JPH02298096A - Circuit board for mounting electronic parts provided with insulating moisture-proof film - Google Patents

Abstract

PURPOSE:To prevent the peeling of an insulating moisture-proof film in spite of a continuous use under severe conditions and to improve the strength of a coating around lead pins by coating both surfaces of a circuit board including electronic components with a soft synthetic resin insulating moisture-proof film and further laminating a hardened film made of a resin of ultraviolet ray hardening type while covering projected front ends of the lead pins on the surface of the circuit board. CONSTITUTION:Both surfaces of a circuit board 1 mounting electronic component 2 by utilizing lead pins 3 are coated, including the electronic component 2, with a synthetic resin insulating moisture-proof film 5 whose Young's modulus is 25kgf/mm<2> or below under a tension speed 1mm/min. and 0 deg.C and whose stretching at the time of broke is 50% or more. On the surface of circuit board 1 from which front ends of lead pins 3 are projecting, a hardened film 6 made of a resin of ultraviolet ray hardening type is laminated while covering at least the front ends of lead pins 3. As a result, particularly, the excessive concentration of stress on the lead pins is restrained. In addition, a follow-up property for the deformation of the circuit board itself can be attained and also the complete covering of the front edge parts of pins can be attained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a circuit board mounted with electronic components sealed with an insulating moisture-proof film.

[Conventional technology]

Circuit boards equipped with electronic components such as ICs and LSIs are often used in computers, OA equipment, etc., but recently, circuit boards equipped with these electronic components are being used for the above-mentioned applications where they are deployed in a well-prepared environment. It is expanding not only to the fields of computers and OA equipment, but also to fields such as home electronics, automobiles, toys, and even industrial robots.

In such household electronic devices, ICs and LSIs are often deployed and used in harsh environments, rather than being deployed in a sufficiently prepared environment like computers and office automation equipment. For this reason, in order to protect circuit boards on which electronic components are mounted from sudden changes in temperature, humidity, condensation, insects, dust, etc., improve their reliability, and maintain the functionality of electronic devices, circuit boards on which electronic components are mounted are The entire board, including electronic components, is coated with an insulating and moisture-proof film.

[Problem that the invention seeks to solve]

As this type of insulating and moisture-proof coating, a multilayer structure coating using a combination of resin such as epoxy resin, urethane resin, butadiene resin, and silica powder or synthetic fiber has been proposed (Japanese Patent Laid-Open No. 12789/1989). In addition, when forming such a multilayer structure film, there is a problem in terms of production efficiency, so paints made of acrylic resin, alkyd resin, phenol resin, amine resin, etc. that can be formed into a single layer film are used. A method of forming a film using a type of sealant has also been proposed. However, the sealing materials used in both of the above proposed methods do not provide sufficient sealing, so when the obtained circuit board with an insulating and moisture-proof film is incorporated into an electronic device, the area around the lead pins may become damaged due to continued use. Problems have arisen in that the coating peels off from the lead pins, creating voids, and cracks and cracks occur in the insulating and moisture-proof coating. In some cases, the insulating moisture-proof coating may partially peel off from the board surface of the circuit board, and the moisture that has entered through the peeled part may cause short circuits on the board, or
A problem arises in that poor sealing on the sharp edges of the lead pins can lead to short circuits and other damage to the circuit board.

This invention was made in view of these circumstances, and the insulating moisture-proof coating does not peel off even after continued use under harsh conditions.In particular, the area around the lead pins is sufficiently covered, and the coating is The purpose of the present invention is to provide a circuit board on which electronic components are mounted with an insulating moisture-proof film that is easy to form.

[Means for solving problems]

In order to achieve the above object, the electronic component mounting circuit board with an insulating moisture-proof film of the present invention has an electronic component mounted circuit board that uses lead pins to mount electronic components, with both surfaces of the circuit board containing electronic components. Young's modulus at 0°C2 tensile speed 1-/min is 25 kgf/mm2 or less, and elongation at break is 50%.
A cured film of an ultraviolet curable resin is laminated on the substrate surface of the circuit board which is covered with the above synthetic resin insulating moisture-proof film and from which the tips of the lead pins protrude, covering at least the tips of the lead pins. The structure is that

[Effect]

As a result of repeated research to overcome the above-mentioned problems, the present inventors discovered that there is a problem with the high hardness specification of the conventional insulation and moisture-proof treatment layer. That is, until now, it has been believed that the higher the hardness of the film, the more advantageous it is for the sealing material for electronic component mounting substrates, as is the case with the sealing materials for semiconductor chips, in terms of film strength. Therefore, for example, even with the above-mentioned conventional paint-based sealants, the target is a hardness comparable to that of a multilayer structure film using epoxy resin or silica powder, and a Young's modulus (0°C) of at least 50 kgf. /mm
The goal has been to form a highly hard film whose elongation at break does not exceed 30% at most.

However, the research conducted by the present inventors has revealed that, due to the high Young's modulus of the high-hardness insulating moisture-proof coating,
The lead pin may be bent due to stress concentration due to thermal expansion and contraction of the film caused by the operating atmosphere, and the bending stress may cause the solder to peel off and the lead pin to lift up and come off, or due to the low elongation of the film, It has become clear that cracks and peeling occur due to insufficient ability to follow the deformation of the substrate due to changes or external forces. Therefore, it has been found that a soft type of film is more advantageous. In addition, the sealing materials used in both of the conventional proposed methods cannot completely cover the sharp edges of the lead pins, and this coverage has been a problem. C9 A flexible synthetic resin insulating and moisture-proof coating that breaks the conventional technical wisdom of having a Young's modulus of 25 kgf/mm" or less at a tensile speed of 1 ma+/min and an elongation at break of 50% or more, which covers circuit boards and electronic components. In particular, if the area around the lead pin is coated with a cured film of ultraviolet curable resin that can be attached to the tip of the lead pin and cured instantly, the coating on the tip of the lead pin will be coated with the above-mentioned soft coating. We have discovered that when combined with an insulating and moisture-proof coating made of synthetic resin, it becomes completely perfect.In other words, by making the insulation and moisture-proof coating a coating with the characteristics described above,
In addition, by coating the lead pins in a laminated manner with a cured film of ultraviolet curable resin, excessive stress concentration on the lead pins, especially in electronic components mounted on the circuit board, is suppressed, and the ability to follow the deformation of the circuit board itself is also achieved. Moreover, complete coverage of the pin tip edge is also achieved.

Next, this invention will be explained in detail.

FIG. 1 is a partial cross-sectional view of an example of the present invention. That is, 1 is a circuit board, 2 is an electronic component such as an IC or LSI mounted on the circuit board, and 3 is a lead pin thereof. This lead pin 3 is inserted into a mounting hole 4 provided on the circuit board 1, and its tip protrudes from the back surface of the board 1.

5 is an insulating moisture-proof film that covers both sides of the circuit board l including the electronic components 2, and 6 is an ultraviolet ray film laminated to cover the tips of the lead pins 3 protruding from the back side of the board 1. It is a cured film of curable resin. As shown in FIG. 2, the cured film 6 of the ultraviolet curable resin may cover only the tip portion of the protruding lead pin.

The synthetic resin insulation moisture-proof coating covering both substrate surfaces is 0°
Young's modulus at C2 tensile speed im/min is 25 kgf/
mm" or less, preferably 0.1 to 20 kgf/mm",
More preferably, the film exhibits a Young's modulus of 2 to 17 kgf/mm" and an elongation at break of 50% or more, preferably 100 to 3000%. If this value is not met, the film is Similar to the sealing material film in the above, excessive stress concentration on the lead pins or cracking or peeling due to insufficient followability to the substrate may lead to the problem of circuit breakdown of the substrate.

A film satisfying such numerical conditions can be formed.

Representative examples of synthetic resins (including rubber) include butadiene rubber, isoprene rubber, butyl rubber, 2-ac-1-nor resin, vinyl resin, urethane resin, etc., which may be used alone or in combination. The above synthetic resin must not only satisfy the above numerical conditions, but also satisfy the necessary insulation properties, moisture resistance, cleanliness (no inclusion of miscellaneous ions involved in corrosion), and adhesion to substrate electronic components, etc. It has been demanded. therefore,
When the above-exemplified synthetic resin alone cannot satisfy these conditions or when even better properties are desired, the above-exemplified synthetic resin may be subjected to a modification treatment as necessary. Examples of such modification treatments include graft polymerizing butadiene rubber with acrylic alkyl esters or vinyl monomers, blending styrene resin with the graft polymer, or copolymerizing acrylic acid with ethyl acrylate. Examples include grafting processing, blending processing, etc. In addition, other components such as a tackifier may also be blended with the synthetic resin. In this case, the blending amount should be 300 parts or less based on 100 parts by weight (hereinafter abbreviated as "parts") of the synthetic resin, considering the physical properties of the synthetic resin and the Young's modulus and elongation of the resulting film. Such a synthetic resin, which is preferably set, is applied to both surfaces of the circuit board to form a film by an appropriate film processing method such as a dipping method, a potting method, or a spraying method. In this case, the thickness of the film is 5 to 2000 μm, especially 10 to 10 μm.
From the viewpoint of effectiveness, it is preferable to set the thickness to 00 μm. The coating is coated once or twice or more to form a single layer film of a predetermined thickness.

Even if both sides of the circuit board are coated with a synthetic resin insulating moisture-proof film that meets the above numerical conditions, if the sealing properties at the tips of the lead pins are incomplete, a short circuit will occur from that area, and the circuit board will be destroyed. lives here. Therefore, a cured film of ultraviolet curable resin is formed. As such an ultraviolet curable resin, commercially available ultraviolet curable resins can be used. Examples of the prepolymer include polyester acrylate, polyurethane acrylate, epoxy acrylate, polyethyl acrylate, oligoacrylate, alkyd acrylate, and polyol acrylate. Furthermore, examples of photopolymerizable monomers include monofunctional acrylates. As the photopolymerization initiator, radical reaction type and ion reaction type are used, such as acetophenones, benzophenone, Michler's ketone, benzyl, benzoyl, benzoin ether, benzyl dimethyl ketal,
Examples include tetramethylthiuram monosulfide, thioxansones, and azo compounds. In addition, various additives such as storage stabilizers, fillers, and viscosity modifiers are used in combination with the solvent 1.

In the electronic component-mounted circuit board with an insulating moisture-proof film of the present invention,
A film is formed using the above raw materials, for example, in the following manner. That is, a soft synthetic resin insulating moisture-proof film is coated on both sides of the circuit board including the electronic components by the method described above, and this is immersed in an ultraviolet curable resin. In this case, the protruding portions of the lead pins may be immersed, or the entire substrate may be immersed. next,
After such immersion treatment, it is immediately cured by ultraviolet irradiation. As a result, the tip edge of the ribbon is completely covered, and as a result, the electronic component mounting circuit board with the insulating and moisture-proof coating of the present invention is obtained. Of course, the same effect as described above can also be obtained by forming a soft synthetic resin insulation moisture-proof coating after treating the pin tip edge portion of the lead pin with an ultraviolet curable resin.

(Effects of the Invention) As described above, in the electronic component-mounted circuit board with an insulating moisture-proof film of the present invention, both surfaces of the circuit board include electronic components.
・It is coated with a soft synthetic resin insulation moisture-proof coating that is completely different from conventional insulation moisture-proof coatings, and a cured coating of ultraviolet curable resin is layered on the surface of the circuit board, covering the protruding tips of the lead pins. Therefore, the soft synthetic resin insulating moisture-proof coating alleviates stress concentration on the lead pin portion of the electronic component, and at the same time effectively prevents peeling of the insulation moisture-proof coating due to insufficient response to expansion and contraction of the board. Furthermore, the edges of the lead pins are completely covered. Therefore, circuit damage such as short circuits and connection failures is less likely to occur, and reliability is excellent. Further, the above-mentioned insulating moisture-proof coating has a single layer structure and is easy to form.

Next, examples will be described.

[Example 1] <Synthesis of sealing material for insulating moisture-proof film> In toluene, 80 parts of butadiene rubber was graft-polymerized with 20 parts of methyl methacrylate, and the obtained rubber was further graft-polymerized with 35 parts of styrene. The resulting polymer was subjected to demonomer treatment under reduced pressure (1 mHg) at 130° C. for 1 hour to obtain a sealing material.

<Synthesis of ultraviolet curable resin> Bisphenol A epoxy resin (Epicote 1002
, manufactured by Yuka Shell Co., Ltd.) 640 parts, acrylic acid 68.4 parts,
0.3 parts of triethylamine was charged into a 1i flask equipped with two condensing tubes, a stirrer, and a heating device, and the reaction was carried out at 120°C for 4 hours to produce an epoxy acrylate having an acid value of 10 or less. Next, 30 parts of the above epoxy acrylate, 20 parts of dipentaerythritol hexaacrylate, 20 parts of tetrahydrofurfuryl acrylate, and 10 parts of benzyl dimethyl ketal were mixed at a temperature of 60°C for 1 hour to obtain a uniform acrylic resin-based ultraviolet curing mold. Turn fat into resin.

1. Formation of an insulating moisture-proof film) Using toluene as the sealing material. A toluene solution with a concentration of 30% by weight was prepared. Then, the phenol board on which the bottle insertion type LSI was mounted was immersed in the above toluene solution, and an insulating moisture-proof film containing the LSI was deposited on both surfaces of the circuit board with a thickness of about 150 μm. Subsequently, this was dried with hot air at 60° C. for 15 minutes, and then the lead pin protruding side of the circuit board on which the insulation and moisture-proofing coating was formed was immersed in the ultraviolet curable resin liquid and pulled out. Next, use a lkw mercury lamp (80w/cm) at 15c.
Ultraviolet irradiation was performed for 20 seconds from a distance of m, then 60°
After curing for 30 minutes at C, the intended electronic component-mounted circuit board with an insulating and moisture-proof film was obtained.

[Comparative Example 1] Copolymerization was performed using 100 parts of methyl acrylate and 10 parts of acrylic acid in toluene, and the resulting polymer was subjected to demonomer treatment according to Example 1 to obtain a sealing material.

This sealing material was applied to the circuit board with a thickness of 150 μm as in Example 1.
As a second step, the same ultraviolet curable resin as in Example 1 was coated to form an insulating and moisture-proof film.

[Comparative Example 2] A film was formed on both sides of a substrate using only the same sealing material used in Example 1. However, immersion in the ultraviolet curable resin was omitted.

[Comparative Example 3] A film was formed on both sides of a substrate using only the same ultraviolet curable resin used in Example 1. However, the formation of a film using a sealing material prior to the film formation using an ultraviolet curable resin was omitted.

The following evaluation tests were conducted on the electronic component mounting circuit boards with insulating and moisture-proof coatings obtained in the above Examples and Comparative Examples.

〔Evaluation test〕

(Young's modulus, elongation) A toluene solution of the above film forming material was cast onto a table to a thickness of 1
A film of 00 μm was prepared, and the tensile speed of 0” C1 was 1 k//.
Young's modulus and elongation at break were measured.

Heat Cycle Resistance〉 The circuit board was subjected to a heat cycle test between -30°C and 80°C, and changes in the board were examined after the rapid temperature change was repeated 20 times. Alternatively, the case where cracks occurred in the insulation moisture-proof film was marked as ×.

<Lead pin tip coverage> 13. Between one set of pins of LSf mounted on the circuit board.
Apply 5DCV, immerse in 3% salt (phenolphthalein), and check the presence or absence of leakage current at the tip of the pin by the presence or absence of color development (reddish-purple) in the solution.If no color develops, it is set as O, and if color develops, check for leakage current at the tip of the pin. is ×.

The above results are shown in Table 1.

As is clear from the table above, the example product has good heat cycle resistance and lead pin tip coverage, and even after repeated use under harsh conditions, the insulation and moisture-proof coating does not peel off, resulting in long-term reliability. It can be seen that it is rich in

[Brief explanation of drawings]

FIG. 1 is a partial cross-sectional view of one embodiment of an electronic component mounting circuit board with an insulating moisture-proof film of the present invention, and FIG. 2 is a partial cross-sectional view of another embodiment. l...Circuit board 2...Electronic component 3...Lead pin 5...Insulating moisture-proof coating 6...Ultraviolet curing resin cured coating

Claims (1)

[Claims] (1) Both surfaces of the circuit board on which electronic components are mounted using lead pins are heated to 0°C with the electronic components included.
Young's modulus at a tensile speed of 1 mm/min is 25 kgf/m
m^2 or less and an elongation at break of 50% or more, the substrate surface of the circuit board is coated with a synthetic resin insulating moisture-proof coating having an elongation at break of 50% or more, and at least the tips of the lead pins are coated on the board surface of the circuit board from which the tips of the lead pins protrude. A circuit board equipped with an electronic component equipped with an insulating moisture-proof film, characterized in that a cured film of an ultraviolet curable resin is formed in a laminated state.