 mL of cold ethanol. The crystals were then dried in a vacuum drying oven at 80 °C for 24 hours to obtain N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether.

[0033] Preparation of 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine: Under dry nitrogen protection, 400 mL of anhydrous dimethyl sulfoxide was added to a 1000 mL three-necked round-bottom flask equipped with a mechanical stirrer, thermometer, and constant-pressure dropping funnel. Stirring was started at 250 rpm. 18.4 g of cyanuric chloride was added to the solvent, and the reaction flask was cooled and stirred in an ice-salt bath until the internal temperature of the reaction solution stabilized at -2 °C. In another 500 mL Erlenmeyer flask, 41.4 g of anhydrous potassium carbonate was dissolved in 200 mL of anhydrous dimethyl sulfoxide, and then 45.6 g of methylparaben was added to this solution. The mixture was manually stirred to form a homogeneous suspension. This suspension was transferred to a constant-pressure dropping funnel. Under vigorous stirring and while maintaining an internal temperature below 5 °C, the suspension was slowly added dropwise to the cyanuric chloride solution over a period of 2 hours. After the addition was complete, the ice-salt bath was removed, and the reaction mixture was allowed to naturally warm to 25°C, where it was stirred for 5 hours. Subsequently, the reaction system was placed in an oil bath, heated to 50°C, and stirred for 9 hours. After the reaction was complete, the reaction solution was cooled to room temperature and filtered under reduced pressure through a diatomaceous earth-lined sand core funnel to remove the inorganic salts generated during the reaction. The filtrate was collected. In a 5000 mL beaker, 2000 mL of a 1 mol / L hydrochloric acid aqueous solution was prepared beforehand. The filtrate was slowly poured into the hydrochloric acid solution with stirring, immediately producing a large amount of white precipitate. Stirring was continued for 30 minutes to ensure complete precipitation. The precipitate was filtered through a Buchner funnel, and the white precipitate was collected. The filter cake was washed with copious amounts of deionized water until the filtrate was neutral as determined by pH paper. The wet filter cake was transferred to a 2000 mL beaker, and 500 mL of a 10% sodium hydroxide aqueous solution was added. The mixture was heated and stirred in an 80°C water bath for 3 hours to carry out the hydrolysis reaction. After hydrolysis, the solution was cooled to 25°C, and then concentrated hydrochloric acid was slowly added dropwise with stirring to adjust the pH to 2.0. At this point, the product reprecipitated as a white flocculent precipitate. The mixture was filtered again, and the filter cake was washed with deionized water until neutral. The filter cake was transferred to a watch glass and dried in a vacuum drying oven at 100°C for 24 hours to obtain 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine.

[0034] Preparation of heat-shrinkable polyimide film to reduce the heat-affected zone of laser drilling: 500g of N-methylpyrrolidone was added to a 2L jacketed glass reactor filled with dry, high-purity nitrogen (oxygen content <10ppm). Mechanical stirring was started at 200rpm, and the material temperature was controlled to 20℃ using circulating water in the jacket. 70g of 4,4'-diaminodiphenyl ether and 10g of the previously prepared N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether were added sequentially to the solvent, and stirred for 30 minutes until all solids were completely dissolved, yielding a homogeneous diamine solution. While maintaining stirring and temperature control, a solid mixture of 40g of pyromellitic dianhydride and 30g of biphenyltetracarboxylic dianhydride was divided into four equal portions. One portion of the dianhydride mixture was added to the reactor every 15 minutes, ensuring the temperature of the reaction system did not exceed 30℃ during each addition. After all the dianhydrides were added, the reaction temperature was set to 28°C, and the reaction was continuously stirred at this temperature for 18 hours, finally yielding a pale yellow, viscous polyamic acid resin solution with a solid content of approximately 20 wt% and a viscosity (at 25°C) of approximately 12000 cP. 6 g of the previously prepared 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine powder was added to this resin solution. The stirring speed was increased to 300 rpm and stirred continuously for 2 hours to ensure complete dispersion and uniform mixing of the crosslinking agent. The uniformly mixed resin solution was then forced through a 1 μm PTFE filter under nitrogen pressure of 0.1 MPa to remove any air bubbles and small particles that might be present in the solution. Using a precision slit coater, the filtered resin solution was uniformly coated onto an 18 μm thick electrolytic copper foil (roughened surface), with the wet film coating thickness set to 100 μm. The coated wet copper foil was placed in a programmable hot air circulating oven for step-by-step heat treatment. The heat treatment procedure was as follows: First stage: temperature increased from room temperature to 90℃ at a rate of 5℃ / min, and held at 90℃ for 60 minutes; Second stage: temperature increased from 90℃ to 180℃ at a rate of 2℃ / min, and held at 180℃ for 45 minutes; Third stage: temperature increased from 180℃ to 250℃ at a rate of 2℃ / min, and held at 250℃ for 75 minutes; Fourth stage: temperature increased from 250℃ to 310℃ at a rate of 2℃ / min, and held at 310℃ for 90 minutes. After heat treatment, the oven heating was turned off, and the sample was allowed to cool naturally to 25℃. The resulting copper-clad laminate sample was then obtained.

[0035] Example 2

[0036] The difference between this embodiment and Example 1 lies in the preparation of N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether: Under dry nitrogen protection, 500 mL of anhydrous N-methylpyrrolidone was added to a 1000 mL three-necked round-bottom flask. Stirring was started, with the speed controlled at 300 rpm. 25.0 g of 4,4'-diaminodiphenyl ether was added to the solvent, and stirring was carried out at 25°C until the solid was completely dissolved. The reaction flask was placed in an ice-salt bath and cooled to 3°C. While maintaining this temperature, 31.3 g of 2-chlorobenzoxazole was slowly added dropwise over 30 minutes using a constant-pressure feeding funnel, controlling the reaction solution temperature below 10°C. After the addition was complete, the ice-salt bath was removed, and the reaction system was allowed to naturally warm to 25°C, and the reaction was continued at this temperature with stirring for 3 hours. Afterward, the reaction solution was heated to 88°C and refluxed with stirring for 13 hours. After the reaction was complete, the mixture was cooled to 25°C and poured into 3000g of ice water with stirring. The solid was collected by filtration, washed three times with 1000mL of deionized water, and then washed twice with 500mL of ethanol. The crude product was dried in a vacuum oven at 80°C for 12 hours. The crude product was dissolved in 200mL of N,N-dimethylformamide, heated to 60°C to dissolve, and then 200mL of ethanol was slowly added. The mixture was allowed to stand at room temperature to crystallize. The solution was filtered, washed with ethanol, and dried under vacuum at 80°C for 24 hours to give N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether.

[0037] Preparation of 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine: Under dry nitrogen protection, 400 mL of anhydrous dimethyl sulfoxide was added to a 1000 mL three-necked round-bottom flask. Stirring was started at 250 rpm. 16.6 g of cyanuric chloride was added to the solvent, and the mixture was cooled to -2 °C. In another container, 37.3 g of anhydrous potassium carbonate was dissolved in 200 mL of anhydrous dimethyl sulfoxide, and 41.0 g of methylparaben was added to form a suspension. While stirring and maintaining an internal temperature below 5 °C, the suspension was added dropwise to the cyanuric chloride solution over 2 hours. After the addition was complete, the temperature was raised to 25 °C and stirred for 5 hours, then heated to 50 °C and reacted for 9 hours. After cooling, the reaction solution was filtered, and the filtrate was poured into 2000 mL of 1 mol / L hydrochloric acid to precipitate the precipitate. The precipitate was filtered and washed with water until neutral. The solid was transferred to 500 mL of 10% sodium hydroxide solution and hydrolyzed by stirring at 80 °C for 3 hours. After cooling, concentrated hydrochloric acid was added dropwise to adjust the pH to 2.0. The mixture was filtered, washed with water until neutral, and dried under vacuum at 100 °C for 24 hours to give 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine.

[0038] Preparation of heat-shrinkable polyimide film to reduce the heat-affected zone of laser drilling: In a 2L reactor filled with dry nitrogen, 450g of N-methylpyrrolidone was added and stirred at 200rpm at 18°C. Then, 65g of 4,4'-diaminodiphenyl ether and 8g of the previously prepared N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether were added sequentially, and stirred for 30 minutes until dissolved. Under controlled temperature (<32°C), 35g of pyromellitic dianhydride and 25g of biphenyltetracarboxylic dianhydride were added in batches. After the addition was complete, the mixture was stirred at 26°C for 20 hours to obtain a polyamic acid resin solution. Then, 4g of the previously prepared 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine was added to the solution, and stirred at 300rpm for 1.5 hours. The adhesive solution was filtered through a 1μm filter membrane and then coated onto an 18μm electrolytic copper foil, resulting in a wet film thickness of 100μm. The heat treatment procedure was as follows: The temperature was increased from room temperature to 85℃ at a rate of 5℃ / min and held for 70 minutes; then increased to 182℃ at a rate of 2℃ / min and held for 40 minutes; finally, the temperature was increased to 248℃ at a rate of 2℃ / min and held for 80 minutes; and then increased to 305℃ at a rate of 2℃ / min and held for 100 minutes. The mixture was then allowed to cool naturally to 25℃.

[0039] Example 3

[0040] The difference between this embodiment and Example 1 lies in the preparation of N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether: Under dry nitrogen protection, 500 mL of anhydrous N-methylpyrrolidone was added to a 1000 mL three-necked round-bottom flask. Stirring was started, with the speed controlled at 300 rpm. 30.0 g of 4,4'-diaminodiphenyl ether was added to the solvent and stirred at 25°C until dissolved. The mixture was cooled to 3°C, and 37.6 g of 2-chlorobenzoxazole was slowly added dropwise over 30 minutes, maintaining the temperature below 10°C. After the addition was complete, the temperature was raised to 25°C and stirred for 3 hours. Then, the mixture was heated to 88°C and refluxed for 13 hours. After the reaction, the mixture was cooled to 25°C and poured into 3000 g of ice water. The mixture was filtered, and the solid was washed successively with 1000 mL of water and 500 mL of ethanol. The crude product was dried under vacuum at 80°C for 12 hours, and then recrystallized with 200 mL of N,N-dimethylformamide and 200 mL of ethanol. The crystals were dried under vacuum at 80°C for 24 hours to obtain N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether.

[0041] Preparation of 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine: Under dry nitrogen protection, 400 mL of anhydrous dimethyl sulfoxide was added to a 1000 mL three-necked round-bottom flask. Stirring was started at 250 rpm. 20.2 g (0.11 mol) of cyanuric chloride was added, and the mixture was cooled to -2 °C. 45.4 g of anhydrous potassium carbonate was dissolved in 200 mL of anhydrous dimethyl sulfoxide, and 49.9 g of methylparaben was added to prepare a suspension. The suspension was added dropwise to the cyanuric chloride solution at below 5 °C over 2 hours. Subsequently, the temperature was raised to 25 °C and stirred for 5 hours, then heated to 50 °C and reacted for 9 hours. After filtration, the filtrate was poured into 2000 mL of 1 mol / L hydrochloric acid to precipitate a solid. After filtration and washing with water, the solid was hydrolyzed in 500 mL of 10% sodium hydroxide solution at 80 °C for 3 hours. The solution was cooled and acidified to pH 2.0, filtered and washed with water, and then vacuum dried at 100°C for 24 hours to obtain 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine.

[0042] Preparation of heat-shrinkable polyimide film to reduce the heat-affected zone of laser drilling: In a 2L reactor under dry nitrogen, 550g of N-methylpyrrolidone was added and stirred at 200rpm at 22°C. Then, 75g of 4,4'-diaminodiphenyl ether and 12g of the previously prepared N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether were added sequentially, and stirred for 30 minutes to dissolve. Under controlled temperature (<28°C), 45g of pyromellitic dianhydride and 35g of biphenyltetracarboxylic dianhydride were added in batches. After the addition was complete, the mixture was stirred at 29°C for 15 hours to obtain a polyamic acid resin solution. Then, 8g of the previously prepared 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine was added, and stirred at 300rpm for 2.5 hours. The solution was filtered through a 1μm filter membrane and coated onto an 18μm electrolytic copper foil, resulting in a wet film thickness of 100μm. Heat treatment procedure: Increase the temperature from room temperature to 95°C at 5°C / min and hold for 50 minutes; increase the temperature to 178°C at 2°C / min and hold for 50 minutes; increase the temperature to 252°C at 2°C / min and hold for 70 minutes; increase the temperature to 315°C at 2°C / min and hold for 80 minutes. Allow to cool naturally to 25°C.

[0043] Comparative Example 1

[0044] The difference between this comparative example and Example 1 is that 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine and N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether are not added.

[0045] Comparative Example 2

[0046] The difference between this comparative example and Example 1 is that 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine is not added.

[0047] Comparative Example 3

[0048] The difference between this comparative example and Example 1 is that N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether is not added.

[0049] The performance of the heat-shrinkable polyimide films for reducing the heat-affected zone of laser drilling obtained in Examples 1-3 and Comparative Examples 1-3 was tested in accordance with national and industry standard testing specifications.

[0050] All tests were conducted under standard conditions of 23±2℃ and 50±10% relative humidity. All samples were tested after standing for 24 hours in the standard environment of 23±2℃ and 50±10% relative humidity.

[0051] The heat shrinkage rate test was performed using a thermomechanical analyzer. The peeled film was cut into 10mm × 10mm square samples and placed flat on the instrument's sample stage. The samples were heated from 30℃ to 250℃ in a nitrogen atmosphere at a heating rate of 5℃ / min. The samples were then held at the target temperature of 150℃ for 10 minutes. The percentage change in the sample's size relative to its initial size at the end of the isothermal test was recorded. The arithmetic mean of the results in the machine's travel direction and its perpendicular direction was taken as the heat shrinkage rate of the sample.

[0052] Static mechanical property testing was performed using a universal testing machine. The film was cut into dumbbell-shaped specimens with a total length of 100 mm, a gauge length of 25 mm, and a width of 10 mm. Uniaxial tension was applied at a constant crosshead speed of 10 mm / min, and the stress-strain curves were recorded in real time until the specimen broke. The tensile strength, elastic modulus, and elongation at break were calculated and reported from the curves. Each group of samples was tested at least 5 times and the average value was reported.

[0053] Dynamic thermomechanical property testing was performed using a dynamic thermomechanical analyzer with a thin-film stretching fixture. The sample was held under constant tension and scanned from 30°C to 400°C under nitrogen protection conditions at a frequency of 1 Hz, a heating rate of 3°C / min, and a nitrogen atmosphere. The relationship curves between storage modulus and temperature, as well as the curves of mechanical loss factor changing with temperature, were recorded. The temperature corresponding to the peak value of the main peak of the loss factor curve was defined as the glass transition temperature of the sample. Thermal stability testing was performed using a thermogravimetric analyzer. Approximately 10 mg of sample was accurately weighed and placed in an alumina crucible. Under a nitrogen atmosphere at a flow rate of 50 mL / min, the sample was heated from 50°C to 800°C at a heating rate of 10°C / min. The change in sample mass with temperature was continuously recorded. The temperature at which the sample mass loss reached 5% of the original mass was recorded as the thermal decomposition temperature of the material.

[0054] Dielectric performance testing was performed using an impedance analyzer. The film sample was cut into a 30mm diameter disc, and 20mm diameter circular gold electrodes were uniformly deposited on its upper and lower surfaces by vacuum evaporation to form a parallel plate capacitor. The capacitance and loss values ​​were measured at room temperature and a test frequency of 1MHz, and the dielectric constant and dielectric loss factor of the material were calculated.

[0055] The quality assessment of laser drilling was performed using a combination of an ultraviolet picosecond laser processing system and a laser confocal microscope. The laser wavelength was 355 nm, the average power was set to 0.8 W, the repetition frequency was 100 kHz, and the scanning speed was 1200 mm / s. Ten through holes with a diameter of 50 μm were machined on the sample surface. The laser confocal microscope was used to perform a three-dimensional morphological scan on each hole. The height of the highest protrusion of the copper foil relative to the original surface and the depth of the largest depression in the resin layer on the circumference of each hole were measured. The arithmetic mean of the ten measurements was calculated as the average height of the copper protrusion and the average depth of the resin depression for the sample.

[0056] The peel strength test is conducted according to the standard method for copper clad laminates. Using a universal testing machine, the copper foil on the sample is first made into three parallel lines with a width of 3 mm and a length of at least 10 cm through photolithography and etching. A special fixture is used to clamp the copper strips at a 90-degree angle and peel them at a speed of 50 mm / min. The force value curve within a stroke of at least 50 mm during the peeling process is recorded. The average force value within the range of the curve is calculated and divided by the width of the copper strip to obtain the peel strength of the sample. The result is the average value of the test values ​​of the three copper strips.

[0057] The performance test data above are shown in Table 1.

[0058] Table 1 Performance Test Results

[0059]

[0060] The test results in Table 1 above clearly show that, based on the comprehensive performance test data comparison and analysis of Examples 1-3 and Comparative Examples 1-3, the technical solution proposed in this invention successfully solves the core technical problems faced by existing polyimide films in laser precision drilling applications: namely, the excessive processing deformation caused by insufficient material thermal stability, and the comprehensive performance imbalance caused by single performance improvement.

[0061] Specifically, the test results of Comparative Example 1 (without any added modified compounds) clearly reveal the inherent limitations of traditional polyimide films. Its thermal shrinkage rate at 150°C is as high as 0.35%, which directly leads to an average copper protrusion of 22µm on the hole wall and an average resin indentation depth of 25µm after laser drilling, with a total defect of up to 47µm. This far exceeds the industry's stringent standard of less than 15µm required for high-density interconnects, proving that simply optimizing process parameters cannot fundamentally solve the bottleneck of mismatch in the thermophysical properties of materials. The results of Comparative Example 2 (only BODA containing benzoxazole was added) and Comparative Example 3 (only the triazine crosslinking agent TCTP was added) show that while a single molecular modification strategy can improve performance to some extent (e.g., the heat shrinkage rate of Comparative Example 2 decreased to 0.15% and that of Comparative Example 3 decreased to 0.20%), the improvement is limited. The laser drilling defects are still as high as 34µm and 38µm, respectively. Moreover, the two methods show different focuses and shortcomings in mechanical properties and heat resistance. For example, Comparative Example 2 improved its modulus (4.5GPa) and Tg (300℃) but its strength (220 MPa) was insufficient, while Comparative Example 3 showed the opposite. This highlights that a single modification route cannot synergistically resolve the contradiction between "inhibiting heat shrinkage" and "maintaining strength and toughness".

[0062] In stark contrast, Examples 1-3 achieved a breakthrough synergistic optimization of material properties by synergistically introducing N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether (BODA) and crosslinkable 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine (TCTP), thereby systematically solving the above-mentioned problems.

[0063] First, in addressing the fundamental issue of thermal shrinkage and laser processing deformation, the embodiments reduce the thermal shrinkage rate to below 0.05% (as low as 0.03%), thereby stably controlling laser drilling defects within an extremely low range of 19-21µm (17µm in the optimal embodiment 3). This demonstrates that the rigid molecular chain framework provided by BODA and the three-dimensional cross-linked network constructed by TCTP produce a dual effect of "anchoring" and "locking," greatly suppressing the movement and retraction of polymer chains under instantaneous laser thermal shock at the molecular scale, thus achieving control over material deformation from the source.

[0064] Secondly, in addressing the issue of overall performance imbalance, the embodiments demonstrate comprehensive performance improvements: tensile strength (≥238 MPa) and elastic modulus (≥5.0 GPa) are simultaneously enhanced, while glass transition temperature (≥310℃) and thermal decomposition temperature (≥550℃) are significantly increased. This proves that the two modifiers enhance rigidity and heat resistance without causing a surge in brittleness (elongation at break remains above 30%), achieving a balance between rigidity and toughness. Furthermore, the dielectric properties (dielectric constant approximately 3.1-3.2, loss factor approximately 0.0025) and peel strength (≥8.2 N / cm) of the embodiments remain at excellent levels, indicating that this modification strategy does not compromise the fundamental electrical properties and interfacial bonding reliability of polyimide as a high-frequency circuit substrate.

[0065] In summary, this invention is not a simple improvement on existing materials or processes, but rather a novel polyimide film material created through original molecular structure design and synergistic mechanism. This material possesses extremely low thermal shrinkage, excellent laser processing adaptability, superior mechanical properties, high heat resistance, and good overall practicality, fundamentally overcoming the bottleneck of material thermal defects in high-precision micro-hole processing.

Claims

1. A process for preparing a heat-shrinkable polyimide film to reduce the heat-affected zone of laser drilling, characterized in that the steps include... include: S1. By weight, in a dry, nitrogen-filled reactor, add 400-600 parts of N-methylpyrrolidone and stir at 15-25°C. Then, add 60-80 parts of 4,4'-diaminodiphenyl ether and 5-15 parts of N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether and stir. Add 30-50 parts of pyromellitic dianhydride and 20-40 parts of biphenyltetracarboxylic dianhydride and continue stirring at 25-30°C to obtain a polyamic acid resin solution. S2. Add 3-10 parts of 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine to the polyamic acid resin solution, stir and mix, filter to remove air bubbles, and then coat it on the surface of electrolytic copper foil to obtain a wet film. Heat treat the wet film and let it cool naturally to room temperature. The preparation method of the N,N'-bis(2-benzoxazolyl)-4,4'-diaminodiphenyl ether includes: A1. Under the protection of dry nitrogen, anhydrous N-methylpyrrolidone and 4,4'-diaminodiphenyl ether were added to a round-bottom flask and stirred at room temperature to obtain a reaction mixture; the reaction mixture was cooled to 0-5°C, and 2-chlorobenzoxazole was added while stirring continuously. The mixture was then heated to room temperature and stirred continuously to obtain a reaction solution. A2. Heat the reaction solution to 85-90℃ and reflux to obtain a mixture. Cool the mixture to room temperature and pour it into ice water with stirring. Filter to collect the solid and wash it with deionized water and then with ethanol to obtain a crude product. Dry the crude product in a vacuum drying oven at 78-82℃ and recrystallize it with a mixed solvent of N,N-dimethylformamide and ethanol.

2. The preparation process of the heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling according to claim 1, characterized in that, In step S1, the reaction is continued to be stirred at 25-30℃ for 12-18 hours.

3. The preparation process of the heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling according to claim 1, characterized in that, In step S2, the specific steps for heat treatment of the wet film are as follows: keep it at 80-100℃; then raise the temperature to 175-185℃ and keep it there; continue to raise the temperature to 245-255℃ and keep it there; finally raise the temperature to 300-320℃ and keep it there.

4. The preparation process of the heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling according to claim 1, characterized in that, In step A1, the stirring time should continue for 2-4 hours.

5. The preparation process of the heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling according to claim 1, characterized in that, In step A2, the reflux reaction time is 12-14 hours.

6. The preparation process of the heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling according to claim 1, characterized in that, The preparation method of the 1,3,5-tris(4-carboxyphenoxy)-2,4,6-triazine includes: B1. Under dry nitrogen protection, anhydrous dimethyl sulfoxide and cyanuric chloride are added to a three-necked round-bottom flask and cooled to -5~0℃ to obtain an anhydrous dimethyl sulfoxide solution of cyanuric chloride; anhydrous potassium carbonate is dissolved in anhydrous dimethyl sulfoxide and methylparaben is added, and stirred to obtain a suspension; the suspension is added dropwise to the anhydrous dimethyl sulfoxide solution of cyanuric chloride to obtain a reaction mixture; the reaction mixture is heated to room temperature and stirred, then heated to 48-52℃ to continue the reaction to obtain a reaction solution; B2. Filter the reaction solution to obtain a filtrate. Pour the filtrate into an aqueous hydrochloric acid solution while stirring, filter again, and wash the precipitate with water until neutral to obtain a wet filter cake. Transfer the wet filter cake to an aqueous sodium hydroxide solution, heat to 78-82℃ and stir to obtain a reaction solution. Cool the reaction solution to room temperature, adjust the pH to 1.8-2.2 while stirring, filter to obtain a crude product, wash the crude product with deionized water to obtain the product, and vacuum dry the product at 98-102℃.

7. The preparation process of the heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling according to claim 6, characterized in that, In step B1, the reaction continues for 8-10 hours.

8. The preparation process of the heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling according to claim 6, characterized in that, In step B2, the reaction is carried out at 78-82℃ with stirring for 2-4 hours.

9. A heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling, characterized in that, The heat-shrinkable polyimide film for reducing the heat-affected zone of laser drilling is prepared according to any one of claims 1-8.

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