CN112351592A - Method for preparing flexible circuit board based on liquid crystal polymer film - Google Patents
Method for preparing flexible circuit board based on liquid crystal polymer film Download PDFInfo
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- CN112351592A CN112351592A CN202011328400.7A CN202011328400A CN112351592A CN 112351592 A CN112351592 A CN 112351592A CN 202011328400 A CN202011328400 A CN 202011328400A CN 112351592 A CN112351592 A CN 112351592A
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- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K3/00—Apparatus or processes for manufacturing printed circuits
- H05K3/0011—Working of insulating substrates or insulating layers
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- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K3/00—Apparatus or processes for manufacturing printed circuits
- H05K3/0011—Working of insulating substrates or insulating layers
- H05K3/0014—Shaping of the substrate, e.g. by moulding
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- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K3/00—Apparatus or processes for manufacturing printed circuits
- H05K3/02—Apparatus or processes for manufacturing printed circuits in which the conductive material is applied to the surface of the insulating support and is thereafter removed from such areas of the surface which are not intended for current conducting or shielding
- H05K3/06—Apparatus or processes for manufacturing printed circuits in which the conductive material is applied to the surface of the insulating support and is thereafter removed from such areas of the surface which are not intended for current conducting or shielding the conductive material being removed chemically or electrolytically, e.g. by photo-etch process
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Abstract
The invention discloses a method for preparing a flexible circuit board based on a liquid crystal polymer film, which comprises the following steps: firstly, preparing a metal foil and a prefabricated film of a liquid crystal polymer material with the tensile modulus of 0.5-6.0 GPa and the bending modulus of 1.0-11.0 GPa; adjusting the tensile modulus of the prefabricated film from 0.5-6.0 GPa to 1.8-3.5 GPa through a modulus adjusting procedure; and the bending modulus of the prefabricated film is adjusted from 1.0-11.0 GPa to 2.0-8.0 GPa through a modulus adjustment procedure to obtain a finished film; wherein the modulus adjusting procedure comprises the steps of carrying out heat treatment on the prefabricated film under the condition of temperature T and cooling the prefabricated film, and controlling the time T of the heat treatment and the rate v of the coolingt(ii) a Then, carrying out hot-press molding on the metal foil and the finished film to obtain a molded metal-clad plate; and finally, etching a circuit on the metal conducting layer of the metal-clad plate according to a circuit design drawing, and carrying out hole turning and electroplating on the circuit to obtain the flexible circuit board. The invention canThe problem of among the prior art cover metal sheet and have the pore-forming defect is solved.
Description
Technical Field
The invention belongs to the technical field of flexible circuit board preparation, and mainly relates to a method for preparing a flexible circuit board based on a liquid crystal polymer film.
Background
With the rapid development of communication equipment and technology, the production and preparation of high-performance high-frequency substrate materials become an indispensable part for the development of high-frequency wireless communication. The flexible circuit board is a single-layer or multi-layer flexible circuit board formed by a flexible metal-clad plate through a Flexible Printed Circuit (FPC) process, and a circuit is formed through steps of hole turning/punching, metallized via hole passing, developing, etching and the like according to a circuit design drawing. When the multilayer circuit board is prepared, the liquid crystal polymer film can be directly used as a bonding layer to be laminated with the metal foil to form a metal-clad plate, and the liquid crystal polymer film and the metal foil can also be sequentially bonded together by adopting a low-dielectric adhesive sheet.
In the prior art, most of traditional flexible circuit board base materials are polyimide films as insulating base materials of metal-clad plates, but the polyimide films have high hygroscopicity and poor high-frequency dielectric properties, so that the requirements of the wireless communication industry on high-frequency and high-speed application are difficult to meet. The liquid crystal polymer is an aromatic polyester polymer material composed of rigid molecular chains, has excellent characteristics of low dielectric constant and dielectric loss (the dielectric constant is about 3 in the frequency range of 1-60 GHz, the dielectric loss is less than 0.003), low hygroscopicity (< 0.04%), low thermal expansion coefficient and the like, and is widely concerned with high-frequency flexible substrates prepared based on liquid crystal polymer films.
The molecular chain of the liquid crystal polymer is composed of rigid liquid crystal units, optically anisotropic nematic liquid crystal can be formed in a melt state, and due to the existence of the local ordered structure and the intrinsic rigid chain structure, the thermotropic liquid crystal polymer material is easily cured on the surface firstly in the processes of extruding and forming a film from the melt state to form a skin-core structure. The skin-core structure ensures that the liquid crystal polymer film has a rigid skin layer on one hand, and is not beneficial to hot pressing with materials such as copper foil and the like; on the other hand, the liquid crystal polymer film is layered in the thickness direction, and defects appear when the circuit board is subjected to via hole and electroplating, so that the communication connection of the circuit is influenced.
At present, liquid crystal polymer films can be obtained by techniques such as a film blowing method, a T-die extrusion stretching method, a laminate stretching method, and a calendering method. In evaluating whether the obtained liquid crystal polymer film is suitable for the production of flexible circuit boards, consideration is usually given to the mechanical anisotropy in the film plane, the magnitude of the thermal expansion coefficient, and the like. The film is considered to satisfy the use requirements when the corresponding parameters fall within the set ranges, however, many of the parameter sets are obtained from the viewpoint of preparing a flexible circuit board based on a polyimide film, and the intrinsic characteristics of the liquid crystalline polymer are not well considered. When the circuit board is actually prepared, particularly in the process of processing via holes, a plurality of films meeting parameter indexes also have processing defects. Under the condition, the prior art often puts forward a more rigorous range for parameter indexes to carry out production and preparation after searching out corresponding parameters.
Another approach in the prior art is: in order to prevent severe delamination and cracking of the multilayer substrate, a requirement is placed on the coefficient of thermal expansion in the thickness direction of the film, which is proposed to be satisfactory for the production of flexible circuit boards with a thermal expansion in the thickness direction of less than 270 ppm/DEG C. However, in this method, the thermal expansion coefficient in the thickness direction is obtained by a mathematical formula based on the bulk expansion coefficient of the film, and does not directly correspond to the structure in the thickness direction.
Disclosure of Invention
Technical problem to be solved
The method is mainly used for solving the problem that in the prior art, in the process of preparing the flexible circuit board based on the liquid crystal polymer film, defects are easy to occur when the metal plate forming holes are covered flexibly.
(II) technical scheme
A method for preparing a flexible circuit board based on a liquid crystal polymer film comprises the following steps:
firstly, preparing a metal foil and a prefabricated film of a liquid crystal polymer material with the tensile modulus of 0.5-6.0 GPa and the bending modulus of 1.0-11.0 GPa; wherein the melting point of the prefabricated film is Tm(ii) a Adjusting the tensile modulus of the prefabricated film from the range of 0.5-6.0 GPa to the range of 1.8-3.5 GPa through a modulus adjusting procedure; and the bending modulus of the prefabricated film is adjusted from the range of 1.0-11.0 GPa to the range of 2.0-8.0 GPa through a modulus adjustment procedure to obtain a finished film; wherein the modulus adjusting procedure comprises the steps of carrying out heat treatment on the prefabricated film under the condition of temperature T and cooling the prefabricated film, and controlling the time T of the heat treatment and the rate v of the coolingt；
Then, carrying out hot-press molding on the metal foil and the finished product film to obtain a molded metal-clad plate;
and finally, etching a circuit on the metal conducting layer of the metal-clad plate according to a circuit design drawing, and carrying out hole turning and electroplating on the circuit to obtain the flexible circuit board.
Optionally, with respect to TmAnd the range of T is: t ism-70℃～Tm+40℃。
Optionally, with respect to TmAnd the range of T is: t ism-70℃～Tm-5 ℃; the range of t is: 1-60 hours; v. oftLess than 10 deg.C/min.
Optionally, with respect to TmAnd the range of T is: t ism-5℃～Tm+40 ℃; the range of t is: 0.2 to 30 hours, vtLess than 3 deg.C/min.
Alternatively, TmThe melting point range of (A) is: 200 to 400 ℃.
Optionally, the hot press forming comprises: directly carrying out hot-press molding on the metal foil and the finished film; or, adding an adhesive layer between the metal foil and the finished film for hot-press molding.
Optionally, the hot press forming comprises: melting point T relative to the finished filmm', the hot pressing temperature range is: t ism’-10℃～Tm’+15℃。
Optionally, before hot press forming, cleaning and drying the metal foil and the finished film; the exhaust is performed after the hot press molding.
Optionally, the surface roughness of the metal foil is less than 3.0 μm.
Alternatively, the thickness range of the monolayer finished film: 8-400 μm; thickness range of single-layer metal foil: 3 to 60 μm.
(III) advantageous effects
By adopting the method for preparing the flexible circuit board based on the liquid crystal polymer film, the prepared flexible circuit board has excellent pore-forming performance and good bending resistance, and the qualification rate of flexible circuit board products is greatly improved.
The method for preparing the flexible circuit board based on the liquid crystal polymer film is simple to operate, easy to realize modulus adjustment of the elegant and quiet polymer film, has very good practicability, is beneficial to industrial preparation of the high-frequency flexible circuit board, and also provides reference for screening of insulating base materials before the flexible circuit board is prepared.
Drawings
FIG. 1 is a schematic cross-sectional view of a metal-clad plate according to an embodiment of the present invention;
fig. 2 is a schematic cross-sectional view of a single-layer flexible circuit board according to an embodiment of the invention;
fig. 3 is a schematic cross-sectional view of a multi-layer flexible circuit board according to an embodiment of the invention.
FIG. 4 is a process diagram of heating a preformed film using a heated roller in accordance with one embodiment of the present invention.
Description of reference numerals:
1. a liquid crystal polymer film; 2. a metal conductive layer; 3. a circuit; 4. a first through hole; 5. a first electroplated layer; 6. a second through hole; 7. a second electroplated layer; 8. a third type hole; 9. a third electroplated layer; 10. prefabricating a film; 11. a heating roller; 12. and heating the device cavity.
Detailed Description
In order that the objects, technical solutions and advantages of the present invention will become more apparent, the present invention will be further described in detail with reference to the accompanying drawings in conjunction with the following specific embodiments.
Fig. 1 is a schematic cross-sectional view of a metal-clad plate according to an embodiment of the present invention; the metal-clad plate is composed of a liquid crystal polymer film 1 and metal conductive layers 2 covering the two side surfaces of the liquid crystal polymer film.
Fig. 2 is a schematic cross-sectional view of a single-layer flexible circuit 3 board manufactured according to an embodiment of the present invention; the single-layer flexible circuit 3 board is obtained by further etching a circuit 3 on the metal conductive layer 2 of the metal-clad board shown in fig. 1, performing hole turning to form a first through hole 4, and then manufacturing a first electroplated layer 5 on the wall of the first through hole 4.
FIG. 3 is a schematic cross-sectional view of a multi-layer flexible circuit 3 board manufactured according to an embodiment of the present invention; the multilayer flexible circuit 3 board is formed by laminating, compounding, hole turning and electroplating a plurality of layers of liquid crystal polymer films 1 and a plurality of layers of metal conducting layers 2, and a second through hole 6 penetrating through the whole circuit 3 board is formed in the multilayer flexible circuit 3 board; a second electroplated layer 7 is manufactured on the wall of the second through hole 6; further, a third-type hole 8 and a third-type plating layer 9 formed on the wall of the third-type hole 8 are formed in each layer of the liquid crystal polymer film 1.
The invention provides a method for preparing a flexible circuit 3 board based on a liquid crystal polymer film 1, which is described in detail and specifically comprises the following steps:
(1) preparing a metal foil and preparing a prefabricated film 10 of a liquid crystal polymer material with the tensile modulus of 0.5-6.0 GPa and the bending modulus of 1.0-11.0 GPa.
As the material of the metal conductive layer 2 in the embodiment of the present invention, the material of the metal foil may be a metal such as gold, silver, copper, nickel, or aluminum, or an alloy containing these metals; from the viewpoint of usability, workability and cost, it is preferable to use a copper foil; the copper foil may be prepared by a rolling method or an electrolytic method.
The surface roughness of the metal foil influences the bonding strength with the liquid crystal polymer film 1 and the transmission effect of the circuit 3, the roughness is too low, the adhesion between the metal foil and the liquid crystal polymer film 1 is low, and the peeling strength of a circuit 3 board is low; too high roughness may result in skin effect and reduced high frequency characteristics of the circuit 3 board. Comprehensively, the surface roughness of the metal foil can be less than 3 μm, or 0.5-2 μm for reducing skin effect.
The metal foil is too thin, and wrinkles are easily generated when the metal foil is compounded with the liquid crystal polymer film 1; the thickness of the metal foil is too thick, and the processing efficiency for preparing the circuit 3 board may be reduced. In general, the thickness of the metal foil may be 3 to 60 μm, and preferably 8 to 40 μm.
In the embodiment of the present invention, the liquid crystal polymer resin material used for preparing the prefabricated film 10 is a liquid crystal copolyester or polyesteramide material which can be melt-extruded, and may be a pure liquid crystal copolyester or polyesteramide material, or a blend of two or more liquid crystal copolyesters or polyesteramides, or a modified material of one or more liquid crystal copolyesters or polyesteramides, or a blend containing a liquid crystal copolyester or polyesteramide material.
In these liquid crystal copolyesters, a polymer containing at least 6-hydroxy-2-naphthoic acid and/or p-hydroxybenzoic acid as a repeating unit may be selected, or (i) a copolymer containing repeating units of 6-hydroxy-2-naphthoic acid and p-hydroxybenzoic acid, (ii) a copolymer containing repeating units of at least one aromatic hydroxycarboxylic acid of 6-hydroxy-2-naphthoic acid and p-hydroxybenzoic acid, at least one aromatic diol of 4, 4' -dihydroxybiphenyl and hydroquinone, and at least one aromatic dicarboxylic acid of terephthalic acid, isophthalic acid and 2, 6-naphthalenedicarboxylic acid may be selected.
The liquid crystal polymer material can be a thermoplastic liquid crystal polymer material with a melting point within the range of 200-400 ℃, and can be 230-360 ℃ for the purposes of heat resistance and processability of the film. The liquid crystal polymer material is prepared by reducing the temperature of a liquid crystal polymer melt to 25 ℃ at a cooling speed of 50 ℃/min by using a differential scanning calorimeter, preserving the temperature for 5min, then heating at a heating speed of 20 ℃/min, recording the peak temperature of a main endothermic peak in the heating process, and finally obtaining the melting point of the liquid crystal polymer material.
Optionally, the liquid crystal polymer material adopts: the intrinsic viscosity is 2-12 dL/g, or 5-9.5 dL/g, wherein the intrinsic viscosity measurement conditions are as follows: the liquid crystalline polymer material was dissolved in pentafluorophenol at a concentration of 0.1% at 60 ℃.
The liquid crystal polymer material may contain a thermoplastic resin such as a fluororesin, polyphenylene sulfide, polyether ether ketone, polyamide, polyester amide, polyethylene terephthalate and modified resins thereof, and polyolefin, as long as the effects of the embodiments of the present invention are not impaired; various additives such as slipping agents and antioxidants; fillers such as glass and ceramics.
In the embodiment of the present invention, the liquid crystal polymer film 1 can be obtained by extrusion molding of the liquid crystal polymer material. In the embodiments of the present invention, the prefabricated film forming method is not limited, and includes, but is not limited to, a blown film method, a T-die extrusion stretching method, a laminate stretching method, a calendering method, and the like, alternatively, a multilayer coextrusion blown film method, a T-die extrusion stretching method, a laminate stretching method, or alternatively, a multilayer coextrusion blown film method and a laminate stretching method are used. In the multilayer coextrusion blown film method and the laminate stretching method, on one hand, stress can be simultaneously applied along the transverse direction and the longitudinal direction of the film, and a prefabricated film with more balanced in-plane physical properties is obtained; on the other hand, the protective layer material on the surface layer of the liquid crystal polymer film has a heat preservation effect on liquid crystal polymers, and can weaken the skin-core effect in the film forming process.
When the multilayer co-extrusion blown film is used for preparation, a melt parison of the liquid crystal polymer film is extruded from an annular neck mold, and the preparation of the film can be realized by selecting a proper protective (co-extrusion) layer material and controlling a traction ratio (stretching along the extrusion direction of the film, namely longitudinal direction) and a blow-up ratio (similar to stretching perpendicular to the extrusion direction of the film, namely transverse direction). The traction ratio can be 0.8-50, optionally 1.0-30, or optionally 1.0-20; the blow-up ratio may be 0.8 to 40, optionally 1.2 to 30, or optionally 2 to 25.
When the laminate is stretched, a multilayer film containing a liquid crystal polymer layer is obtained, and then stretching is performed in the longitudinal direction and the transverse direction of the film. The longitudinal stretching ratio can be 1.2-20, optionally 1.5-15, or optionally 1.5-10; the transverse stretching ratio can be 1.5-20, optionally 1.8-15, or optionally 2.0-8.
In any of the above processes, the ratio of the draw ratio (or draw ratio) in the machine direction to the blow ratio (or draw ratio) in the cross direction may optionally be less than 4.0, or may optionally be less than 3.0.
The tensile modulus of the prepared prefabricated film 10 is required to be within the range of 0.5-6.0 GPa, the bending modulus is within the range of 1.0-11.0 GPa, and the difference value of the tensile modulus in different directions in the film surface is less than 20%. Reaching the specific parameter range value, and adopting the film forming methodThus obtaining the product. The tensile modulus measurement of the film is obtained by using a universal testing machine according to the test method of GB/T1040.3-2006 and the flexural modulus measurement of the film is obtained according to the test method of GB/T9341-2008, and regarding the difference value of the tensile modulus in different directions in the film surface, the invention adopts the steps of measuring at intervals of 10 degrees from the longitudinal direction of the film, and the maximum value of the measured values is set as EmaxThe minimum value is set to EminAnd the average value is set to EaThe percentage error in the tensile modulus of the film is defined as: (E)max-Emin)/Ea×100％。
In the embodiment of the invention, the liquid crystal polymer material is subjected to drying pretreatment before film forming extrusion molding, so that the phenomena of bubbles, degradation and the like in the film processing process are prevented, the drying form is not limited, the pretreatment temperature and time are determined according to the properties of processing materials, and the water content of the liquid crystal polymer after the drying treatment can be selected to be not higher than 800ppm or can be selected to be not higher than 600 ppm. Specifically, for example, the liquid crystal polymer material is dried in an air blast or vacuum drying oven at a glass transition temperature or lower for more than 5 hours.
(2) Adjusting the tensile modulus of the prefabricated film 10 from the range of 0.5-6.0 GPa to the range of 1.8-3.5 GPa through a modulus adjusting procedure; in addition, the bending modulus of the prefabricated film 10 is adjusted from the range of 1.0-11.0 GPa to the range of 2.0-8.0 GPa through a modulus adjustment procedure, and a finished film is obtained; wherein the modulus adjusting step comprises heat-treating the prefabricated film 10 at a temperature T and cooling the film, and controlling the time T of the heat treatment and the rate v of the coolingt。
Therefore, the liquid crystal polymer film 1 prepared by the film forming process often cannot meet the requirements for preparing the flexible circuit 3 board, and the performance of the film needs to be adjusted and improved through a modulus adjusting process. The modulus and the delamination effect of the film are adjusted by means of heat treatment under high temperature conditions (the liquid crystal polymer molecular chain has the mobility). The heat treatment temperature, the heat treatment time and the cooling rate after the heat treatment are important for the adjustment effect. The higher the heat treatment temperature, the higher the mobility of the molecular chain, the more obvious the adjustment effect, but the higher the temperature, the higher the mechanical property improvement brought by the extrusion stretching process is lost. The longer the heat treatment temperature is, the more sufficient time is provided for the molecular chain to carry out structure adjustment, and the better the adjustment effect of the modulus is. If the cooling rate after completion of the heat treatment is too fast, a serious skin-core structure may still be formed due to the mobility of the molecular chains at that time.
For the above reasons, the heat treatment temperature T needs to be within a reasonable range, and at the same time, the heat treatment time and the cooling rate after the heat treatment are adjusted according to the heat treatment temperature, so as to adjust the tensile modulus and the flexural modulus of the prefabricated film 10 to the set range values.
Optionally, preformed film 10 has a melting point TmThe melting point range of (A) is: 200 to 400 ℃. Alternatively, the melting point is T relative to the preformed film 10mThe heat treatment temperature T ranges from: t ism-70℃～Tm+40 ℃. In the embodiment of the invention, the implementation can be divided into two temperature intervals, wherein one low-temperature interval is as follows: t ism-70℃～Tm-5 ℃; and a high temperature zone: t ism-5℃～Tm+40 ℃, in the specific implementation process, the heat treatment time and the cooling rate after the treatment are also adjusted correspondingly.
Specifically, one of the temperature intervals is: melting point T relative to prefabricated film 10mThe heat treatment temperature T ranges from: t ism-70℃～Tm-5 ℃; or alternatively Tm-60 ℃ to Tm-10 ℃; accordingly, the range of t is: 1-60 hours; or optionally 0.8-20 hours, or optionally 0.8-10 hours; accordingly, vtLess than 10 deg.C/min, or alternatively less than 6 deg.C/min, or alternatively less than 5 deg.C/min.
Another temperature interval is: melting point T relative to prefabricated film 10mThe heat treatment temperature T ranges from: t ism-5℃～Tm+40 ℃; or alternatively Tm-5 ℃ to Tm+20 ℃; accordingly, the range of t is: 0.2-30 hours, or 0.5-20 hours, or 0.5-10 hours; accordingly, vtLess than 3 deg.C/min, or optionallyIs less than 2 deg.C/min, or alternatively less than 1.5 deg.C/min.
In addition, the modulus adjusting process can adjust the tensile modulus and the flexural modulus of the prefabricated film 10 to set range values, through the modulus adjusting process, the melting point of the liquid crystal polymer film can be increased by 5-50 ℃, the heat resistance of the liquid crystal polymer film is better, and meanwhile, the requirement for preparing a flexible circuit board is met, and the modulus difference value of different directions in the film surface is required to be less than 10% or is optionally less than 5% for a finished film prepared after the modulus adjusting process.
In the embodiment of the present invention, the mode and equipment of the heat treatment are not limited, and the modulus adjustment process may be performed with or without stretching the preform film 10 in a heating device such as a heating roller 11, a heating plate, a heating box, a hot air circulating furnace, or a high temperature air drying box.
In the embodiment of the invention, the heating device is adopted, and the heating modes are mainly divided into direct heating and indirect heating modes, wherein the indirect heating mode can adopt resistance heating/electric heating (such as a heating box, a hot air circulating furnace and a high-temperature blast drying box): the resistance material in the heat treatment device is used as a heating element, generates heat after being electrified, and heats the sample in a heat radiation or heat convection mode through the heating element. The carrier carrying the prefabricated film 10 is heated by direct heating (for example, a heating roller 11 and a heating plate are adopted), the heat is conducted to the prefabricated film 10 through the carrier to heat the prefabricated film 10, and the gradual cooling of the liquid crystal polymer film can be realized by controlling the temperature of different areas of the carrier, so that the purpose of controlling the cooling rate is achieved. Optionally, let in the inside dry nitrogen gas of circulation and let the inside temperature of device more even at heating device, the power through control electrical heating can control the cooling rate after heating temperature and heating: firstly, the heating power is controlled, and secondly, the flow of low-temperature nitrogen gas blown into the device and the discharge rate of high-temperature nitrogen gas can be controlled to control the cooling rate, namely, the heat exchange rate of the heating device is controlled.
Fig. 4 is a process diagram of heating the prefabricated film 10 by the heating roller 11 using the heating roller 11. As shown in fig. 4, the plurality of heating rollers 11 are located in the heating device cavity 12, the surface of the prefabricated film 10 directly contacts the surface of the heating rollers 11 to receive the heat transferred by the heating rollers 11, the heating temperature of different areas on the prefabricated film 10 is adjusted by controlling the heating power of different heating rollers 11, so that the prefabricated film 10 can be locally controlled in temperature, circulating dry nitrogen is introduced into the heating device cavity 12 to make the temperature in the heating device cavity 12 more uniform, in addition, the prefabricated film 10 is cooled after heating is completed by mainly introducing low-temperature nitrogen which enters from one side of the heating device cavity 12, the low-temperature nitrogen absorbs heat to become high-temperature nitrogen which is discharged from the other side of the heating device cavity 12, the gas temperature in the heating device cavity 12 is controlled by controlling the charging flow rate of the low-temperature nitrogen and the discharging flow rate of the high-temperature nitrogen, that is, the cooling rate is indirectly controlled, the purpose of cooling the prefabricated film 10 is achieved.
In the embodiment of the present invention, the heating method may also be induction heating, arc heating, electron beam heating, infrared heating, or the like.
When used for a high-frequency circuit 3 board, the thicker the film thickness is, the smaller the transmission loss of the signal is, and the thinner the film is, the mechanical properties of the film are reduced, but the increased film thickness reduces the flexibility of the film, so the film thickness should be in a proper range, alternatively, the thickness range of a single-layer finished film: 8-400 μm; or optionally 12-200 μm.
(4) And (3) carrying out hot-press molding on the metal foil and the finished film to obtain a molded metal-clad plate (shown in figure 1).
Optionally, the hot press forming comprises: directly carrying out hot-press molding on the metal foil and the finished film; or, adding an adhesive layer between the metal foil and the finished film for hot-press molding.
In the embodiment of the invention, an adhesive layer made of a low dielectric material can be arranged according to the use requirement of the circuit 3 board, and the adhesive layer is arranged between the liquid crystal polymer film 1 and the metal foil and used for improving the peeling strength between the liquid crystal polymer film 1 and the metal foil. The structure and type of low dielectric adhesive used is not limited thereto. As methods for preparing and using these adhesives, reference may be made to, for example, patent documents: CN110876231, CN105683266, CN109868102, CN108882501, JPWO2018030026a1, WO2019244980a1, TW201728445 and the like, and an epoxy-based adhesive, an acrylic adhesive, an olefin copolymer or a copolymer of an olefin and an unsaturated carboxylic acid or an unsaturated carboxylic acid anhydride and the like are not described herein again.
Alternatively, in the hot press forming, the melting point T relative to the finished filmm', the hot pressing temperature range is: t ism’-10℃～Tm' +15 ℃. The key points of the hot pressing process condition control are hot pressing temperature, hot pressing pressure and hot pressing time. The hot-pressing temperature is selected according to the actual melting point T of the liquid crystal polymer film after the modulus adjustment processm' determination, from the viewpoint of the pasting effect and the process control, the hot pressing temperature may be Tm’-10℃～Tm' +15 ℃. The hot pressing pressure may be 70 to 300kg/cm2Or alternatively 90 to 240kg/cm2. The hot pressing time may be greater than 0.1 seconds, or alternatively greater than 0.5 seconds.
If an adhesive layer is added, the hot pressing temperature is selected according to the heat distortion temperature of the adhesive layer. When the metal-clad plate is obtained, exhaust treatment can be carried out, particularly when the heat treatment temperature is higher than the melting point of the liquid crystal polymer and/or an adhesive layer is used, volatile substances can be generated in the hot pressing process, and the defects of bubbling and the like of the copper-clad plate can be prevented by carrying out exhaust treatment.
Optionally, before hot press forming, cleaning and drying the metal foil and the finished film; the surface of the liquid crystal polymer film 1 is pretreated by plasma treatment, corona discharge or other chemical methods.
Alternatively, the hot pressing may be a roll-to-roll method, a flat plate type vacuum hot pressing method, or a double belt pressing method.
In the process of coating the metal conducting layer 2, the flatness of the copper foil and the liquid crystal polymer film 1 needs to be ensured, and wrinkles are prevented from occurring. The copper foil and the liquid crystal polymer film 1 can be kept tensioned in the hot pressing process, and the tension is 0.1-0.3 kg/mm2Too large a draft will be produced on the film, and too small a draft will hardly achieve the desired effect.
In the hot-pressing equipment, the hardness of the laminated surface in contact with the surface of the copper foil or the liquid crystal polymer film 1 is more than 75 degrees, and can be selected to be 80-90 degrees. The laminated surface is too soft, which causes poor bonding effect between the copper foil and the liquid crystal polymer film 1 during hot pressing, and the laminated surface is too hard, which leaves an impression on the surface of the copper foil or the liquid crystal polymer film 1.
(5) After obtaining the metal-clad plate, the metal-clad plate is subjected to dry film pasting, exposure and development, a circuit 3 is etched on the metal conducting layer 2 of the metal-clad plate according to a circuit 3 design drawing, and the circuit 3 is subjected to hole transferring and electroplating to obtain a flexible circuit 3 plate (shown in figures 2 and 3). Wherein, pore-forming is carried out on flexible clad metal plate, including the through-hole that runs through whole circuit 3 board to and run through the blind hole that is used for connecting metal conducting layer 2 of 3 boards of single-layer circuit. The hole can be in the shape of a circle, a rectangle, a triangle and the like, and the hole rotating mode comprises mechanical hole rotating, infrared laser hole drilling, ultraviolet laser hole drilling, hole punching, plasma etching, chemical etching and the like. The diameter of the pores may be in the range of 0.5 to 100 μm.
And after pore forming, cleaning the pores by using Hall source plasma to remove processing scraps, dirt and the like on the surface, and then electroplating the surfaces of the formed pores to metalize the pores, wherein the thickness of the electroplated metal conducting layer 2 is 0.1-30 mu m. The conductive layer may also be deposited on the surface of the hole prior to electroplating using techniques such as plasma deposition, magnetron sputtering, and the like.
The flexible circuit 3 board prepared by the embodiment of the invention has the soldering temperature higher than 260 ℃, and the dielectric constant of the liquid crystal polymer substrate is less than 3.0 and the dielectric loss factor is less than 0.0025 under the test frequency of 10GHz by using a resonant cavity perturbation method.
The method for evaluating the performance of the prepared flexible circuit 3 board in the embodiment of the invention comprises the following steps: (1) layering effect: the invention uses a scanning electron microscope to observe the layering effect of the liquid crystal polymer film 1 in the thickness direction, and the experimental method comprises the following steps: cutting a strip-shaped liquid crystal polymer film 1 with the width of 5mm, placing the film in liquid nitrogen for 30s, then performing brittle fracture to obtain a cross section of the film, and observing the cross section by using a scanning electron microscope under the condition that the accelerating voltage is 2 kV. (2) As for the dielectric property of the film, the dielectric property of the film is tested by using a resonant cavity perturbation method, and the test frequency is 10 GHz. (3) Peel strength: a peeling test piece with the width of 5mm is manufactured by using the prepared flexible circuit 3 board, the bonding surface of the liquid crystal polymer film 1 and the metal layer is uncovered from the edge under the room temperature condition, the liquid crystal polymer film 1 layer is fixed on a flat plate by using a double-sided adhesive tape, the metal conducting layer 2 is peeled off towards the 90-degree direction at the speed of 50mm/min, the peeling load is recorded by using a digital display push-pull dynamometer, the peeling length is more than 50mm, and the peeling strength is calculated according to the average value of the load in the peeling process.
Example 1
Selecting liquid crystal copolyester of p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid with the melting point of 280 ℃ as the liquid crystal high polymer material, and drying the liquid crystal high polymer material in a vacuum drying oven for 5 hours at the drying temperature of 150 ℃, wherein the water content of the dried resin is lower than 400 ppm. The liquid crystal polymer is subjected to melt extrusion film forming processing by using a multilayer co-extrusion film blowing device, and a film with the melting point of 280 ℃ and the thickness of 25 mu m is obtained under the conditions that the traction ratio is 10 and the blow-up ratio is 4. And (3) placing the film in a high-temperature air-blast drying oven, treating for 6 hours at the temperature of 260 ℃, finishing the modulus adjusting process at the cooling rate of 3 ℃/min after the treatment is finished, and changing the melting point of the film to 298 ℃.
Two electrolytic copper foils having a thickness of 10 μm and a surface roughness of about 1.5 μm were selected as the metal conductive layers 2. The surfaces of the liquid crystal polymer film 1 and the copper foil are cleaned with ultrapure water, and then surface dehydration treatment is performed with an argon-containing ion gas. Stacking the copper foil/liquid crystal polymer film 1/copper foil in sequence, and heating to 295 deg.C and 100kg/cm by flat hot press with surface hardness of 90 °2And laminating the liquid crystal polymer film 1 and the copper foil together under the condition that the hot pressing time is 5s to obtain the double-sided copper-clad metal-clad plate, wherein as shown in figure 1, metal conducting layers 2 are laminated on both sides of the liquid crystal polymer film 1.
As shown in fig. 2, the metal conductive layer 2 on the surface of the metal-clad plate is etched according to a circuit design drawing, a first through hole with the diameter of 10 μm is formed on the liquid crystal polymer film 1 by laser drilling, the first through hole is cleaned by using hall source plasma, and finally a first electroplated layer with the thickness of 2.5 μm is deposited on the inner wall of the first through hole by electroplating, so that the flexible circuit board is manufactured.
Example 2
The difference from example 1 is that: the thickness of the liquid crystal polymer film 1 was 50 μm.
Example 3
The difference from example 1 is that: the time for the modulus-adjusting step was 8 hours.
Example 4
The difference from example 1 is that: the time for the modulus-adjusting step was 10 hours.
Example 5
The difference from example 1 is that: the temperature in the modulus-adjusting step was 280 ℃ and the time was 2 hours.
Example 6
The difference from example 1 is that: the traction ratio during multilayer coextrusion film blowing is 8, and the blow-up ratio is 3.5.
Example 7
The difference from example 1 is that: an adhesive layer is arranged between the liquid crystal polymer film 1 and the copper foil.
Example 8
The difference from example 1 is that: the temperature during hot pressing was 300 ℃ and the pressure was 120kg/cm2。
Example 9
The difference from example 8 is that: this example was used to prepare a multilayer flexible circuit 3 board, as shown in fig. 3. The multilayer flexible circuit 3 board is formed by laminating, compounding, hole turning and electroplating a plurality of layers of liquid crystal polymer films 1 and a plurality of layers of metal conducting layers 2, and a second through hole 6 penetrating through the whole circuit 3 board is formed in the multilayer flexible circuit 3 board; a second electroplated layer 7 is manufactured on the wall of the second through hole 6; further, a third-type hole 8 and a third-type plating layer 9 formed on the wall of the third-type hole 8 are formed in each layer of the liquid crystal polymer film 1. After the finished film is manufactured, a plurality of finished films and copper foils are processed according to the following steps: and laminating and compounding the copper foil/the finished film/the copper foil in sequence, and then performing hole turning and electroplating.
Comparative example 1
The difference from example 1 is that: the traction ratio during multilayer coextrusion film blowing is 30, and the blow-up ratio is 2.
Comparative example 2
The difference from example 1 is that: the time for the modulus-adjusting step was 0.5 hour.
Comparative example 3
The difference from example 1 is that: the cooling rate at the end of the modulus adjustment process was 30 ℃/min.
For the prepared flexible circuit board, the thickness delamination condition, tensile modulus, flexural modulus and melting point of the liquid crystal polymer film 1, and the dielectric property, peel strength and pore-forming property of the flexible circuit board are characterized, and the results are listed in table 1.
TABLE 1
In table 1, in the column of the hole forming performance test results of the flexible circuit board: :
o: the flexible circuit board has excellent pore-forming performance
A tangle-solidup: better pore-forming performance of flexible circuit board
Gamma rays: the flexible circuit board has poor hole forming performance and serious defects
In table 1, evaluation of the delamination effect test results in the column: :
o: no delamination was observed in the thickness direction of the film
A tangle-solidup: with a small number of layers in the thickness direction (mainly the surface layer)
Gamma rays: the delamination phenomenon in the thickness direction of the film is obvious
The dielectric constants in the above examples and comparative examples are both 2.8-2.9, and the dielectric loss factors are both less than 0.0025, which shows that the flexible circuit board prepared by the method for preparing the flexible circuit board provided by the embodiment of the invention has excellent high-frequency characteristics.
As can be seen from the results of examples 1-8 in Table 1, the circuit boards have good hole forming performance and excellent peel strength under different processing conditions by using the method provided by the invention.
It can be seen from comparative example 1 that if the tensile modulus and flexural modulus of the pre-formed film are not within the ranges defined in the present invention, the liquid crystal polymer film 1 satisfying the requirements could not be obtained by the modulus adjustment process; furthermore, the prepared flexible circuit board has obvious layering effect and poor pore-forming performance.
Based on the above analysis, when the heat treatment is performed in the modulus adjustment step, the heat treatment temperature, the heat treatment time, and the temperature reduction rate after the heat treatment all affect the treatment result in the modulus adjustment step, and therefore, the tensile modulus and the flexural modulus of the preform film can be adjusted to the set values within reasonable ranges by all of these three parameters. As can be seen from comparative examples 2 to 3, the temperature of the heat treatment was Tm-70℃～Tm-5 ℃, in which case t is in the range: 1-60 hours; v. oftThe tensile modulus and the flexural modulus of the prefabricated film can be adjusted to set range values only when the temperature is less than 10 ℃/min, and the prepared flexible circuit board can achieve better pore-forming performance. In comparative examples 2 to 3, the heat treatment time is short, and the cooling rate is high, which are not within the range defined in the embodiment of the present invention, and the skin-core delamination in the film thickness direction cannot be completely eliminated. The reason why the cooling rate cannot be too fast is that the nematic liquid crystal structure in the liquid crystal polymer melt state is very easy to solidify, and if the cooling rate is too fast, the surface layer of the liquid crystal polymer film is firstly solidified and then is the core layer, so that a skin-core structure is formed.
In summary, as shown in the results in table 1, based on the scanning electron microscope results of the liquid crystal polymer film 1 in the thickness direction and the results of the pore-forming performance and bending resistance test of the corresponding film, it is shown that the liquid crystal polymer film 1 (with the tensile modulus ranging from 1.8 GPa to 3.5GPa and the bending modulus ranging from 2.0 GPa to 8.0 GPa) having the specific in-plane tensile modulus and bending modulus has a significantly reduced delamination effect in the thickness direction, and the prepared flexible copper clad laminate has good usability and a high yield. The tensile modulus and the flexural modulus of the film can be adjusted to the ranges through the modulus adjusting procedure in the embodiment of the invention, and in the modulus adjusting procedure, the important point is that the heat treatment temperature, the heat treatment time and the cooling rate after the heat treatment are controlled within reasonable ranges. The method for preparing the flexible circuit board based on the liquid crystal polymer film 1 provided by the embodiment of the invention limits the parameters and the preparation method, and by adopting the preparation method, the prepared flexible circuit board has excellent pore-forming performance and good bending resistance, greatly improves the qualification rate of flexible circuit board products, is beneficial to the industrialized preparation of high-frequency flexible circuit boards, and also provides reference for screening of insulating base materials before the preparation of the flexible circuit boards.
In addition, in the prior art, when the liquid crystal polymer film 1 is prepared, the liquid crystal polymer film 1 with the tensile modulus ranging from 1.8 GPa to 3.5GPa and the bending modulus ranging from 2.0 GPa to 8.0GPa is difficult to prepare by one-step molding by adopting the conventional method, or the product can be prepared only by adopting a relatively complex preparation method under very strict process conditions, the preparation method provided by the embodiment of the invention provides a simple, convenient and easy modulus adjustment method with simple process, the method of adjusting the modulus step by step is adopted, the prefabricated film with a specific parameter range is prepared, the parameter range of the prefabricated film is larger (the tensile modulus needs to be within the range of 0.5-6.0 GPa, and the bending modulus is within the range of 1.0-11.0 GPa), therefore, the method is easy to realize, the initial parameters are adjusted to the target parameters by the modulus adjusting procedure, the operation is simple, the realization is easy, and the method has good practicability.
Furthermore, the preparation method provided by the embodiment of the invention limits the parameter range of the prefabricated film (the tensile modulus needs to be in the range of 0.5-6.0 GPa, and the bending modulus needs to be in the range of 1.0-11.0 GPa), because the adjustable range of the modulus adjusting process is limited, if the performance index of the prefabricated film is not in the set range, the liquid crystal polymer film 1 for the flexible circuit board required by the target is difficult to obtain.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A method for preparing a flexible circuit board based on a liquid crystal polymer film is characterized by comprising the following steps:
preparing a metal foil and a prefabricated film of a liquid crystal polymer material with the tensile modulus of 0.5-6.0 GPa and the bending modulus of 1.0-11.0 GPa; wherein the melting point of the prefabricated film is Tm；
Adjusting the tensile modulus of the prefabricated film from the range of 0.5-6.0 GPa to the range of 1.8-3.5 GPa through a modulus adjusting procedure; and the bending modulus of the prefabricated film is adjusted from the range of 1.0-11.0 GPa to the range of 2.0-8.0 GPa through a modulus adjustment procedure to obtain a finished film; wherein the modulus adjusting step comprises heat-treating the prefabricated film at a temperature T and cooling the prefabricated film, and controlling the time T of the heat treatment and the rate v of the coolingt；
Carrying out hot-press molding on the metal foil and the finished product film to obtain a molded metal-clad plate;
and etching a circuit on the metal conducting layer of the metal-coated plate according to a circuit design drawing, and carrying out hole turning and electroplating on the circuit to obtain the flexible circuit board.
2. The method for preparing a flexible circuit board based on the liquid crystal polymer film according to claim 1, comprising:
relative to the TmAnd the range of T is as follows: t ism-70℃～Tm+40℃。
3. The method for preparing a flexible circuit board based on the liquid crystal polymer film according to claim 2, comprising:
relative to the TmAnd the range of T is as follows: t ism-70℃～Tm-5 ℃; the range of t is: 1-60 hours; v istLess than 10 deg.C/min.
4. The method for preparing a flexible circuit board based on the liquid crystal polymer film according to claim 2, comprising:
relative to the TmAnd the range of T is as follows: t ism-5℃～Tm+40 ℃; the range of t is: 0.2 to 30 hours, vtLess than 3 deg.C/min.
5. The method for preparing a flexible circuit board based on the liquid crystal polymer film according to claim 1, comprising:
the T ismThe melting point range of (A) is: 200 to 400 ℃.
6. The method for preparing the flexible circuit board based on the liquid crystal polymer film according to claim 1, wherein the hot press molding comprises:
directly carrying out hot-press molding on the metal foil and the finished film; alternatively, the first and second electrodes may be,
and adding an adhesive layer between the metal foil and the finished film for hot press molding.
7. The method for preparing the flexible circuit board based on the liquid crystal polymer film according to claim 1, wherein the hot press molding comprises:
relative to the melting point T of the finished filmm', the hot pressing temperature range is: t ism’-10℃～Tm’+15℃。
8. The method for preparing a flexible circuit board based on the liquid crystal polymer film according to claim 1, comprising:
cleaning and drying the metal foil and the finished film before the hot-press molding; and exhausting after the hot press forming.
9. The method for preparing a flexible circuit board based on the liquid crystal polymer film according to claim 1, comprising:
the surface roughness of the metal foil is less than 3.0 μm.
10. The method for preparing a flexible circuit board based on the liquid crystal polymer film according to claim 1, comprising:
thickness range of the single layer of the finished film: 8-400 μm;
thickness range of single-layer metal foil: 3 to 60 μm.
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