DE8626486U1 - Fuse for direct assembly of printed circuit boards - Google Patents

Description

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Patent Attorneys 1685.3 KÄ/Be

Wenzel & Kaiköff
Fläßkühle 6
PO Box 2448
5810 Witten/Rühf

Applicant: Wickmänh-Werke GmbH

5810 Witten/Ruhr

Designation: Fuse for direct

Assembly of printed circuit boards

The invention relates to a fuse for the direct assembly of printed circuit boards, with two connections and at least one fusible conductor encased in an extinguishing agent between the connections, in which the extinguishing agent and parts of the connections are surrounded by a plastic mass.

The protection of components on printed circuit boards or the entire circuit of a printed circuit board using fuses is generally desirable in order to limit damage in the event of short circuits and similar circuit errors so that other components are not affected.

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The fuses available for this purpose can be replaceable using a fuse insert, or the fuses are soldered in place, so that after their switching function a corresponding BaU group must be replaced or repaired.

The reduction in the size of the electronic components used to assemble printed circuit boards is not recorded in the same way. This is understandable because the switching-off function of a fuse is associated with a high energy surge, which often leads to the generation of an arc with a temperature of almost 3000° C, which must be extinguished in a very short time. The space or materials available for this purpose cannot be reduced at will if a certain electrical power is still to be switched off.

The smallest fuse type is a cylindrical body with a diameter of approx. 6 mm and a length of approx. 9 mm, with the two connections protruding from one end face*. In addition, a so-called picofuse is known, which is designed as an elongated cylindrical body with a length of approx. 7 mm and wire connections on both sides in the manner of small resistors.

Both types are suitable for assembling circuit boards if a type of circuit board is used in which the electrical components penetrate the board and are soldered from the underside. Another type of assembly is not possible in that the fuses themselves are manufactured using soft solder and thus wave soldering is excluded, in which the entire fuse would be exposed to excessive heat.

The miniaturization of electronic components mentioned above has now led to a printed circuit board form

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in which the components are on the same side of the circuit board as the connections and the soldering * In principle, two types can be distinguished, the so-called wave bath process and the reflow process. In the wave bath process, all components must be attached to the circuit board except by soldering, which is usually done by gluing. In an upside-down position, the entire circuit board is then passed over a solder bath wave and the individual connections are soldered. In the reflow process, the circuit board is essentially aligned horizontally and the components are pressed into small drops of solder paste that is previously applied to the contact points. * The circuit board then runs in the same , 15 layer through a heat curtain so that the solder paste runs to the solder. There are no regulations for these two processes.

corresponding fuses.

It is therefore the object of the invention to create a fuse of the type mentioned at the beginning, which is suitable for surface mounting for the wave bath or reflow^ process.

To solve this problem, the invention proposes that each connection is provided with a flat soldering surface, that both soldering surfaces lie within one and the same plane, and that both soldering surfaces are placed together and are sufficiently large so that the fuse has a stable equilibrium position when standing on the connections.

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The thermal balance of a fuse determines the intended characteristics and cut-off accuracy. The proximity of the fuse to the circuit board caused by the above-mentioned processes causes an increasing heating of the circuit board at this point, which emanates from the fuse and has a repercussive effect on it. As the thermal coupling to the circuit board can only be predicted imprecisely, as it is, among other things,

1 depends on the nature of the circuit board, the surrounding components and also on the solder load, it is difficult to comply with the values provided for the fuse. For particularly favorable cases, the given clearance i 5 is completely sufficient. Favorable conditions include, for example, the attachment of a fuse according to the invention to a circuit board in a very easily accessible and ventilated place. In the previously claimed form, the fuse is suitable for both

10 dfl.s suitable for both the reflow process and the wave bath process } in the latter case, a given surface element of one of the connections is used for the bondingj which can be located outside or inside the contact area of the circuit board at this point. It occurs ·■

15 merely ensures that the fuse is securely held in the overhead position according to the invention.

k especially for the reflow process it is advisable

I the connections that protrude from the plastic mass

ⁿ 20 ^{of the} fuse, bent into a J-shape so that the soldering surfaces are below the plastic mass.

I This achieves the most compact design. If

$ on the space requirement of the fuse according to the

I finding less attention must be paid to the

I;! 25 soldering surfaces are outside the plastic mass, so

' for example, so-called "gull wing" connections

£ can be used.

For further thermal insulation of the fuse 30 from the circuit board, a plurality of alternating lateral incisions can be provided at each connection in the section from the plastic mass to the soldering surface, which lead to a zigzag-shaped current path and thus heat path of the connections. In this way

35 In this way, a relatively long t connection is created in a compact design, which is thermally

J2 is favorable. The incisions must be as wide

I should be chosen so that no

5
Solder bridges can occur.

In addition to good thermal insulation, the connections
by such lateral incisions greatly reduced plastic
Formability. This property can be used to improve

the thermal decoupling between the fuse | and the circuit board can be exploited if, in the wave bath process, the fuse is first connected to the circuit board ■

plate must be glued. If the adhesive points

in the area of the plastic mass, their dimensions and adhesive strength should be as small as possible. \ After soldering in the manner described with the help of .!

of the wave bath process, the fuse is
the circuit board is lifted half to one millimeter,

so that the adhesive joint is destroyed. The connections are plastically deformed so that the soldering is preserved
remains and after this operation the fuse
is clearly thermally decoupled from the circuit board*

In the following, embodiments of the invention are described, which
shown in the drawing, explained in more detail; in the

Show drawing: >

Figure 1 is a side view of a fuse according to the invention with the &iacgr;

Plastic compound curved connections, j

&Iacgr; Figure 2 is a front view of the fuse |

according to Figure 1, }

Figure 3 is a view according to Figure 1 of a '

further implementation example for a
Schmelzsichej-i-ny according to the invention,

Fig. 4 is a plan view of another AUS-

Example of the invention in adhesive surfaces on the connections and

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Figure 5 is a view according to Figure 1 of another

Design example with J-shaped bent connections and an adhesive surface on the fuse body.

The embodiment of a fuse according to the invention shown in Figures 1 and 2 consists of a fuse body made of a plastic mass 1 which encloses the fuse element ends of two connections 2. Between the ends of the connections 2 there is a fuse element 3 _/ which is applied, for example, by bonding, i.e. by a type of cold pressure welding process. This process can be carried out for gold, silver, aluminum and silver alloys, provided that the fuse element and the connections have the same or a very similar material, possibly by plating. Around the fuse element there is a sheath made of an extinguishing agent 4 which creates both the cavities for receiving the melted fuse element 3 and the provision of extinguishing gas in the event of a shutdown.

The free ends of the connections are bent under the plastic compound 1. The facing away surfaces lie in one plane and form soldering surfaces 5, which are particularly suitable for the reflow process. The side areas of the connections are each provided with three alternating incisions 6, so that these areas have special properties. On the one hand, the thermal insulation of the actual fuse within the plastic compound 1 is achieved from the soldering surfaces 5 because the heat flow path is longer than without the side incisions 6, and on the other hand, this design offers the possibility of attaching the plastic compound 1 with the fusible conductor 3 located therein particularly easily at different heights to the circuit board. This can be done before or after soldering to the circuit board, but in particular after soldering.

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As a rule, every circuit board has a highest component that determines the installation dimensions under neighboring circuit boards or in housings. The fuse according to the invention can be moved away from it in each individual case at least to the level of the highest component of the circuit board, so that the thermal insulation from the circuit board is optimal due to the maximum distance. Of course, there are natural limits here that cannot be exceeded, but also do not need to be exceeded, since above a certain distance of the fuse from the circuit board the gain in thermal decoupling is negligible. The exemplary embodiment of a fuse according to the invention shown in Figures 1 and 2 is in reality approx. 10 mm long and 6 mm wide. If distances of 3 to 5 mm from the circuit board are achieved, sufficient thermal decoupling is present.

^The embodiment shown in Figure 3 is constructed in the same way inside the plastic compound 1. The connections 12 are, however, designed in the manner of a slight offset, so that the soldering surfaces 5 are located far outside the plastic compound 1. This design is particularly suitable for the wave bath process. Consequently, an adhesive point 9 is provided on the underside of the plastic compound 1, to which the fuse is glued to the corresponding circuit board (not shown) before passing through the wave bath. In thermal terms, this embodiment is only suitable for particularly favorable installation positions in which good ventilation of the fuse is guaranteed.

In the embodiment according to Figure 4, instead of the adhesive point 9 on the underside of the plastic mass, the respective end of the connections 22 is designed differently.

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&igr; point 10 is present, which can be in the insulation area of the circuit board or in the conductor area. It only

S pointed out that at this point a bonding before the

Soldering is possible. Due to the lack of connection between the plastic mass 1 and the circuit board underneath, the thermal decoupling is more favorable than in the embodiment shown in Figure 3, so that with this type correspondingly more difficult installation positions can be mastered.

10

In the embodiment according to Figure 5, an adhesive point 9 is again provided indirectly on the plastic compound 1, and there are also connections 32 which can be stretched in their side areas with the help of alternating lateral incisions 6. This elasticity is used to force the adhesive point 9 open again after the soldering surfaces 5 have been soldered to the corresponding circuit board. This later installation position is shown in dashed lines, in which the displacement of the entire fuse body compared to the initial position can be clearly seen. The side areas of the connections 32 are plastically deformed. Due to the twisted sections, breakage of the connections 32 is virtually impossible with these small deformations. Instead of the incisions 6, there can of course also be correspondingly multiple S-shaped connections, which also applies to the embodiment according to Figures 1 and 2.

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Claims (8)

Protection claims 1. Fuse for direct assembly of printed circuit boards, with two connections and at least one fusible conductor encased in an extinguishing agent between the connections, in which the extinguishing agent and parts of the connections are surrounded by a plastic mass, characterized in that each connection (2) is provided with a flat soldering surface (5), that both soldering surfaces (5) lie within one and the same plane, and that both soldering surfaces (5) together are placed and sufficiently large that the fuse has a stable equilibrium position when standing on the connections (5). 2. Fuse according to claim 1, characterized in that the soldering surfaces (5) are located substantially beneath the plastic mass (1). 3. Fuse according to claim 2, characterized in that each connection (2) emerges laterally from the plastic mass (1) and is bent in a J-shape. 4. Fuse according to claim 1, characterized in that the soldering surface (5) 4 14 Il 414 I) Il 14 4*4 4 «II are arranged outside the plastic mass (1), and that the common plane of the soldering surfaces (5) is at the same level as the underside of the plastic mass (1) or is lower than it. 5. Fuse according to one of the preceding claims, characterized in that each soldering surface (5) is subdivided into itself, i.e. consists of several surface elements. 6. Fuse according to claim 5, characterized in that at least one surface element (10) serves as an adhesive surface for carrying out wave soldering. 7. Fuse according to one of the preceding claims, characterized in that each connection (2) in the section from the plastic mass (1) to the soldering surface (5) is provided with alternating incisions (6) to form a long heat conduction path. 8. Fuse according to claim 3 and 7, characterized characterized in that the plastic mass (1) extends down to the level of the soldering surfaces (5) at least at one point and serves as an adhesive point (9) for carrying out wave soldering, and that after soldering the adhesive point (9) can be destroyed by lifting the plastic mass (1) from the circuit board with plastic deformation of the connections. 35