JPH1032372A - Protective structure of printed board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lessen a time required for melting a fuse so as to prevent a board and a device from being overheated by a method in which a part of a network is set thinner than the rest of the network. SOLUTION: A copper foil 5 at a heat release fusing spot 10 is reduced to 45 to 80% of the rest of the copper foil 5 in thickness by etching or mechanical processing. A solder 6 is filled up into a recess 7 formed by etching or the like. The copper coil 5 becomes small in sectional area and high in resistance at the heat release fusing spot 10 because it is made thin by etching or the like. Therefore, when an overcurrent flows through a network, heat concentrates on the heat release fusing spot 10. Tin contained in the solder 6 and the copper coil 5 are mixed into alloy which becomes quickly low in fusing point, so that the copper foil 5 is more quickly fused at the heat release fusing spot 10 than a case in which the copper foil 5 is not set as thin as prescribed. Therefore, a time required for melting the copper foil 5 at the heat release fusing spot 10 is more shortened with a reduction in the thickness of the copper foil 10, so that a board or a device mounted on it can be surely protected against overheating.

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 protection structure having a fuse function.

[0002]

2. Description of the Related Art A printed circuit board is a printed circuit board in which electrical wiring for connecting electrogram components is represented by wiring conductors on an insulating substrate by an appropriate method.
It has holes used for insertion and connection of component terminals, and the holes are plated with through holes to connect the upper and lower conductor layers of the insulating substrate. 2. Description of the Related Art Conventionally, when an overcurrent flows on a printed board, a circuit generates heat, and the board and mounted elements are overheated. As a countermeasure against these problems, a method of providing an abnormality detection circuit to cut off the supply of current, a method of providing a cooling mechanism in the heat generating portion, and a method of forming a heat generating portion on a separate substrate so as not to affect other portions. Methods and the like have been used. However, in any of the methods, since the size of the substrate itself is increased, it may be difficult to carry out the method for a device having a limited mounting position such as a vehicle. As another countermeasure, there is a technique of providing a fuse portion in a part of a circuit. For example, as shown in FIG.
It is like having a narrow width as 0. However, in such a printed circuit board protection structure, since the fuse is not melted unless the temperature rises to 1084 ° C., which is the melting point of copper, the board and the mounted elements are overheated during that time. As a method for solving the above problems, Japanese Patent Application Laid-Open No. 57-160192 and Japanese Patent Application Laid-Open
Japanese Patent No. 8792 discloses a technique in which a low melting point metal layer such as tin or solder is provided in a heat-blown portion of a circuit network. The purpose of this is to form an alloy using the interdiffusion between the low-melting-point metal and the conductor caused by the rise in the temperature of the heat-fused part when an overcurrent flows, and to reduce the melting point, thereby fusing quickly. It is. When tin having a melting point of 232 ° C. is used for the low melting point metal and copper foil is used for the conductor layer, alloying is 4
It occurs when the temperature exceeds 00 ° C, and is melted at about 600 ° C.

[0003]

However, even if this technique is used, the time until alloying and fusing is long, and during this time, the substrate or the element may be overheated. The present invention has been made in view of the above-described circumstances, and it is an object of the present invention to provide a printed circuit board protection structure capable of shortening the time required to blow a fuse and preventing overheating of a board or an element. Make it an issue.

[0004]

The above-mentioned problems are solved by a printed circuit board protection structure having the following features. That is, according to the present invention, in a printed circuit board protection structure in which a low melting point metal layer is provided in a part of a circuit network made of conductors constituting the printed circuit board, the thickness of the circuit network is partially reduced. It is characterized in that it is made thinner than the thickness of the other part of the net, and it is desirable that the thickness be reduced to 45 to 80% than the thickness of the other part. By reducing the thickness of a part of the circuit network and reducing the cross-sectional area, the resistance value of the heat fusing portion increases, and the heat generation increases. As described above, by concentrating the heat on the heat-fused portion, the alloy of the low-melting-point metal and the conductor, the melting point can be quickly reduced, and the fusing time is shortened. Therefore, overheating of the substrate and the element can be prevented. According to a second aspect of the present invention, in a printed circuit board protection structure including a low melting point metal layer in a part of a circuit network formed of conductors constituting a printed circuit board, an insulating substrate portion in contact with a lower surface of the circuit network is provided. It is characterized as a heat insulating layer. By providing a heat insulating layer, heat is concentrated on the heat-fused portion, and an alloy of a low-melting-point metal and a conductor, and the melting point is rapidly reduced. Since the fusing time is shortened, overheating of the substrate and the element can be prevented. According to a third aspect of the present invention, in a printed circuit board protection structure in which a low melting point metal layer is provided in a part of a circuit network made of a conductor constituting the printed circuit board, a part of the circuit network is curved or formed at an acute angle. It is characterized by being shaped. Since the portion having the changed shape generates the most heat, the alloy of the low melting point metal and the conductor and the lowering of the melting point are accelerated. Since the fusing time is shortened, overheating of the substrate and the element can be prevented.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION

[First Embodiment] A protection structure for a printed circuit board according to a first embodiment of the present invention will be described below with reference to FIGS. FIG. 12 shows a configuration example of the printed circuit board 20. The process of forming the printed circuit board 20 is as follows. A copper foil 5 as a conductor layer is attached on the insulating substrate 4. Masking is performed on a portion to be left as a circuit network, and the copper foil 5 other than the circuit network is removed by etching. After the removal, the upper surface is covered with the photoresist 3. After that, the photoresist 3 on the circuit network on which the element 1 is to be installed is removed, and the element 1 is placed at that location. The element 1 is temporarily fixed with the adhesive 11 and completely fixed with the solder 2. Between elements 1, solder 2 and copper foil 5 as a conductor layer
And current flows between the elements 1. Also, in order to prevent a short circuit, the element 1
Is covered with a coating agent 12. The thickness of the copper foil 5 is specified by JIS (Japanese Industrial Standards) as two types, 0.035 mm and 0.070 mm. Here, the thickness is set to 0.070 mm. Also,
As the insulating substrate 4, a phenol resin based on paper was used from among many usable substrates. FIG. 1 shows a copper foil 5 which is a circuit network connecting the elements 1.
FIG. 2 shows a longitudinal sectional view of a heat fusing section 10 formed in a part of the heat generating fusing section. FIG. 2 shows the relationship between the thickness of the copper foil 5 and the fusing time in the heat fusing portion 10 shown in FIG. The copper foil 5 in the heat fusing portion 10 is thinned to 45 to 80% of the thickness of other portions by etching or machining. The solder 6 is provided in the formed recess 7. The cross-sectional area of the copper foil 5 of the heat fusing portion 10 is reduced by thinning. Generally, the resistance R [Ω] of a conductor is represented by R =
It is represented by ρ * l / A. ρ is the specific resistance [Ωcm]
l is the length of the conductor [cm], and A is the cross-sectional area of the conductor [c
m ² ], which shows that the resistance value R increases as the cross-sectional area A decreases. Therefore, when an overcurrent flows through the circuit network, heat concentrates on the heat-fusing portion 10. When the heat is concentrated, the alloy and the lowering of the melting point quickly occur between the tin in the solder 6 and the copper foil 5, and the copper foil 5 is blown more quickly than in the case where the predetermined thickness is not set. As shown in FIG. 2, as the thickness of the copper foil 5 becomes thinner, the fusing time is shortened. However, when the thickness becomes 45% or less of the thickness of the other part of the circuit network, the continuous time is prolonged near the upper limit of the normally used current. There is a risk of fusing when energized. Also 8
At 0% or more, there is little effect on shortening the fusing time,
The conventional problem of overheating of the substrate will not be solved.

[Second Embodiment] A method of protecting a printed circuit board according to a second embodiment of the present invention will be described below with reference to FIGS. FIG. 3 is a vertical cross-sectional view of the heat fusing portion 10 formed on a part of the copper foil 5 which is a circuit network connecting the elements 1. FIG. 4 shows a comparison between the fusing time of the present embodiment and the conventional fusing time. Exothermic fusing section 10
Is formed on the insulating substrate 4 corresponding to the above, and a copper foil 5 is attached to the upper surface thereof. At this time, a concave shape corresponding to the convex portion 8 of the insulating substrate 4 is formed on the lower surface of the copper foil 5 in contact with the insulating substrate 4. It is desirable that the thickness of the copper foil 5 of the projection 8 is also set to 45 to 80% of the thickness of the other part of the circuit network. The solder 6 is provided on the thinned copper foil 5. When an overcurrent flows in the circuit network, heat concentrates on the copper foil 5 on the protrusion 8, so that an alloy and a low melting point quickly occur between tin and the copper foil 5 in the solder 6, and It melts faster than when no 8 is formed. As shown in FIG. 4, when the same material as that of the insulating substrate 4 is used for the convex portion 8, the fusing time is about 2 times less than the conventional case.
Can be reduced by half an hour. In addition to the method of making the insulating substrate 4 itself convex, there is a method of disposing a material having a large heat capacity, for example, alumina or the like on the insulating substrate 4. In this case, the length can be further reduced by slightly less than one minute as compared with the case where the same material as that of the insulating substrate 4 is used for the projection 8.

[Third Embodiment] A method for protecting a printed circuit board according to a third embodiment of the present invention will be described below with reference to FIGS. FIG. 5 is a vertical cross-sectional view of the heat fusing portion 10 formed on a part of the copper foil 5 which is a circuit network connecting the elements 1. FIG. 6 shows a comparison between the fusing time of the present embodiment and the conventional fusing time. Exothermic fusing section 10
An air layer 9 is formed on the insulating substrate 4 by machining or a sublimable substance or a resin that can be dissolved and removed at a low melting point.
Is formed, and a copper foil 5 is adhered on the upper portion. The solder 6 is provided on the upper surface of the copper foil 5 in the portion having the air layer at the bottom.
When an overcurrent flows in the circuit network, the air layer 9 plays a role of a heat insulating layer, so that heat concentrates on the copper foil 5 above the air layer 9, and between the tin in the solder 6 and the copper foil 5. The alloy and the lowering of the melting point occur quickly. Therefore, the fusing is performed earlier than in the conventional protection structure in which the air layer 9 is not provided. Instead of providing the air layer 9, a heat insulating member may be provided at this location.
As shown in FIG. 6, the provision of the air layer 9 can shorten the fusing time by about 3 minutes compared to the related art.

[Fourth Embodiment] FIGS. 7 to 10
A method for protecting a printed circuit board according to the fourth embodiment of the present invention will be described with reference to FIG. FIG. 7 shows a conventional view in which a heat-fused portion 10 formed on a part of a copper foil 5 which is a circuit network connecting the elements 1 is viewed from above. FIGS. 8 and 9 are views of the heat-fusing portion 10 of the present embodiment as viewed from above. FIG. 10 shows a comparison between the fusing time of the present embodiment and the conventional fusing time. The heat fusing portion 10 which was conventionally a straight line is formed so as to have a curved shape shown in FIG. 8 and an acute angle shape shown in FIG. 9, and the solder 6 is provided at the deformed portion. When an overcurrent flows through the network, heat concentrates on the curved or sharp corners. Due to the heat generation, an alloy and a low melting point are rapidly generated between the tin in the copper foil 5 and the solder 6, and the melting is quicker than in a straight line. As shown in FIG. 10, when the heat-fusing portion 10 is curved or made to have an acute angle, compared with a conventional straight line,
The fusing time can be reduced by about 1-2 minutes. Further, it has been found that the fusing time can be further shortened when the angle is made sharper than when it is curved. Claims 1 to 1 in the claims
The invention described in 3 can be applied not only to the heat-dissipating sections 10 having the same width as shown in FIG. 7, but also to the heat-dissipating sections 10 having a small width as shown in FIG. Note that the photoresist 3 on the heat-fused portion 10 according to the present embodiment is removed in advance.

[0009]

As described in detail above, according to the first to third aspects of the present invention, it is possible to concentrate heat on the heat-fused portion. Formation occurs quickly, and the fusing can be performed earlier than before. Therefore, overheating of the substrate and the mounted element can be reliably prevented.

[Brief description of the drawings]

FIG. 1 is a view showing a vertical cross-sectional view of a heat fusing portion according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between a thickness of a copper foil and a fusing time in a heat fusing portion shown in FIG. 2;

FIG. 3 is a view showing a vertical cross-sectional view of a heat fusing portion according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a comparison between a fusing time according to a second embodiment of the present invention and a conventional fusing time.

FIG. 5 is a view showing a longitudinal sectional view of a heat fusing portion according to a third embodiment of the present invention.

FIG. 6 is a diagram showing a comparison between a fusing time according to a third embodiment of the present invention and a conventional fusing time.

FIG. 7 is a diagram showing a heat-fused portion of a conventional printed circuit board viewed from above.

FIG. 8 is a view of a heat-fused portion according to a fourth embodiment of the present invention as viewed from above.

FIG. 9 is a view of a heat-fused portion according to a fourth embodiment of the present invention as viewed from above.

FIG. 10 is a diagram showing a comparison between a fusing time according to a fourth embodiment of the present invention and a conventional fusing time.

FIG. 11 is a view showing a conventional narrow heat fusing portion.

FIG. 12 shows a configuration example of a printed circuit board.

[Explanation of symbols]

 4 Insulating board 5 Copper foil 6 Solder 9 Air layer

Claims (3)

[Claims] In a protective structure for a printed circuit board, wherein a low melting point metal layer is provided in a part of a circuit network made of conductors constituting the printed circuit board, the thickness of a part of the circuit network is changed to another part of the circuit network. A printed circuit board protection structure characterized by being thinner than the thickness. 2. A printed circuit board protection structure in which a low melting point metal layer is provided on a part of a circuit network made of a conductor constituting the printed circuit board, wherein an insulating substrate portion in contact with a lower surface of a part of the circuit network is defined as an insulating layer. A protective structure for a printed circuit board. 3. A printed circuit board protection structure in which a low melting point metal layer is provided in a part of a circuit network made of conductors constituting the printed circuit board, wherein the part of the circuit network is curved or acute-angled. The printed circuit board protection structure.