Printed circuit board with at least one embedded precision resistor and method for producing a printed circuit board

Abstract

Printed circuit board (1) with at least two connection points (3) and at least one precision resistor (2) for measuring a current flowing between the connection points (3), wherein the precision resistor (2) extends between the connection points (3) in the printed circuit board (1), characterized in that the precision resistor (2) is designed as a flat wire, wherein a bottom side and at least one of the edge sides of the precision resistor (2) are covered at least in sections with insulating compound (4), a top side of the precision resistor (2) is at least in sections flush with a top side of at least one of the edge sides of an adjacent layer of insulating compound (4), and the precision resistor (2) is welded to the connection points (3).

Description

[0001] The invention relates to a printed circuit board with at least two connection points and at least one precision resistor for measuring a current flowing between the connection points, as well as a method for producing such a printed circuit board.

[0002] A printed circuit board according to the preamble of claim 1 is known from DE 10 2008 022 787 A1.

[0003] Further examples of resistance measurements relating to printed circuit boards are disclosed in US 2009 / 0 044 403 A1 and US 2005 / 0 057 865 A1.

[0004] Such circuit boards with a precision resistor as part of a measuring device for the current flowing between the connection points are known from the prior art. The precision resistor, which may be made of a solid, electron-beam-welded composite material made of copper and resistance alloys, is soldered onto an existing circuit as a surface-mount component (SMD) or connected via bond wires. By measuring the voltage between the connection points and based on the known electrical resistance value, which generally changes only slightly in the relevant temperature range of 20 to 60°C due to the extremely low temperature coefficient of the materials used, the current flowing between the connection points can be determined with high accuracy.

[0005] The production of these circuit boards is comparatively complex and expensive. The contacting of the precision resistors at the connection points is often susceptible to vibration, which can reduce the lifespan of the circuit board. Furthermore, the trend in many technical applications (e.g., automobiles, smartphones, etc.) is toward minimizing size ("downsizing"), which particularly limits the installation space of the electrical components used, such as circuit boards.

[0006] The invention is therefore based on the object of producing a printed circuit board of the type mentioned at the outset more cost-effectively, so that the printed circuit board has a reduced installation space while preferably having a longer service life.

[0007] The object of the invention is achieved by the printed circuit board according to claim 1, comprising at least two connection points and at least one precision resistor for measuring a current flowing between the connection points, wherein the precision resistor extends between the connection points in the printed circuit board. By arranging the precision resistor in the printed circuit board, the installation space is reduced, while at the same time the manufacturing effort and the sensitivity of the printed circuit board to external influences such as vibrations are significantly reduced. This allows the printed circuit board to be manufactured more cost-effectively, more compactly, and more durable than conventional products.

[0008] According to the invention, the precision resistor is designed as a flat wire, wherein a bottom side and at least one of the edge sides of the precision resistor are covered at least in sections with insulating compound, a top side of the precision resistor is at least in sections flush with a top side of at least one of the edge sides of an adjacent layer of insulating compound and the precision resistor is welded to the connection points.

[0009] It may be advantageous if the printed circuit board has at least one layer of insulating material, wherein the thickness of the at least one layer of insulating material is in the range from 50 to 2000 µm, preferably in the range from 100 to 1800 µm, particularly preferably in the range from 800 to 1600 µm.

[0010] It can also prove useful if circuit electronics and / or power electronics are arranged on the circuit board. The inventive manufacturing technology for circuit boards is particularly suitable for use in the field of circuit electronics and / or power electronics, since large amounts of heat and current can be managed, for example, via conductor wires and / or preformed conductor parts embedded in the circuit board.

[0011] According to an advantageous embodiment of the invention, the precision resistor meets at least one of the following requirements: - The precision resistor contacts the connection points. - The precision resistor has a resistance value in the range of 0.1 to 300 mOhm, preferably in the range of 1 to 100 mOhm. - The precision resistor comprises a variance of less than + / - 5%, preferably a variance of less than + / - 2%, preferably a variance of + / - 1% or less. - The temperature coefficient of the electrical resistance of the precision resistor for the temperature range between 20 and 60 °C is in the range of 0.1 ppm / K to 200 ppm / K, preferably in the range of 0.5 ppm / K to 100 ppm / K, preferably in the range of 1 ppm / K to 50 ppm / K. - The precision resistor consists of metal, preferably of at least one of the elements copper (Cu), manganese (Mn), nickel (Ni), chromium (Cr), aluminum (AI), silicon (Si) or tin (Sn), preferably of an alloy containing at least one of the elements copper (Cu), manganese (Mn), nickel (Ni), chromium (Cr), aluminum (AI), silicon (Si) or tin (Sn), for example of alloys known under the brand names “Manganin”, “Zeranin” or “Isaohm”. - The precision resistor is at least predominantly, preferably completely, embedded in the circuit board. - An upper side is covered at least in sections, preferably completely, with insulating compound. - The top sides and / or the bottom sides and / or at least one of the edge sides of the circuit board and the precision resistor are aligned parallel to each other. - The precision resistor can be manufactured or is manufactured using the extrusion process. - The precision resistor is designed as a flat wire. - The precision resistor extends essentially in one plane. - The precision resistor comprises a rectangular cross-section, wherein preferably the side of the cross-section with the larger extension points towards the surface of the circuit board. - The precision resistor has a thickness in the range of 10 to 2000 µm, preferably in the range of 50 to 1000 µm, preferably in the range of 100 to 500 µm.

[0012] There are a wide variety of design options for the precision resistor, which are intended to be encompassed by the scope of the invention. The decisive factor is that the precision resistor runs between the connection points in the circuit board, whereby the design of the precision resistor as a flat wire appears to be the most appropriate for cost reasons or for manufacturing reasons. The design of the precision resistor as a flat wire also has the advantage that the contacting at the connection points can be made via large contact surfaces, thus resulting in only minimal power losses that could adversely affect current measurement.

[0013] It may prove advantageous if the circuit board has a voltage measuring device that meets at least one of the following requirements: - The voltage measuring device is designed to measure a voltage at the connection points, preferably at the contact points between the precision resistor and the connection points. - The voltage measuring device implements an HDI (High Density Interconnect) circuit, preferably with at least one of the following features: ◯ The precision resistor and / or the connection points are covered by at least one layer of insulating compound. ◯ A hole for contacting the precision resistor through at least one layer of insulating material is preferably designed as a laser hole. ◯ The precision resistor is contacted by at least one layer of insulating compound, preferably via microvias. ◯ The precision resistor is galvanically contacted.

[0014] The design of the voltage measuring device as an HDI (High Density Interconnect) circuit proves to be particularly advantageous in conjunction with particularly low-ohm precision resistors, since the contacting can be carried out very precisely.

[0015] The object of the invention is also achieved by a method according to claim 6 for producing a printed circuit board with at least one precision resistor extending in the printed circuit board for measuring a current flowing between connection points, comprising the steps: - Contacting the precision resistor at the connection points and welding the precision resistor to the connection points. - Covering the precision resistor with an electrical insulating compound, so that a bottom side and at least one of the edge sides of the precision resistor are at least partially covered with the insulating compound and an upper side of the precision resistor is at least partially flush with an upper side of at least one of the edge sides of an adjacent layer of insulating compound. - Electrically isolating the connection points from each other so that a current flowing between the connection points flows through the precision resistor.

[0016] According to an advantageous embodiment, the method comprises at least one of the following steps: - Pressing the precision resistor with a prepreg made of insulating compound. - Machining out the connection points and / or machining out conductor tracks, preferably for a voltage measuring device, by etching from a copper foil attached to the surface of the circuit board, wherein the etching of the copper foil preferably takes place after the precision resistor has been contacted at the connection points. - Producing a voltage measuring device, preferably by implementing an HDI (High Density Interconnect) circuit on the printed circuit board, preferably with at least one of the following steps: ◯ Covering the precision resistor and / or the connection points with at least one layer of insulating compound. ◯ Producing a hole for contacting the precision resistor through at least one layer of insulating material, preferably as a laser hole. ◯ Contacting the precision resistor through at least one layer of insulating material, preferably via microvias. ◯ Galvanic contacting of the precision resistor.

[0017] Further advantageous developments of the invention result from combinations of the features disclosed in the claims, the description and the drawings. Short description of the characters Fig. 1 shows a schematic sectional view of a printed circuit board according to the first embodiment of the invention, wherein the printed circuit board has a substrate with a layer of insulating compound and a precision resistor of the order of 100 mOhm embedded therein, wherein conductor tracks with end connection points run on the top side of the printed circuit board and the connection points are welded to the precision resistor. Fig. 2 shows a schematic sectional view of a printed circuit board according to the second embodiment of the invention, wherein the printed circuit board has a substrate with two layers of insulating material and a precision resistor in the order of magnitude of 1 mOhm embedded in the lower layer, wherein conductor tracks with end connection points run between the upper and lower layers of insulating material and the connection points are welded to the precision resistor, wherein the precision resistor is galvanically contacted through the upper layer of insulating material via conductor tracks and microvias to implement a voltage measuring device in the form of an HDI circuit.

[0018] The representations of the circuit boards in the figures are schematic in nature. Therefore, the proportions between the individual components of the circuit boards may not be realistic. Detailed description of the preferred embodiments

[0019] The preferred embodiments of the invention are described in detail below with reference to the figures. First embodiment (Fig. 1)

[0020] The first embodiment of the invention according to Fig. 1 relates to a printed circuit board 1 with two connection points 3 and a precision resistor 2 for measuring a current flowing between the two connection points 3. According to the invention, the precision resistor 2 extends between the connection points 3 in the printed circuit board 1. The printed circuit board 1 has a layer of insulating compound 4 with a thickness of approximately 1000 µm, in which the precision resistor 2 is embedded. The underside, all edges, and a large part of the top side of the precision resistor 2 are completely covered with insulating compound 4. The top side of the precision resistor 2 runs, at least in sections, in particular at the ends connected to the connection points 3, flush with an upper side of the surrounding and adjacent layer of insulating compound 4.

[0021] Circuit electronics and / or power electronics, for example, are arranged on the printed circuit board 1 (not shown). The precision resistor 2, welded to both connection points 3, is designed as a flat wire made of "Manganin" (brand name) and has a resistance value of approximately 100 mOhm with a variance of + / - 5% or less. The temperature coefficient of the electrical resistance of the precision resistor 2 for the temperature range between 20 and 60 °C is approximately 5 ppm / K. The cross-section of the precision resistor 2, designed as a flat wire, is rectangular, approximately 100 µm thick and approximately 500 µm wide, with the side of the cross-section with the larger extension facing the surface of the printed circuit board 1. This results in large contact areas with the precision resistor 2 at both connection points 3.

[0022] A voltage measuring device (not shown) is designed to tap a voltage at the connection points 3 and measure it directly at the contact points between the precision resistor 2 and the connection points 3. The distance between the two contact points is precisely defined. Due to the known and very constant electrical resistance of the precision resistor 2 over the relevant temperature range, the current flowing between the connection points 3 can be determined very precisely.

[0023] A method according to the invention for producing the printed circuit board 1 with the precision resistor 2 extending in the printed circuit board 1 for measuring the current flowing between connection points 3 comprises the following steps: Step a: Contacting the precision resistor 2 made of manganin, which is designed as a flat wire and has, for example, a thickness of approximately 100 µm and a width of 500 µm, with a copper foil, which has, for example, a thickness of approximately 35 µm, at predetermined connection points 3 by welding. Step b: Applying a layer of insulating compound 4 to the side of the foil connected to the precision resistor 2 by pressing it with a prepreg of insulating compound. Step c: Machining out the connection points 3 and / or machining out conductor tracks, e.g. for a voltage measuring device, in one operation by etching from the copper foil attached to the surface of the circuit board 1 or to the layer of insulating compound 4, in order to electrically isolate the connection points 3 from one another, so that a current flowing between the connection points 3 flows through the precision resistor 2.

[0024] In the following, the same reference numerals are used for similar elements to avoid repetition of the description. Second embodiment (Fig. 2)

[0025] The second embodiment of the invention according to Fig. 2 is essentially based on the first embodiment described previously in connection with Fig.1. The circuit board 1 according to the second exemplary embodiment of the invention is intended in particular for high-precision current measurements in the low-resistance range and comprises two layers consisting essentially of insulating compound 4. The structure of the lower layer of the circuit board 1 of the second exemplary embodiment is essentially the same as that of the circuit board 1 of the first exemplary embodiment. The dimensions and resistance value of the precision resistor 2, at approximately 300 µm thick and approximately 1400 µm wide or 1 mOhm, are different from the first exemplary embodiment. Therefore, a differently designed voltage measuring device 5 is used for voltage measurement, as described below.

[0026] The lower layer 11 of the circuit board 1 is covered with a layer of insulating compound 4, which covers the precision resistor 2 and the connection points 3. On the upper side of the upper layer 12 of the circuit board 1 or the upper layer of insulating compound 4, which has a thickness of preferably approximately 100 µm, a circuit diagram with corresponding connections 52 to the microvias 51 for voltage measurement is etched from a copper foil with a thickness of preferably approximately 18 µm according to the previously described scheme. The voltage measuring device 5 implements an HDI high-density interconnect circuit, with laser holes machined through the upper layer of insulating compound 4 for contacting the precision resistor 2. The precision resistor 2 is galvanically contacted via these holes using microvias 51.The spacing of microvias 51 is precisely defined, allowing the resistance value to be precisely determined via the length of the precision resistor 2 located between them. The voltage measurement combined with the precisely determined resistance value allows the current flowing between connection points 52 and microvias 51 to be precisely calculated.

[0027] The invention is not limited to the above embodiments. Further advantageous developments of the invention arise in particular through the additional use of conductor wires or molded parts embedded in the circuit board, preferably in combination with etched conductor tracks. This allows electronic components on the circuit board to be interconnected.

Claims

[1] Printed circuit board (1) with at least two connection points (3) and at least one precision resistor (2) for measuring a current flowing between the connection points (3), wherein the precision resistor (2) extends between the connection points (3) in the printed circuit board (1), characterized by in that the precision resistor (2) is designed as a flat wire, wherein a bottom side and at least one of the edge sides of the precision resistor (2) are covered at least in sections with insulating compound (4), an upper side of the precision resistor (2) is at least in sections flush with a top side of at least one of the edge sides of an adjacent layer of insulating compound (4), and the precision resistor (2) is welded to the connection points (3). [2] Printed circuit board (1) according to claim 1, characterized by that the printed circuit board (1) has at least one layer of insulating material (4). [3] Printed circuit board (1) according to one of the preceding claims, characterized by that circuit electronics and / or power electronics are arranged on the printed circuit board (1). [4] Printed circuit board (1) according to one of the preceding claims, characterized by that the precision resistor (2) meets at least one of the following requirements: a. The precision resistor (2) contacts the connection points (3). b. The precision resistor (2) has a resistance value in the range of 0.1 to 300 mOhm. c. The precision resistor (2) covers a variance of less than + / - 5%. d. The temperature coefficient of the electrical resistance of the precision resistor (2) for the temperature range between 20 and 60 °C is in the range of 0.1 ppm / K to 200 ppm / K. e. The precision resistor (2) is made of metal. f. The precision resistor (2) is at least predominantly embedded in the circuit board (1). g. An upper side of the precision resistor (2) is covered at least in sections with insulating compound (4). h. The top surfaces of the circuit board (1) and the precision resistor (2) are aligned parallel to each other. i. The precision resistor (2) can be manufactured or is manufactured by extrusion. j. The precision resistor (2) extends essentially in one plane. k. The precision resistor (2) has a rectangular cross-section. I. The precision resistor (2) has a thickness in the range of 10 to 2000 µm. [5] Printed circuit board (1) according to one of the preceding claims, characterized by that the printed circuit board (1) has a voltage measuring device (5) which meets at least one of the following requirements: a. The voltage measuring device (5) is designed to measure a voltage at the connection points (3). b. The voltage measuring device (5) implements an HDI (High Density Interconnect) circuit. [6] Method for producing a printed circuit board (1) with at least one precision resistor (2) extending in the printed circuit board (1) and designed as a flat wire for measuring a current flowing between connection points (3), comprising the steps: a. Contacting the precision resistor (2) to the connection points (3) and welding the precision resistor (2) to the connection points (3). b. Covering the precision resistor (2) with an electrical insulating compound (4) such that a bottom side and at least one of the edge sides of the precision resistor (2) are at least partially covered with the insulating compound (4) and an upper side of the precision resistor (2) is at least partially flush with an upper side of at least one of the edge sides of an adjacent layer of insulating compound (4). c. Electrically isolating the connection points (3) from one another so that a current flowing between the connection points (3) flows through the precision resistor (2). [7] Method according to claim 6, characterized by at least one of the following steps: a. Pressing the precision resistor (2) with a prepreg made of insulating compound. b. Working out the connection points (3) by etching from a copper foil attached to the surface of the circuit board (1). c. Producing a voltage measuring device (5) on the circuit board with at least one of the following steps: i. Covering the precision resistor (2) with at least one layer of insulating compound (4). ii. Producing a bore for contacting the precision resistor (2) through at least one layer of insulating compound (4). iii. Contacting the precision resistor (2) through at least one layer of insulating compound (4). iv. Galvanic contacting of the precision resistor (2).

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