Patent Application
20150041200
The invention refers to a soldering method and a corresponding soldering device for soldering a printed circuit board to an electric component, such as a current sense resistor (shunt), using a solder.
When conventionally equipping printed circuit boards with SMD components (SMD: Surface Mounted Device), the heat required to fuse the solder is provided in an oven which is heated by a separate heat source. The disadvantage of this known soldering method is firstly that a separate heat source is required to heat the solder. A further disadvantage of this known soldering method is that the entire printed circuit board with soldering points and the components is usually heated in the oven which is unnecessary and can be harmful to heat-sensitive components.
Furthermore, so-called resistance soldering is known from the state of the art in which an electrical current flows through the actual soldering point with the solder, whereby the electrical heat loss causes the solder to melt. So far, however, resistance soldering has not yet been used to equip printed circuit boards with electrical components. This is also because separate connections would be necessary to conduct the electrical current through the soldering point.
It is known from DE 10 2009 031 227 A1 that a heating current can be applied to an electrical conductor in order to solder the conductor with a printed circuit board substrate using the resultant heat loss. However, the conductor is not an electronic component. This known soldering method is therefore not suitable for the soldering of an electronic component.
A soldering method is furthermore known from U.S. Pat. No. 4,582,975 in order to connect an integrated circuit with a printed circuit board. However, the heat required to melt the solder here is generated by a separate electrical heater in the integrated circuit. The disadvantage of this known soldering method is therefore the necessity of a corresponding modification of the integrated circuit.
The invention is therefore based on the object of creating an appropriately improved soldering method and a corresponding soldering device.
This object is solved by an inventive soldering method and by a corresponding soldering device in accordance with the independent claims.
The invention comprises the general technical teaching to generate the heat necessary to melt the solder by running an electrical current through the component to be assembled, whereby the electrical current in the component generates an electrical heat loss which passes from the component to the solder, thereby causing the solder to melt.
Unlike the resistance soldering referred to at the beginning, the electrical heat loss is not generated directly in the soldering point or the solder, but in the component to be assembled. This offers the advantage that no separate electrical connections are required for the application of electrical current to the component because the connections can be used to apply the electrical current to the component which are also used during operation of the printed circuit board layout with the electrical component.
The component is preferably a passive component such as a resistor. However, the invention is not restricted to passive components (e.g. resistors) with respect to the component to be assembled, but can basically also be realised with other types of components which generate heat when an electrical current is applied to them which can be used to melt the solder.
However, in a preferred embodiment of the invention, the component to be assembled is a resistor, which comprises a resistance element made of a resistance material (e.g. Manganin®) and two connectors made of a conducting material (e.g. copper), whereby the resistance element is connected electrically between the two connectors so that the electrical current is introduced via one of the two connectors in the resistor and flows from here through the resistance element into the other connector, from where the electrical current is then dissipated from the resistor. Such low-ohm current sense resistors are known from the state of the art and are described for example in EP 0 605 800 A1, so that full reference is made to the content of this publication in terms of the structure and functioning of the resistor described here.
In agreement with the conventional SMD soldering method, the inventive soldering method provides for the solder to be applied for example in the form of soldering paste on soldering pads (connection areas) of the printed circuit board and/or to the connectors of the resistor, whereby the solder adheres to the soldering pads. Finally, the printed circuit board is assembled with the resistor so that the solder is between the soldering pads of the printed circuit board and the connectors of the resistor. Electrical current is then applied to the resistor so that the electrical heat loss arising in the resistance element is transferred to the solder via the connectors of the resistor, thereby causing the solder to melt. It is advantageous here that the resistance material of the resistance element and also the conducting material of the connectors have a high thermal conductivity which leads to a corresponding good transfer of heat from the resistance element to the solder. A cooling phase then follows the energizing of the resistance element in which the resistor with the printed circuit board cools together with the solder so that the solder becomes rigid and connects the connectors of the resistor electrically and mechanically with the soldering pads of the printed circuit board.
If the component to be assembled is a current sense resistory, the soldering pads of the printed circuit board preferably form voltage taps in order to measure a drop in voltage across the resistance element of the resistor. The soldering pads of the printed circuit board which serve as voltage taps are preferably arranged here such that the solder is in contact with the connectors of the resistor directly at the transition between the connectors and the resistance element. This is advantageous because the voltage measured then virtually exclusively reflects the voltage drop across the resistance element without this measured value being falsified by a drop in voltage across the connectors.
In addition, an electronic measuring circuit is preferably also assembled on the printed circuit board in order to measure the voltage drop across the resistance element of the resistor. Such measuring circuits are known and described, for example, in EP 1 363 131 A1 so that so that full reference is made to the content of this publication in terms of the structure and functioning of the measuring circuit described here. It is merely to be mentioned at this point that the measuring circuit can be an ASIC (Application Specific Integrated Circuit). When assembling the electronic measuring circuit on the printed circuit board, a connection is also created between the connectors of the resistor via the corresponding soldering pads of the printed circuit board and the measuring circuit so that the measuring circuit can measure the voltage drop across the resistance element of the resistor.
In the preferred embodiment the two connectors and the resistance element of the resistor are each plate-shaped as described, for example, in EP 0 605 800 A1. The resistance element is preferably thinner than the adjacent connectors here, whereby the resistance element is preferably set back in relationship to the printed circuit board. This is a good idea to prevent the solder flowing onto the resistance element during the soldering process but contacting the respective connector exclusively. If the solder were to flow onto the resistance element, a parallel connection would occur on the outer side edges of the resistance element via the solder, causing the geometrically determined resistance value of the resistance element to be falsified, thereby leading to a corresponding measurement error. The thinner resistance element therefore preferably closes flush with the adjacent connectors on the side facing away from the printed circuit board so that the resistance element on the side facing the printed circuit board is set back in relationship to the surface of the thicker connector. The solder therefore preferably has no direct contact with the resistance element, namely before, during and/or after the actual soldering process.
In a preferred embodiment of the invention the soldering temperature which reflects the desired temperature of the solder is closed-loop controlled. For this control, a desired setpoint value is determined for the soldering temperature, whereby the setpoint value depends on the composition of the respective solder. During the actual soldering process the actual value of the soldering temperature is constantly measured. Any deviation between the setpoint value of the soldering temperature and the measured actual value of the soldering temperature is determined. The electrical energization of the component is then set as dependent on the deviation between the setpoint and the actual value so that the actual value of the soldering temperature is adjusted to the setpoint value. In practice, the power of the electrical current flowing through the component is varied using this control mechanism.
Furthermore, in a preferred embodiment of the invention, the soldering temperature during the soldering process is varied in accordance with a stipulated temperature-time profile so that the temporal curve of the soldering temperature follows the stipulated temperature-time profile. The soldering temperature in accordance with the desired temperature-time profile can either be set as part of an open-loop control or closed-loop control.
The soldering temperature is preferably the temperature of the solder. However, it is frequently impossible to measure the temperature of the solder itself. In these cases it is alternatively possible in the invention to measure the temperature of the resistance element or of the connectors of the resistor in order to derive the temperature of the solder therefrom. The term of soldering temperature used in the invention is therefore to be understood generally and is not restricted to the temperature of the soldered connection itself.
The conducting material of the connectors of the resistor is preferably copper or a copper alloy so that the conducting material has a specific electrical resistance which is as low as possible. This is important so that the measurement of the electrical voltage drop across the resistance element is falsified as little as possible by the drop in voltage within the connectors.
By contrast, the resistance material of the resistance element can, for example, be a copper alloy such as a copper manganese alloy or a copper-manganese-nickel alloy (e.g. Cu84Ni4Mn12, i.e. Manganin®). However, in terms of the resistance material the invention is not restricted to the above materials mentioned by way of example.
However, in the preferred embodiment of the invention, the resistance material of the resistance element has a higher specific resistance than the conducting material.
It is furthermore to be mentioned that the connectors are preferably connected mechanically fixedly with the resistance element, in particular by a welded seam which can be manufactured for example by electron beam welding. It is advantageous here for the connection between the connectors and the resistance element to be heat-resistant and not to dissolve in the inventive soldering method.
It is furthermore to be mentioned that the resistance material of the resistance element is preferably low ohmic and therefore, for example, has a specific electrical resistance which is smaller than 2·10−4 Ω·m, 2·10−5 Ω·m or even smaller than 2·10−6 Ω·m.
By contrast, the conducting material of the connectors has a specific electrical resistance which is smaller than 10−5 Ω·m, 10−6 Ω·m or even smaller than 10−7 Ω·m.
It is furthermore to be mentioned that the connectors and the resistance element of the resistor in the invention are preferably plate-shaped, as described for example in EP 0 605 800 A1, whereby the plate-shaped connectors or the plate-shaped resistance element may preferably be planar or bent.
It is also to be mentioned that to melt the solder the component is energized with an electrical current which is sufficiently large to generate the heat necessary to melt the solder. The component is therefore energized during the soldering process preferably with a current of more than 200 A, 500 A, 1000 A or even more than 2000 A.
Finally, it is to be mentioned with respect to the inventive soldering method that the component to be assembled is preferably an SMD component which is mounted by surface mounting on the printed circuit board.
However, the invention comprises not only the above described soldering method but also a corresponding soldering device to solder the printed circuit board with the electrical component, whereby a heating device is provided in the form of a current source which energizes the component with the electrical current in order to melt the solder by the electrical heat loss arising in the component.
In addition, the inventive soldering device preferably has a temperature sensor in order to measure an actual value of the soldering temperature, whereby the soldering temperature reflects the temperature of the solder. The temperature sensor is preferably connected with a controller which triggers the current source as dependent on a deviation between a stipulated setpoint value of the soldering temperature and the measured actual value of the soldering temperature and adjusts the actual value of the soldering temperature to the stipulated setpoint value of the soldering temperature.
In addition, the inventive soldering device can have a control unit which provides a temperature-time profile for the soldering temperature, whereby the control unit triggers the controller or the current source in accordance with the temperature-time profile. The control unit can therefore set the setpoint value for the soldering temperature in accordance with the set temperature-time profile depending on the time or directly control the current source accordingly.
Finally, the invention also comprises a printed circuit board layout with a printed circuit board and an electrical component which is soldered with the printed circuit board by means of a solder. The inventive printed circuit board layout differs from conventional printed circuit board layouts in that the solder is melted by an electrical energization of the component which is reflected in the finished soldered connection and distinguishes the inventive printed circuit board layout from conventional printed circuit board layouts.
Other advantageous further developments of the invention are described in the dependent claims or explained in greater detail below together with the description of the preferred embodiments of the invention using the figures. The figures show as follows:
Furthermore,
In addition, the solder device comprises a current source 14 which is connected with the two connectors 9, 10 of the current sense resistor 8 and which energizes the current sense resistor 8 with a solder current ILÖT, whereby the solder current ILÖT may have a current strength of 1000 A, for example. The solder current ILÖT firstly enters the connector 10 and then flows through the resistance element 11 and the connector 8 back to the current source 14. The solder current ILÖT generates an electrical heat loss in the resistance element 11 which passes through the connectors 9, with a heat current Q to the soldering paste 6, 7 and causes it to melt, as is shown graphically in
It can also be seen from the enlarged representation in
The diagrammatic representation in
In a first step S1, the measuring circuit 3 is mounted on the printed circuit board 2.
In a further step S2, the soldering paste 6, 7 is attached to the soldering pads 4, 5 of the printed circuit board 2.
In a step S3, the printed circuit board layout 1 is then joined with the current sense resistor 8.
The current sense resistor 8 is then connected in a step S4 to the current source 14 so that the current sense resistor 8 can then be energized with the soldering current ILÖT in a step S5 in order to melt the soldering paste 6, 7.
Finally, the printed circuit board 1 with the soldering paste 6, 7 and the current sense resistor 8 is then cooled in a step S6, so that the melted soldering paste 6, 7 becomes rigid and creates an electrical and mechanical connection between the soldering pads 4, 5 and the connectors 9, 10 of the current sense resistor 8.
In a step S7, the current sense resistor 8 is then isolated from the current source 14.
The inventive soldering device comprises a temperature sensor 16 which measures an actual value TIST of the soldering temperature. For example, the temperature sensor 16 can directly measure the temperature of the soldering paste 6, 7. However, the temperature sensor 16 usually measures the temperature of the connectors 9, 10 in the area of the side edges 12, 13, which is considerably easier from a technical point of view.
In addition, the inventive soldering device with the control circuit shown comprises a control unit 17 which provides a temperature-time profile for a desired setpoint value TSOLL of the soldering temperature.
The measured actual value TIST of the soldering temperature is then entered into a subtracter 18 together with the time-dependent setpoint value TSOLL which calculates a setpoint to actual value deviation ΔT and inputs this to a controller 19.
Depending of the deviation ΔT between setpoint and actual value, the controller 19 generates an adjustment variable I* for the current source 14 so that the current source 14 adjusts the soldering current ILÖT accordingly, whereby the actual value TIST of the soldering temperature is controlled to the stipulated setpoint value TSOLL for the soldering temperature.
A special feature of this embodiment is that an open-loop controller 20 is provided instead of the closed-loop controller 19, whereby the open-loop controller 20 controls the current source 14 without a feedback in accordance with the set temperature-time profile.
Finally,
The invention is not restricted to the above-described preferred embodiments. Rather, a large number of versions and modifications are possible which similarly make use of the inventive concept and which therefore fall within the protective area. In addition, the invention also claims protection for the subject matter and features of the dependent claims irrespective of the features of the claims referred to.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2012 001 883.5 | Feb 2012 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2012/005350 | 12/21/2012 | WO | 00 |
