RU2323555C1 - Method for manufacture of printed circuit board - Google Patents

Abstract

FIELD: various branches of microelectronics and manufacture of printed circuit boards, in particular, manufacture of multilayer printed circuit boards.

SUBSTANCE: a current-conducting pattern of the electric circuit is formed on the dielectric base in succession by the method of photolithography with production of a layer of metalloresist, the dielectric base is punched and through holes are obtained on the preset sections, and a metal-containing coating is applied by the galvanic method. After that the layer of the current-conducting coating is protected, and the coating is removed from the non-sanctioned sections corresponding to the photolithographic circuit by the method of chemical pickling. Finally the lands are fused. Solutions of cuprous chloride are used as the pickling agent for chemical pickling, and a tin-bismuth alloy obtained from electrolyte based on the salts of thin and bismuth is used as an unremoved layer of metalloresist with a brightener by galvanic method at a current density of 1 to 6 A/sq.dm. EKOMET-L 6 is used as the brightener.

EFFECT: developed method for manufacture of the printed circuit board characterized by a high accuracy of reproduction of the electric circuit pattern providing a high adhesion of the layer of metalloresist to the substrate, lands and to the walls of the punched holes of the printed circuit board preserving the ability of soldering by a tin-containing alloy during a long period of time (about 6 months).

2 cl, 2 ex, 1 tbl

Description

The present invention relates to the field of technology for the manufacture of printed circuit boards (PCBs), and can be used to produce double-sided PCBs used in any field of radio electronics and radio instrument making.

A method for manufacturing a printed circuit board is known as an analogue, which includes forming on a dielectric base a conductive drawing of an electrical circuit in series by photolithography to obtain a photoresist layer and applying a metal-containing coating by the galvanic method, followed by protecting the conductive coating layer and removing the coating from unauthorized sites (RF patent No. 2058437, IPC C25D 3/56, publ. BI 11/96).

The disadvantage of the analogue is the insufficiently high quality of the coating and the lack of the ability to accurately reproduce the pattern.

Known as a prototype is a method of manufacturing a printed circuit board (OST 107.460092.004.01-86), including surface mounting by forming on a dielectric base a conductive drawing of an electrical circuit in series by photolithography to obtain a photoresist layer, perforating the dielectric base to obtain through holes in predetermined areas and applying a metal-containing electroplating followed by protection of the conductive coating layer and removal of the coating from unauthorized parties weights corresponding to the photolithographic scheme by chemical etching.

The disadvantages of the prototype are the high toxicity of the method due to the use of lead electrolyte, insufficiently high equal thickness, which is why the flatness of the circuit pattern is not high, and the accuracy of the reproduction of the circuit pattern due to the rapid oxidizability of the layer of such a coating and loss of solderability, which excludes the use of galvanic coating as a finish coating alloy providing high reproducing accuracy.

The objective of the inventors is to develop a method for manufacturing a printed circuit board, characterized by high accuracy of reproducing a drawing of an electrical circuit, providing equal thickness of a layer of electroplated coating with a thickness of 20-40 microns and flatness of the conductive surface of the board, as well as solderability, which lasts for a longer time (from 6 months or more ) and environmental cleanliness of production.

A new technical result of increasing the accuracy of reproducing a drawing of an electrical circuit, ensuring equal thickness of a layer of galvanic coating with a thickness of 20-40 microns, including in the holes, and flatness of the conductive surface of the board, as well as ensuring high solderability that lasts for a longer time (up to 6 months and more) and increase the environmental cleanliness of production.

An additional technical result of the invention is to provide improved coating quality by improving the uniformity and equal thickness of the coating on the surface of the substrate and in the holes.

These tasks and new technical results are achieved by the fact that in the known method, including surface mounting by forming a conductive pattern of an electrical circuit on a dielectric base, using a photolithography method sequentially to obtain a photoresist layer, perforating the dielectric base to obtain through holes in predetermined areas and applying a metal-containing coating by galvanic method with subsequent protection of the conductive coating layer and removal of the coating from unauthorized sections corresponding to the photolithographic scheme, the method of chemical etching, in accordance with the proposed method as the etching agent for chemical etching using solutions of chlorine copper, as a protective unremovable coating layer using a tin-bismuth alloy obtained by the galvanic method from an electrolyte based on tin and bismuth salts with a bright-forming complex additive based on a complex organic compound, in the following ingredients, in g / l:

Tin sulfate 30-50 Bismuth sulphate 0.8-1.5 Sulfuric acid 100-140 Sodium chloride 0.4-0.8 Surfactant (Syntanol-ALM) 10-12 Formalin (40%) 4-6 Brightening Additive 30-45

at a current density of 1-6 A / dm ² , using tin anodes in cases made of chlorine fabric.

In addition, ECOMET-L6 additive is used as a bright-forming additive.

The essence of the proposed method is as follows.

Initially, surface mounting is carried out by forming a conductive drawing of an electrical circuit on a dielectric base. To do this, first, a pattern of conductors and contact pads (photoresist) is formed on a substrate of dielectric material by photolithography, and then the holes and the surface of the substrate are metallized to obtain a copper layer (metal resist). The uniformity of the obtained layer both in the holes and on the surface of the substrate is provided with higher quality than in the prototype due to the use of a bright-forming, leveling additive - ECOMET-L6, as shown in the experiment.

Further, in accordance with a predetermined pattern, perforation is performed to obtain through holes in the base of the board in its individual sections. Metallization is carried out in a galvanic manner to obtain a metal-containing coating layer based on a tin-bismuth alloy, followed by protecting the conductive coating layer in accordance with a predetermined scheme by applying a protective layer of insulating material (for example, marking paint). Then, by chemical etching, the metal coating is removed from unauthorized sites corresponding to the photolithographic scheme by immersion in a solution of a chemical reagent (alkaline solutions based on chlorine copper).

As experimentally shown, the galvanic method needs less control over its parameters, is more stable, economical and simple compared to other methods of applying the specified alloy. It should be noted that the electrolyte used in the proposed method was not used to obtain the final protective coating in the manufacture of PP in the art.

The use of a tin-bismuth alloy, as experimentally identified, allows to increase the environmental friendliness of production (low toxicity class compared to the prototype), in addition, the resulting coating layers are characterized by greater uniformity, flatness and do not require additional reflow operation in infrared rays, which reduces the complexity of the method. The use of a bright-forming additive makes it possible to obtain a high-quality tin-bismuth alloy layer with greater uniformity and improved planarity (flatness), which makes it possible to even out the coating layer and eliminate the need to melt it in infrared rays, which is traditionally used to even out the layer thickness of such a coating. Anodes are proposed to be used in cases made of impregnated fabric due to the reduction of sludge formation, which could negatively affect the quality of the coating, because sludge particles could be involved in the coating layer and reduce the accuracy of the reproduction of the conductive pattern. At the same time, chlorine tissue functions as a filter, preventing the ingress of mechanical particles from the anodes into the electrolyte.

The current density is selected experimentally as optimal to achieve the required characteristics of the PP coating.

Thus, the use of all the conditions and operations of the proposed method, as well as chemical additives, improves the accuracy of reproducing the electrical circuit pattern, high adhesion of the metal resist layer to the substrate - contact pads and to the walls of the perforation holes of the PP, the uniform thickness of the galvanic coating layer with a thickness of 20-40 microns, including holes, planarity of the circuit, higher quality of the coating, which retains the ability to solder with tin-containing alloy for a long time (about 6 months ant, and more than a prototype.

The possibility of industrial implementation of the proposed method is confirmed by the following performance examples.

Example 1

The proposed method is implemented in the following sequence.

Initially, samples were prepared, under the conditions of this example, by cutting blanks from a foil-coated dielectric, specifically fiberglass, grade SF 2-35G-1,5, GOST 10316-78. Then, perforation was performed on the base to produce technological, base and through holes in predetermined sections of the dielectric base using CNC drilling machines. After that, preliminary chemical-galvanic metallization of the holes and the surface of the workpieces was carried out. The conductive pattern of the circuit on a dielectric base was formed by the method of photolithography using dry photoresist. Then, according to the planned topography, the printed circuit copper was galvanically deposited to the required thickness. Before etching the circuit board, a protective layer of offset ink was applied to protect the metallization in authorized places. During the subsequent etching, the metallization layer (copper layer) was removed from unauthorized sections of the PP. Finally, a given conductive circuit was obtained in accordance with the requirements of the design documentation.

The operation of galvanic deposition on a printed circuit of a protective metal resist of a tin-bismuth alloy is carried out at room temperature, at a current density of 0.8-1 A / dm ² , for 15-18 minutes, in an electrolyte with the following ratio of ingredients, g / l:

Tin sulfate thirty Bismuth sulphate 1,5 Sulfuric acid one hundred Sodium chloride 0.8 Surfactant (Syntanol-ALM) 10 Formalin (40%) 6 Brightening additive "Ekomet L-6" 45

As a result of electrolysis, a uniform layer of tin - bismuth alloy is deposited in this electrolyte, which does not require additional reflow, which made it possible to significantly simplify the method and its duration.

Example 2

In the conditions of example 1, the obtained alloy coating on PP in an electrolyte of the following composition of components, g / l:

Tin sulfate fifty Bismuth sulphate 0.8 Sulfuric acid 140 Sodium chloride 0.4 Surfactant (Syntanol-ALM) 12 Formalin (40%) four Brightening additive "Ekomet L-6" thirty

The test results of the coating are summarized in table.

So, as examples of the implementation of the proposed method showed, an increase in the accuracy of reproducing a drawing of the electrical circuit, ensuring high adhesion of the metal resist layer to the substrate - contact pads and to the walls of the holes of the perforation of the PP, the equal thickness of the galvanic coating layer with a thickness of 20-40 microns, planarity, higher the quality of the coating, which retains the ability to solder tin alloy for a long time (about 6 months) than that of the prototype.

Examples The thickness of the layer of metal resist, microns Current density, a / dm ² Electrolyte base for galvanic alloy production Availability of additives Flatness (planarity) Operations to improve the quality of the coating (uniform thickness and uniformity), suitable for soldering Prototype example Uneven on the surface (high error ~ 30%) 1-3 Tin lead Technological "Peptone" High accuracy ~ 30% The need for additional reflow in the infrared rays Example 1 of the proposed method 20-40 0.8-1.0 Tin bismuth The bright-forming leveling "ECOMET-L6". Accuracy of coating thickness 5% It is absent due to the manifestation of the leveling effect of the bright-forming additive ECOMET-L6 Example 2 of the proposed method 20-40 1-6 Tin bismuth The bright-forming leveling "ECOMET-L6". Accuracy of coating thickness 5% - "-

Claims (2)

1. A method of manufacturing a printed circuit board for surface mounting by forming a dielectric base of an electrical circuit in series by photolithography to obtain a photoresist layer, perforating the dielectric base with through holes in predetermined areas and applying a metal-containing coating by the galvanic method, followed by protecting the conductive coating layer and removing coatings from unauthorized sites corresponding to the photolithographic scheme, by chemical etching method, characterized in that solutions of chloric copper are used as an etching agent for chemical etching, and a tin-bismuth alloy obtained by the galvanic method from an electrolyte based on tin and bismuth salts with a bright-forming complex additive based on a complex is used as a protective non-removable layer of a metal-containing coating organic compound, with the following ingredients, g / l: Tin sulfate 30-50 Bismuth sulphate 0.8-1.5 Sulfuric acid 100-140 Sodium chloride 0.4-0.8 Surfactant (Syntanol-ALM) 10-12 Formalin (40%) 4-6 Brightening Additive 30-45 at a current density of 1-6 A / dm ² , using tin anodes in cases made of chlorine fabric. 2. The method according to claim 1, characterized in that as a bright-forming additive, ECOMET-L6 is used.