Method for thermally conductive joining of two joining partners

Abstract

According to a first aspect of the invention, a method for thermally conductive joining of at least two joining partners is proposed. In this method, after the application of a viscous medium, in particular a bead of adhesive, to at least one of the joining partners, the joining partners are pressed together. According to the invention, this pressing process is supported by vibration, such that the pressing force required to displace the viscous medium is reduced compared to a pressing force without vibration.

Description

[0001] The invention relates to a method for thermally conductive joining of two components. The components are, for example, a printed circuit board and a heat sink. State of the art

[0002] Thermal interface materials for the thermal bonding of printed circuit boards to metal housings, for example in engine control units, are typically dispensed in manufacturing in the form of adhesive beads (zigzag beads) onto the cooling surface of the metal housing parts. When the printed circuit board is joined between the housing parts, the thermal interface material is compressed to the smallest possible thickness in order to minimize thermal resistance.

[0003] If the joining forces are too great, the circuit board can bend significantly and damage can occur.

[0004] German patent application DE 10 2016 216 672 A1 discloses an electrical device comprising a housing, at least one printed circuit board, and at least one heat sink. The housing encloses a cavity in which the printed circuit board is mounted. The housing has a cover, the cover having at least one, two, or three pressure elements, in particular pressure bolts or pressure pins, mounted transversely to a flat extension of the cover so as to be longitudinally displaceable. The pressure element is designed to be inserted into the cavity and to press, at least indirectly or directly, against the printed circuit board, thereby pressing the printed circuit board against the heat sink. Disclosure of the invention

[0005] It can therefore be considered an object of the invention to provide a method for thermally conductive joining of two joining partners in which the joining forces are reduced and yet a predetermined gap dimension is achieved.

[0006] According to a first aspect of the invention, a method for thermally conductive joining of at least two joining partners is proposed. In this method, after the application of a viscous medium, in particular a bead of adhesive, to at least one of the joining partners, the joining partners are pressed together. According to the invention, this pressing process is supported by vibration, such that the pressing force required to displace the viscous medium is reduced compared to a pressing force without vibration.

[0007] In particular, no other fasteners, such as rivets, screws, or similar, are provided to join the components.

[0008] Vibration, in this context, means that at least one of the joining partners is set into mechanical vibration, particularly in the area where the viscous medium has been applied. This can be achieved, for example, by using an actuator that introduces mechanical vibrations at a predefinable frequency into at least one of the joining partners while a pressing force is applied.

[0009] The invention is therefore based on the finding that, through vibration, the pressing force required to displace the viscous medium is reduced compared to a pressing force without vibration. This means that a lower pressing force is required to achieve a specific gap between the two joining partners than without vibration. Furthermore, it has been found that the vibration according to the invention improves both process quality and cycle time. The lower pressing force also prevents high mechanical stress on one or both joining partners. Advantageously, smaller gaps between the joining partners can be achieved.

[0010] In a preferred embodiment of the invention, the viscous medium is configured as a so-called thermal interface material (TIM). The viscous medium can, for example, be an adhesive, in particular a thermally conductive adhesive. The adhesive can comprise a thermally conductive material (e.g., metal particles or metal oxide particles) and thus conduct heat particularly efficiently. Alternatively, the viscous medium can be configured as a thermally conductive medium without adhesive properties, for example, as a thermal paste.

[0011] In an advantageous embodiment, the joining partners are designed as a printed circuit board and a heat sink, which are thermally connected using the viscous medium. Printed circuit boards are particularly susceptible to damage because they can deflect considerably during pressing, depending on the shape of the joining partner and the properties of the viscous medium.

[0012] In an advantageous embodiment, ultrasonic vibrations with frequencies between 20 kHz and 1 GHz are introduced into the joining area, where the viscous medium is applied, during the pressing process. Alternatively, mechanical vibrations, for example in a frequency range of 20 to 200 Hz, can be introduced.

[0013] The application of vibration is intended to influence the flow behavior of the viscous medium, so that not only does the pressing force cause a distribution of the viscous medium, but the distribution is further enhanced by the vibration. The pressing force and / or the main direction of the vibration can preferably be oriented essentially perpendicular to a main direction of extension of at least one joining partner. This results in a particularly efficient pressing process.

[0014] In a preferred embodiment of the invention, a third joining partner is provided. The second joining partner, which is preferably configured as a printed circuit board, is pressed between the first and third joining partners in this example. For instance, the first joining partner can be configured as a housing base and the third joining partner as a housing cover. In this way, for example, an electronic device can be implemented in a housing, e.g., an engine control unit.

[0015] According to a second aspect of the invention, an electrical device is proposed. The electrical device comprises a housing with a lid and a base, and at least one printed circuit board. The housing encloses a cavity, and the printed circuit board is received in the cavity. According to the invention, the printed circuit board is thermally connected to the base and / or the lid of the housing, at least in an edge region, according to a method according to the first aspect. With such devices, the invention advantageously allows so-called no-go zones—that is, areas on the printed circuit board that would have to remain free of electrical or electronic components in conventional joining methods because the risk of damage would be too high—to be significantly smaller. Brief description of the characters

[0016] With reference to the attached figures, embodiments of the invention are described in detail. Fig. Figure 1 shows an assembly process according to the state of the art, in which a printed circuit board is pressed between a lid and a base of a housing. Fig. Figure 2 shows an assembly process according to a possible embodiment of the invention, in which a printed circuit board is pressed between a lid and a base of a housing, the pressing process being supported by vibration. Fig. Figure 3 shows a finished housing with a printed circuit board pressed into it, wherein the printed circuit board was thermally connected to the housing according to a possible embodiment of a method according to the invention. Fig. Figure 4 shows an experimental setup and several measurement curves, where a pressing force was measured over time. Fig. Figure 5 shows two further measurement curves, where a pressing force was measured over time. Preferred embodiments of the invention

[0017] In the following description of exemplary embodiments of the invention, identical elements are designated by the same reference numerals, and a repeated description of these elements may be omitted. The figures represent the subject matter of the invention only schematically. Fig. Figure 1 shows an assembly process for an electrical device 10' according to the prior art. Here, a printed circuit board 2 is pressed between a cover 1 and a base 2 of a housing. Before the actual pressing, a bead of adhesive made of a viscous medium 4 is applied to an edge region of the printed circuit board 2. A pressing force 7', 8', directed essentially perpendicular to a main dimension of the printed circuit board 2, is exerted on the edge region from both sides. The pressing forces 7', 8' cause the viscous medium 4 to spread in the dimension of the printed circuit board 2 until a predetermined gap is established between the printed circuit board 2 and the cover 1. While the pressing force 7', 8' is applied, the printed circuit board 2 can expand or contract, depending on the thickness of the adhesive bead, the properties and viscosity of the viscous medium 4 of which the adhesive bead consists (e.g.,(a thermally conductive adhesive) and the pressing force F cause it to bend significantly (as indicated by line 2'). This can lead to damage to the circuit board 2.

[0018] Fig. Figure 2 shows an assembly process for an electrical device 10 according to one possible embodiment of the invention. Here, too, a printed circuit board 2 is pressed between a cover 1 and a base 2 of a housing. Before the actual pressing, a bead of adhesive 4 is applied to an edge region of the printed circuit board 2. A pressing force 7, 8, directed essentially perpendicular to a main dimension of the printed circuit board 2, is exerted on the edge region from both sides. The pressing forces 7, 8 distribute the viscous medium 4 in the dimension of the printed circuit board 2 until a predetermined gap is established between the printed circuit board 2 and the cover 1. To assist the process, vibration is introduced into the cover 1, the viscous medium 4, and the printed circuit board 2 in the area where the viscous medium 4 is located.For this purpose, an actuator 6 is provided which exerts a mechanical vibration on the cover 1 in the designated area. The vibration is transmitted to the viscous medium 4 and the circuit board 2.

[0019] Experiments have shown that the introduced vibrations influence the flow behavior of the viscous medium 4, and that smaller pressing forces 7, 8 are required compared to the pressing forces without vibration to achieve a specific gap between the circuit board 2 and the cover 1. The deflection of the circuit board 2 (again indicated by line 2') is significantly lower in this case, and damage to the circuit board can be avoided.

[0020] Fig. Figure 3 shows an electrical device 10 assembled in this manner, with a printed circuit board 2 pressed between a base 3 and a cover 1, the pressing process being assisted by vibration. A viscous, in particular thermally conductive, medium 4 is applied between the printed circuit board 2 and the cover 1.

[0021] Fig. Figure 4 illustrates an experimental setup for determining the effect of vibration during the pressing of two joining partners with a viscous medium placed between them. Fig. 4 a) shows a measurement diagram 100, in which a respective pressing force 108 was measured and recorded over time for several pressing operations. Fig. Figure 4 b) shows the corresponding experimental setup 110. The joining partners 101 and 102 are first pressed together in such a way that a constant pressing speed is achieved when the viscous medium 104, for example a thermal paste, is displaced between them. Thus, at a specific time T (here after approximately 5.9 seconds), a specific gap between the joining partners 101 and 102 is reached during each measurement. The measured pressing forces initially increase to a maximum value K and then decrease again. The respective maximum pressing force at time T can be determined and compared from the measurement curves 62 and 64. Furthermore, a residual force K' can be determined after a specific waiting time t'.

[0022] The test measurements showed that when the joining partners 101, 102 were subjected to a vibration generated in this setup by an actuator 106 located below the joining partner 102, both the measured maximum pressing forces and the residual forces after a waiting period of 3 seconds were significantly lower than without vibration. Thus, bar 80 in diagram 100 shows a range of the measured maximum pressing forces without vibration. Bar 40 in diagram 100 shows a range of the measured maximum pressing forces with vibration between 3500 and 4500 revolutions per minute. While maximum pressing forces of 33.1 Newtons were measured without vibration, a maximum of 18.5 Newtons was measured with vibration. The residual force after a specific waiting period of t' = 3 seconds was approximately [value missing] without vibration.18 Newtons, with vibration loading a maximum of approximately 5 Newtons.

[0023] Fig. Figure 5 shows two measurement curves, 210 and 220, one of which was recorded during an alternative execution of the experiment. The experimental setup corresponds to that described in Figure 5. Fig. 4 b) the setup shown. For the upper curve 210, after pressing and a certain waiting time t' (e.g., approx. 3 seconds), the vibration was stopped and the residual force F, which essentially corresponds to the restoring force exerted by the viscous medium 104 on the joining partners 101, 102, was measured. This residual force F remained essentially constant after pressing over a measurement period of 60 minutes.

[0024] In the lower curve 220, after pressing, vibration was applied at a constant frequency (75 Hz or 4500 revolutions per minute in the example shown), and the residual force F, which essentially corresponds to the restoring force exerted by the viscous medium on the joining partners, was measured. The residual force F decreased essentially continuously over a measurement period of 60 minutes. After a time t0 of approximately 15 minutes, the restoring force even assumed essentially negative values, indicating that the viscous medium 104 was pulling the joining partners towards each other, thus further reducing the gap between the joining partners 101 and 102. An examination of the joining point (not shown) revealed that the continued vibration caused the viscous medium to be distributed much more widely and thinly between the joining partners 101 and 102.

[0025] This makes it advantageous to achieve even further reduced gap dimensions. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2016 216 672 A1

[0004]

Claims

[1] Method for the thermally conductive joining of at least two joining partners (1, 2, 3 101, 102), wherein the joining partners (1, 2, 3 101, 102) are pressed together after the application of a viscous medium (4, 104), in particular an adhesive bead, to at least one of the joining partners (1, 2, 3 101, 102), characterized by that the pressing process is supported by vibration, such that a pressing force for displacing the viscous medium is reduced compared to a pressing force without vibration. [2] Method according to claim 1, wherein the joining partners are designed as a printed circuit board (2) and a heat sink (1) which are thermally conductively connected by means of the viscous medium (4, 104). [3] Method according to one of the preceding claims, wherein the flow behavior of the viscous medium (4, 104) is influenced by the vibration application. [4] Method according to one of the preceding claims, wherein the viscous medium (4, 104) is designed in particular as a thermally conductive material (TIM) or as a thermally conductive adhesive. [5] Method according to one of the preceding claims, wherein during the pressing process, ultrasonic vibrations are introduced by the vibration application at least into a joining region in which the viscous medium (4, 104) is applied. [6] Method according to one of the preceding claims, wherein the pressing force (7, 8, 108) is oriented substantially perpendicular to a main extension direction of at least one joining partner (1, 2, 3 101, 102). [7] Method according to one of the preceding claims, wherein a third joining partner (3) is provided and a second joining partner (2), in particular a printed circuit board, is pressed between the first and third joining partners (1, 3), wherein the first joining partner (1) is designed in particular as a housing base and the third joining partner (3) is designed in particular as a housing cover. [8] Electrical device (10), comprising a housing with a cover (1) and a base (3) and at least one printed circuit board (2), wherein the housing encloses a cavity and wherein the printed circuit board (2) is received in the cavity, wherein the printed circuit board (2) is thermally conductively connected to the base (3) and / or the cover of the housing (1) at least in an edge region according to a method according to one of claims 1 to 7.

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