Patent Application
20240334593
The present patent document claims the benefit of United Kingdom Patent Application No. 2304705.3, filed Mar. 30, 2023, which is hereby incorporated by reference in its entirety.
The present disclosure relates to a holding down construction for a printed circuit board and a corresponding printed circuit board assembly.
It is known to press printed circuit board (PCB) based power electronics devices to a heat sink for cooling, whereby the power electronics devices to be cooled may be designed as surface-mounted devices (SMD) or through-hole technology (THT) devices and located on the underside of the PCB. To compensate for height tolerances, it is further known to press such PCB based power electronics devices against a heat sink through a thermal interface material. As such thermal interface material reduces the cooling performance compared to a direct contact, there is a desire to reduce the thickness of such thermal interface material to the extent possible.
The use of holding down clamps represents an effective means to limit the deformation of a printed circuit board. Preventing bending of the printed circuit board alone already leads to an increase in lifetime and reliability. Further, by limiting the deformation of a printed circuit board through a holding down clamp, the pressure between a power electronics device connected to the printed circuit board and a heat sink may be increased. The higher the pressure between the heat sink and the component to be cooled, the greater the thermal conductivity and thus the cooling effect of the heat sink. The increased pressure further allows to limit the thickness of the thermal interface material and keep such thickness within a predefined tolerance range.
Printed circuit boards with power electronics devices used in electric drives located in aircrafts need to be robust against temperature changes, with temperatures varying between −40° C. and 150° C. Because a printed circuit board includes several different materials stacked upon each other, the bending of a printed circuit board is unavoidable with temperature change because of different coefficients of thermal expansion of the different materials. The displacement of the printed circuit board may increase the distance between the critical components from the heat sink, setting up high requirements towards the thermal interface material and potentially distorting the board critically.
The present disclosure provides a holding down construction that holds down a printed circuit board over a wide temperature range in a manner that addresses the forementioned issues or at least provides a useful alternative to known holding down constructions for printed circuit boards.
The scope of the present disclosure is defined solely by the appended claims and is not affected to any degree by the statements within this summary. The present embodiments may obviate one or more of the drawbacks or limitations in the related art.
According to a first aspect, a holding down construction for a printed circuit board is provided. The holding down construction includes a grid structure including first ribs and second ribs arranged in a crossed manner, wherein the first ribs and the second ribs form intersections, and wherein the structure has an upper side and a lower side. The holding down construction further includes screwing posts attached to the structure and configured to be screwed against a printed circuit board and/or another component such as a heat sink, and contact structures attached to the lower side of the structure and configured to lie against and exert a force against the printed circuit board.
Aspects of the disclosure are thus based on the idea of providing a holding down construction in the form of a grid structure to which screwing posts and contact structures are attached (including integrated). The grid structure is naturally flexible and allows to provide a counterforce against any deformation of the printed circuit board caused by a temperature change or by other reasons in an efficient manner, wherein the counterforce is exerted on the circuit board through the contact structures of the holding down construction that may be formed as pins. Bending away of the printed circuit board from a heat sink may thus be efficiently prevented or reduced. At the same time, the screwing posts allow to connect the holding down construction to the printed circuit board and/or a heat sink, providing sufficient load for the fixation.
A further advantage associated with the present holding down construction lies in that, by using a grid structure, a lighter holding down construction with less material is provided for compared to a solid holding down construction.
The holding down construction may be formed as one part, wherein the screwing posts and the contact structures are integrated into the holding down construction. The holding down construction may be formed by a non-conductive material such as a polymer, in particular for high-voltage applications. However, at noncritical areas such as at the screwing posts metallic components such as metallic compression limiters may be applied for improved screwing of the holding down construction to a printed circuit board assembly.
It should be noted that the side of the holding down construction facing a printed circuit board is referred to as the lower side of the holding down construction, regardless of the actual spatial orientation of the holding down construction.
According to an aspect of the disclosure, the holding down construction is formed elastically, wherein, when the printed circuit board bends towards the holding down construction, the first ribs and/or the second ribs are elastically deflected at least along sections, thereby providing a counterforce acting against the bending of the printed circuit board. The counterforce is provided through the contact structures of the holding down construction located at the ribs. By having a flexible grid structure that includes ribs, a strong counterforce may be created by deflection of the ribs, wherein the counterforce may be proportional to the deflection.
According to a further aspect, the holding down construction is elastically deformable by a temperature change, wherein, upon an increase and/or decrease in temperature comparted to an assembly temperature, the first ribs and/or the second ribs increase their pressure towards the printed circuit board at least along sections. The increased pressure on the printed circuit board is provided through the contact structures of the holding down construction. This aspect turns a problem into a solution in that a temperature change, which leads to an undesired deformation of the printed circuit board, at the same time automatically increases the downward force by the holding down construction on the printed circuit board, thereby counteracting the deformation of the printed circuit board.
To increase the pressure towards the printed circuit board upon a temperature change, a plurality of different constructions of the ribs are possible. The temperature change may lead to a lengthening or shortening of ribs or sections of the ribs, which causes the respective sections or ribs to bend towards the printed circuit board, thereby increasing the pressure exerted through the contact structures on the printed circuit board. Specific forms of the ribs or sections of the ribs are required to achieve that a lengthening or shortening translates into an increased downward pressure.
In some embodiments, at least one section of the first rib and/or at least one section of the second rib is V-shaped towards the printed circuit board. The V-shape may both be at the lower side and at the upper side of the rib or, alternatively, be at the lower side of the rib only, in which case the upper side of the rib may be flat. The V-shape leads automatically to an increased downward pressure if the section forming the V-shape lengthens due to an increased temperature.
In some embodiments, at least one section of the first rib and/or at least one section of the second rib has a curved shape, the curved shaped section being convex towards the printed circuit board. The curved shape may be both at the lower side and at the upper side of the rib or, alternatively, be at the lower side of the rib only, in which case the upper side of the rib may be flat. The curved shape leads automatically to an increased downward pressure if the section forming the curved shape lengthens due to an increased temperature. The form of the curve of the curved shape may be circular.
In some embodiments, at least one section of the first rib and/or at least one section of the second rib forms a curved V-shape towards the printed circuit board, the curved V-shaped section including two subsections convexly curved towards the printed circuit board. Such form may also be referred to as arch shaped. The curved V-shape leads automatically to an increased downward pressure if the section forming the curved V-shape lengthens due to an increased temperature.
In some embodiments, at least one section of the first rib and/or at least one section of the second rib forms a diamond shape, the diamond shaped section including an upper V-shaped subsection, a lower V-shaped subsection, and an inner hole between the upper V-shaped subsection and the lower V-shaped subsection. A bending down and increased pressure of the rib section may be reached both when the temperature lowers and when the temperature rises compared to the assembly temperature. More particularly, when the temperature is lower than the assembly temperature, the upper V-shaped section bends downwards and provides an extra downward force, and when the temperature is higher than the assembly temperature, the lower V-shaped section bends downwards and provides an extra downward force.
In some embodiments, at least one section of the first rib and/or at least one section of the second rib is formed with a T-shape, the T-shaped section including a horizontal subsection extending in the direction of the respective rib and a perpendicular subsection extending towards the printed circuit board. Further, the horizontal subsection is attached at its longitudinal ends to a bimaterial that includes two materials joined together and has a different coefficient of thermal expansion. For example, the bimaterial is a bimetal. Such bimaterial converts a temperature change into a mechanical displacement as the different coefficient of thermal expansion cause the bimaterial to bend, wherein the bending is transferred on the horizontal subsection of the T-shaped section such that an additional downward force of the perpendicular subsection is created.
In such embodiments, by choosing if the inner material or the outer material of the bimaterial has a higher coefficient of thermal expansion, an increased downward pressure may be created either when the actual temperature is below the assembly temperature or when the actual temperature is above the assembly temperature.
According to some embodiments, the at least one section of the first rib or the at least one section of the second rib extends between two of the screwing posts or between two of the intersections. Naturally, an elastic, bendable section needs to be bounded at its ends by stiffer elements between which the bending takes place. Such stiffer elements may be the screwing posts at which the holding down construction is fixed against a printed circuit board and heat sink. Alternatively, such stiffer elements may be formed by other areas of the holding down construction, such as the intersections of the first ribs and second ribs that include an increased rigidity.
In some embodiments, the at least one section of the first rib or the at least one section of the second rib forms or is connected at its point closest to the printed circuit board to one of the contact structures configured to lie against and exert a force against the printed circuit board. This ensures that the increased downward pressure is directly transferred onto the printed circuit board, thereby preventing or reducing bending of the printed circuit board towards the holding down construction.
It may be provided that the holding down construction is configured to be mounted on the printed circuit board at an assembly temperature under a preload that provides a force acting on the printed circuit board even when the printed circuit board is not bending towards the holding down construction. Providing a preload ensures that even at assembly temperature the printed circuit board and electric modules attached to the underside of the printed circuit board are pressed against a heat sink for efficient heat transfer.
In some embodiments, the screwing posts are attached to and arranged spaced to each other along the first ribs. The screwing posts and the first ribs may be formed of a non-conductive material. However, metal components such as compression limiters may be added to the screwing posts. The screwing posts may be at positions along the first ribs located in the middle between neighboring intersections of the first ribs and the second ribs.
Further, the contact structures may be arranged along the second ribs. For example, the contact structures may be located between intersections of the first ribs and the second ribs.
In some embodiments, the contact structures configured to lie against and exert a force against the printed circuit board may include at least one pushing pin configured to punctually assert a pressure against the printed circuit board. Accordingly, the ribs assert a force on the printed circuit board at individual points or small areas that allows to assert a high-pressure at these points against the printed circuit board.
The first ribs and the second ribs may run perpendicularly with respect to each other. However, the first ribs and the second ribs may alternatively be angled with respect to each other under an angle different than 90 degrees.
The first ribs, which run parallel to each other, are not necessarily completely identical. There may be some ribs formed differently than other ribs. For example, at least one or most of the ribs may run straight. One or several other ribs may have a different form from a straight line, and, e.g., may include curved sections. The form of the ribs may be configured to run around electric components located on the printed circuit board.
The second ribs may also run parallel to each other and may also do not need to be completely identical. However, in some embodiments, all second ribs run straight.
The holding down construction may be designed as a 3D printed part. However, other forms of manufacture such as casting or injection molding are possible as well. Also, the holding down construction may include several parts assembled together.
In a second aspect, a printed circuit board assembly includes: a printed circuit board having an upper side and a lower side; electric modules having an upper side and a lower side; a heat sink; and a holding down construction for the printed circuit board. The electric modules are connected at their upper sides to the lower side of the printed circuit board. The electric modules are thermally coupled at their lower sides to the heat sink. Additionally, the holding down construction is in accordance with the first aspect and arranged on the upper side of the printed circuit board.
In certain embodiments, the electric modules each include a power semiconductor element. The electric contacts of the electric modules are located at the upper side and electrically connected to respective electric contacts at the lower side of the printed circuit board. For example, each electric module includes three electric contacts in the form of solder pads at the upper side that provide for a gate voltage, a drain voltage, and a source voltage for the power semiconductor element. At the lower side, the electric module may form a thermal interface that is electrically insulated from the power semiconductor element. To this end, one or several ceramic layers may be included in the electric module. The thermal interface of the electric module may be thermally coupled to a heat sink. A thermal interface material may be arranged between the thermal interface of the electric module and the heat sink.
The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and/or combined with any other feature or parameter described herein.
The disclosure is explained in more detail on the basis of embodiments with reference to the accompanying drawings in which:
Aspects and embodiments of the present disclosure are now discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art.
The holding down clamp 100 includes a grid structure 1. The grid structure includes first ribs 10, 10a and second ribs 20 arranged in a perpendicular manner. Alternatively, the first ribs and the second ribs may be arranged in an angled manner with an intersection angle different than 90 degrees. The first ribs 10, 10a and the second ribs 20 form intersections 13. The structure 100 and, accordingly, the first and second ribs 10, 10a, 20 include an upper side 11 and a lower side 12.
The holding down clamp 100 further includes screwing posts 2. The screwing posts 2 are configured to be screwed against a printed circuit board and a heat sink, as discussed with respect to
The holding down clamp 100 further includes contact structures 3 attached to the lower side 12 of the grid structure 1. The contact structures 3 are configured to lie against a printed circuit board and exert a force on the printed circuit board, as discussed with respect to
In certain embodiments, the first ribs may include two ribs 10 that run straight. The straight ribs 10 include straight sections 15 that extend between the intersections 13. With the straight ribs 10, the screwing posts 2 are each located in the middle between two intersections 13.
The first ribs further include a rib 10a that does not run completely straight, even though still having the same longitudinal extension as the other ribs 10 and still running overall parallel to the other ribs 10. The rib 10a includes concavely curved sections 15a. The curved sections 15a each extend between two neighboring screwing posts 2.
It is further pointed out that both on the upper side 11 and on the lower side 12 of the second ribs 20 a web 14, 16 is formed that increases the stiffness of the second ribs 20. The pushing pins 3 extend from the web 14 arranged on the lower side 12 of the second ribs 20.
At the intersections 13, holes 16 may be formed in the grid structure 1. The holes are encircled by parts of the mentioned webs 14, 16.
The holding down clamp 100 may be formed by 3D-printing.
According to
Electric modules 5, which may be power semiconductor modules, are attached with their upper side 51 to the lower side 42 of the printed circuit board 4 as shown in
As may be seen in
Further, in embodiments, a construction of the holding down clamp is provided in which the pressure of the pushing pins 3 onto the circuit board 4 is even increased if the temperature changes compared to an assembly temperature, thereby even more effectively producing a counterforce acting against bending upwards of the printed circuit board 4. This is achieved in embodiments by a downward curving shape of the ribs or sections of the ribs, the downward curving shape leading to an increased downward pressure if the rib lengthens due to a temperature increase.
Several constructions that provide for an increased pressure of a pushing pin 3 onto the printed circuit board 4 in case of a temperature change are discussed with respect to
Such effect is achieved in a similar manner in the embodiment of
Such effect is achieved in a similar manner in the embodiment of
It should be understood that the above description is intended for illustrative purposes only and is not intended to limit the scope of the present disclosure in any way. Also, those skilled in the art will appreciate that other aspects of the disclosure may be obtained from a study of the drawings, the disclosure and the appended claims. All methods described herein may be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. Various features of the various embodiments disclosed herein may be combined in different combinations to create new embodiments within the scope of the present disclosure. In particular, the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. Any ranges given herein include any and all specific values within the range and any and all sub-ranges within the given range.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2304705.3 | Mar 2023 | GB | national |
