WO2020204745A1

WO2020204745A1 - Method for forming copper conductors for printed circuit board

Info

Publication number: WO2020204745A1
Application number: PCT/RU2019/000215
Authority: WO
Inventors: Екатерина Евгеньевна АЛЯСОВА; Алексей Викторович ОСИН
Original assignee: Общество С Ограниченной Ответственностью "Русоксид" (Ооо "Русоксид")
Priority date: 2019-04-05
Filing date: 2019-04-05
Publication date: 2020-10-08

Abstract

The invention relates to methods for forming conductors on a surface and can be used for preparing semiconductor modules, one-sided printed circuit boards and two-sided printed circuit boards, and hybrid integrated circuits. A method for forming copper conductors on the surface of an aluminum substrate includes two steps of anodizing a surface of the aluminum substrate, between which the topology of the substrate is formed. During the second anodizing step, the temperature of the solution is raised gradually to a value 2 to 10 times greater than the value of the temperature of the solution used during the first anodizing step. Thereafter, pores of the aluminum oxide obtained are filled with a dielectric material, after which two copper-plating steps are carried out to form the conductors. During the second copper-plating step, the current strength is raised at least 10 times in relation to that during the first copper-plating step. The invention makes it possible to produce printed circuit boards with a high breakdown voltage and a layer of copper conductors which have a thickness of at least 280-310 µm for the high-quality installation of electronic parts during use of the printed circuit board in power electronics.

Description

METHOD FOR FORMING COPPER CONDUCTORS FOR PCB

The invention relates to methods for forming conductors on a surface and can be used in radio engineering and electronics, in particular, power electronics, automotive electronics, in the manufacture of semiconductor modules, one-sided and double-sided printed circuit boards, hybrid integrated circuits.

The main problem to be solved by the claimed invention is the formation of a layer of copper conductors with a thickness of at least 280-310 microns on the surface of the dielectric layer of aluminum oxide, which in turn is formed on an aluminum substrate for a printed circuit board with high adhesion of the copper layer to the dielectric layer of aluminum oxide , as well as the elimination of problems of poor-quality installation of electronic elements on the surface of the printed circuit board, such as various soldering defects, welding through the copper layer, or defects associated with insufficient adhesion of the copper layer to the surface of the dielectric layer of the printed circuit board, which leads to the separation of the copper layer during the soldering process electronic components or when using a printed circuit board.

Another problem addressed by the present invention is overheating of the substrate of the printed circuit board, which reduces the life of electronic devices using such boards. To increase the service life of electronic devices, it is necessary to ensure efficient heat removal from printed circuit boards, on which powerful electronic components are located, in connection with which new technical and design solutions for the manufacture of printed circuit boards and methods for their cooling are currently emerging. So, as substrates for printed circuit boards, either various dielectric materials are used, such as fiberglass and getinax, or aluminum, the surface of which is coated dielectric material. In particular, aluminum is used with a porous dielectric layer of aluminum oxide formed on its surface, the pores of which are filled with a dielectric material.

As dielectric materials suitable for filling the pores of alumina, organosilicon compounds, polyamides, polyimides, and similar polymer compounds are widely used.

RF patent N * 2471020 discloses a method of applying a copper galvanic coating on parts made of aluminum and its alloys, including anodic oxidation in an aqueous solution containing orthophosphoric and sulfuric acids, electroplating from a sulfuric acid electrolyte of copper plating. Anodic oxidation is carried out at an anodic current density of 1-2 A / dm ² , and before applying the coating by electrodeposition, a sublayer is applied from a solution of the composition, g / l:

copper sulfate 5-water - 200-250;

sulfuric acid - 50-70;

hydrofluoric acid - 10-15;

water - the rest,

within 2-3 minutes at room temperature and a cathode current density of 1-2 A / dm ² .

The disadvantage of this known method is that the use of electrolyte solutions based on phosphoric and sulfuric acid in the process of anodic oxidation does not allow the oxidation process to be carried out at high voltages, for example, 100-150 V, and to obtain a layer of anodic aluminum oxide with a thickness of more than 100 μm. The copper coating obtained on the surface of the substrate has good adhesion to the base of the aluminum substrate; however, the thickness of such a coating does not exceed 9 μm, which does not allow high-quality mounting of electronic elements on the surface of such a substrate. RF patent N ^! 2214483 discloses a method for electrolytic copper plating of aluminum products, including coating in an electrolyte containing, g / l:

CuS0 _{4 *} 5H ₂ 0 -45-55;

Na ₄ P ₂ O7 * 10H ₂ O - 200-240;

KN0 ₂ - 10-15,

at a temperature of 55-65 ° C, while the products are preliminarily kept in the electrolyte, and the coating is applied in two stages of electrolysis with interruption of the current between them. The preliminary exposure in the electrolyte is carried out for 1-3 minutes. At the first stage of electrolysis, a copper layer is applied at a current density of 0.2-0.4 A / dm ² , a break is made for 1-3 minutes, and the second stage of electrolysis is carried out at a current density of 0.3-1.0 A / dm ² ... The resulting coating is evenly distributed over the surface of the aluminum substrate, but the thickness of such a coating does not exceed 12 μm, which also does not allow high-quality installation of electronic elements on the surface of such a substrate and the use of such a substrate for the production of power printed circuit boards.

The objective of the present invention is to develop a method for forming copper conductors on the surface of a porous dielectric layer of alumina formed on an aluminum substrate by anodic oxidation and impregnated with a dielectric material, which will provide high adhesion of a copper layer with a thickness of at least 280-310 μm to the surface of dielectric alumina, and also high breakdown voltage. This makes it possible to obtain printed circuit boards with a high breakdown voltage and a layer of copper conductors with a thickness of at least 280-310 microns, and on the surface of such a printed circuit board it is possible to carry out high-quality assembly of electronic elements, as well as to use such a printed circuit board in power electronics.

The technical result consists in increasing the adhesion of copper conductors to the dielectric layer of aluminum oxide on the surface of the aluminum substrate, as well as providing the possibility of conducting high-quality mounting of electronic elements on the surface of a printed circuit board including copper conductors formed by the method according to the invention.

The problem is solved, and the claimed technical result is achieved due to the method of forming copper conductors on an aluminum substrate with a dielectric layer of aluminum oxide formed on it. A dielectric layer is formed on the surface of the aluminum substrate by deep porous anodic oxidation of aluminum (anodization) in an acidic solution at an anodization voltage value selected from the range of 100-180 V in two stages of anodization. After the first stage of anodizing with the first anodizing parameters, including at least anodizing voltage, anodizing time and solution temperature, a photoresist is applied to the surface of the obtained alumina, the necessary topology of the substrate is formed, and then the second stage of anodizing is carried out, with the second anodizing parameters including at least anodizing voltage, anodizing time and solution temperature. The pores of the obtained anodic porous alumina are impregnated with a dielectric material. Next, a layer of copper with a thickness of 280-310 microns is applied. At the second stage of anodizing, the temperature of the solution is increased stepwise to a value 2-10 times higher than the temperature of the solution used in the first stage of anodizing. The copper layer is electroplated in two stages with the parameters of copper plating, including at least the current strength of copper plating, the temperature of the copper plating solution and the time of copper plating to obtain a copper layer with a thickness of 280-310 μm, while at the second stage of copper plating the current strength is increased by at least 10 times compared with that at the first stage of copper plating.

Thanks to the proposed method of forming copper conductors on the surface of an aluminum substrate for a printed circuit board, high adhesion of a sufficiently thick copper layer to the porous surface of dielectric aluminum oxide, where the pores filled with dielectric material. Taken together, this makes it possible to produce printed circuit boards with a high breakdown voltage and a layer of copper conductors with a thickness of at least 280-310 μm. On the surface of such a printed circuit board, it is possible to carry out high-quality assembly of electronic elements, as well as to use such a printed circuit board in power electronics.

The problem is solved, and the claimed technical result is also achieved in preferred embodiments of the invention, some of which are listed below.

Thus, an aluminum alloy of grade A1100H18 with a thickness of about 3.0 mm, preferably with a surface roughness Ra of not more than 0.3 μm, can be used as a substrate.

It is preferable if the surface of the aluminum substrate is pretreated in an alkali solution, followed by clarification in a nitric acid solution, or a mixture of solutions of nitric and hydrofluoric acids.

In one embodiment of the invention, anodizing is carried out in a 2-6% solution of oxalic acid, the first stage of anodizing being carried out at a solution temperature of 3-10 ° C, and the second stage of anodizing at a temperature of 20-28 ° C. In this case, it is preferable if the anodizing time in the first anodizing step is at least 10 minutes. It is also preferred if the anodizing time in the second anodizing step is at least 45 minutes.

In a preferred embodiment of the invention, an auxiliary layer of anodic porous alumina with a thickness of 20-30 μm is formed prior to the application of the photoresist.

After the completion of the second stage of anodizing, the pores of the obtained aluminum oxide can be additionally modified (etched) in a solution of phosphoric acid at a temperature of 30-50 ° C for 5-15 minutes. It is preferable if, after the second stage of anodization, the pores of the obtained alumina are activated in a nitric acid solution at a temperature of 15-25 ° C for 1.5-3.5 minutes.

The pores are impregnated with a dielectric material, in particular, using ultrasound with a frequency of, for example, at least 20 kHz. After such impregnation with the use of ultrasound, it is advisable to dry the dielectric material with gentle heating, for example, up to 250-290 ° C, and subsequent smooth cooling.

At the first stage of copper plating, it is preferable to choose the value of the copper plating current not more than 2 A / dm ² , the temperature of the copper plating solution 20-22 ° C, and the copper plating time in the range of 20-40 minutes, with the formation of a copper layer 10-20 μm thick. It is also preferable if, in the second stage of copper plating, the magnitude of the copper plating current is selected at no more than 20 A / dm ² , the solution temperature is 38-42 ° C, and the copper plating time is in the range of 150-170 minutes, with the formation of a copper layer with a thickness of 280-310 μm ...

After the completion of the copper plating process, it is possible to additionally carry out annealing of the resulting billet with gentle heating, for example, from 20 to 250 ° C, and sharp cooling, for example, to 20 ° C.

Another object of the present invention is a printed circuit board comprising an aluminum substrate with a layer of copper conductors formed on the aluminum substrate by the method described above or any of its particular variants. The claimed printed circuit board is characterized by high breakdown voltage and the presence of a layer of copper conductors with a thickness of at least 280-310 microns, which allows high-quality assembly of electronic elements on the surface of such a printed circuit board, in particular, when using such a printed circuit board in power electronics.

Further possible embodiments of the invention are disclosed in more detail with reference to the drawings and some examples.

Brief Description of Drawings FIG. 1 is a micrograph of the surface of an aluminum substrate with a layer of anodic alumina with an ordered pore structure.

FIG. 2 is a micrograph of the surface of an aluminum substrate with a layer of anodic alumina with an ordered pore structure.

FIG. 3 is a micrograph of the surface of an aluminum substrate with a layer of anodic alumina with an ordered pore structure.

FIG. 4 is a graphical representation of the pore structure of anodic alumina.

FIG. 5 is a graphical representation of the pore structure of anodic alumina.

FIG. 6 is a micrograph of the surface of an aluminum substrate with a layer of anodic aluminum oxide, where the pores are modified in a solution of phosphoric acid to form a funnel.

FIG. 7 is a micrograph of the surface of an aluminum substrate with an anodic alumina layer, where the pores are activated in a nitric acid solution.

According to the present invention, a method for forming copper conductors on the surface of an aluminum substrate for a printed circuit board includes several steps.

First, the surface of the aluminum substrate is anodized to form a dielectric layer of aluminum oxide on it. Then the pores of aluminum oxide are impregnated with a dielectric material, and then a copper layer is applied to the surface of the dielectric layer by the copper plating process to obtain a copper layer, in particular, with a thickness of 280-310 μm.

Anodizing is carried out in solution at an anodizing voltage value selected in the range from 100 to 180 V, and anodizing, in turn, includes two stages of anodizing, between which a photoresist is applied to the surface of the obtained aluminum oxide, and the necessary topology of the substrate is also formed. The first anodizing step is carried out with first anodizing parameters including at least anodizing voltage, anodizing time and solution temperature, to obtain alumina.

The second stage of anodizing is carried out with second anodizing parameters, including at least anodizing voltage, anodizing time and solution temperature, to obtain alumina of the required porosity. In the second stage of anodizing, the temperature is stepped up to a higher value than the temperature of the solution in the first stage of anodizing. In particular, the temperature of the solution at the second stage of anodizing is 2-10 times higher than the temperature of the solution at the first stage of anodizing.

As a result, a porous dielectric layer of aluminum oxide is formed on the surface of the aluminum substrate.

This completes the anodization, and as mentioned above, then the pores of the formed aluminum oxide layer are impregnated with a dielectric material.

After filling the pores of the aluminum oxide with a dielectric material, the stage of copper plating is transferred, in which a copper layer is applied to the surface of the said dielectric layer by the copper plating process to obtain a copper layer with a thickness of 280-310 μm. Moreover, the copper plating process also includes two stages of copper plating.

The first stage of copper plating is carried out with the first parameters of copper plating, including at least the strength of the copper plating current, the time of copper plating, the temperature of the solution and the composition of the electrolyte solution.

The second stage of copper plating is carried out with the second parameters of copper plating, including at least the current strength of copper plating, the time of copper plating, the temperature of the solution and the composition of the electrolyte solution, and the current strength of the copper plating in the second stage of copper plating is at least 10 times higher than the current strength of the copper plating in the first stage of copper plating.

As a result, a dense, highly adhesive structure from a porous oxide layer is formed on the surface of the aluminum substrate. aluminum, the pores of which are filled with a dielectric material, and a sufficiently thick layer of copper conductors.

In a preferred embodiment of the invention, the surface of the aluminum substrate is pretreated for 5-15 minutes in an alkali solution (7-13% sodium hydroxide solution) at a temperature of 25-35 ° C, washed and clarified in a solution of 15-25% nitrogen acid for 2-5 minutes at a temperature of 25-35 ° C.

For preliminary preparation of the surface of an aluminum substrate, the method of acid etching and degreasing in a mixture of nitric acid in an amount of 350-600 g / l and hydrofluoric acid in an amount of 80-120 g / l at a temperature of 15-30 ° C for 15-45 seconds. In addition, as studies have shown, in order to achieve better adhesion at the anodizing stage, it is preferable that the surface of the prepared aluminum substrate be sufficiently smooth, in particular, characterized by a roughness parameter Ra of no more than 0.3 μm.

As an electrolyte for anodizing, you can use, for example, 2-6% aqueous solution of oxalic acid.

It is preferable if said first anodizing step is carried out at a solution temperature of 3-10 ° C, and said second anodizing step at a temperature of 20-28 ° C.

The duration of the first anodizing step can be at least 10 minutes.

Upon completion of the first stage of anodizing, an auxiliary layer of anodic, possibly (but not necessarily) porous, aluminum oxide with a thickness of about 20-30 μm is obtained on the surface of the aluminum substrate, which provides the surface roughness of the substrate necessary for confident fixation of a layer of dry film photoresist on it, and also prevents the destruction of the dielectric layer of aluminum oxide during subsequent acid etching. Then, by photolithography, the topology of dielectric substrates is formed - the bases for future printed circuit boards. First, those areas that should not be subjected to subsequent anodizing (i.e. the second stage of anodizing), including areas for further machining (milling), areas between printed circuit boards, etc. are covered with a dry film photoresist. And then they actually form the topology of dielectric substrates

Blanks of printed circuit boards with photolithographically formed areas for anodizing are sent to the second stage of anodizing, as a result of which a layer of porous anodic aluminum oxide with a thickness of about 120-160 μm is formed.

To obtain a porous alumina with low porosity and a pore diameter of 100-150 nm along the entire length of the pores, the second stage of anodizing is carried out in the same electrolyte and applying the same voltage that was used in the first stage of anodizing. At the beginning of the process, the temperature of the electrolyte solution is 20-22 ° C, and then the temperature is raised stepwise by 2 ° C every 30-90 minutes. The duration of the second stage of anodizing is 45-250 minutes.

The value of the anodizing voltage at the first stage of anodizing and the second stage of anodizing is selected in the range of 100-180 V.

In the process of anodizing (in fact, porous anodic oxidation), cells, or clusters, anodizing 350-500 nm in size with a certain pore geometry are formed. As a result, a layer of anodic porous alumina is obtained with a minimum porosity of about 5-10% at the interface between aluminum and alumina and a higher porosity of 15-20% in the surface layer and an ordered pore structure with a diameter of 100-150 nm, which implies a large amount of alumina phase and a low content of pores with a diameter of less than 100 nm (see Fig. 1, 2, 3).

The increase in porosity in the upper part of the oxide at the oxide-chromium-copper interface is explained by the effect of pore etching at stepwise increase in temperature and subsequent treatment in acid solutions.

By the method of scanning electron microscopy on metallographic thin sections, the geometric parameters of the aluminum pore were determined, which are reflected in Table 1 and in Fig. 4, 5.

Table 1

Table 1 introduces the following designations:

P - porosity,%

D - anodizing cell size, nm

d - pore wall thickness, nm

dl - pore diameter, nm

d2 is the distance between the centers of the pores. The porosity is calculated based on the geometric parameters of the pore using the formula (1): p ~ X 100% (1) After the completion of the second stage of anodizing, the substrate is thoroughly washed in deionized water, the layer of dry film photoresist is removed.

Additionally, although not necessarily, the pores of the obtained alumina are modified (etched) in a solution of 3-10% phosphoric acid at a temperature of 30-50 ° C for 5-15 minutes. As a result of this modification, the pore openings acquire a funnel-like shape (see Fig. B). Modification of the pores to achieve a funnel shape ensures effective penetration of the dielectric material into a narrow portion of the pores.

After modification (if used), the substrate is thoroughly washed with deionized water, dried with gentle heating to 120 ° C for 1-2 hours, kept at 120 ° C for 30-45 minutes and slowly cooled at room temperature to 25-40 ° FROM.

To expand the pores after the completion of the second stage of anodizing, instead of modification, the process of pore activation can be carried out in a 5-10% solution of nitric acid at a temperature of 15-25 ° C for 1.5-3.5 minutes. The activation of the pores makes it possible to slightly expand the pores of aluminum oxide without disturbing the structure of its surface, thereby ensuring effective filling of the pores with a dielectric material.

The cooled blanks of printed circuit boards are placed in an ultrasonic bath filled with a dielectric material, porous anodic alumina is impregnated with it for 15-30 minutes at room temperature, in a preferred embodiment, when ultrasound is applied with a frequency of at least 20 kHz.

As the dielectric material, organosilicon, polyamide, polyimide varnishes and similar polymer compounds having dielectric properties can be used. The uniform distribution of the dielectric material in the pores of the anodic aluminum oxide makes it possible to increase the breakdown voltage of the dielectric layer. The pores of alumina are filled by the following mechanism.

In the case of a constant pore diameter, a capillary effect of pore filling is observed, enhanced by ultrasonic cavitation (Konovalov's law).

Upon completion of the impregnation, the blanks of the printed circuit boards are carefully wiped with a lint-free cloth and placed in a polymerization oven, where the dielectric material is dried at temperatures up to 250-290 ° C with gentle heating and cooling, as a result of which the dielectric material becomes solid.

After the end of the polymerization process, the remaining varnish is mechanically cleaned from the front surface of the aluminum substrate with a dielectric layer of aluminum oxide, the surface is degreased with ethyl alcohol and, preferably, a sublayer of chromium with a thickness of 0.5-1 microns and copper 0.8-2 microns is vacuum deposited in a conventional way magnetron sputtering to form a contact surface for subsequent galvanic copper deposition. Upon completion of the sputtering process, a copper layer is formed on the surface of the aluminum substrate by a conventional electroplating method.

To carry out the process of copper plating, in particular, galvanic copper plating, an electrolyte based on sulfuric acid and copper sulfate is used. Various additives can be present in the electrolyte to provide gloss and uniformity of the coating.

Copper plating is carried out in two stages.

In the first stage of copper plating, the magnitude of the copper plating current is preferably not more than 2 A / dm ² . The time of copper plating at the first stage of copper plating is chosen in the range of 20-40 minutes, and the temperature of the solution is 20-22 ° C. As a result of the first stage of copper plating, a reinforced copper layer is formed (relative to the copper layer applied by vacuum deposition) with a coating thickness of 10-20 microns.

In the second stage of copper plating, the current strength of the copper plating is preferably not more than 20 A / dm ² . The time of copper plating in the second stage of copper plating is selected in the range of 150-170 minutes, and solution temperature - 38-42 ° С. The composition of the electrolyte of the second stage of copper plating is shown in Table 2. As a result of the second stage of copper plating, the thickness of the copper layer increases to 280-310 microns.

Options for the composition of the electrolyte and the conditions for the process of galvanic copper plating, according to the invention, are shown in table 2.

table 2

Upon completion of the copper plating process, the required topology of the printed circuit board is formed on the obtained copper layer with a thickness of 280-310 microns by the acid etching method generally accepted in the technology of printed circuit boards.

To increase the adhesion of the obtained copper conductors to the dielectric base, the process of calcination is carried out with a smooth heating from 20 to 250 ° C and sharp cooling to 20 ° C. The preferred calcination mode is shown in Table 3.

Table 3

In the process of calcining, the resulting copper layer is annealed. As a result, the ductility of copper increases. The layer of dielectric material, which impregnates the pores of the anodic aluminum oxide, melts, and at the same time, the metal sublayer of chromium (if used) is partially immersed in the dielectric material and fixed in it, which significantly increases the adhesion of copper conductors to the dielectric layer and allows you to confidently use the resulting substrates in the installation of various electronic components.

Double-sided and one-sided printed circuit boards with the structure of copper conductors formed according to the claimed invention have a breakdown voltage of more than 4-6 kV, which makes it possible to use them in power electronics, the manufacture of semiconductor modules, in automotive electronics, etc.

Further, examples 1-6 disclose possible specific embodiments of the invention.

Example 1

The surface of a sheet of aluminum grade A1100H18 with a thickness of 3 mm and a size of 245x330 was treated for 5 minutes in a solution of 10% sodium hydroxide at a temperature of 30 ° C. After washing the sheet clarified in a solution of 20% nitric acid for 5 minutes at a temperature of 30 ° C. After surface treatment, the substrate was anodized in an electrolyte based on 3% oxalic acid at a voltage of 120 V for 10 minutes at a temperature of 5 ° C. As a result, a 20-μm-thick primary aluminum oxide layer was formed on the substrate surface.

After washing and drying, a dry film photoresist was applied to the PCB blank, the required topology of the dielectric substrate was formed by photolithography, and the second stage of anodizing was carried out in an electrolyte based on 3% oxalic acid at a voltage of 120 V for 180 minutes and an initial electrolyte temperature of 20-22 ° C. First, anodizing was carried out for 60 minutes, after which the electrolyte temperature was increased stepwise every 60 minutes to a value of 26-28 ° C. As a result, a highly ordered porous structure of anodic aluminum oxide with a thickness of 120 μm was formed.

The resulting oxide was thoroughly washed in deionized water, the layer of dry film photoresist was removed by conventional methods, and the oxide of porous anodic alumina was modified in a 5% orthophosphoric acid solution at 40 ° C for 5 minutes.

After modification, the porous alumina was thoroughly washed with deionized water, dried with gentle heating to 120 ° C for 1.5 hours, kept at 120 ° C for 30-45 minutes, and slowly cooled at room temperature to 25-40 ° C. The cooled blanks of printed circuit boards were placed in an ultrasonic bath filled with a dielectric organosilicon varnish KO-921, and porous anodic alumina was impregnated with it for 20 minutes at room temperature at an ultrasonic vibration frequency of at least 20 kHz. Upon completion of the impregnation, the blanks of the printed circuit boards were removed from the varnish, carefully wiped with a lint-free cloth and placed in a polymerization oven, where the dielectric silicone varnish was dried with gentle heating. to a temperature of 280 ° C and subsequent smooth cooling to room temperature.

After the end of the polymerization process, the remaining varnish was mechanically removed from the front surface of the technological workpiece, it was degreased with ethyl alcohol, and the conventional method of magnetron sputtering was carried out by vacuum deposition of a chromium sublayer with a thickness of 0.7 μm and copper 1 μm. Upon completion of the spraying process, a copper layer with a thickness of at least 300 μm was formed by electroplating in two stages according to the modes indicated in Table 4.

Table 4

After obtaining a copper layer with a thickness of 300 μm, the required topology was formed on it by the acid etching method generally accepted in the technology of printed circuit boards, and then calcination was carried out according to the mode indicated in Table 3.

Example 2 The surface of a sheet of aluminum grade A1100H18 with a thickness of 3 mm and a size of 245x330 was treated in a mixture of solutions of nitric acid in an amount of 600 g / l and hydrofluoric acid in an amount of 80 g / l at a temperature of 30 ° C for 45 seconds. After surface treatment, the substrate was anodized in an electrolyte based on 3% oxalic acid at a voltage of 130 V for 15 minutes at a temperature of 10 ° C. As a result, a primary layer of aluminum oxide with a thickness of 25 μm was formed on the substrate surface.

After washing and drying, a dry film photoresist was applied to the PCB blank, the required topology of the dielectric substrate was formed by photolithography, and the second stage of anodizing was performed in an electrolyte based on 2% oxalic acid at a voltage of 130 V for 200 minutes and an initial electrolyte temperature of 20-22 ° C. First, anodizing was carried out for 60 minutes, after which the electrolyte temperature was increased stepwise every 60 minutes to a value of 26-28 ° C. As a result, a highly ordered porous structure of anodic aluminum oxide with a thickness of 130 μm was formed. The resulting oxide was thoroughly washed in deionized water, the dry film photoresist layer was removed by conventional methods, and the pores of the porous anodic alumina oxide were activated in 5% nitric acid solution at 25 ° C for 3.5 minutes. After activation, the surface of porous alumina was thoroughly washed with deionized water, dried with gentle heating to 120 ° C for 1.5 hours, kept at 120 ° C for 30-45 minutes, and slowly cooled at room temperature to 25-40 ° C. The cooled blanks of printed circuit boards were placed in an ultrasonic bath filled with a dielectric organosilicon varnish KO-921, and porous anodic alumina was impregnated with it for 20 minutes at room temperature at an ultrasonic vibration frequency of at least 20 kHz. Upon completion of the impregnation, the PCB blanks were removed from the varnish, carefully wiped with a lint-free cloth and placed in a polymerization oven, where they were dried. dielectric organosilicon varnish at a temperature with gentle heating up to 280 ° C and subsequent smooth cooling to room temperature.

After the end of the polymerization process, the remaining varnish was mechanically removed from the front surface of the technological workpiece, it was degreased with ethyl alcohol, and the conventional method of magnetron sputtering was carried out by vacuum deposition of a chromium sublayer with a thickness of 0.7 μm and copper 1 μm. Upon completion of the spraying process, a copper layer with a thickness of 280 μm was formed by electroplating in two stages according to the modes indicated in Table 5.

Table 5

After obtaining a copper layer with a thickness of 280 μm, the required topology was formed on it using the acid etching method generally accepted in the technology of printed circuit boards, and then calcination was carried out according to the mode specified in Table 3.

Example 3 The surface of a sheet of aluminum grade A1100N18 with a thickness of 3 mm, size 245x330 was treated for 5 minutes in a solution of 12% sodium hydroxide at a temperature of 28 ° C, after washing the sheet was clarified in a solution of 25% nitric acid for 3 minutes at a temperature of 25 ° C ... After surface treatment, the substrate was anodized in an electrolyte based on 2% oxalic acid at a voltage of 150 V for 12 minutes at a temperature of 7 ° C. As a result, a 28-μm-thick primary aluminum oxide layer was formed on the substrate surface.

After washing and drying, a dry film photoresist was applied to the PCB blank, the required topology of the dielectric substrate was formed by photolithography, and the second stage of anodizing was carried out in an electrolyte based on 2% oxalic acid at a voltage of 150 V for 220 minutes and an initial electrolyte temperature of 20-22 ° C. First, anodizing was carried out for 60 minutes, after which the temperature of the electrolyte was increased stepwise every 60 minutes to a value of 2b-28 ° C. As a result, a highly ordered porous structure of anodic aluminum oxide with a thickness of 145 μm was formed. The resulting oxide was thoroughly washed in deionized water, the dry film photoresist layer was removed by conventional methods, and the pores of the porous anodic alumina oxide were activated in a 10% nitric acid solution at 25 ° C for 2 minutes. After activation, the surface of porous alumina was thoroughly washed with deionized water, dried with gentle heating to 120 ° C for 1.5 hours, kept at 120 ° C for 30-45 minutes and slowly cooled at room temperature to 25-0 ° C. The cooled blanks of printed circuit boards were placed in an ultrasonic bath filled with a dielectric organosilicon varnish KO-921, and porous anodic alumina was impregnated with it for 20 minutes at room temperature at an ultrasonic vibration frequency of at least 20 kHz. Upon completion of the impregnation, the blanks of the printed circuit boards were removed from the varnish, carefully wiped with a lint-free cloth and was placed in a polymerization oven, where the dielectric organosilicon varnish was dried at a temperature with gentle heating to 280 ° C and subsequent gradual cooling to room temperature.

After the end of the polymerization process, the remaining varnish was mechanically removed from the front surface of the technological workpiece, it was degreased with ethyl alcohol, and the conventional method of magnetron sputtering was carried out by vacuum deposition of a chromium sublayer with a thickness of 0.7 μm and copper 1 μm. Upon completion of the spraying process, a copper layer with a thickness of 310 μm was formed by electroplating in two stages according to the modes indicated in Table 6.

Table b

After obtaining a copper layer with a thickness of 300 μm, the required topology was formed on it by the acid etching method generally accepted in the technology of printed circuit boards, and then calcination was carried out according to the mode indicated in Table 3. Example 4

The surface of a sheet of aluminum grade A1100H18 with a thickness of 3 mm and a size of 245x330 was treated for 15 seconds in a mixture of solutions of nitric acid in an amount of 350 g / l and hydrofluoric acid in an amount of 120 g / l at a temperature of 15 ° C for 15 seconds. After surface treatment, the substrate was anodized in an electrolyte based on 2.2% oxalic acid at a voltage of 180 V for 10 minutes at a temperature of 3 ° C. As a result, a primary aluminum oxide layer 30 μm thick was formed on the substrate surface.

After washing and drying, a dry film photoresist was applied to the PCB blank, the required topology of the dielectric substrate was formed by photolithography, and the second stage of anodizing was carried out in an electrolyte based on 2.2% oxalic acid at a voltage of 180 V for 170 minutes and an initial electrolyte temperature of 20 -22 ° C. First, anodizing was carried out for 60 minutes, after which the electrolyte temperature was increased stepwise every 60 minutes to a value of 26-28 ° C, as a result of which a highly ordered porous structure of anodic alumina with a thickness of 160 μm was formed. The resulting oxide was thoroughly washed in deionized water, the layer of dry film photoresist was removed by conventional methods, and the oxide of porous anodic alumina was modified in a 5% phosphoric acid solution at 0 ° C for 5 minutes. After modification, the porous alumina was thoroughly washed with deionized water, dried with gentle heating to 120 ° C for 1.5 hours, kept at 120 ° C for 30-45 minutes, and slowly cooled at room temperature to 25-40 ° C. The cooled blanks of printed circuit boards were placed in an ultrasonic bath filled with a dielectric organosilicon varnish KO-921, and porous anodic aluminum oxide was impregnated with it for 20 minutes at room temperature at an ultrasonic vibration frequency of at least 20 kHz. Upon completion of the impregnation, the blanks of the printed circuit boards were removed from the varnish, thoroughly wiped with a lint-free with a cloth and placed in a polymerization oven, where the dielectric organosilicon varnish was dried at a temperature with gentle heating to 280 ° C and subsequent gradual cooling to room temperature.

After the end of the polymerization process, the remaining varnish was mechanically removed from the front surface of the technological workpiece, it was degreased with ethyl alcohol, and the conventional method of magnetron sputtering was carried out by vacuum deposition of a chromium sublayer with a thickness of 0.7 μm and copper 1 μm. Upon completion of the spraying process, a 300 μm thick copper layer was formed by electroplating in two stages according to the modes indicated in Table 7.

Table 7

After obtaining a copper layer with a thickness of 300 μm, the required topology was formed on it by the acid etching method generally accepted in the technology of printed circuit boards, and then calcination was carried out according to the mode indicated in Table 3. The obtained substrates can be used for the production of printed circuit boards, the breakdown voltage of which is more than 6 kV DC with an area of copper conductors of 0.15-0.2 dm ² .

Example 5

To determine the breakdown voltage, a technique was used according to the technical specifications MSFA.758700.001 TU.

For testing, samples of electronic boards were taken, in which standard substrates with a dielectric layer of aluminum oxide 70 and 90 μm thick were used as a substrate - samples 1 and 2, respectively, and substrates obtained according to the above examples 1-4 (respectively, samples 3-b ) with a copper contact area of 0.15 dm ² . Before testing, samples of printed circuit boards were dried in an oven at a temperature of 80-100 ° C for at least 20 minutes and kept at a temperature of 15 to 20 ° C in a test room for at least 4 hours.

The tests were carried out using a breakdown device providing a direct current supply with a voltage from 100 V to 6000 V DC. During the measurements, the voltage value at which the breakdown occurred (a sharp increase in the leakage current more than 1 mA) was recorded. One probe of the voltage source was connected to the base of the sample, the other was connected in turn with all the electrical circuits of the printed circuit board (sample). The voltage rise rate was 10-15 seconds, the holding time at the test voltage was 2 seconds. Voltage was applied between selected points of the sample.

The breakdown of the insulating (dielectric) layer was determined by a sharp increase in the leakage current of more than 1 mA, which indicated a loss of insulation resistance.

The test results are shown in Table 8.

Table 8

From the data given in table 8, it follows that the highest breakdown voltage (4-6 kV) corresponds to samples 3, 4, 5, 6. Therefore, the samples obtained according to examples 1-4 when used as a substrate for printed circuit boards have good dielectric properties and can withstand voltages up to 6 kV.

Example 6

To determine the adhesion strength of a layer of copper conductors, the technique specified in clause 7.2.25 of MSFA.758700.001 TU was used.

The pull-off strength of the contact pads is taken as the force perpendicular to the surface of the printed circuit board necessary to separate the contact pad from the base material. The tests were carried out on circular contact pads separated from the adjacent conductors. An adhesion tester (adhesion meter) was used to pull off. The force was applied until the contact pad was completely detached. The tensile strength of the contact pads was taken as the minimum result obtained when ten tested contact pads were detached from the base material.

The tests were carried out on the samples obtained in examples 1-4

(respectively samples 3-6).

As shown by adhesion tests, the samples obtained in accordance with the present invention show high values of the adhesion strength. Thus, when using the claimed method of forming copper conductors on the surface of the dielectric layer of alumina, high adhesion of a sufficiently thick copper layer to the porous surface of dielectric alumina, where the pores are filled with a dielectric material, is ensured. Taken together, this makes it possible to produce printed circuit boards with a high breakdown voltage and a layer of copper conductors with a thickness of at least 280-310 μm. On the surface of such a printed circuit board, it is possible to carry out high-quality assembly of electronic elements, as well as to use such a printed circuit board in power electronics.

Claims

CLAIM

1. A method of forming copper conductors on the surface of an aluminum substrate for a printed circuit board, in which:

anodizing the surface of the aluminum substrate is carried out to form a dielectric layer of aluminum oxide on it, while anodizing is carried out in solution at an anodizing voltage value selected in the range from 100 to 180 V, and anodizing includes:

a first anodizing step with first anodizing parameters including at least anodizing voltage, anodizing time and solution temperature, to produce alumina,

applying a photoresist to the surface of the obtained aluminum oxide,

formation of the necessary topology of the substrate, and

the second stage of anodizing with second anodizing parameters, including at least anodizing voltage, anodizing time and solution temperature, to obtain aluminum oxide of the required porosity, and at the second stage of anodizing the temperature is stepwise increased to a value 2-10 times higher than the solution temperature at the first stage anodizing,

impregnate the pores of aluminum oxide with a dielectric material, and apply a copper layer on the surface of the dielectric layer by the copper plating process to obtain a copper layer with a thickness of 280-310 microns, and the copper plating process includes:

the first stage of copper plating with the first parameters of copper plating, including at least the current strength of the copper plating, the time of copper plating, the temperature of the solution and the composition of the electrolyte solution, and the second stage of copper plating with the second parameters of copper plating, including at least the strength of the current of copper plating, the time of copper plating, the temperature of the solution and the composition electrolyte solution, while the copper plating current in the second copper plating stage is at least 10 times higher than the copper plating current in the first copper plating stage.

2. The method according to claim 1, characterized in that aluminum alloy of grade A1100H18 is used as the material of the aluminum substrate.

3. The method according to claim 1 or 2, characterized in that the surface roughness Ra of the aluminum substrate is not more than 0.3 μm.

4. A method according to claim 1, characterized in that the surface of the aluminum substrate is pretreated in an alkali solution followed by clarification in a nitric acid solution, or pretreated with a mixture of solutions of nitric and hydrofluoric acids.

5. The method according to claim 1, characterized in that the anodizing is carried out in a 2-6% solution of oxalic acid, the first stage of anodizing being carried out at a solution temperature of 3-10 ° C, and the second stage of anodizing at a temperature of 20-28 ° C.

6. The method according to claim 1, characterized in that the anodizing time in the first anodizing step is at least 10 minutes.

7. The method according to claim 1, characterized in that the anodizing time in the second anodizing step is at least 45 minutes.

8. The method according to claim 1, characterized in that before applying the photoresist, an auxiliary layer of anodic porous aluminum oxide with a thickness of 20-30 μm is formed.

9. The method according to claim 1, characterized in that after the completion of the second stage of anodizing the pores of the obtained alumina are modified (etched) in a solution of phosphoric acid at a temperature of 30-50 ° C for 5-15 minutes.

10. The method according to claim 1, characterized in that after the second stage of anodizing, the pores of the obtained alumina are activated in a nitric acid solution at a temperature of 15-25 ° C for 1.5-3.5 minutes.

11. A method according to claim 1, characterized in that the pores are impregnated with a dielectric material using ultrasound with a frequency of at least 20 kHz.

12. The method according to claim 11, characterized in that after impregnation with the use of ultrasound, the dielectric material is dried with gentle heating to 250-290 ° C and gradual cooling.

13. The method according to claim 1, characterized in that at the first stage of copper plating, the value of the copper plating current is not more than 2 A / dm ² , the temperature of the copper plating solution is 20-22 ° C, and the copper plating time is in the range of 20-40 minutes, with the formation copper layer 10-20 microns thick.

14. The method according to claim 1, characterized in that at the second stage of copper plating, the value of the copper plating current is selected at no more than 20 A / dm ² , the solution temperature is 38-42 ° C, and the copper plating time is in the range of 150-170 minutes, with the formation copper layer with a thickness of 280-310 microns.

15. The method according to claim 1, characterized in that after the completion of the copper plating process, the resulting billet is calcined with gradual heating from 20 to 250 ° C and sharp cooling to 20 ° C.

16. A printed circuit board comprising an aluminum substrate with a layer of copper conductors formed on an aluminum substrate by the method according to any one of claims. 1-15.