RU2395938C1 - Method for manufacturing of printed circuit boards - Google Patents

Abstract

FIELD: radio engineering.

SUBSTANCE: in method for manufacturing of printed circuit boards transition holes are filled with melt of alloy having melt temperature below temperature of dielectric destruction, from which printed circuit board is made, and prior to application of protective metal resistive coating, electric circuit is covered with copper or other metal, having high electric conductivity.

EFFECT: improvement of ecological purity of the process, manufacturing of printed circuit boards, which do not contain water, salts of metals and gases that are ingressed in process of chemical and galvanic metallisation in dielectric base and in metal coating.

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Description

The invention relates to the field of electronics and can be used in the manufacture of printed circuit boards used in the design of electronic equipment for aircraft and space construction.

Currently, almost all circuits of radio equipment are made in the form of a metal pattern on a dielectric basis by selectively etching sections of copper foil glued to the base of the dielectric. The areas of the foil that should not be etched and which form the desired pattern of the radioelectric circuit are protected from the action of the etching solution by a coating resistant to it - photoresist [1]. After etching and removing the photoresist from the conductive paths, a drawing of an electrically conductive circuit is obtained.

The main disadvantage of this method is that all processes (etching, activation, coating) are carried out in aqueous solutions of salts. Therefore, water and salts are present in the fiberglass base, especially between the layers of fiberglass in the transition and technological holes, which inevitably leads to a violation of the electrically conductive bonds between the layers, especially when the temperature changes. Electrolysis products are also present in the coatings themselves, since galvanic coatings are porous. In this case, the formation of galvanic pairs “copper - tin-lead alloy” is possible, which can lead to the destruction of electrically conductive paths, the appearance of jumpers between conductors, and during operation at temperatures below 0 ° C, and to the destruction of electrically conductive paths.

The same disadvantages have a method of manufacturing printed circuit boards in accordance with GOST 23770-79 [2, 3].

As a prototype, a method for manufacturing printed circuit boards from foil and non-foil fiberglass with vias was chosen, in which instead of chemical metallization, gas metallization is carried out by thermal decomposition of organometallic compounds [4]. The known method avoids the ingress of moisture and salts into the fiberglass, especially in the holes, as it involves the application of a protective electrically conductive coating, after which copper is galvanically applied. However, this method has several disadvantages. The starting compounds, nickel and cobalt dicyclopentadienyl compounds, are not commercially available. The decay process is carried out in a stream of hydrogen, i.e. is explosive. Coatings over 3 μm exfoliate, and for large areas of the surface to be coated it is difficult to control the thickness of the electrically conductive metal coating. It is mandatory to use a catalyst that is applied from aqueous solutions, and although the residence time in them is small, a certain amount of water and catalyst remains in the existing holes, especially if there are scores during drilling.

The objective of the invention is to improve the environmental cleanliness of the process, the manufacture of printed circuit boards that do not contain a dielectric base, as well as in the metal coating of water, metal salts and gases that are involved in chemical and galvanic metallization.

The specified technical result is achieved by the fact that in the method of manufacturing printed circuit boards, including the operation of drilling vias, selective etching of individual sections of copper foil, applying a protective metal resistive coating, vias are filled with a melt alloy having a melting temperature below the temperature of the destruction of the dielectric from which the printed circuit board is made , and before applying a protective metal-resistive coating, the electrical circuit is coated with copper or another metal having im high electrical conductivity.

The method is as follows. Transitional and technological holes are drilled on a foil dielectric plate on both sides, and they are made not through holes, but up to a metal foil located on the back side of a fiberglass plate. Then, on both sides of this foil plate, an electrical circuit is made using any of the known methods (photolithography, laser beam, milling). Cuttings of wire with a diameter less than the diameter of the hole are inserted into the holes obtained after drilling to the entire depth. The wire should have a melting point below the temperature of the destruction of the dielectric plate. Then, a dielectric plate with electrical circuits on both sides and pieces of wire inserted into the vias is heated to the melting temperature of the wire, after which a metal coating, for example, copper, is applied to the electrical circuit tracks and contact pads by cold spraying [5], and then a metal-resistive protective coating , for example tin-lead, tin-bismuth, a rose alloy and others, by any known method.

It was experimentally established that it is advisable to insert the wire with a thickness of a foil-coated dielectric plate of more than 2 mm or more. With a thickness of less than 2 mm, filling the holes with wire pieces is excluded, the holes are filled in the process of cold spraying.

It was experimentally established that the particle size of the powder of the sprayed material should not exceed 20-25 microns. With large sizes, the porosity increases from 1-2% to 5-7%.

Information sources

1. Fedulova A.A., Kotova E.A., Yavich E.R. Multilayer printed circuit boards. M .: Sov. Radio, 1977. S. 248.

2. GOST 23770-78 "Printed circuit boards. Typical technical processes of chemical and galvanic metallization. "

3. RF patent No. 2213435. A method of manufacturing printed circuit boards. Bulletin No. 27, 2003

4. RF patent No. 2329620. A method of manufacturing printed circuit boards. Bulletin No. 20, 2008 (prototype).

5. O. Klyuyev, A. Kashirin. Cold spraying of metal coatings. Science and life. No. 3. 2005, p. 82.

Claims (3)

1. A method of manufacturing printed circuit boards, including the operation of drilling vias, selective etching of individual sections of copper foil, applying a protective metal-resistive coating, characterized in that the vias are filled with a melt alloy having a melting point below the temperature of the dielectric of which the printed circuit board is made, and Before applying a protective metal-resistive coating, the electrical circuit is coated with copper or another metal with high electrical conductivity. 2. The method according to claim 1, characterized in that the metallization of the electrical circuit is carried out by the method of "cold" spraying. 3. A method of manufacturing printed circuit boards, including the operation of drilling vias, selective etching of individual sections of copper foil, applying a protective metal-resistive coating, characterized in that the vias are filled with a spray metal when the dielectric thickness is less than 2 mm, and the circuitry is coated with copper before applying a protective metal-resisting coating or other metal having high electrical conductivity.