JP2001111214A - Method of forming protective film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problems in prior art where all small holes of many printed boards having many small holes, especially through-holes, e.g. BGA, CSP, F/CBGA boards must be planarized uniformly in short time to meet the today's demands for making lighter, thinner, more compact, high-density and multilayer products, and where the conventional methods are inferior in quality, yield and cost. SOLUTION: After sandwiching a board having holes filled with a solder resist between an upper and lower resin layers from which solvents have been removed by heating, flat solder resist films with no rise are formed by the press method, required portions are cured by UV, unwanted uncured portions are developed and removed, the solder resist is heat set to fill up the holes, and at the same time, patterns are formed to form protective films.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a printed circuit board used for electronic equipment, electric equipment, computers, communication equipment and the like. Furthermore, the present invention relates to a method for forming a protective film on a printed circuit board.

[0002]

2. Description of the Related Art In a known method, a solder resist is applied to the entire surface of a printed circuit board by a printing method, cured by exposing to ultraviolet light, and then dissolving an unnecessary portion of the solder resist with an alkali to fill through holes. It is carried out. In this method, the cured solder resist becomes convex or concave at the location of the through-hole and does not have a uniform surface.Also, bubbles are accumulated inside the solder resist film, and the solder resist film is peeled off and the hole cannot be filled. is there.
Generally, it was used in this condition, but according to recent demands for lightness and size reduction and improvement in quality accuracy, air bubbles accumulate on the flat solder resist surface without through-holes or depressions in through-holes or inside of through-holes. There is a need to fill in the gaps. In order to respond to the request, after applying the solder resist, heating or applying vibration to flatten the coated surface of the solder resist, removing bubbles by leaving under reduced pressure, ultraviolet curing, mechanically ( For example, the raised portion is removed by buffing to make it a flat surface. In any case, such a method is reaching its limits both in terms of quality and productivity.

[0003]

According to the demands of the era of lightness, shortness, high density, and multi-layer, a printed circuit board having a large number of pores, especially a through-hole (eg, a BGA board, a CSP board, an F / CBGA) Substrate), it is necessary to uniformly fill all the pores of a large number of substrates in a short time and flatten it. Conventional methods have problems in terms of quality, yield, and cost. There is a demand for the development of a technology that solves this problem.

[0004]

According to the present invention, in a final step of forming a printed circuit board, a resin layer obtained by evaporating a solvent by heat from a solder resist is applied to a printed board in which solder resist is filled in through holes by a printing method. This is a method of forming a flat protective film with no or little swelling, which does not depend on the thickness of the filling material, by pressing with a press method. A pattern is formed by exposure to ultraviolet light and development and removal of unnecessary portions, and the pattern is thermally cured to form a solder mask film. The resulting product is a printed circuit board protected by a uniform and flat solder mask film.

According to the present invention, in forming a protective film using a solder mask on a printed board, (1) filling a through-hole of the printed board with a solder resist ink by a printing method, and (2) after applying the solder resist ink to a flat plate ,
A solvent layer is formed by evaporating the solvent from the solder resist ink. (3) Pressing the resin layer on a printed circuit board. (4) A method of forming a protective film by ultraviolet exposure, development, and heat curing. The present invention is a method for forming a protective film by forming a resin film by evaporating a solvent from a photocurable solder resist by heat. The present invention is a method for forming a protective film in which a solder resist is a photocurable epoxy-modified acrylic resin or a photocurable acrylic-modified epoxy resin.

[0006]

Embodiments of the present invention will be described below in detail. The board with through holes used in the present invention is described in "Printed Circuit Technology Handbook" (edited by the Printed Circuit Society)
The reference was made. The substrate having a through hole according to the present invention is specifically a BGA substrate, a CSP substrate, an F / C-
A BGA substrate.

The printed circuit board used in the present invention is a commercially available copper foil having a thickness of 5 to 70 μm and an insulating base material such as epoxy resin, polyimide resin, bismaleimide triazine (BT).
Resin, PPE resin, or a prepreg impregnated with glass fiber, glass cloth, or paper impregnated with the resin was overlaid to produce a copper foil double-sided board or a multilayer board. The thickness of the substrate is 0.05 to 2.4 mm. Next, through holes were formed with a drill or a laser, and conduction was ensured by plating.

In the present invention, a printed circuit board filled with a solder resist is produced by a printing method. The solder resist used in the present invention is a photo-curable resin, such as an epoxy-modified acrylic resin and an acrylic-modified epoxy resin. It is generally diluted with a solvent to adjust the viscosity of the solder resist. As a commercially available product, for example, PSR-4000 manufactured by Taiyo Ink Manufacturing Co., Ltd. is used.

The method of forming a resin layer according to the present invention employs screen printing, a roll coater, a spray coater, a curtain coater, and a dip coater. The solvent is evaporated from the solder resist applied to a flat plate (for example, a stainless steel plate or a Teflon plate) by heat to form a solder resist layer without a solvent (hereinafter, referred to as a resin layer). Specifically, conditions suitable for evaporating the solvent from the solder resist are performed at 70 to 90 ° C. and a reduced pressure of normal pressure to 100 hPa. If the temperature exceeds 90 ° C., the solder resist partially decomposes or undergoes a polymerization reaction. If the temperature is lower than 70 ° C., the rate of solvent evaporation is extremely slow or the solvent does not evaporate, which is not preferable.
The resin layer is applied so as to have a thickness of 20 to 100 μm. If the thickness is less than 20 μm, the adhesion to the solder resist filled in the surface of the substrate or the through-hole is poor. If the thickness is more than 100 μm, the bubbles in the through-hole do not escape, and there is a problem that air bubbles easily accumulate.

A flat solder resist film can be formed on the substrate of the present invention by pressing the resin layer from above and / or below the substrate surface with a press molding machine. That is, by applying a printing method first, a light-curable solder resist in which a solvent has been evaporated is pressure-bonded to the substrate in which the through-holes have been filled, thereby forming a smooth and well-adhered solder resist film. The conditions for pressing the resin layer are as follows:
By setting the resin temperature to 50 to 100 ° C., a suitable fluidity (for example, 20 to 50 centipoise at 100 ° C.) can be provided without decomposing or polymerizing the solder resist in which the solvent has evaporated. If the temperature is higher than 100 ° C., the solder resist is decomposed or polymerized, and thus undesired portions cannot be developed and removed in a subsequent step, which is not preferable. In the operation of the press, the atmosphere of the press can be reduced from normal pressure to reduced pressure (for example, ≦ 50 mmHg). Considering workability and productivity, a large number (for example, 5
(Up to 20 sheets) can be overlapped and pressed. The pressing pressure is ² to ²⁰ kg / cm ² and the time is 3 to
30 minutes.

The pattern formation of the present invention is performed by ultraviolet irradiation and development. A necessary portion on the printed board is irradiated with ultraviolet rays (for example, 365 nm) to be cured, and an unnecessary portion is developed and removed with an alkali solution (for example, an aqueous solution of sodium carbonate) to form a pattern.

In the last step of the present invention, a solder resist pattern obtained by curing with ultraviolet rays is thermally cured to produce a completely cured product. Thermal curing conditions are 120-
By heating at 180 ° C. for 60 to 120 minutes, a cured protective film is formed. With this method, a printed board having through holes completely filled and having a flat protective film can be manufactured. Unlike conventional products, the printed circuit board protected by the solder resist obtained by this method completely fills through holes (no bubbles in the through holes) and has good surface smoothness (specifically, Is
The fluctuation width of the surface irregularities is about 0 to 15 μm), and the adhesion between the substrate and the solder resist film (for example, a stud pull test or a cellophane test) is good.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. The BGA substrate used in the present invention is 12 μm
Copper foil BT resin double-sided board (Mitsubishi Gas Chemical Co., Ltd. product;
It is a collective substrate having copper-plated through-holes (inner diameter: 0.2 mm, 250 holes in a 3 cm square) or holes plated with copper (thickness: 0.4 mm).

FIG. A photo-curable solder resist (manufactured by Taiyo Ink Mfg. Co., Ltd .: PSR-4000) is filled into the through-hole portion of this assembly substrate by a screen printing method. In addition, the photocurable solder resist was applied to the surfaces of two stainless steel plates for pressing by a curtain coater method. The collective substrate and the two stainless steel plates were heated at 80 ° C. for 10 minutes to form a solder resist in which the solvent was evaporated. Place the collective substrate on the stainless steel plate on which one resin layer is formed, place the other stainless steel plate of the resin layer upside down, and overlap the board surface and the resin surface

FIG. 2 shows a press machine (1) while heating (90 ° C.)
(0 Kg / cm ² for 10 minutes) to form a solder resist film. Subsequently, a necessary portion was light-cured by irradiating ultraviolet rays, and the unnecessary portion was dissolved and removed with a 1% aqueous sodium carbonate solution.
Subsequently, the solder resist film was placed in a dryer and heated at 150 ° C. for 60 minutes to completely cure the solder resist film, thereby forming a protective film.

FIG. The cured solder resist film had a smooth surface over the entire surface, the thickness of the solder resist on the base resin was about 30 μm, and there were no irregularities easily generated above and below the through holes. Also, in order to check the filled state inside the 50 through holes, the through hole was cut with a cutting machine and the filled state of the solder resist on the cross section was visually observed. Could not be confirmed.

[0014]

According to the present invention, by filling all the pores in a short time and forming a smooth solder resist film on a printed circuit board having a large number of pores, especially through holes, the quality and the yield can be improved. However, by manufacturing a substrate that is also advantageous in terms of cost, it is possible to meet the demands of the age of lightness, shortness, high density, and multi-layering.

[Brief description of the drawings]

FIG. 1 BGA aggregate substrate with copper plated through holes or holes

FIG. 2 shows a stainless steel plate having a resin layer formed on both sides of the plate.
And layout drawing to press from below

FIG. 3 shows a printed circuit board having a smooth solder resist film filled in a through hole.

FIG. 4 is a printed circuit board having a solder resist film having voids and irregularities inside through holes.

[Explanation of symbols]

 1: Insulation layer 2: Copper pattern 3: Copper plated through hole 4: Hole 5: Resin layer 6: Stainless steel plate 7: Solder resist film 8; Pressing direction by press 9: Cavity 10: Unevenness of solder resist film

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5E314 AA25 AA27 AA32 AA33 BB11 BB12 BB13 CC06 DD06 DD07 FF01 GG24 5E317 AA24 BB01 BB11 CD21 CD23 CD27 CD31 CD32 GG16

Claims (3)

[Claims] In the formation of a protective film using a solder mask on a printed board, (1) filling a through hole of the printed board with solder resist ink by a printing method; and (2) applying the solder resist ink to a flat plate,
Forming a resin layer by evaporating the solvent from the solder resist ink. (3) Pressing the resin layer on a printed circuit board. (4) A method of forming a protective film, which comprises performing ultraviolet exposure, development, and heat curing. 2. The method according to claim 1, wherein the resin layer is formed by evaporating the solvent by heating the photocurable solder resist. 3. The method according to claim 2, wherein the solder resist is a photocurable epoxy-modified acrylic resin or a photocurable acrylic-modified epoxy resin.