Welding Defect Inspection Method

Abstract

The welding defect detection method of the present invention includes: a threshold resistance setting step (S100) that measures the resistance of the welded part in the sample group to derive a threshold resistance value that becomes an evaluation criterion for weak welding; a resistance measurement step (S200) that measures the resistance value of the welded part to be inspected; and a step (S300) that determines weak welding when the resistance value measured in the resistance measurement step exceeds the threshold resistance value. Among them, in the threshold resistance setting step (S100) and the resistance measurement step (S200), a micro-resistance measuring instrument with a resolution from nano-ohm unit to micro-ohm unit is used to measure the resistance. The welding defect inspection method of the present invention has excellent effects on welding defects caused by weak welding.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0023657, filed on Feb. 26, 2020, and the entire contents of the Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a method for inspecting welding defects, and more particularly, to a method for inspecting whether there are welding defects caused by weak welding on a welded portion between electrode tabs and between an electrode tab and an electrode lead in a lithium secondary battery. Background Art

[0003] Generally, unlike a primary battery that cannot be charged, a secondary battery means a battery that can be charged and discharged, and is widely used in electronic devices such as mobile phones, laptop computers, cameras, or electric vehicles. In particular, a lithium secondary battery has a larger capacity than a nickel-cadmium battery or a nickel-metal hydride battery, and because of its high energy density per unit weight, the use of lithium secondary batteries is increasing rapidly.

[0004] In addition, lithium secondary batteries are classified according to the structure of an electrode assembly having a positive electrode / separator / negative electrode structure. Representative examples thereof include: a jelly roll electrode assembly in which a long sheet-type positive electrode and a negative electrode are wound with a separator disposed therebetween; a stacked electrode assembly in which a plurality of positive electrodes and negative electrodes cut into a predetermined size unit are sequentially stacked with a separator disposed therebetween; and a stacked / foldable electrode assembly in which a dual monomer or a full monomer is wound, in the dual monomer or full monomer, a positive electrode and a negative electrode of a predetermined unit are stacked with a separator disposed therebetween.

[0005] In recent years, pouch-type batteries having a stacked or stacked / folded electrode assembly embedded in a pouch-shaped battery case made of an aluminum laminate have attracted much attention because of their low manufacturing cost and light weight, and their usage is gradually increasing.

[0006] Lithium secondary batteries mainly use lithium-based oxides and carbon materials as positive electrode active material and negative electrode active material, respectively. A lithium secondary battery includes: an electrode assembly in which a positive electrode plate and a negative electrode plate coated with a positive electrode active material and a negative electrode active material, respectively, are placed with a separator disposed therebetween; and an external material that seals and stores the electrode assembly together with an electrolyte.

[0007] At this time, a plurality of positive electrode connection tabs extending from a plurality of positive electrode plates and a plurality of negative electrode connection tabs extending from a plurality of negative electrode plates are formed in the electrode assembly, and the plurality of positive electrode connection tabs and the plurality of negative electrode connection tabs are respectively connected to a positive electrode lead and a negative electrode lead by welding. Here, the plurality of positive electrode connection tabs and the plurality of negative electrode connection tabs form electrode connection tabs, and the positive electrode lead and the negative electrode lead form electrode leads.

[0008] Similarly, when the electrode connection tabs and the electrode leads are welded, if poor welding is performed between the electrode connection tabs and between the electrode connection tabs and the electrode leads, welding defects may occur. Therefore, a process for checking for welding defects caused by such poor welding is required.

[0009] Conventionally, in order to check for welding defects caused by poor welding on the welded portion, the tensile strength is measured by pulling the welded portion in opposite directions. However, when using this method, the electrode connection tabs or the electrode leads may be damaged during the process of measuring the tensile strength. Therefore, a comprehensive investigation cannot be carried out.

[0010] Korean Patent 2017-0125707 discloses a technique that divides the welded portion generated by connecting an electrode foil element and a terminal into a plurality of segments, measures the resistance of each segment by applying current to each segment, and then determines that a welding defect exists if the measured resistance is greater than a threshold resistance. However, in the above document, the correlation between the resistance and the tensile strength in the welded portion is used when deriving the threshold resistance. Therefore, the process of measuring the tensile strength of the welded portion is essential for deriving the threshold resistance.

[0011] However, conventionally, measuring the tensile strength of the welded portion by pulling the welded portion in opposite directions may cause errors. Therefore, the reliability of the threshold resistance value derived from the correlation between the tensile strength and the resistance is not high.

[0012] Therefore, a technique is required that relates to a method for checking for poor welding of a welded portion with excellent detection ability while allowing a comprehensive investigation. Summary of the Invention

[0013] Technical Problem

[0014] Thus, the present invention is designed to solve the above problems, and an object of the present invention is to provide a method for checking for poor welding of a welded portion with excellent detection ability while allowing a comprehensive investigation.

[0015] Technical Solution

[0016] A method for inspecting welding defects for achieving the above object of the present invention includes: a threshold resistance setting step (S100) that measures the resistance of the welded parts of a sample group and derives a threshold resistance value that becomes an evaluation criterion for weak welding; a resistance measurement step (S200) that measures the resistance value of the welded part to be inspected; and a determination step (S300) that determines the welded part as a weak welding if the resistance value measured in the resistance measurement step exceeds the threshold resistance value, wherein the threshold resistance setting step (S100) and the resistance measurement step (S200) include: measuring the resistance using a micro-resistance measuring instrument having a resolution in nano-ohm to micro-ohm units.

[0017] In a specific example, the resistance measurement step (S200) includes: measuring the resistance by allowing two resistance measurement probes to contact the welded part.

[0018] In a specific example, the method may further include: measuring the total resistance of the welded part by allowing one resistance measurement probe to contact one end of the welded part and allowing another resistance measurement probe to contact the other end of the welded part.

[0019] In a specific example, the resistance measurement probe includes a voltage probe and a current probe.

[0020] In a specific example, the resistance measurement step (S200) includes: measuring the resistance of the welded part by a four-wire measurement scheme.

[0021] In a specific example, the resistance measurement step (S200) includes: measuring the resistance of the welded part by a DC scheme.

[0022] In a specific example, the threshold resistance setting step (S100) includes: a data construction step (S110) that measures the micro-resistance of the sample group in nano-ohm and micro-ohm units and stores the measured micro-resistance; and a threshold resistance value derivation step (S120) that derives a threshold by processing the data accumulated by the data construction step (S110) using a statistical scheme.

[0023] In a specific example, the number of objects in the sample group is equal to or greater than 100,000.

[0024] In a specific example, the resistance values of the sample group form a normal distribution curve.

[0025] In a specific example, the threshold resistance value is the average value + 6δ.

[0026] In a specific example, the welded portion is formed by ultrasonic welding. At this time, the resistance is measured by allowing the resistance measurement probe to contact the interface of the welded portion.

[0027] In a specific example, the welded portion is formed by laser welding. At this time, the resistance is measured by allowing the resistance measurement probe to contact the outer peripheral surface of the welded portion.

[0028] The method for inspecting welding defects of the present invention is preferably applied to the welded portion between the electrode lead and the electrode tab of the pouch secondary battery or the welded portion between the electrode tabs.

[0029] Advantageous Effects

[0030] According to the welding defect inspection method of the present invention, the resistance of a sample group is measured, a threshold resistance value is set from the normal distribution curve of the measured resistance values, and when measuring the resistance of the sample group and the welded portion to be inspected, the resistance is accurately measured by using a micro-resistance measuring instrument having a resolution in the nano-ohm to micro-ohm level, thereby showing excellent detection ability for weak welding defects. Brief Description of the Drawings

[0031] Figure 1 is a flowchart schematically showing the method for inspecting welding defects of the present invention.

[0032] Figure 2 is a view showing the correlation between welding strength and resistance.

[0033] Figure 3 is a normal distribution curve of the resistance values of the sample group according to an embodiment of the present invention.

[0034] Figure 4 is a schematic view showing the method for measuring resistance according to an embodiment of the present invention.

[0035] Figure 5 is a schematic view showing the method for measuring resistance according to another embodiment of the present invention.

[0036] Figure 6 is a schematic view of the resistance measurement probe of the present invention.

[0037] Figure 7 is a schematic view showing the method for measuring the resistance of a welded portion formed by ultrasonic welding according to an embodiment of the present invention.

[0038] Figure 8 is a schematic view showing the method for measuring the resistance of a welded portion formed by laser welding according to an embodiment of the present invention. Detailed Description of the Invention

[0039] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The terms and words used in this specification and claims should not be construed as limited to ordinary or dictionary terms, and the inventor may appropriately define the concept of the terms so as to best describe his invention. The terms and words should be construed to have meanings and concepts consistent with the technical idea of the present invention.

[0040] Accordingly, the embodiments described in the specification and the configurations described in the drawings are only the most preferred embodiments of the present invention and do not represent all the technical ideas of the present invention. It should be understood that there may be various equivalents and modifications that replace them when this application is filed.

[0041] Moreover, throughout the specification, when an element is referred to as "including" an element, it should be understood that the element may also include other elements, unless specifically stated otherwise.

[0042] Figure 1 is a flowchart of a method for inspecting welding defects according to the present invention. Referring to Figure 1 , the method for inspecting welding defects according to the present invention includes: a threshold resistance setting step (S100) for measuring the resistance of the welded portion of a sample group and deriving a threshold resistance value as an evaluation criterion for weak welding; a resistance measurement step (S200) for measuring the resistance value of the welded portion to be inspected; and a determination step (S300) for determining the welded portion as weak welding if the resistance value measured in the resistance measurement step exceeds the threshold resistance value, wherein the threshold resistance setting step (S100) and the resistance measurement step (S200) include measuring the resistance using a micro-resistance measuring instrument having a resolution in the unit of nano-ohm to micro-ohm.

[0043] The inventor of the present invention has found that the resistance value of a welded portion with low welding strength is greater than that of a welded portion with normal welding strength, which has led to the present invention. Referring to Figure 2 , the resistance value of a welded portion welded with a normal welding strength of 22 kgf or more is less than that of a welded portion welded with a weak welding strength of less than 22 kgf. Conventionally, there is a technique for inspecting welding defects by measuring the resistance of a welded portion and comparing the measured resistance value with a threshold resistance value. However, in this conventional technique, the method for deriving the threshold resistance value is not specific, or because the correlation between the tensile strength and the resistance of the welded portion is used when deriving the threshold resistance value, a process for measuring the tensile strength of the welded portion is required.

[0044] However, the present invention is characterized in that, while not relying on the correlation between the tensile strength and the resistance of the welded portion, a statistical scheme is introduced when deriving the threshold resistance value, and the resistance is accurately measured by measuring the resistance at a level up to nano-ohm to micro-ohm levels by increasing the resolution.

[0045] That is, when measuring the resistance of an object forming a large number of sample groups, the measured resistance values form a normal distribution curve. In this normal distribution curve, from the perspective of statistical probability, an object with a large deviation can be easily assumed to be defective, and thus a predetermined deviation is determined as the threshold resistance value. In addition, this scheme is based on the premise that the data for the sample group is reliable. Therefore, in the process of setting the threshold, a micro-resistance measuring instrument capable of accurately measuring the resistance of the sample group at a level up to nano-ohm to micro-ohm is used, and when measuring the resistance of the object to be inspected, a micro-resistance measuring instrument capable of accurately measuring the resistance at a level up to nano-ohm to micro-ohm is used. Similarly, in the present invention, when deriving the threshold resistance value, the threshold resistance value is statistically derived from a large number of sample groups. Therefore, different from the conventional technology, it is not necessary to separately measure the tensile strength of the welded portion to set the threshold resistance value.

[0046] First, the threshold resistance setting step (S100) will be described.

[0047] The threshold resistance setting step (S100) according to an embodiment of the present invention includes: a data construction step (S110) that measures the micro-resistance of a sample group in nano-ohm and micro-ohm units and stores the measured micro-resistance; and a threshold resistance value derivation step (S120): The threshold resistance value derivation step derives a threshold by processing the data accumulated by the data construction step (S110) using a statistical scheme.

[0048] The data construction step (S110) includes a process of measuring the resistance of the welded portion of an object forming a sample group. At this time, the number of objects in the sample group is at least 100,000, preferably 200,000, and in terms of reliability, it is preferable to have as many sample group objects as possible.

[0049] In the data construction step (S110), the resistance of the object of the sample group is measured by using a micro-resistance measuring instrument having a resolution in nano-ohm (nΩ) to micro-ohm (uΩ) units. This is to accumulate more reliable data. In addition, the micro-resistance measurement method is performed in the same manner as the scheme for measuring the resistance of the welded portion in the resistance measurement step (S200).

[0050] The threshold resistance value derivation step (S120) includes deriving a threshold resistance value by processing the data accumulated by the data construction step (S110) using a statistical scheme. The statistical processing method according to an embodiment of the present invention is to obtain a normal distribution curve of the resistance values of the objects in the sample group, and the sum of the mean value + 6δ values in the normal distribution curve is used as the threshold resistance value.

[0051] Figure 3 An example of a distribution curve of the resistance of a sample group according to an embodiment of the present invention is shown. Refer to Figure 3 , when the sample group shows a normal distribution curve as in Figure 3 , most of the objects have values close to the mean value (μ), and the number of objects with values significantly deviated from the mean value (μ) is small. Therefore, the objects with values significantly deviated from the mean value can be randomly assumed to be defective. Specifically, the probability of an object having a deviation of 1δ (standard deviation) is approximately 32%, the probability of an object having a deviation of 2δ is approximately 5%, the probability of an object having a deviation of 3δ is approximately 0.3%, the probability of an object having a deviation of 4δ is approximately 0.01%, the probability of an object having a deviation of 5δ is approximately 0.001%, and the probability of an object having a deviation of 6δ is approximately 0.0000001%.

[0052] Thus, in an embodiment of the present invention, the sum of the mean value and 6δ is set as the threshold resistance value.

[0053] Hereinafter, the resistance measurement step (S200) will be described in detail.

[0054] Figure 4 is a schematic diagram showing a method of measuring resistance according to an embodiment of the present invention. Refer to Figure 4 , in the resistance measurement step (S200) of the present invention, the micro resistance measurement instrument for resistance measurement includes two resistance measurement probes 100, and the resistance is measured by allowing the two resistance measurement probes to contact the welding portion 30.

[0055] The resistance of the entire welding portion 30 can be measured by allowing one resistance measurement probe to contact one end 31 of the welding portion and allowing the other resistance measurement probe to contact the other end of the welding portion.

[0056] In addition, as shown in Figure 4 (b), the welding portion 30 can be divided into a welding portion 33 of the electrode tab 20 portion and a welding portion 34 of the electrode lead 10, as shown in Figure 4As shown, it is possible to allow both resistance measurement probes to contact the welded portion 33 of the electrode tab 20. In contrast, it is possible to allow both resistance measurement probes to contact the welded portion 34 of the electrode lead 10 portion. Alternatively, as Figure 5 shown, it is possible to allow one resistance measurement probe to contact the welded portion 33 of the electrode tab 20, and it is possible to allow the remaining one resistance measurement probe to contact the welded portion 34 of the electrode lead 10, thereby measuring the resistance. In the above embodiments, in terms of the detection ability for detecting weak welding, it is most preferable to measure the resistance in such a form that both resistance measurement probes contact the welded portion 33 on the electrode tab 10.

[0057] Figure 6 Schematic diagram showing the resistance measurement probe of the present invention. Referring to Figure 6 , the resistance measurement probe 100 of the present invention includes a current probe 110 and a voltage probe 120. The current probe applies a current to the welded portion to be measured, and the voltage probe measures the voltage. In this way, the resistance of the welded portion can be measured.

[0058] In the present invention, the resistance is measured by allowing two resistance measurement probes to contact the welded portion. In this way, the resistance of the welded portion can be measured by a four-wire measurement scheme. Since the four-wire resistance measurement scheme is less affected by contact resistance compared with the two-wire resistance measurement scheme, in the four-wire resistance measurement scheme capable of measuring resistance even in nano-ohm units, micro-resistance can be measured more accurately.

[0059] In a specific example, the resistance measurement step (S200) includes measuring the resistance of the welded portion by a DC scheme. Compared with the AC scheme, the DC scheme has the advantage of being able to perform high-precision resistance measurement.

[0060] The welding defect inspection method of the present invention can be widely applied to the welded portions of secondary batteries, and can be applied to welded portions according to various welding schemes. That is, the welding defect inspection method of the present invention can be applied to the welded portions between electrode tabs and between electrode tabs in a battery pack, the welded portions between electrode tabs and electrode leads, and the welded portions between electrode leads and bus bars, and can also be applied to welded portions formed by ultrasonic welding and welded portions formed by laser welding, etc.

[0061] Figure 7 is a schematic diagram showing a method of measuring the resistance of a welded portion formed according to an ultrasonic welding scheme. Referring to Figure 7, the welded portion 30 formed by the ultrasonic welding solution has a line or surface form. In this way, the contact position 40 of the resistance measurement probe for resistance measurement is located on the boundary line of the welded portion. The resistance of the entire welded portion can be measured by allowing the resistance measurement probe to contact the boundary line of the welded portion. There are 4 contact positions 40 of the resistance measurement probe. Here, the current probe contacts two of the contact positions 40, and the voltage probe contacts the remaining two of the contact positions 40.

[0062] Figure 8 is a schematic diagram showing a method of measuring the resistance of a welded portion formed according to a laser welding solution. Refer to Figure 8 , the welded portion 30 formed by the laser welding solution has a dot shape. In order to measure the resistance of the entire welded portion, it is desirable to determine the contact position 40 of the resistance measurement probe on the outer peripheral surface with a virtual line obtained by connecting the outermost points among the points as the boundary. There are 4 contact positions 40 of the resistance measurement probe. Here, the current probe contacts two of the contact positions 40, and the voltage probe contacts the remaining two of the contact positions 40.

[0063] According to the welding inspection method of the present invention, the resistance of a sample group is measured, a threshold resistance value is set from the normal distribution curve of the measured resistance values, and when measuring the resistance of the sample group and the welded portion to be inspected, the resistance is accurately measured by using a micro-resistance measurement instrument with a resolution at the nano-ohm to micro-ohm level, thereby showing excellent detection ability for weak welding defects.

Claims

1. A method for inspecting welding defects, comprising: A threshold resistance setting step (S100) that measures the resistance of the welded parts of a sample group and derives a threshold resistance value that serves as an evaluation criterion for weak welding; A resistance measurement step (S200) that measures the resistance value of the entire welded part to be inspected; And A determination step (S300) that determines the welded part as a weak welding if the resistance value measured in the resistance measurement step exceeds the threshold resistance value, wherein the threshold resistance setting step (S100) and the resistance measurement step (S200) include: measuring the resistance using a micro-resistance measuring instrument with a resolution in the nano-ohm to micro-ohm unit, wherein the threshold resistance setting step (S100) includes: A data construction step (S110) that measures the micro-resistance of the sample group in nano-ohm and micro-ohm units and stores the measured micro-resistance; and A threshold resistance value derivation step (S120) that derives the threshold resistance value by processing the data accumulated by the data construction step (S110) using a statistical scheme.

2. The method according to claim 1, wherein, The resistance measurement step (S200) includes: measuring the resistance by allowing two resistance measurement probes to contact the welded part.

3. The method according to claim 2, further comprising: The total resistance of the welded part is measured by allowing one resistance measurement probe to contact one end of the welded part and allowing another resistance measurement probe to contact the other end of the welded part.

4. The method according to claim 2, wherein, The resistance measurement probe includes a voltage probe and a current probe.

5. The method according to claim 1, wherein, The resistance measurement step (S200) includes: measuring the resistance of the welded part by a 4-wire measurement scheme.

6. The method according to claim 1, wherein, The resistance measurement step (S200) includes: measuring the resistance of the welded part by a DC scheme.

7. The method according to claim 1, wherein, The number of objects in the sample group is equal to or greater than 100,000.

8. The method according to claim 1, wherein The resistance values of the sample group form a normal distribution curve.

9. The method according to claim 8, wherein, The threshold resistance value is the average value + 6δ.

10. The method according to claim 3, wherein, The welded part is formed by ultrasonic welding.

11. The method according to claim 10, wherein, The resistance is measured by allowing the resistance measurement probe to contact the boundary line of the welded part.

12. The method according to claim 3, wherein, The welded part is formed by laser welding.

13. The method according to claim 12, wherein, The resistance is measured by allowing the resistance measurement probe to contact the outer peripheral surface of the welded part.

14. The method according to claim 1, wherein, The welded part is one selected from the welded part between the electrode lead and the electrode tab of a pouch secondary battery and the welded part between the electrode tabs of a pouch secondary battery.

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