Method and apparatus for determining the quality of a sintered paste layer

Abstract

Method for determining the quality of a sintered paste layer (2) comprising the following process steps: a) Applying a first section (21) of the sintered paste layer (2) to a non-electrically conductive area (32) of a surface (30) of a substrate (3) wherein the first section (21) of the sintered paste layer (2) has a meandering shape; b) Measuring the electrical resistance of the sintered paste layer (2) between two measuring points and determining the specific electrical resistance of this sintered paste layer (2) during a process in which the substrate (3) is processed in a plant.

Description

[0001] The invention describes a method for determining the quality of a sintered paste layer and a device for this purpose. Such a sintered paste layer is particularly suitable for the pressure sintering of two components, especially two components of a power electronic switching device. The components can, by way of example, be a substrate and a semiconductor device or a connecting device. Here and in the following, a sintered paste layer is understood to mean any layer of a sinterable paste arranged on a surface. It is irrelevant whether this layer is freshly applied, in the drying process, dried, undergoing sintering, or whether the sintering process is already complete.

[0002] From the prior art, exemplified in DE 10 2018 130 719 B3, a method for determining the quality of preferably a sintered paste layer is known, comprising the following process steps: a) Applying a moist sintered paste to a bonding partner over a surface, thereby creating a moist sintered paste layer; the bonding partner can, for example, be a substrate, a power semiconductor device, or a terminal element of a power semiconductor module; b) Drying the moist sintered paste layer and converting it into a sintered paste layer that is at least partially dry; c) Moving a test instrument along a movement profile with a main direction of movement parallel to the surface and creating a trench in the sintered paste layer with a mean first depth and measuring the force required for this movement; d) Determining the quality of the sintered paste layer based on the force measurement.

[0003] JP 5 922 561 B2 discloses a preparation state evaluation system for a conductive paste, comprising: a combinatorial sample preparation device for using the mixing ratio of two or more types of paste samples to form a conductive paste as at least one parameter, and producing a plurality of pastes differing in composition on a substrate at predetermined positions by mixing two or more types of paste samples to form the conductive paste while changing the parameter; a firing agent for forming a plurality of fired bodies by firing the plurality of conductive pastes; means for measuring the electrical conductivity of the plurality of fired bodies; means for analyzing the compositions of the plurality of fired bodies;and means for correcting the mixing ratios of the two or more types of paste samples for each fired body based on a result of the composition analysis and evaluating the several conductive pastes from the relationship between the mixing ratio after correction and the electrical conductivity, producing a conductive material.

[0004] XU, Di Erick [et al.]: Real-time resistance monitoring during sintering of silver paste. In: Journal of alloys and compounds, Vol. 731, 2018, pp. 504–514. ISSN 0925-8388 discloses a method for investigating the silver sintering process using real-time resistance monitoring. The compound resistance signals show events such as a resistance increase to over 10 GΩ, an abrupt resistance drop from over 10 GΩ to below 1 kΩ, and a gradual resistance drop to below 1 mΩ. Based on cross-sections of samples at different stages of sintering and differential scanning calorimetry (DSC), a correlation between the resistance signal and solvent evaporation, capping agent degradation, and silver sintering is revealed.

[0005] In light of these circumstances, the invention aims to present an alternative method for determining the quality of a sintered paste layer and a device for applying this method, wherein the method is accessible for in-situ application.

[0006] This problem is solved according to the invention by a method for determining the quality of a sintered paste layer comprising the following process steps: a) Applying a first section of the sintered paste layer to a non-electrically conductive area of ​​a substrate surface, wherein the first section of the sintered paste layer has a meandering shape; b) Measuring the electrical resistance of the sintered layer between two measuring points and determining the specific electrical resistance of this sintered paste layer during one cycle of a treatment process in which the substrate is processed in a plant.

[0007] In process step a), it can be particularly advantageous to simultaneously apply a first, a second and a third section of the sintered paste layer, wherein the second and third sections are each applied to an electrically conductive contact area of ​​the substrate and wherein the sintered paste layer is formed continuously, and wherein the respective contact area is preferably only partially, i.e. not completely, covered and thus only in a covering section.

[0008] In this process step b), it is further advantageous if the first contact area is used as a first measuring point and a second contact area is used as a second measuring point, and these measuring points are preferably located on a contact section of the respective contact area that is not covered with sintered paste.

[0009] In principle, it is advantageous if the treatment method is selected from one of the following: drying of the sintered paste layer or pressureless temperature application to a pre-dried sintered paste layer or pressure and simultaneous temperature application to a pre-dried sintered paste layer.

[0010] Regarding the dimensions, it can be advantageous if the first section of the sintered paste layer has a constant thickness between 20µm and 150µm, preferably between 50µm and 100µm.

[0011] It can also be advantageous if the first section of the sintered paste layer has a constant width between 0.2mm and 5mm, preferably between 1mm and 2mm.

[0012] It can also be advantageous if at least 100, preferably at least 250 and particularly preferably at least 500 sintered metal particles are present in each cross-section of the sintered paste layer, and wherein preferably a maximum of 1500, preferably a maximum of 1000 sintered metal particles are present in each cross-section.

[0013] On the one hand, it can be advantageous to determine a first and a second target value during the measurement, within which the sintered paste layer exhibits a desired quality for the treatment process. This method is suitable for determining the first and second target values ​​as precisely as possible, which are particularly appropriate for a defined sintered paste layer configuration, especially with regard to its thickness or solvent content. The method, and thus the respective measurement, can be applied multiple times, and the process can be interrupted or terminated at various points to compare the quality at that time with other methods. The first and second target values ​​obtained in this way can then be applied, for example, in the in-situ variant of the method described below.

[0014] On the other hand, it may be preferable if, during the treatment procedure, the treatment procedure is terminated immediately upon reaching an initial target value of the specific resistance or after an additional time offset.

[0015] On the other hand, it may be preferable if, during the treatment procedure, upon reaching a third target value of the specific resistance, which lies between the first and second target values, the treatment procedure is terminated immediately or after a further time offset.

[0016] In both of the latter variants, it may also be preferable if, in process step a), a further sintered paste layer is applied to an electrically conductive area of ​​the substrate surface. The process is thus applied in-situ, while not only the sintered paste layer but also the additional sintered paste layer is treated. This additional sintered paste layer is not used for measurement but is processed during the manufacture of a product or an intermediate product.

[0017] The problem is further solved according to the invention by a device for carrying out the method according to one of the preceding claims, comprising a device for applying heat to the substrate, a device for determining the specific resistance of the sintered paste layer, and a device for controlling the temperature application to the substrate. Control here also includes terminating the temperature application.

[0018] It can be advantageous if the device for applying temperature is designed as a process chamber or as a heating plate.

[0019] It may also be preferable if the system has a device for applying pressure to the substrate.

[0020] Further explanations of the invention, advantageous details and features, will become apparent from the following description of the invention contained in the Fig. 1 to 5 schematically illustrated embodiments of the invention, or of respective parts thereof. Fig. Figure 1 shows a top view of a section of a substrate for carrying out the process. Fig. Figure 2 shows this substrate during the execution of the process. Fig. Figure 3 shows a further embodiment of a substrate with an arranged sintered paste layer. Fig. Figure 4 shows a substrate in a side view according to Fig. 3. Fig. Figure 5 shows a side view of an embodiment of a device according to the invention.

[0021] Fig. Figure 1 shows a top view of a section of a substrate 3 for carrying out the method according to the invention. The substrate 3 shown can, in principle, be configured as a printed circuit board or a partially metal-coated industrial ceramic, such as, for example, aluminum oxide, aluminum nitride, or silicon nitride. Such industrial ceramics form electrical insulating layers of conventional power electronic substrates. As is customary, such substrates can be divided into sub-substrates by breaking, i.e., separating, the insulating ceramic along break lines 300, 302. For processing reasons, there is typically a circumferential edge region of the substrate 3 that is not populated with power semiconductor components; see also Figure 1. Fig. 3, and is discarded as part of the singulation process into sub-substrates.

[0022] The surface 30 of the power electronic substrate 3 shown here has non-conductive and electrically conductive regions 32, 34. The electrically conductive regions 34 are metal layers, which are typically used for the arrangement of power semiconductor devices. These are surrounded by the surface of the insulating ceramic, which forms the non-conductive region 32.

[0023] According to the invention, this substrate 3 has a first and a second contact area 42, 43 in the edge region. Each contact area 42, 43 has a contact section 420 and a cover section 422. A contact section 420 serves as a measuring tap for the electrical connection with a measuring device, while the cover section 422 serves to provide a surface on which a section of the sintered paste layer is placed, cf. Fig. 2.

[0024] Fig. Figure 2 shows the substrate 3 according to Fig. 1 when the process is carried out in a system. A sintered paste layer 2 is applied beforehand to the edge region of the substrate 3, which is divided into three continuous, uninterrupted sections 21, 22, 23. A first section 21 is arranged on the non-electrically conductive area 32 and is meander-shaped, while a second and third section 22, 23 are formed at the respective ends of this section. The second section 22 is arranged on a cover section 422 of the first contact area 42, while a third section 23 is arranged on a cover section of the second contact area 43.

[0025] The first section 21 of the sintered paste layer 2 has a constant thickness of 80µm and a width of 1.2mm.

[0026] This results in a measuring arrangement consisting of the first section 21, the second section 22 of the sintered paste layer 2 which is electrically connected to it, the first section 21 of the sintered paste layer 2, the third section 23 of the sintered paste layer 2 which is electrically connected to the second contact area 43.

[0027] The measuring device consists of a voltage source 50, a current meter 52, a voltage meter 54, and an evaluation unit (not shown) and is electrically connected to the two contact areas 42 and 43. The method according to the invention can be carried out in all described variants using the measuring arrangement and the measuring device. This measuring device thus forms the device for determining the specific electrical resistance of the sintered paste layer 2, more precisely its first section 21.

[0028] Also shown is another sintered paste layer 20, arranged on an electrically conductive area 34 of the substrate 3, on which a power semiconductor component is arranged in later steps of the production of a power electronic circuit arrangement, cf. example. Fig. 5.

[0029] In an in-situ process, the degree of drying of the subsequent sintered paste layer 20 can thus be determined relative to that of the sintered paste layer 2, and the drying process can be terminated at the appropriate time. Such a drying process is typically carried out in a process chamber equipped with a device for homogeneous temperature application to the entire substrate 3.

[0030] A typical value for a first target value of the specific electrical resistance of the first section 21 of the sintered paste layer 2 to achieve a particularly advantageous drying state of this sintered paste layer is 1.1 ohms (mm²). 2 / m). A typical corresponding value for the second setpoint is 0.4 ohms (mm). 2 / m). A typical corresponding value for the third setpoint is 0.8 ohms (mm). 2 / m), is therefore not necessarily the average of the first and second target values.

[0031] The window between the first and second target values ​​is chosen such that the sintered paste layer 2 has a degree of dryness which, in a subsequent pressure sintering connection between the further sintered paste layer 20 and a power semiconductor device, results in a connection of high quality and durability.

[0032] Fig. Figure 3 shows a further embodiment of a substrate 3 with an arranged sintered paste layer 2, while Fig. Figure 4 shows the side view of this substrate 3 when cut along a section line AA.

[0033] Substrate 3 comprises four sub-substrates, one of which is shown in more detail. This sub-substrate has five electrically insulated metal layers 34, which form the conductive traces of the sub-substrate. Further sintered paste layers 20 are arranged on two of these metal layers 34, each intended for connecting power semiconductor devices to the metal layers. The break lines for separating substrate 3 into four sub-substrates are also shown.

[0034] In the surrounding edge region of substrate 3 around the sub-substrates already determined here, there is again a first and a second contact area, as described below. Fig. 2 described, arranged. Furthermore, the sintered paste layer 2 is arranged circumferentially on the edge of the substrate 3 and is again formed in a meandering shape. This creates the longest possible measuring section. Furthermore, the sintered paste layer 2 has a thickness 210, which, in combination with its width 212, results in a cross-section of the sintered paste layer containing between 200 and 300 sintered metal particles 214. Advantageously—but not necessarily, and thus in Fig. As shown in Figure 4, both sintered paste layers have the same thickness.

[0035] Fig. Figure 5 shows a side view of an embodiment of a device according to the invention, here designed as a sintering press. This has a lower punch 60 and an upper punch 70, wherein a device 6 for applying heat to the substrate 3 is arranged in the lower punch 60. The substrate 3 is arranged on a surface 600 of the lower punch 60 by means of a workpiece carrier 8. A PTFE film 72 is arranged between the substrate 3 and the upper punch 70. The upper punch 70, as a device 7 for applying pressure to the substrate 3, interacts with the lower punch 60 in a manner customary in the art.

[0036] Substrate 3 is essentially identical to the one described above. Fig. 4, however, here the drying of the sintered paste layers 2, 20 is complete and a power semiconductor device 200 is already arranged on the further sintered paste layer 20. By applying pressure and temperature using the upper 70 and lower punch 60 as well as by means of the device 6, both sintered paste layers 2, 20 are sintered. During this sintering process, the progress of the sintering process is evaluated in-situ by measuring the electrical resistance of the first section of the sintered paste layer 2, see also Fig. 3. When a corresponding third target value of the specific electrical resistance is reached, the sintering process is terminated.

[0037] The measuring device used here corresponds to that shown in the description. Fig. 2 and is not explicitly shown.

Claims

[1] Method for determining the quality of a sintered paste layer (2) comprising the following process steps: a) Applying a first section (21) of the sintered paste layer (2) to a non-electrically conductive area (32) of a surface (30) of a substrate (3) wherein the first section (21) of the sintered paste layer (2) has a meandering shape; b) Measuring the electrical resistance of the sintered paste layer (2) between two measuring points and determining the specific electrical resistance of this sintered paste layer (2) during a process in which the substrate (3) is processed in a plant. [2] Method according to claim 1, comprising the process step of simultaneous application, during process step a), of a first, a second and a third section (21, 22, 23) of the sintered paste layer (2), wherein the second and third section (22, 23) is each applied to an electrically conductive contact area (42, 43) of the substrate (3) and wherein the sintered paste layer (2) is formed continuously, and wherein the respective contact area (42, 43) is preferably only partially covered, and thus only in a covering section (422). [3] Method according to claim 2, wherein in method step b) the first contact area (42) is used as a first measuring point and a second contact area (43) is used as a second measuring point and wherein these measuring points are preferably located on a contact section (420) of the respective contact area (42,43) that is not covered with sinter paste. [4] Method according to any of the preceding claims, wherein the treatment method is selected from any of the following: • Drying of the sintered paste layer (2), • Pressureless temperature application to a pre-dried sintered paste layer (2), • Pressure and simultaneous temperature application to a pre-dried sintered paste layer (2). [5] Method according to one of the preceding claims, wherein the first section (21) of the sintered paste layer (2) has a constant thickness (210) between 20µm and 150µm, preferably between 50µm and 100µm. [6] Method according to one of the preceding claims, wherein the first section (21) of the sintered paste layer (2) has a constant width (212) between 0.2mm and 5mm, preferably between 1mm and 2mm. [7] Method according to one of the preceding claims, wherein at least 100, preferably at least 250 and particularly preferably at least 500 sintered metal particles (214) are present in each cross-section of the sintered paste layer and wherein preferably a maximum of 1500, preferably a maximum of 1000 sintered metal particles (214) are present in each cross-section. [8] Method according to claims 1 to 7, wherein during the measurement a first and a second target value are determined within which the sintered paste layer (2) has a quality desired within the treatment process. [9] Method according to claims 1 to 7, wherein during the treatment process, upon reaching a first target value of the specific resistance, the treatment process is terminated immediately or after an additional time offset. [10] Method according to claims 1 to 7, wherein during the treatment process, when a third target value of the specific resistance is reached, which lies between the first and second target values, the treatment process is terminated immediately or after a further time offset. [11] Method according to claim 9 or 10, wherein in process step a) a further sintered paste layer (20) is applied to an electrically conductive area (34) of the surface (30) of the substrate (3). [12] Device for carrying out the method according to one of the preceding claims comprising a device (6) for applying temperature to the substrate (3), a device (5) for determining the specific resistance of the sintered paste layer (2) and a device for controlling the temperature application to the substrate. [13] Device according to claim 12, wherein the device (6) for temperature application is designed as a process chamber or as a heating plate. [14] Device according to claim 12 or 13, wherein the system includes a device (7) for applying pressure to the substrate (3).

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