Title of Invention: Welding Defect Inspection Method
technical field
[One]
This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0023657 dated February 26, 2020, and all contents disclosed in the literature of the Korean patent application are incorporated as a part of this specification.
[2]
The present invention relates to a method for inspecting a welding defect, and more particularly, to a method for inspecting whether a welding portion between an electrode tab and an electrode tab or a welding portion between an electrode tab and an electrode lead is weakly welded in a lithium secondary battery.
background
[3]
In general, a secondary battery means a battery that can be charged and discharged, unlike a primary battery that cannot be charged, and is widely used in electronic devices such as mobile phones, notebook computers, camcorders, etc. or electric vehicles. In particular, since the lithium secondary battery has a larger capacity than a nickel-cadmium battery or a nickel-hydrogen battery and has a high energy density per unit weight, the degree of its utilization is rapidly increasing.
[4]
On the other hand, lithium secondary batteries are classified according to the structure of the electrode assembly of the positive electrode/separator/negative electrode structure. Typically, a jelly-like structure in which a long sheet-type positive electrode and negative electrode are wound with a separator interposed therebetween. A roll electrode assembly, a stacked electrode assembly in which a plurality of positive and negative electrodes cut in units of a predetermined size are sequentially stacked with a separator interposed therebetween, a bi-cell in which positive and negative electrodes of a predetermined unit are stacked with a separator interposed therebetween; or and a stack/folding type electrode assembly having a structure in which full cells are wound.
[5]
Recently, a pouch-type battery having a structure in which a stack-type or stack/folding-type electrode assembly is embedded in a pouch-type battery case of an aluminum laminate sheet has attracted a lot of attention due to low manufacturing cost, small weight, easy shape deformation, etc. and its use is gradually increasing.
[6]
Such a lithium secondary battery mainly uses a lithium-based oxide and a carbon material as a positive electrode active material and a negative electrode active material, respectively. A lithium secondary battery includes an electrode assembly in which a positive electrode plate and a negative electrode plate to which the positive electrode active material and the negative electrode active material are applied, respectively, are disposed with a separator interposed therebetween, and a packaging material for sealing and housing the electrode assembly together with an electrolyte.
[7]
At this time, a plurality of positive electrode tabs extending from the plurality of positive electrode plates and a plurality of negative electrode tabs extending from the plurality of negative electrode plates are formed in the electrode assembly, and the plurality of positive electrode tabs and the plurality of negative electrode tabs are respectively a positive electrode lead and a negative electrode The lead is welded together. Here, a plurality of positive electrode tabs and a plurality of negative electrode tabs constitute an electrode tab, and a positive electrode lead and a negative electrode lead constitute an electrode lead.
[8]
In this way, when welding the electrode tab and the electrode lead, if the welding between the electrode tab and the electrode tab and between the electrode tab and the electrode lead is weakly performed, a welding defect occurs. process is needed
[9]
As a method for inspecting weak welding defects of a weld, conventionally, a method of measuring the tensile strength by pulling the welded part in the opposite direction with respect to the welded object was used. However, in this method, since the electrode tab or the electrode lead is damaged in the process of measuring the tensile strength, there is a limit in which full investigation is impossible.
[10]
Korean Patent Application Laid-Open No. 2017-0125707 discloses that a welding part for joining an electrode foil element and a terminal is divided into a plurality of zones, a current is applied to each zone to measure resistance, and then the measured resistance is higher than the critical resistance. In a large case, a technique for detecting a welding defect is disclosed. However, since the above document uses the correlation between tensile strength and resistance at the welded portion in deriving the critical resistance, a process of measuring the tensile strength of the welded portion is inevitably necessary to derive the critical resistance.
[11]
However, as described above, a widely used method of measuring the tensile strength of a weld is based on a method of pulling the weld in the opposite direction with respect to the welded object. The threshold resistance value derived from the relationship has a problem of low reliability.
[12]
Therefore, there is a need to develop a technology for an inspection method with excellent detection power while allowing full investigation in inspecting whether a weld is weakly welded.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[13]
Accordingly, the present invention has been devised to solve the above problems, and it is to provide an inspection method with excellent defect detection power while allowing full investigation in detecting weak welding defects of welds.
means of solving the problem
[14]
A welding defect inspection method of the present invention for achieving the above object includes a threshold resistance setting step (S100) of measuring the resistance of a weld portion of a sample group and deriving a threshold resistance value that is a criterion for determining weak welding; A resistance measurement step of measuring the resistance value of the welding part to be inspected (S200); and when the resistance value measured in the resistance measurement step exceeds the threshold resistance value, determining as weak welding (S300), wherein the threshold resistance setting step (S100) and the resistance measurement step (S200) are , characterized in that the resistance is measured using a micro-resistance meter having a resolution of nano-ohm to micro-ohm units.
[15]
In one specific example, the micro resistance measuring device includes two resistance measuring probes, and measuring resistance by contacting the two resistance measuring probes to a welding part.
[16]
In one specific example, one resistance measuring probe is brought into contact with one end of the welding part and the other resistance measuring probe is brought into contact with the other end to measure the resistance of the entire welding part.
[17]
In one specific example, the resistance measuring probe includes a voltage probe and a current probe.
[18]
In one specific example, the resistance measurement step ( S200 ) is to measure the resistance of the welded portion in a 4-wire measurement method.
[19]
In one specific example, the resistance measurement step ( S200 ) is to measure the resistance of the welding part in a DC method.
[20]
In one specific example, the threshold resistance setting step (S100) may include: a data construction step (S110) of measuring a micro-resistance in a nano-ohm to a micro-ohm unit with respect to a sample group and storing it; and a threshold resistance value deriving step (S120) of deriving a threshold value by processing the data accumulated by the data building step (S110) in a statistical manner.
[21]
In one specific example, the number of individuals in the sample population is at least 100,000.
[22]
In one specific example, the resistance values ​​of the sample group form a normal distribution curve.
[23]
In one specific example, the threshold resistance value is an average value + 6δ.
[24]
In one specific example, the weld is formed by ultrasonic welding. In this case, it is preferable to measure the resistance by contacting the resistance measuring probe to the interface of the welding part.
[25]
In one specific example, the weld is formed by laser welding. At this time, it is preferable to measure the resistance by contacting the resistance measuring probe to the outer peripheral surface of the welding part.
[26]
The welding defect inspection method of the present invention is preferably applied to a welding portion between an electrode tab and an electrode lead of a pouch-type secondary battery or a welding portion between an electrode tab and an electrode tab.
Effects of the Invention
[27]
In the welding defect inspection method of the present invention, the resistance of the sample group is measured in the critical resistance setting step, the critical resistance value is set from the normal distribution curve of the measured resistance value, and the resistance of the sample group and the welding part to be inspected is measured. , Since the resolution is nano-ohm to M, the resistance is precisely measured by using an ohm-level micro-resistance meter, so that the detection power of weak welding defects is very excellent.
Brief description of the drawing
[28]
1 is a flowchart of a welding defect inspection method of the present invention.
[29]
2 is a graph showing the correlation between welding strength and resistance.
[30]
3 is a schematic diagram of a normal distribution curve of resistance values ​​of a sample group according to an embodiment of the present invention.
[31]
4 is a schematic diagram illustrating a resistance measurement method according to an embodiment of the present invention.
[32]
5 is a schematic diagram illustrating a resistance measurement method according to another embodiment of the present invention.
[33]
6 is a schematic diagram of a resistance measuring probe of the present invention.
[34]
7 is a schematic diagram illustrating a method of measuring the resistance of a welded portion by ultrasonic welding according to an embodiment of the present invention.
[35]
8 is a schematic diagram illustrating a method of measuring the resistance of a welding part by laser welding according to an embodiment of the present invention.
Best mode for carrying out the invention
[36]
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, the terms or words used in the present specification and claims should not be construed as being limited to conventional or dictionary meanings, and the inventor should properly understand the concept of the term in order to best describe his invention. Based on the principle that it can be defined, it should be interpreted as meaning and concept consistent with the technical idea of ​​the present invention.
[37]
Therefore, the configuration shown in the embodiments and drawings described in the present specification is only the most preferred embodiment of the present invention and does not represent all the technical spirit of the present invention, so at the time of the present application, various It should be understood that there may be equivalents and variations.
[38]
Throughout the specification, when a part "includes" a certain component, it means that other components may be further included, rather than excluding other components unless otherwise stated.
[39]
[40]
1 is a flowchart of a welding defect inspection method according to the present invention. Referring to FIG. 1 , the welding defect inspection method of the present invention includes a threshold resistance setting step (S100) of measuring the resistance of a welding part of a sample group and deriving a threshold resistance value that is a criterion for determining weak welding; A resistance measurement step of measuring the resistance value of the welding part to be inspected (S200); and when the resistance value measured in the resistance measurement step exceeds the threshold resistance value, determining as weak welding (S300), wherein the threshold resistance setting step (S100) and the resistance measurement step (S200) are , characterized in that the resistance is measured using a micro-resistance meter having a resolution of nano-ohm to micro-ohm units.
[41]
The inventors of the present invention found that, when the electrical resistance of a weld having a weak welding strength was measured, it was greater than the electrical resistance value of a weld having a normal welding strength, and led to the present invention. Referring to FIG. 2 , it can be seen that the resistance value of the normal weld with a welding strength of 22 kgf or more is smaller than the resistance value of the weak weld with a welding strength of less than 22 kgf. Conventionally, there has been a technique of measuring the resistance of the welded portion and comparing the measured resistance value with the critical resistance value to determine the welding defect, but the conventional technique does not have a specific method for deriving the critical resistance value, or the critical resistance value In deriving , a process of measuring the tensile strength of the weld was required because the correlation between the tensile strength and resistance of the weld was used.
[42]
However, in deriving the critical resistance value, the present invention does not depend on the correlation between the tensile strength of the weld and the resistance, but introduces a statistical method and increases the resolution in measuring the resistance to nano-ohm to micro-ohm units. It has the characteristic of measuring the resistance precisely by measuring the micro-resistance.
[43]
That is, when resistance is measured for individuals constituting a large sample group, the measured resistance values ​​form a normal distribution curve. An individual with a large deviation from the normal distribution curve can be estimated as defective in terms of statistical probability, A predetermined deviation is set as the threshold resistance value, and since this method is based on the premise that the data for the sample group is reliable, the precision of the sample group to the nano-ohm to micro-ohm unit is used in the process of setting the threshold value. A micro-resistance meter that can measure resistance is used, and a micro-resistance meter that can precisely measure resistance in nano-ohm to micro-ohm units is used even when measuring resistance on an object to be tested. As described above, in the present invention, since the critical resistance value is statistically derived from a large sample group in deriving the critical resistance value, it is not necessary to separately measure the tensile strength of the weld to set the critical resistance value unlike the prior art. none.
[44]
First, the threshold resistance setting step ( S100 ) will be described.
[45]
According to one embodiment of the present invention, the threshold resistance setting step (S100) includes: a data construction step (S110) of measuring a micro-resistance in a nano-ohm to a micro-ohm unit for a sample group and storing it; and a threshold resistance value deriving step (S120) of deriving a threshold value by processing the data accumulated by the data building step (S110) in a statistical manner.
[46]
The data building step ( S110 ) includes a process of measuring the resistance of the welding part with respect to the objects constituting the sample group. In this case, the number of individuals in the sample group is at least 100,000, preferably 200,000, and it is preferable in terms of reliability that the number of possible sample groups is large.
[47]
In the resistance measurement in the data building step S110, resistance measurement is performed on the sample group objects using a micro-resistance meter having a resolution of nano-ohm (nΩ) to micro-ohm (uΩ) units. This is to accumulate more reliable data. In addition, the method of measuring the fine resistance is performed in the same way as the method of measuring the resistance of the welding part in the resistance measuring step ( S200 ), as will be described later.
[48]
In the step of deriving the threshold resistance value ( S120 ), the data accumulated by the data building step ( S110 ) is processed in a statistical manner to derive the threshold resistance value. A statistical processing method according to an embodiment of the present invention is to obtain a normal distribution curve of resistance values ​​of individuals in a sample group, and set a value of +6δ as a critical resistance value in the normal distribution curve.
[49]
3 is an example of a distribution curve of resistance of a sample group according to an embodiment of the present invention. Referring to FIG. 3 , when a sample group exhibits a normal distribution curve as shown in FIG. 3 , most individuals have a value close to the mean value (u), and there are few individuals with a large deviation from the mean value (u). Therefore, from a probabilistic point of view, an individual with a large deviation from the average value can be estimated as defective. Specifically, the probability that an individual with a deviation of 1δ (standard deviation) appears is about 32%, the probability that an individual with a deviation of 2δ appears is about 5%, and the probability that an individual with a deviation of 3δ appears is about 0.3%, and the probability of an individual with a deviation of 3δ is about 0.3%, of 4δ. The probability that an individual having a deviation appears is about 0.01%, the probability that an individual having a deviation of 5δ appears is about 0.001%, and the probability that an individual having a deviation of 6δ appears is about 0.0000001%.
[50]
Accordingly, in an embodiment of the present invention, the threshold resistance value is set to a value obtained by adding a value of 6δ to an average value.
[51]
[52]
Hereinafter, the resistance measurement step ( S200 ) will be described in detail.
[53]
4 is a schematic diagram of a resistance measuring method according to an embodiment of the present invention. Referring to FIG. 4 , the micro resistance measuring device used for measuring resistance in the resistance measuring step ( S200 ) of the present invention includes two resistance measuring probes 100 , and welding the two resistance measuring probes to a welding part 30 . Measure the resistance by contacting
[54]
At this time, one resistance measuring probe is brought into contact with one end 31 of the welding part, and the other resistance measuring probe is brought into contact with the other end 32 of the welding part, so that the entire resistance of the welding part 30 can be measured.
[55]
In addition, as shown in FIG. 4(b), the welding part 30 may be divided into a welding part 33 of the electrode tab 20 part and a welding part 34 of the electrode lead 10 part, as shown in FIG. All of the resistance measuring probes may be brought into contact with the welding portion 33 of the electrode tab 20 . Alternatively, both of the two resistance measuring probes may be brought into contact with the welding part 34 of the electrode lead 10 part, and as shown in FIG. 5 , one resistance measuring probe is on the welding part 33 of the electrode tab 20 part, and the other One resistance measuring probe may be in contact with the welding part 34 of the electrode lead 10 to measure resistance. Among the various embodiments, it was most preferable in terms of detection power for detecting weak welding to measure resistance in a form in which both of the two resistance measuring probes were in contact with the welding portion 33 of the electrode tab 10 portion.
[56]
6 is a schematic diagram of a resistance measuring probe of the present invention. Referring to FIG. 6 , the resistance measuring probe 100 of the present invention includes a current probe 110 and a voltage probe 120 . The current probe applies a current to a welding part to be measured, and the voltage probe measures a voltage. Accordingly, the resistance of the weld can be measured.
[57]
According to the present invention, two resistance measuring probes are brought into contact with a welding part to measure the resistance, and accordingly, the resistance of the welding part can be measured in a 4-wire measuring method. Since the 4-wire resistance measurement method is not affected by the contact resistance compared to the 2-wire resistance measurement method, it is possible to measure the fine resistance more precisely.
[58]
In one specific example, the resistance measuring step ( S200 ) may measure the resistance of the welding part in a DC method. The DC method has the advantage of being able to measure resistance with high precision compared to the AC method.
[59]
The welding defect inspection method of the present invention can be widely applied to a welding part of a secondary battery, and can be applied to a welding part according to various welding methods. That is, the welding defect inspection method of the present invention can be applied to the welding part between the electrode tab and the electrode tab, the welding part between the electrode tab and the electrode lead, the welding part between the electrode lead and the bus bar in the battery pack, etc., and the welding part by ultrasonic welding, laser welding, etc. It can be applied to all welding parts, etc.
[60]
7 is a schematic diagram illustrating a method of measuring resistance to a welded portion according to an ultrasonic welding method. Referring to FIG. 7 , a welded portion 30 formed by the ultrasonic welding method has the shape of a line or a plane. Accordingly, the contact position 40 of the resistance measuring probe for measuring the resistance is located on the boundary line of the welding part. The resistance of the entire weld can be measured by contacting the resistance measuring probe on the weld boundary. There are a total of four contact positions 40 of the resistance measuring probe, two of which are contacted with a current probe and the other two with a voltage probe.
[61]
8 is a schematic diagram illustrating a method for measuring resistance to a welded portion according to a laser welding method. Referring to FIG. 8 , the welding part 30 formed by the laser welding method has the shape of a dot. In order to measure the resistance of the entire welded portion, it is preferable to determine the contact position 40 of the resistance measuring probe on the outer peripheral surface of the false line connecting the outermost points of these points as a boundary. There are a total of four contact positions 40 of the resistance measuring probe, two of which are contacted with a current probe and the other two with a voltage probe.
[62]
In the welding inspection method according to the present invention, the resistance is measured for a sample group in the critical resistance setting step, the critical resistance value is set from a normal distribution curve of the measured resistance value, and when the resistance is measured for the sample group and the welding part to be inspected , Since the resolution is nano-ohm to M, the resistance is precisely measured using an ohm-level micro-resistance meter, so that the detection power of weak welding defects is very excellent.
Claims
[Claim 1]
A threshold resistance setting step (S100) of measuring the resistance of the welding part of the sample group and deriving a threshold resistance value that is a criterion for determining weak welding; A resistance measurement step of measuring the resistance value of the welding part to be inspected (S200); and when the resistance value measured in the resistance measurement step exceeds the threshold resistance value, determining as weak welding (S300), wherein the threshold resistance setting step (S100) and the resistance measurement step (S200) are , A welding defect inspection method, characterized in that the resistance is measured using a micro-resistance meter having a resolution of nano-ohm to micro-ohm units.
[Claim 2]
The welding defect inspection method according to claim 1, wherein in the measuring the resistance (S200), the resistance is measured by contacting two resistance measuring probes to the welding part.
[Claim 3]
The welding defect inspection method according to claim 2, wherein one resistance measuring probe is brought into contact with one end of the welded portion and the other resistance measuring probe is brought into contact with the other end of the welded portion to measure the resistance of the entire welded portion.
[Claim 4]
3. The method of claim 2, wherein the resistance measuring probe includes a voltage probe and a current probe.
[Claim 5]
The welding defect inspection method according to claim 1, wherein the resistance measuring step (S200) measures the resistance of the welded portion in a 4-wire measuring method.
[Claim 6]
The welding defect inspection method according to claim 1, wherein the resistance measuring step (S200) measures the resistance of the welding part in a direct current method.
[Claim 7]
The method of claim 1 , wherein the threshold resistance setting step ( S100 ) comprises: a data construction step ( S110 ) of measuring a micro-resistance in a nano-ohm to a micro-ohm unit with respect to a sample group and storing the measurement; and a threshold resistance value deriving step (S120) of deriving a threshold value by processing the data accumulated by the data building step (S110) in a statistical manner.
[Claim 8]
8. The method of claim 7, wherein the number of individuals in the sample group is at least 100,000.
[Claim 9]
The welding defect inspection method according to claim 7, wherein the resistance values ​​of the sample group form a normal distribution curve.
[Claim 10]
The welding defect inspection method according to claim 9, wherein the critical resistance value is an average value + 6δ.
[Claim 11]
The welding defect inspection method according to claim 3, wherein the welding part is formed by ultrasonic welding.
[Claim 12]
The welding defect inspection method according to claim 11, wherein the resistance is measured by contacting the resistance measuring probe with the interface of the welding part.
[Claim 13]
4. The method of claim 3, wherein the welding part is formed by laser welding.
[Claim 14]
14. The method of claim 13, wherein the resistance is measured by contacting the resistance measuring probe with an outer circumferential surface of the welding part.
[Claim 15]
The welding defect inspection method according to claim 1, wherein the welding portion is selected from a welding portion between an electrode tab and an electrode lead of a pouch-type secondary battery and a welding portion between the electrode tab and the electrode tab.