JPH01270386A - Manufacture of printed wiring board - Google Patents

Abstract

PURPOSE:To enable conductor circuits formed on both the sides of a board to be electrically connected easily with each other and decrease the board in thermal deterioration by a method wherein an electron ray setting conductive paste is applied onto a side wall of a through-hole so as to make a central axis section of the through-hole provided to an insulating board hollow, and the conductive paste is irradiated with electron rays from both the sides of the board to be set. CONSTITUTION:A conductive paste is composed of a fine conductive powder, an electron ray setting binder, and an additive, and a solvent is added, if necessary. And, the operating condition basing on the fluidity and the adhesion of the conductive paste is adequately adjusted corresponding to the diameter and the length of a through-hole so as to enable the paste to adhere uniformly to the inner wall of the through-hole preventing the through-hole from being filled with the paste. Next, the conductive paste is irradiated with electron rays in such a manner that a board is irradiated with electron rays at least once from the front and then from the rear once. By these processes, the conductive paste can be sufficiently set even if an electron ray accelerator of a low voltage is employed.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention is a method for manufacturing a printed wiring board used in electronic equipment, etc. The present invention relates to a method of manufacturing a printed wiring board in which electrical connections between circuits are made using a conductive paste that is applied to the side walls of through holes formed in the board and cured.

(Prior Art) Conventionally, printed wiring boards having conductive circuits on both sides of an insulating substrate,
That is, it is well known that in order to electrically connect four circuits on the front and back sides of a double-sided printed wiring board, the walls of through holes provided in the board are plated. However, this method is (1)! :l! The manufacturing process is complicated, (2) it requires advanced technology, and (3) the manufacturing cost is high.

As an alternative method, a method of embedding conductive paste in the through-hole and connecting the front and back conductors has been adopted since around 1970, and this method is mainly used for printed wiring boards for consumer electronics. The conductive paste used in this method is
The main ingredients are conductive powder such as copper, gold, or carbon black, thermosetting resin, and a solvent. After filling the hole, it usually requires heating to remove the solvent and heat treatment to harden the resin.
(several tens of minutes at around 0°C) is required. However, even with this hole filling method using thermosetting conductive paste, (1)
There are problems such as air bubbles remaining in the paste after hardening deteriorate the reliability of electrical connections, and (2) the substrate is damaged or deformed by heating, and solutions to these problems have been long awaited. In order to compensate for these drawbacks, in recent years ultraviolet curable resins,
Alternatively, the use of a conductive paste based on an electron beam curable resin is being considered, but in the former case, the ultraviolet rays do not penetrate into the interior due to reflection by conductive particles, making it difficult to completely cure the paste. Further, electron beam irradiation equipment with the latter pressure is required, and its industrialization has been hindered due to the size and cost of the equipment. That is, - boat thickness (1,6
In order to harden the silver paste filled in the through holes provided in the substrate (mm), an accelerating voltage of at least 1 megavolt or more is theoretically required. This purpose can only be achieved using a voltage electron beam accelerator, and industrialization is practically difficult due to the problem of substrate deterioration.

(Problem to be Solved by the Invention) While researching various types of pastes that are cured by electron beams, the present inventors attached electron beam-curable conductive paste to the side wall of the through-hole so that the central axis was hollow. In such cases, the present inventors have discovered that it is possible to sufficiently cure the conductive paste even by using a low-voltage electron beam accelerator, which has already been widely used for the purpose of curing paint films.

[Structure of the invention]

(Means for Solving the Problems) The present invention applies an electron beam curing type conductive paste to the side wall part so that the central axis part of a through hole provided in an insulating substrate becomes hollow, and then applies an electron beam hardening type conductive paste from both sides of the substrate. A method for manufacturing a printed wiring board, comprising a step of obtaining electrical connection between conductive circuits on both sides of the board by curing the conductive paste by irradiating the wire with a wire, and optionally containing a solvent. An electron beam curable conductive paste may also be used.

This step may be performed after or before providing the insulating substrate on which conductive circuits are already formed on both sides.

In the present invention, insulating substrates include resin laminates made of phenolic resin, glass base cloth/epoxy resin, polyester resin, flexible substrates made of polyimide or polyester, ceramic substrates, metal substrates coated with insulation, etc. Any substrate commonly used in the electronics industry can be used. In addition, methods for forming conductive circuits on the front and back surfaces of these insulating substrates include etching a copper foil laminated in advance;
Methods using electroless plating and/or electrolytic plating, methods of printing conductive paste, methods of wiring conductor wires covered with insulation, etc. are known, and any of these methods,
Alternatively, any combination of two or more of these methods may be used.

In the present invention, the through-hole provided in the insulating substrate may be a through-hole made by any generally known method such as molding, punching, drilling, etc.

The conductive paste used in the present invention is composed of conductive fine powder, an electron beam curable binder, and additives, and a solvent is added as necessary. There are no particular restrictions on the conductive fine powder, including gold, silver, platinum, copper, nickel, chromium,
Fine metal powders such as palladium, aluminum, tungsten, molybdenum or alloys thereof, or inorganic or organic fine powders coated with these metals or alloys, fine powders of oxides such as tin, titanium, iron, etc. Carbon black such as acetylene black, furnace black, thermal black, channel black, graphite refined from natural graphite such as flaky graphite and earthy graphite, or artificial graphite, or surface-treated materials or vinyl monomers. or a mixture thereof can be used. There are no particular restrictions on the electron beam curable binder, and polyallyl compounds such as diallyl orthophthalate and diallyl isophthalate, unsaturated polyesters, poly(meth)acrylates of bisphenol A type epoxy compounds,
Epoxy poly(meth)acrylates such as poly(meth)acrylate of novolak phenolic epoxy compounds, polyurethane compounds and β-hydroxyethyl (
Reaction products with meth)acrylates, poly(meth)acrylates, polyester polyols poly(meth)acrylates, polyether polyols poly(
meth)acrylates, divinyl compounds, phenoxyethyl (meth)acrylate, tetrahydrofurfuryl (
One or more compounds selected from compounds having an ethylenically unsaturated double bond such as meth)acrylate, (meth)acrylic acid alkyl ester, styrene, and α-alkylstyrene can be used. There are no particular restrictions on additives, including polymer compounds inert to electron beams, non-conductive inorganic fine powders such as silica and talc, fluidity improvers, adhesion promoters, antioxidants, and dispersants. etc. are used depending on the purpose. There are no particular restrictions on the solvents that may be added as necessary to adjust fluidity properties, and include methyl ethyl ketone, ethyl cellosolve, ethyl cellosolve acetate, ethyl acetate, toluene, xylene,
Isopropyl alcohol, ethanol, acetone, butyl carpitol, butyl carpitol acetate, carpitol acetate, butyl cellosolve, butyl cellosolve acetate or mixtures thereof can be used.

In the present invention, the conductive paste is generally applied to the inner wall of the through-hole by inserting a bottle with the conductive paste attached into the through-hole and then pulling the bottle out, or by penetrating the conductive paste by a normal screen printing method. One of the methods used is to print on one end of the hole and then vacuum suction from the opening at the other end. Moreover, it is also possible to perform the above-mentioned operation from both the front and back sides, if necessary.

In either case, it is important that the conductive paste adheres uniformly to the inner wall of the through hole and that the through hole is not blocked. It is necessary to adjust the operating conditions for flowability and deposition accordingly. If the flow characteristics of the conductive paste are incorrectly selected, undesirable phenomena may occur, such as the paste flowing before hardening, contaminating the substrate or equipment, or strings being pulled when the bottle is pulled out. - For boat fishing, the paste for the present invention preferably has a viscosity of about 50 to 5000 centivoise (25°C).

As a device for electron beam irradiation in the present invention, a so-called low energy type electron beam accelerator with an accelerating voltage of 150 to 300 kilovolts is used. The electron beam irradiation is
Normally, the electron beam is not particularly limited in the inert gas atmosphere and the incident direction is not limited, but according to research by the present inventors, it is sufficiently cured even in a state parallel to the axis of the through hole. Irradiation is carried out at least once from the front side and then once from the back side.
It will be held twice. The dose of the electron beam to be irradiated is 5 to 5 per time.
30 megarads is preferred. A small dose of electron beam will result in incomplete curing of the conductive paste, and an excessive dose or
The number of irradiations is not preferable from the viewpoint of efficiency, substrate deterioration, etc. If the conductive paste you use contains solvent,
Prior to electron beam irradiation, it is necessary to perform drying to remove the solvent. Possible drying methods for solvent removal include room temperature drying, hot air drying, and far-infrared drying, but far-infrared drying is preferable in consideration of efficiency and thermal effects on the substrate. In the case of infrared drying,
Better results can be obtained by irradiating the front and back surfaces once each.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples in any way. Example "Central part" refers to parts by weight.

(Example 1) An electron beam curable conductive paste was manufactured by mixing silver powder and an electron beam curable binder in the proportions shown below, and then kneading with three rolls.

Ingredients Quantity Silver powder (particle size 0.5-10 μ゛) 28 parts Silica powder (manufactured by Nippon Aerosil ■) 2 parts Spirane resin with ethylenic double bonds 10 parts (Spira Tsukull-3000 manufactured by Showa Kobunshi ■) ) Trimethylolpropane triacrylate 2 parts Silane coupling agent 1.5 parts (Shin-Etsu Chemical fiK
BM-503) Separately, conductive circuits are formed on both the front and back surfaces by etching the 'd4 ffa of a 1.6 mm thick double-sided copper-clad glass epoxy board, and the conductor circuits are passed through the front and back copper foil circuits and the intermediate glass epoxy board. A through hole with a diameter of 1.0+v was provided by drilling. The above conductive paste was printed using a 180 mesh stainless steel screen so that the conductive paste was printed on one end of the through hole of this substrate. Next, the through hole was unblocked by the conductive paste by vacuum suction from the reflective end of the through hole, and the conductive paste was applied to the inner wall of the through hole. In addition, at this time, by appropriately adjusting the conditions of screen printing and vacuum suction, it was possible to make sufficient contact between the front and back copper foil circuits and the conductive paste.

Subsequently, this substrate was held horizontally and irradiated with an electron beam from the vertical direction using an electron beam irradiation device Model 150B-15 manufactured by Energy Science Co., Ltd. under conditions of an accelerating voltage of 160 kilovolts in a nitrogen gas atmosphere. It was cured by 20 megaraft from the front side, and then by 20 megaraft from the back side. The resistance value between both ends of the through hole after conductive paste treatment is 50mΩ/hol
e, and after immersing it in a solder bath at 260°C for 10 seconds 5 times, the resistance value is 100 mΩ/h.
It was ole.

(Example 2) An electron beam curable conductive paste was manufactured by mixing silver powder, an electron beam curable binder, and a solvent in the proportions shown below, and then kneading with three rolls.

Ingredients Amount Silver powder (
Particle size 0.5-10μ) 38 parts diallyl isophthalate oligomer 3 parts Tris-
2-acryloyl ethyl ester 3 parts-isocyanurate trimethylolpropane triacrylate 5 parts Epoxy acrylate resin 3 parts (Showa Kobunshi 9#$5P-4010) Silane cuff pulling agent
2 parts (Shin-Etsu Chemical ■KBM-503) Carpitol acetate 9 parts separately,
Through holes with a diameter of 0.8 nun were punched at predetermined positions on a double-sided copper-clad paper phenol substrate having a thickness of 1.6 mm+, and conductor circuits were formed on both the front and back surfaces by etching the copper foil. Next, a pin having a diameter smaller than the diameter of the hole with the conductive paste attached thereto is inserted into the through-hole of this substrate, and then the pin is pulled out to adhere the conductive paste to the side wall of the through-hole. I was able to do that. By appropriately adjusting the operation of the pins, it was easy to ensure sufficient adhesion between the copper foil circuits on the front and back sides and the conductive paste without blocking the through holes.

Subsequently, the front surface and then the back surface of this substrate were heated with a far-infrared lamp to dry the solvent.

At this time, the atmospheric temperature was 180° C. and the drying time was 1 minute. Further, this substrate was subjected to electron beam irradiation under the same conditions as in Example 1.

It was confirmed by microscopic observation of the inner wall of the through hole and ultrasonic treatment using methyl ethyl ketone that the conductive paste inside the through hole was sufficiently hardened by the above treatment.
Further, the resistance value at both ends of the through hole was 50 mΩ/hole.

(Example 3) At a predetermined position on a one-sided copper-clad paper phenolic resin laminated substrate with a thickness of 1.61 mm, a diameter of 1.6 cm was placed. A conductive circuit was formed on one side by drilling a through hole and then etching the copper foil. The same conductive paste as in Example 2 was applied to the through-holes of this substrate and cured. Next, using the screen method, l of the same conductive paste as in Example 1 was applied.
It was printed on the opposite side of the R foil conductor circuit and irradiated with a 10 megaland electron beam from the same side by the method shown in Example 1. After curing, the resistance value of the through hole between the copper foil conductor circuit on the front surface and the conductive beast circuit on the back surface was 50 mΩ/hole.

(Comparative Example) The same conductive paste as in Example 2 was filled into the through hole of the same substrate as in Example 2 using a pin. However, in this comparative example, far-infrared heating and electron beam irradiation were performed under the same conditions as in Example 2, except that the through holes were completely closed with paste. The conductive paste inside the through-hole was only cured at the exposed portion, and the inside remained uncured.

〔Effect of the invention〕

As described above, in the present invention, a conductive paste applied to the inner wall of a through hole provided in an insulating substrate is hardened by electron beam irradiation using a low-energy accelerator, and electrical connection between conductive circuits on both sides of the substrate is made. It is of great practical value in that it enables the production of double-sided printed wiring boards with little thermal deterioration in a small number of steps.

Claims (4)

[Claims] 1. By applying an electron beam curing type conductive paste to the side wall so that the central axis of the through hole provided in the insulating substrate is hollow, and then curing the conductive paste by irradiating electron beams from both sides of the substrate. A method for manufacturing a printed wiring board, comprising the steps of: obtaining electrical connection between conductive circuits on both sides of the substrate. 2. After applying an electron beam curing conductive paste containing a solvent to the side wall of the through hole of the insulating substrate, the solvent is removed in the first step, and then, in the second step, an electron beam is irradiated from both sides of the substrate. A method for manufacturing a printed wiring board, comprising the step of curing the paste to obtain electrical connection between conductive circuits on both sides of the board. 3. Electron beam curable conductive paste is applied to the side walls of an insulating substrate with conductive circuits formed on both sides so that the center axis of the through hole is hollow, and then an electron beam is irradiated from both sides of the substrate to form the above-mentioned A method for manufacturing a printed wiring board, characterized in that electrical connection between conductive circuits on both sides of the substrate is obtained by curing a conductive paste. 4. After applying an electron beam curable conductive paste containing a solvent to the side wall of the through hole of the insulating substrate, the solvent is removed in a first step, and then an electron beam is irradiated from both sides of the substrate in a second step. A method for manufacturing a printed wiring board, characterized in that electrical connection between conductive circuits on both sides of the substrate is obtained by curing the paste.