Patent Application
20180123014
The present invention relates to a thermoelectric device, in particular a sensor device, which is fed by a thermoelectric generator. Another aspect of the invention relates to a method for producing such a thermoelectric device.
The “Internet of things” is considered to be one of the most important future developments in information technology. According to this concept, not only do humans have access to the Internet, but devices are also networked with each other via the Internet or a similar network. One area relates to the automation of production and the household by means of sensor devices which are installed at suitable points in order to detect variables such as, for example, temperature, pressure, illumination, etc., and to make the variables available via the network, for example wirelessly. In order to minimize the effort required for installation, operation, and maintenance, sensor devices are utilized that obtain the electrical energy required for operation from the surroundings, independently of batteries or an electric connection, using so-called “energy harvesters”. In addition to sensor devices comprising solar cells, sensor devices in particular comprising thermoelectric generators are known, which obtain energy from a temperature difference that is present in its surroundings, e.g., for the purpose of installing the sensor device on a heating system.
The thermoelectric generators utilized for this purpose typically consist of an upper substrate and a lower substrate which are connected to each other via pins which are made of a thermoelectric material and are thermally connected in parallel and are electrically connected in series. An electric voltage is generated within the thermoelectric generator by means of the Seebeck effect when there is a temperature difference between the upper substrate and the lower substrate. In order to allow for a close thermal contact, during operation, with two areas having different temperatures, when such a thermoelectric generator is packaged in an electronics housing, the thermoelectric generator is usually rigidly connected on its top side and bottom side comprising thermally conductive materials to housing walls which are in contact with different temperature zones during operation. As a result, however, any impacts, vibrations, or thermomechanical strains are directly transferred to the pins of the thermoelectric generator and can result in damage to the pins and, therefore, to the entire device.
DE 10 2011 075661 A1 discloses a thermoelectric arrangement which comprises a thermoelectric component which is disposed on a carrier and is located under a cover which is also disposed on the carrier. A plate-like, thermally conductive compensating material which has elastic properties, in particular, is disposed between a hot side and/or a cold side of the thermoelectric component and the carrier and the cover.
Thermomechanical strains, impacts or vibrations cause shear forces, in particular, however, which are absorbed only to a limited extent by a compensating material having a limited thickness. It is desirable to integrate a thermoelectric generator into a thermoelectric device such as a sensor device, for example, in a compact manner, and so the shear forces on the pins of the thermoelectric generator are reduced.
A thermoelectric device comprising a printed circuit board, an electrically supplied component such as, e.g., a sensor, disposed on the printed circuit board, a cover which covers the printed circuit board, a thermoelectric generator, and a spring unit is therefore provided. The thermoelectric generator is thermally connected to the printed circuit board and to the cover in order to generate an electric supply voltage for the component from a temperature difference between the printed circuit board and the cover, wherein the spring unit resiliently holds the thermoelectric generator between the printed circuit board and the cover. The thermal connection of the thermoelectric generator to the printed circuit board can also consist of the thermoelectric generator being thermally connected to a metal structure which is formed as part of the printed circuit board. The thermal conductivity of metals is typically one hundred times greater than the thermal conductivity of an electrically insulating base material of a printed circuit board. For example, in a composite material which comprises base material and metal paths and forms the printed circuit board, heat is transported primarily via the metal paths, and so directional guidance is possible.
Another aspect of the invention relates to a method for producing such a thermoelectric device. The production method includes a step of disposing a component such as, for example, a sensor on a printed circuit board, a step of covering the printed circuit board with a cover, a step of thermally connecting a thermoelectric generator to the printed circuit board and to the cover, in order to generate an electric supply voltage for the component from a temperature difference between the printed circuit board and the cover, and a step of providing a spring unit which resiliently holds the thermoelectric generator between the printed circuit board and the cover.
It should be noted that terms related to spatial directions, such as “on”, “cover”, “upper” and “lower”, etc., in the present description merely relate to a relative orientation within the thermoelectric device, unless expressly indicated otherwise. In particular, there is no intention to refer to a preferred orientation of the device with respect to gravity.
The fact that the cover covers the printed circuit board on which the component is mounted allows for a particularly compact design of the thermoelectric device, because the printed circuit board can simultaneously provide electric connections between the component, the thermoelectric generator, etc., and can function as a lower housing wall. Given that the thermoelectric generator is resiliently held by the spring unit between the printed circuit board and the cover, shear forces and other forces occurring in the production of the thermomechanical device due to, for example, reflow soldering or annealing steps; in the further processing when, for example, the thermoelectric device is to be connected to a further substrate; or during operation of the thermoelectric device due to thermomechanical strains, vibrations, impacts, etc., between the printed circuit board and the cover, are compensated for by the spring unit, which reduces the mechanical load on the thermoelectric generator. This makes it possible to continuously establish the thermal connection, according to the invention, of the thermoelectric generator to the printed circuit board and to the cover, by means of highly thermally conductive materials such as, for example, metal materials, in particular without the need to incorporate an e.g., plate-shaped compensating material into the heat path, whereby, overall, a reduction of the mechanical load on the thermoelectric pins of the thermoelectric generator can be achieved while ensuring a consistently good thermal connection.
It should be noted that the reduction of the mechanical load of the thermoelectric generator also advantageously extends to influences which occur due to mechanical and thermal loads during production, such as, for example, sawing the printed circuit board from a larger piece during transportation to the place of use, or during installation at the place of use.
According to one preferred refinement, the spring unit comprises at least one spring which is disposed between the printed circuit board and the thermoelectric generator. This protects the thermoelectric generator, already during the production of the thermoelectric device, against vibrations, for example, when sawing individual printed circuit boards—which comprise pre-installed thermoelectric generators—apart from a larger composite plate. Preferably, the spring is formed essentially from a base material of the printed circuit board. This makes it possible to design the spring in a simple way during the production of the printed circuit board, and so a separate production and assembly of the spring unit is superfluous.
According to one preferred refinement, the printed circuit board comprises a lower, middle, and upper printed circuit board layer, wherein the spring is essentially formed from the upper printed circuit board layer and the middle printed circuit board layer comprises a recess in the area of the spring. This makes it possible to design the spring in a particularly simple way in the case of a low height of the thermoelectric device by way of the recess providing freedom of movement for the spring and the lower printed circuit board providing protection.
According to one preferred refinement, a metal path is formed on the spring, which deflects the heat in a specific direction in order to provide for a thermal coupling to the underside, for example, by means of thermal vias through the printed circuit board. Furthermore, the metal paths can be efficiently formed in a joint process with electric conductive paths.
According to one preferred refinement, at least one metal passage is formed through the printed circuit board and thermally connects the thermoelectric generator to the underside of the printed circuit board. This allows for a particularly good thermal connection to a temperature range which lies under the printed circuit board, outside the thermoelectric device, during operation.
According to one preferred refinement, the spring unit comprises at least one spring which is disposed between the cover and the thermoelectric generator. This allows for particularly good protection of the thermoelectric generator against mechanical influences such as, for example, vibrations via the cover of the thermoelectric device.
According to one preferred refinement of the method according to the invention, the thermal connection includes a step of forming a sacrificial layer which at least partially hinders the resilience of the spring unit, a step of fixing the thermoelectric generator to the spring unit after the formation of the sacrificial unit, and a step of removing the sacrificial layer after the fixing of the thermoelectric generator. Given that the sacrificial layer stabilizes the spring unit during the fixation, so that the stability necessary for the fixation does not need to be provided by the spring unit itself, the spring unit can be designed to be particularly soft and protective for the thermoelectric generator.
Unless expressly mentioned otherwise, the same reference numbers in the figures refer to identical or equivalent elements.
The thermoelectric device 100 comprises a rectangular printed circuit board 102 which is formed in multiple layers, including a lower printed circuit board 121, a middle printed circuit board 122, and an upper printed circuit board 123 comprising an electrically insulating base material such as, e.g., fiber-reinforced plastic. The printed circuit board layers 121-123 are fixedly connected to each other, for example, by means of lamination. An electric conductive path 126 is shown between the upper printed circuit board layer 123 and the middle printed circuit board layer 122, by way of example, wherein further conductive paths—which are not shown, for the sake of clarity—can be formed in different levels on a top side 119 of the printed circuit board 102, which is formed by the upper printed circuit board layer 123, on an underside 118 of the printed circuit board 102, which is formed by the lower printed circuit board layer 121, and in intermediate layers between the printed circuit board layers 121-123. Furthermore, five electric plated through-holes 127 are shown, by way of example, which extend through the middle printed circuit board layer 122 or through the middle printed circuit board layer 122 and the upper printed circuit board layer 123, wherein further plated through-holes—which are not shown, for the sake of clarity—can be provided in order to electrically connect conductive paths in different levels to each other.
The thermoelectric device 100 also comprises a further component 104 disposed on the printed circuit board 102, for example an integrated circuit, a light-emitting diode, or a sensor, which is to be assumed to be a temperature sensor, by way of example, in the present embodiment, but which can also be any other type of sensor, such as, for example, a light sensor, a sound sensor, a field strength sensor, a vibration sensor, a position sensor, an acceleration sensor, a rotation sensor, a pressure sensor, or a moisture sensor, or any other type of electric component. The component 104 is mechanically fixed, for example, by means of adhesion, on the top side 119 of the printed circuit board 102 and is connected to electric plated through-holes 127 in the printed circuit board 102 by means of lead wires 105. In alternative embodiments, the lead wires can also be laid on any type of conductive structures on the substrate, which, in turn, can be electrically conductively connected to electric plated through-holes on another level. The thermoelectric generator 180 is also disposed on the printed circuit board 102. In addition to the component 104 and the thermoelectric generator 180, even further components can be disposed on the printed circuit board 102, such as sensors, microcontrollers, resistors, coils, radio modules, etc., which are not shown in the figures, however, for the sake of simplicity. Such components can be mechanically and electrically connected to the printed circuit board 102, for example via adhesion or bonding, wherein flip chip is also possible. In the present embodiment, components are disposed only on the top side 119 of the printed circuit board 102, although they can also be disposed on the underside 118 in alternative embodiments.
The thermoelectric generator 180 has two temperature sides 181, 182, which are positioned opposite each other, a cold side 181 of which faces away from the printed circuit board 102 and a hot side 182 of which faces the printed circuit board 102 in the present embodiment. It should be noted that, in alternative embodiments, the cold side 181 and the hot side 182 can also be reversed or each of the temperature sides 181, 182 can be operated both as a hot side and a cold side. The thermoelectric generator 180 is designed to supply the thermoelectric device 100, including the component 104, with an electric supply voltage U when there is a predetermined temperature difference between the temperature sides 181, 182. For this purpose, the thermoelectric generator 180 is electrically connected to the printed circuit board 102 by means of flexible wire bonds 184, and so the resultant supply voltage U can be utilized for operating the remaining components or for charging an energy accumulator (not shown).
The thermoelectric generator 180 is fixed via its cold side 181 to the cover 106 of the thermoelectric device 100 by means of heat-conductive paste 132. The cover 106 has a flat box shape, which is open toward the bottom, and has a roof surface 161 which has a matching outline in the projection perpendicularly to the printed circuit board 102, and has four lateral surfaces 162 which extend from the edge of the roof surface 161 perpendicularly downward to the printed circuit board 102, where they are fixedly connected to the printed circuit board 102, e.g., via adhesion. The heat-conducting paste 132 is used for thermally connecting the cold side 181 of the thermoelectric generator 180 to the cover 106 and, simultaneously, for tolerance compensation when the cover 106 is placed onto the printed circuit board 102 during the production of the thermoelectric device 100.
In the area of the thermoelectric generator 180, a recess 114 is formed in the middle printed circuit board layer 122, the outline of which completely encloses the thermoelectric generator in the projection perpendicular to the printed circuit board 102. Along the edge of the recess 114, multiple decoupling slots 113 are formed in the upper printed circuit board, between which thin webs 112 remain, which mechanically and thermally connect an island area 115 of the upper printed circuit board layer 123 enclosed by the decoupling slots 113 to the remaining area of the upper printed circuit board layer 123 located in the projection perpendicular to the printed circuit board 102 outside the recess 114.
In the present embodiment, the recess 114 and the island area 115 are rectangular, by way of example, although they can be designed in other embodiments to be, e.g., circular, or to have other suitable shapes, including different shapes. Furthermore, in the present embodiment, by way of example, four of the decoupling slots 113 are each formed having a trapezoidal shape with the long base side along one of the four lateral edges of the rectangular recess 114, and so, in each case, two leg sides of adjacent trapezoidal decoupling slots 113, extending in parallel to each other, delimit one of four webs 112, each of which extends from one corner of the rectangular recess 114 in the direction of the center of the recess 114.
On the top side 119 of the printed circuit board 102, a heat-conductive metal path 116 is formed essentially on the entire island area 115, on two of the webs 112, and in a levee area 125 extending between these webs, opposite the island area 115. The webs 112 are distinguished by the fact that they have very good heat-conducting properties. The metal path 116 can be formed, e.g., as a part of a structured metallization layer together with electric conductive paths (not shown in the figures) on the top side 119 of the printed circuit board 102, which simplifies the production of the thermoelectric device 100. The metal path 116 can also be formed independently of such conductor tracks, e.g., having a greater thickness, or can be formed of metal having high thermal conductivity. For example, the metal path 116 having a thickness of 18 to 100 μm is made of copper, the thermal conductivity of which, at 350 W/mK, is substantially higher than the thermal conductivity of typical printed circuit board materials. The metal path 116 can additionally be coated with an oxidation protection made of NiPdAu or similar alloys.
The thermoelectric generator 180 is fixed, via its hot side 182 and by means of heat-conductive adhesive or heat conductive paste, to the section of the metal path 116 covering the island area 115. Formed within the levee area 125 is a thermal plated through-hole 117 through the printed circuit board 102, which thermally connects the metal path 116 to the underside 118 of the printed circuit board 102. The thermal plated through-hole 117 can be designed, e.g., in the form of a fully copper-plated sleeve or a copper insert.
Since the recess 114 is formed underneath the island area 115, the aforementioned design allows the island area 115 to be elastically displaced and/or tilted in different spatial directions with respect to the rest of the printed circuit board 102 within limits which can be suitably predetermined by means of the number, arrangement, and dimensions of the webs 112, the elasticity of the base material of the printed circuit board 102, and the thicknesses of the printed circuit board layers 121 to 123. The webs 112 are therefore springs of a spring unit which resiliently holds the thermoelectric generator 180 between the printed circuit board 102 and the cover 106 and simultaneously provides a thermal connection of the thermoelectric generator 180 to the printed circuit board 102.
The number and dimensions of the webs 112 are to be preferably selected in such a way that the spring unit is flexible enough to absorb thermomechanical strains or to reduce vibrations, but is simultaneously stiff enough to fix the thermoelectric generator 180 on the island area 115 during the production of the thermoelectric device 100 by means of adhesion or bonding, for example. Therefore, a parallel connection of multiple thin resilient webs 112 is particularly suitable, and so the individual webs 112 can absorb strains, but the spring unit, overall, is so stiff that the island area 115 is sufficiently stable in its position during installation of the thermoelectric generator 180.
For example, the spring unit has a total spring rate between 5 kN/m and 500 kN/m with respect to horizontal and vertical deflection. An advantageously particularly softly resilient hold of the thermoelectric generator 180 having a spring rate below 5 kN/m can be made possible by way of the island area 115 being supported by a temporary sacrificial layer 130 during the installation of the thermoelectric generator 180. This sacrificial layer 130 can be formed, for example, from a thermally decomposable polymer, a water soluble adhesive, or the like. By means of this temporary stiffening of the spring unit, reliable assembly and wire bonding of the thermoelectric generator 180 is ensured.
For example, in the present embodiment, the thickness of the upper printed circuit board layer 123, from which the resilient webs 112 are formed, is between approximately 0.2 mm and 0.4 mm, the width of the webs 112 is between 0.2 mm and 1 mm, and the length is less than 2 mm, whereby the total spring rate of the spring unit can be reduced to 1 kN/m. In this case, a modulus of elasticity of 30 GPa is assumed for printed circuit boards or glass fiber epoxy system.
-shaped profile as in
In the case of the spring 112 shown in
Depending on the exact geometry and the metal that was selected, the thickness of the metal spring 112 is preferably not less than 0.1 mm, and the width is also not less than 0.1 mm. Despite the modulus of elasticity, which is 130 GPa for copper, for example, and is higher as compared to the printed circuit board material utilized for the spring in the first embodiment, a spring rate of a few kN/m can be achieved by means of a suitable selection of the geometry also in the case of the metal springs 112. For example, a spring rate of ˜10 kN/m can be achieved for one of the spring legs 144 by way of the spring 112 shown in
A production method for a thermoelectric device 100, as shown in
First, in step 902, a spring consisting of webs 112 is formed from a printed circuit board layer which forms the upper printed circuit board layer 123 in the thermoelectric device 100 by punching out decoupling slots 113 which surround an island area 115, as shown in
Subsequently, in step 920, a component 104 to be electrically supplied, such as, e.g., a sensor, and further electronic components are installed on the printed circuit board 102 and are electrically connected to the printed circuit board 102 by means of wire bonding or the like. In the subsequent step 942, which can also take place already within the scope of step 900, however, a sacrificial layer 130 comprising a polymer material is provided in the recess 114 in the printed circuit board 102, which reinforces the spring unit. In step 944, the thermoelectric generator 180 is fixed on the island area 115 held by means of the webs 112 of the spring unit and by the sacrificial layer 130. In step 946, the sacrificial layer 130 is removed, e.g., by the effect of heat or by means of a suitable solvent. In the following step 960, a heat-conductive adhesive paste 132 is applied onto the thermoelectric generator 180 and a cover 106 is installed over the printed circuit board. 102, and so the thermoelectric generator 180 comes into contact with the heat-conductive adhesive paste 132 and adheres on the cover 106. In the end, the thermoelectric generator 180 is resiliently held between the printed circuit board 102 and the cover 106.
The steps 942, 944, 946 and 960, together with further steps, if necessary, form a higher-order step 940 of the thermal connection of the thermoelectric generator 180 to the printed circuit board 102 and to the cover 106, and so, during operation of the thermoelectric device 100 produced therewith, the thermoelectric generator 180 can generate an electric supply voltage U for the component 104 and the thermoelectric device 100 as a whole from a temperature difference between the printed circuit board 102 and the cover 106.
The upper spring 111 can be designed having different shapes and can be made of a metal such as copper or aluminum, as is the case with the springs shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2015 207 857.4 | Apr 2015 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/058955 | 4/22/2016 | WO | 00 |
