Patent Application
20110051388
One aspect of the invention relates to a method for forming a fuse which electrically connects two metal surfaces that are arranged on a printed circuit board next to each other and spaced apart from each other. Furthermore, another aspect of the invention relates to a printed circuit board with such a fuse.
In the event of a fault, circuits arranged on printed circuit boards can result in extreme overtemperatures. To be protected, circuits are, therefore, usually equipped with fuses, temperature switches, current-limiting PTC elements, or similar components.
The present invention aims at presenting a way of reliably achieving as cost-effectively as possible and with as low space requirements as possible that, in the event of a fault, a load current can be interrupted or, at least, be reduced as far as necessary to prevent secondary damage.
This problem is solved by a method according to the invention comprising the feature presented in Claim 1 as well as by a printed circuit board with a fuse according to Claim 8. Advantageous further developments of the invention are the subject matter of subordinate claims.
A fuse according to the invention bridges a gap between two metal surfaces with soft solder material such that an electrical contact can be established between the two metal surfaces, said metal surfaces being arranged on the printed circuit board next to each other. Therein, the soft solder material covers only a part of each of the metal surfaces. A further part of the metal surfaces in the environment of the soft solder material forms receiving regions which receive molten solder material when the fuse responds. The interfacial energy between the solder material and the receiving regions is lower than the interfacial energy between the solder material and the printed circuit board surface between the two bridged metal surfaces. When the solder material fuses, the receiving regions are, therefore, wetted with liquid solder material, with the result that solder material flows off from the printed circuit board region between the two metal surfaces and the electrical contact formed by the solder material is interrupted.
In the method according to the invention, the receiving regions are formed by covering the corresponding regions of the two metal surfaces with a solder-resistant protective coating prior to applying the solder material. Subsequently, liquid solder material which bridges the gap between the two metal surfaces is applied onto the partial surfaces that are not covered by the protective coating. After the solder material has solidified, the protective coating in the environment of the solder material is removed, with the result that receiving regions are produced that can be wetted with molten solder material.
Metallized surfaces of a printed circuit board, for example, metal surfaces that are made of copper, can be wetted with solder material much more easily than customary synthetic resin surfaces of printed circuit boards. The interfacial energy between the solder material and the synthetic resin, in particular epoxy resin, is therefore higher than the interfacial energy between the solder material and a metal surface, in particular copper. When it fuses, the solder material, therefore, flows off from the printed circuit board region between the two metal surfaces and wets the receiving regions.
In order to improve the wettability of the receiving regions, these regions are covered with flux, preferably at least in part. Suitable fluxing agents are, in particular, fluxing agents that are based on natural or modified resins, for example, rosin, to which activation additives, such as acids, more particularly stearic acid, salicylic acid and/or adipic acid, may be added. Such fluxing agents are, for example, called F-SW 31, F-SW 32, F-SW 33, or F-SW 34. It is also possible to use fluxing agents which contain zinc chloride and/or ammonium chloride if they are provided in an organic preparation, for example, higher alcohols or fats.
In order to cause the contact between the two metal surfaces to be interrupted when the solder fuses, it is actually sufficient to provide a receiving region or reservoir on one side of the bridged gap. Preferably, however, receiving regions are provided on either side of the gap that is bridged by the solder material. That means that the two metal surfaces each form a contact region that is covered with solder material as well as a receiving region.
Preferably, the at least one receiving region surrounds the solder material, for example, in the form of a U or a C. This allows achieving that liquid solder material can wet the receiving regions particularly rapidly. This is to advantage in that the fuse can respond with corresponding rapidness.
Together, the receiving regions are, preferably, at least as large as the printed circuit board region between the two metal surfaces that is covered with solder material. This measure is to advantage in that solder material covering the printed circuit board between the two metal surfaces can migrate into the receiving regions almost completely when the cut-out responds.
According to common linguistic usage, a soft solder material is to be interpreted as a solder material the melting point of which is less than 450° C. Preferred use is made of solder material with a considerably lower melting point, for example, less than 250° C., more particularly less than 200° C. Suitable are, for example, tin alloys, in particular tin-lead alloys and/or indium alloys.
Further details and advantages of the invention will be illustrated by means of exemplary embodiments of the invention with reference being made to the accompanying drawings. Therein, equal parts that are corresponding to each other are designated with corresponding reference symbols. In the Figures,
Before discussing example embodiments of the invention, it will be appreciated that the present invention includes methods as well as apparatuses. Methods of the invention may be useful to make apparatuses of the invention. It will therefore be appreciated that in describing a method of the invention description of an apparatus may be had, and vice versa.
When the solder material 1 fuses, the receiving regions 2b are wetted by the solder material 1. As a result, the solder material 1 is drawn off from the printed circuit board region 3 between the two metal surfaces 2 and the electrical contact formed by the solder material 1 is interrupted.
To manufacture the fuse shown in
In order to improve the wettability of the receiving regions 2b, these regions can be covered with flux either partially or completely. The metal surfaces 2 are made of a material that is customary for solder tracks, for example, copper with potential coatings. In the exemplary embodiment shown, the receiving regions 2b and the contact regions 2a are, together, designed as extended end sections of the pcb-tracks 2c. The printed circuit board region 3 that is disposed between the two metal surfaces 2 and is covered with the solder material 1 can, for example, consist of an epoxy resin that is customary for printed circuit boards.
The receiving regions 2b surround the contact regions 2a in the form of a U or a C. When the cut-out trips, molten solder material 1 can, therefore, flow across an advantageously large circumferential surface and into the receiving regions 2b. Together, the receiving regions 2b are, preferably, larger than the surface 3 that is disposed between the two metal surfaces 2 and is covered with solder material 1. In this manner, there is sufficient space to receive the solder material 1 bridging the gap 3 between the two metal surfaces 2 when the cut-out trips. In the schematic diagram of
The fuse described above can, for example, be used in the event of a fault to interrupt a load current of a field-effect transistor, more particularly of a MOSFET, arranged on the printed circuit board. To achieve this, the fuse is thermally coupled to the field-effect transistor, for example, by being arranged in the immediate vicinity of the transistor. During operation, a load current to be switched by the field-effect transistor flows through the fuse. In the event of a fault, a heating of the field-effect transistor causes the solder material 1 to fuse, which will then wet the receiving regions 2b and, therein, flow off from the space between the two contact regions 2a, with the result that the cut-out responds and an electrical contact between the two metal surfaces 2 is interrupted.
In the exemplary embodiment shown in
A special feature of the exemplary embodiment shown in
In considering the above description and attached Figures, it will be appreciated that some example embodiments of the invention have been shown and described. Many other embodiments are possible and within the scope of the invention as claimed. Also, substitutes and equivalents to various elements of invention embodiments will be apparent to those knowledgeable in the art involved. Various steps of example methods could be changed in order, different elements from different embodiments interchanged with one another, and the like.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 040 022.2 | Sep 2009 | DE | national |
