Patent Application
20050046543
1. Field of the Invention
The invention relates to a low-impedance electrical resistor and a process for its manufacture according to the preamble of the independent claims. In particular, the resistors are low-impedance precision resistors for current measurement purposes of the SMD or chip construction type.
2. Background Art
In a process for the manufacture of SMD measurement resistors with resistance values in the milliohm range, a laminate is first formed from a copper metal sheet which serves as substrate and from a film consisting of a Cu-based resistor alloy and a heat-conducting adhesive located between them. On the free top side of the alloy film, connection contact areas for the individual resistors are then photolithographically defined, coated with copper by an electroplating process and covered with nickel. After the electroplating metallization, the structure itself of the resistors and their connection contacts is formed by etching. The resistor structure can have the usual four pole and/or meandering shape. The separation of the resistors can be carried out using a laser cutting installation or, preferably, by breaking the adhesive film, after the metal layers have been perforated on both sides by etching, along the provided separation lines. This known process is relatively expensive. Above all, it is hardly possible to produce resistors with precisely defined resistance values using the etching procedure which was required in the past. It is particularly difficult to structure by etching connection contacts which have been applied by electroplating onto the alloy film, without attacking the alloy metal located under it, and, moreover, the precision is negatively affected by the poorly defined shape of the front surfaces of the connection contacts, which is typical in etching, where the front surfaces are not precisely perpendicular to the surface, rather they run a more or less concave course up to the alloy surface.
In another process for the manufacture of SMD measurement resistors, connection contacts of a resistance films, which are bonded to a substrate and structured in the conventional manner are applied in the form of a paste in the screen printing procedure onto the film, optionally after a electroplating preliminary tin coating of the connection areas, and then remelted into compact “pearls.” Here too the desired precise resistance value can only be achieved by subsequent adjustment. The separation of these resistors in practice is carried out by stamping, the procedure which is also conventionally used for other similar known components.
The invention is based on the problem of providing a process for the manufacture of precision resistors of the considered type, which is simpler than similar known processes and which, in particular, allows for the formation of the connection contacts without etching.
This problem is solved by the process for the manufacture of low-impedance electrical resistors, in which, onto photolithographically defined areas of a layer consisting of a metallic resistor alloy in the form of a metal sheet or a film, a metal is electroplated for the formation of connection contacts for a multitude of individual resistors, and the layer provided with the connection contacts is divided into the individual resistors, including the process steps of photolithographic formation of a cover mask, which is formed by a multitude of parallel strips extending over a surface of the layer, and at mutual identical intervals; electroplating of the layer on only its surface which carries the cover mask, for the deposition of the connection contact metal onto the resistor strip located between the parallel mask strips; and division of the electroplated layer along groups of cutting planes which are perpendicular with respect to the layer's surface and perpendicular with respect to each other, where the cutting planes which are parallel to the connection contact strips in each case divide one of the connection contact strips, while the other cutting planes separate the resistors at their edges which run transversely with respect to the connection contact strips as well as a resistor made by this process.
According to the invention it is possible, in a few simple work steps, to manufacture precision resistors in the milliohm range, which do not require subsequent adjustment, having resistance tolerances of at most ±5%. Neither the connection contacts nor the alloy areas have to be etched, and thus the drawbacks of structuring by the etching technology are avoided, namely the formation of running, nonperpendicular etching flanks, which lead to large variations in the resistance value and poor reproducibility.
One advantageous effect is that, according to the invention, after the photolithographic definition of the separation surfaces, the comparatively thick copper contact layers can be electroplated with high precision, where, in particular, the perpendicular formation, with respect to the main surface of the metal sheet or of a film forming the resistance layer, as well as the position of the flanks turned toward the activator resistor area are important.
The second prerequisite for the manufacture of precision resistors according to the invention is the obtention of a defined width. It is preferred to achieve this by sawing the electroplated resistance layer.
By means of sawing, a substantially higher precision and reproducibility of the resistors is achieved than with other separation processes such as etching, stamping and, for example, in the case of the use of lasers which itself is another possibility. In addition, it is possible to maximize the number of resistors, which can be manufactured for a given useful surface area by sawing.
The process is appropriate, among other purposes, for the manufacture of extremely low-impedance resistances, for example, in the range of approximately 0.5 mΩ to 5 mΩ in large scale manufacture, however, it is also possible to manufacture resistors with even lower or with higher resistance values, for example 0.01-50 mΩ. In a modified construction form with particularly thin resistor films, the resistance value can also be further increased without problem, for example, up to 100 mΩ. Moreover, the resistors are flexible and, depending on the desired load rating, they can be manufactured in sizes which can be as large or as small as desired. Since the resistor manufactured according to the invention substantially consists only of metal and since the organic adhesive used in the above-mentioned processes is either completely omitted or, if it is used at all, does not have to be heat conducting, it has the advantage of high heat resistance and high load capacity. In the application cases which are typical for this resistor it is sufficient to remove the dissipation heat amounts through the connection contacts, for example, into a circuit board on whose surface the resistors are mounted according to the SMD technique.
The invention is further explained with reference to the embodiment examples represented in the drawing figures, which:
FIGS. 1(A) through 1(F) show the different successive steps or stages of the process according to the invention;
According to
In the next step, according to
Before, after, or simultaneously with the photolithographic structuring of the photoresistance layer 2, the bottom side of the metal sheet 1 is covered with a protective film 3, which prevents a metallization of the metal sheet bottom side during the subsequent electroplating.
In the process stage according to
According to
Now the precision resistors which are already finished can be separated. For this purpose, the metal sheet 1 which is provided with the connection contacts is separated along groups of cutting planes which are perpendicular to the metal sheet surface and perpendicular to each other. The cutting planes of one of these two groups run parallel to the copper strips 4 and thus to one of the edges of the metal sheet 1 through the entire metal sheet and, in each case, they are located in the middle of one of the copper strips 4, which as a result are divided in each case into two identical strip parts, along the arrows 7 in
This separation of the resistors is best carried out by sawing the individual cutting planes. Sawing presents the advantage of allowing a very precise maintenance of the desired dimensions of the resistors in each case with planar cutting surfaces which are precisely perpendicular to the metal sheet plane. Precision sawing machines which are suitable for this purpose, which can be oriented (referenced) for example optically, with the electroplated metal sheet, and which operate with high precision in the μm range, in themselves are known and available commercially. For the sawing, the metal sheet is advantageously bonded to a support, and later it is then unproblematic to separate the separated resistors from its adhesive layer. It is advantageous for the electroplated metal sheet to be first sawed along one of the two groups of cutting planes into individual strips, which in turn can then be sawed to form the individual resistors. Depending on the type of sawing machine, it is theoretically also possible to simultaneously saw off or saw apart several strips. From a metal sheet having the purpose mentioned above as an example and dimensions of approximately 300×400 mm, it is possible to cut by sawing more than 10,000 resistors in the manner as has been described here.
The individual resistor which is formed after the last process step is represented schematically (not to scale) in
As can be seen, the resistor, at the mentioned opposite ends, presents planar front surfaces 9 of the connection contacts and of the metal sheet piece 1′ itself, which are aligned precisely perpendicularly to the metal sheet plane. The same applies to the two lateral front surfaces 8 of the connection contacts and the metal sheet piece 1′. The protective lacquer layer 5′ is located between the connection contacts, while the surface of the resistor which is turned away from the contacts may still be covered by the protective film 3′.
The modified resistor represented in
The connection contacts 14 with the tin layers 16 and the protective lacquer layer 15 correspond to the embodiment according to
Instead of the bonded aluminum substrate 18, the ease of handling an optionally very thin resistor film, such as the film 11 in
