KR20120064149A - Reliability assessment method for flexible warming element - Google Patents

Abstract

PURPOSE: A reliability evaluation method for a plate type heating element is provided to predict the lifetime reliability of the heating element by qualitatively analyzing and modeling a voltage acceleration test based on the malfunction mechanism. CONSTITUTION: A reliability evaluation method for a plate type heating element comprises the following steps: setting the testing standard based on the measuring gap and the temperature range of the plate type heating element(S10); measuring the resistance of the plate type heating element applied with the acceleration voltage in the prefixed measuring gap(S14); drawing a graph of the change of the resistance by the time variation during an acceleration lifetime test(S18); and recording the malfunction time of a sample for the plate type heating element having the malfunction by the error of the resistance during the acceleration lifetime test(S28).

Description

Reliability assessment method for flexible warming element

The present invention relates to a method for evaluating the reliability of a planar heating element, and more particularly, to design an acceleration condition for an accelerated life test according to a two level quality function deployment based on a failure mechanism analyzed for the planar heating element. The present invention relates to a method for evaluating the reliability of a planar heating element that performs quantitative analysis on a plurality of samples and establishes an acceleration model that can shorten the test time according to the quantitative analysis result.

Planar heating element is an economical material in the form of mat that has the function of converting electrical energy into thermal energy through a material having electrical resistance, and is widely used in household, commercial, industrial, agricultural, etc. It is an essential material component to replace electric heating.

Referring to FIG. 1, the planar heating element has a structure in which a carbon heating element having an electrode and polyethylene are laminated between upper and lower insulating coatings, and a terminal connected to the electrode is connected to a lead wire.

The surface heating element may be classified into a front coating type of FIG. 2, a partial coating type of FIG. 3, and a hot linear type of FIG.

The planar heating element has a positive temperature coefficient (PTC) characteristic, and the positive temperature coefficient is a power consumption increases with increasing voltage as shown in FIG. 5, the temperature increases, and the increase in temperature increases the resistance to decrease the current density. It is a phenomenon.

As for the planar heating element having the positive temperature coefficient characteristic as described above, the standard for product certification should be strictly applied, but the required lifespan of the planar heating element normally used for building heating is 10 years and the lifespan guarantee under such practical use conditions. It takes a lot of time for the test, and entering the market after actually passing such a test is not appropriate considering the life time of the product in the market.

However, the current reliability life test method for the planar heating element is not properly presented.

Therefore, it is necessary to present an effective reliability evaluation method for the surface heating element.

An object of the present invention is to evaluate the failure mechanism of a planar heating element based on an analysis based on the two-level quality function deployment technique. In providing an evaluation method.

Another object of the present invention is to provide a method for evaluating the reliability of a planar heating element that uses various levels of acceleration voltage and quantitatively analyzes a plurality of samples while performing an accelerated life test and sets an acceleration model for the life.

Reliability evaluation method of the planar heating element according to the present invention comprises the steps of determining the test criteria by setting the temperature range and the measurement interval of the planar heating element having an upper limit and a lower limit; Measuring the resistance of the planar heating element to which an acceleration voltage is applied at a preset measurement interval and performing an accelerated life test; Graphing the resistance change with time change during the accelerated life test; And recording a failure time of a sample of the planar heating element in which a failure occurs due to an abnormality of resistance during the accelerated life test.

Here, a graph showing the correlation of unreliability versus time can be further created as the failure time obtained as a result of the accelerated life test.

로 정의하여 가속 모델을 정의하는 단계를 더 포함할 수 있으며, 상기 a는 일반 상수이고, 상기 b는 재료 상수이며, L은 가속 조건에서의 수명이고, V는 가속 전압으로 정의될 수 있다.And, the life of the planar heating element

The method may further include defining an acceleration model, wherein a is a general constant, b is a material constant, L is a lifetime under acceleration conditions, and V is defined as an acceleration voltage.

으로 정의하는 단계를 더 포함할 수 있으며, V는 가속 전압이고 V₀는 정격 전압으로 정의될 수 있다.And the acceleration coefficient AF

It may further comprise a step, wherein V is the acceleration voltage and V ₀ may be defined as the rated voltage.

로 계산됨이 바람직하다.Then, using the shape parameters and Weibull distribution obtained in the accelerated life test and by using the excerpt test method to determine the target life (B ₁₀₎ and the confidence level (1-β) life expectancy time is

Is preferably calculated as

로 계산되며 B _10use 는 실사용 조건에서에서의 목표 수명으로 정의될 수 있다.
And, the life guarantee time is in consideration of the acceleration factor

B _10use can be defined as the target life under practical conditions.

According to the present invention, as a result of analyzing the failure mechanism, the reliability evaluation conditions for verifying the reliability of the planar heating element can be predicted by quantitative analysis and modeling for the voltage acceleration test for the life test. There is an effect of improving the reliability of the surface heating element.

1 is a schematic view showing a laminated structure of a general planar heating element.
2 is a photograph of a front-coated planar heating element.
3 is a photograph of a partially coated planar heating element.
4 is a photograph of a hot linear planar heating element.
5 is a graph illustrating the positive temperature coefficient characteristics of the planar heating element.
6 is a one step analysis table of two level QFD.
7 is a two stage analysis table of two level QFD.
8 is a graph illustrating the periodic temperature rise and fall of the surface heating element.
9 is a flowchart illustrating a preferred embodiment of a method for analyzing reliability of a planar heating element according to the present invention.
10 is a graph of resistance versus time of a planar heating element.
11 is a graph of accelerated test result life distribution.
12 is a graph illustrating a correlation between voltage and lifespan according to an acceleration model.
13 is exemplary time data when voltage acceleration V / V ₀ = 2 and confidence level (1-β) = 90%.
14 is exemplary time data when voltage acceleration V / V ₀ = 3 and confidence level (1-β) = 90%.
15 is exemplary time data when voltage acceleration V / V ₀ = 2 and confidence level (1-β) = 60%.
16 is exemplary time data when voltage acceleration V / V ₀ = 3 and confidence level (1-β) = 60%.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the following embodiments are provided to those skilled in the art to fully understand the present invention, and may be modified in various forms, and the scope of the present invention is limited to the embodiments described below. It doesn't happen.

In the reliability evaluation method of the planar heating element according to the present invention, evaluation is made by selecting a voltage acceleration test for a life test as a test item.

The reliability evaluation of the planar heating element according to the present invention is based on the failure mechanism analysis, the design, the experimental design, the quantitative analysis, the acceleration model analysis according to the two-level quality function deployment technique (hereinafter referred to as two-level QFD). In order.

Fault Mechanism Analysis

Planar heating elements are classified into a low temperature type and a high temperature type. The two types of planar heating elements classified into a low temperature type and a high temperature type exhibit different failure mechanisms.

-Major failure mechanism of high temperature type (over 400 ℃) plane heater

1) Electro-thermal instability: Planar heating elements increase resistance when the proper temperature is exceeded, and if the resistance increase rate differs for each position, it leads to a runaway local current density, resulting in hot spots and melting and carbonization Causes

2) Disruption of the Potential Barrier at the p-n Junction: Planar heating elements have a nonlinear characteristic of conductivity with respect to voltage, which is due to the presence of the crystal boundary region between the nanoscale carbon crystal and the impurities therebetween. This crystal boundary forms a p-n junction, and the electrons excited by the temperature rise run over the potential barrier to the other side and form electron hole pairs. The positive hole also combines with the closed electrons beyond the electron barrier, destroying the barrier and accelerating congestion. Congestion causes hot spots.

3) Mechanical stress: Tearing due to excessive mechanical stress occurs.

4) Concentration of current density in the outer part: Melting point occurs because the current density in the outer part is higher than the surface heating element.

5) Gold Circle Phenomenon: Gold Won Phenomenon is a phenomenon in which the carbon component is decomposed by external moisture when the electrode is formed, and is decomposed by <Equation 1> below. Performance degradation occurs.

[Reaction Scheme 1]

-Major failure mechanism of low temperature type (400 ℃ or less) planar heating element

The carbon component hardly oxidizes or carbonizes below 400 ° C., and polyethylene (PE), which is a base component on which the carbon paste is applied, deteriorates. Polyethylene is stretched or crimped due to deterioration, resulting in a lower carbon density, which in turn increases resistance.

<Design according to two-level QFD>

In the two-level QFD according to the present invention, a requirement vs failure mode analysis may be performed in one step as shown in FIG. 6, and a failure mode vs test mode analysis may be performed in two steps as shown in FIG. 7. Can be.

First, in the step 1 analysis of FIG. 6, requirement items are classified into performance, component life, temperature, and humidity, and failure mode items are destroyed of contact. ), Corrosion, Delamination, Degradation, and Pin Hole.

It can be seen that the performance degradation is analyzed as the highest failure factor in the first stage analysis of FIG. 6.

In addition, the results of the analysis of the environmental test and the life test for each failure mode presented in step 1 in the second step analysis of FIG. 7 can be confirmed, and the environmental test includes a voltage test, insulation test, moisture resistance test, and thermal shock test. , Cryopreservation test, salt spray test, and life test is voltage acceleration test.

In the two-stage analysis of FIG. 7, the moisture resistance test and the voltage acceleration test were ranked higher, but the voltage acceleration test may be determined to be the highest effective item.

Therefore, the method for evaluating the reliability of the planar heating element according to the present invention discloses an embodiment of a method for improving the reliability of the planar heating element by presenting a method for setting the acceleration modeling by the voltage acceleration test.

Experimental Design

 The voltage acceleration test for the life test of the planar heating element is carried out at a temperature saturation state by applying an acceleration voltage at room temperature and humidity conditions, and the change of resistance may be presented as an evaluation standard of 15% or less of the initial performance.

At this time, the acceleration voltage may be set to 220V, 267V, 293V, it is preferable that the contact point for temperature measurement is formed on the insulating coating layer forming the upper and lower surfaces of the planar heating element.

The acceleration voltage may be set to repeat the on state for 3 minutes and the off state for 4 minutes. Accordingly, the planar heating element may periodically increase or decrease the temperature as shown in FIG. 8.

The system for testing may include a structure for mounting a planar heating element, a power supply and a controller for applying and regulating the acceleration voltage, and a monitoring device for displaying the measurement status may be configured. The structure for mounting the surface heating element is preferably configured to provide an environment of room temperature and humidity.

Quantitative Analysis

According to the present invention, a quantitative analysis may be performed by repeating a cycle while applying an acceleration voltage to a planar heating element according to an acceleration condition set according to the above-described experimental design. Quantitative analysis according to the present invention can be described with reference to the flowchart of FIG. 9 and the graphs of FIGS. 10 and 11.

The system for the life test of the planar heating element is set the upper limit and the lower limit temperature for the life test of the planar heating element and periodically set the measurement time interval ΔT to measure the resistance of the planar heating element (S10). Then set the initial failure time T = 0.

When the life test starts, the failure time T increases in units of ΔT every cycle for measuring the resistance of the surface heating element (S12) and measures the resistance of the surface heating element for each failure time increased by ΔT (S14).

If there is an abnormality in the resistance of the planar heating element measured in the above-described step (S16), it is determined that the planar heating element is faulty, the fault sample number and the fault time T are recorded, and the test is finished.

If there is no abnormality in the resistance of the planar heating element measured in the above-described step (S16), a graph of the resistance versus time of the planar heating element is prepared as shown in FIG.

Referring to FIG. 10, the change in resistance over time may be checked for each sample.

After creating the graph of FIG. 10, the temperature of the planar heating element is measured (S20), and it is determined whether the measured temperature of the planar heating element is within a preset range.

If the temperature of the planar heating element is within the preset range, the voltage is turned on for 3 minutes in advance (S24), and if the temperature is out of the preset range, the voltage is turned off for 4 minutes (S26). This is preferred.

As described above, when the predetermined time turn-on (S24) or the predetermined time turn-off (S26) is finished, the failure time (T) is accumulated again by a preset measurement time interval △ (S12) and the resistance measurement (S14) Repeat.

After the life test of the planar heating element as described above, if the time versus unreliability probability graph is made as the acceleration test result, the distribution may be determined to follow the Weibull Distribution defined by Equation 1. have.

In the above, f (t) is a life distribution function, A is a constant, t is a failure time, m is a shape parameter, μ means a scale parameter.

As a result of the accelerated test as shown in FIG. 11, the life distribution was found to follow the Weibull distribution having a shape parameter m of about 7.

Acceleration tests were performed at three levels of acceleration voltages 220V, 262V and 293V by voltage acceleration, and the environmental conditions were maintained at room temperature and humidity.

In the general case of voltage acceleration, an inverse model is applied and the present invention can also be applied to this model. The general equation according to the inverse model is shown in Equation 2.

Where a is a general constant, b is a material constant, L is the lifetime under acceleration conditions, and V is the acceleration voltage.

Knowing only a and b in Equation 2, the lifetime at the desired voltage can be calculated.

When the acceleration coefficient according to <Equation 2> is obtained, it can be summarized as in <Equation 3>.

Where Vo is the rated voltage and V is the acceleration voltage.

In three levels of acceleration test with acceleration voltages of 220V, 262V and 293V, the material constant b = 2.2 can be found.

Therefore, the acceleration model of the planar heating element finally established is shown in <Equation 4>.

The correlation between the voltage corresponding to the model and the lifetime can be confirmed through FIG. 12.

<Life test time calculation>

Goal B ₁₀ by excerpt After setting the lifetime and confidence level, the acceleration voltage was determined.

Taken warranty inspection method is applied to extract the test scheme to guarantee that the number of failures after taking the n samples from the pilot test for a total test time T c is less than one authentication target B ₁₀ The lifetime, confidence level (1-β) (or confidence level CL), number of samples (n), total test time (T), and number of acceptance judgments (c) should be determined.

In the present invention, the service life of the planar heating element is 5 years, and the test time and the number of samples for guaranteeing the B ₁₀ life of 5 years are determined. The confidence level is 90%.

The method of determining the test time of the sample when the lifetime follows the Weibull distribution is different from that in the general index distribution. When the n samples are tested for t hours and the lot is passed when c is less than c , the following relationship is established by the binomial theorem, if the producer risk is α and the consumer risk is β.

이므로

또는

이다.Where R ₁ (t) is the reliability at time t , failure rate λ and F ₁ (t) is 1-R ₁ (t). R _o (t) is the reliability at time t and failure rate λ ₀ and F ₀ (t) is 1-R ₀ (t). The shape parameter m, look to derive the scale parameter θ is a test time required for the life test in the Weibull distribution, so the reliability of the B ₁₀ life is

Because of

or

to be.

Therefore, it can be summarized as in Equation (7).

In addition, if r = 0 is entered in Equation 5,

또는

이다.

or

to be.

therefore,

Here,

In order to organize lifetimes based on equality conditions,

Becomes

The planar heating element follows the Weibull distribution and has a shape parameter (m) of about 7. If the lifetime distribution is based on the Weibull distribution, the confidence time (1-β), the number of samples n , and the shape parameter m, the test time for each sample guaranteeing the B ₁₀ lifetime is given by Equation 11 below.

In the case of voltage acceleration, the test time t _test considering the acceleration factor AF of Equation 4 may be calculated as in Equation 12 below.

Where t _test is the test time per unit sample in consideration of the acceleration factor, B _10test means B ₁₀ life under accelerated conditions, and B _10use means B ₁₀ life under actual operating conditions.

The final test time equation is shown in Equation 13 below.

=1.36으로 하였다.In the present invention, the number of samples n = 16, confidence level (1-β) = 90%, acceleration voltage

= 1.36.

Accordingly, the test time is 43,800 hours in terms of five years, and 21,900 hours, provided that the annual operating rate of the planar heating element is 50%. The surface heating element designates the upper limit temperature and the lower limit temperature, and when the temperature exceeds the upper limit temperature, the power is cut off to the lower limit temperature, and when the surface heating element is lower than the lower limit temperature, the planar heating element adopts the method of FIG. 8 so that the temperature is maintained within a predetermined range while the power is turned on again.

When the planar heating element is turned on / off at 1 cycle, the measurement result is that the power-on time can be measured at about 40% of 1 cycle as shown in Fig. 8, so that the actual five-year operating time of the planar heating element is Same as>

Therefore, when B _10use is 8,760 hours, the test time is calculated and calculated according to <Equation 13>, the test time per unit sample is 4,629 hours and the sample sample test time is 4,600 hours in the present invention.

For reference, when the voltage acceleration V / V ₀ = 2, confidence level (1-β) = 90%, the time data can be obtained as shown in Figure 13, voltage acceleration V / V ₀ = 3, confidence level (1-β) ) = 90%, the time data can be obtained as shown in FIG.

In addition, when the voltage acceleration V / V ₀ = 2, the confidence level (1-β) = 60%, the time data can be obtained as shown in Figure 15, the voltage acceleration V / V ₀ = 3, the confidence level (1-β) In the case of = 60%, time data can be obtained as shown in FIG.

As a result of analyzing the failure mechanism as described above, by setting the life test of the planar heating element as an item, an acceleration model capable of verifying the reliability of the planar heating element may be presented by quantitative analysis and modeling.

As mentioned above, although preferred embodiment of this invention was described in detail, this invention is not limited to the said embodiment, A various deformation | transformation by a person of ordinary skill in the art within the scope of the technical idea of this invention is carried out. This is possible.

Claims (6)

Setting a test criterion by setting a temperature range and a measurement interval of the planar heating element having an upper limit and a lower limit;
Measuring the resistance of the planar heating element to which an acceleration voltage is applied at a preset measurement interval and performing an accelerated life test;
Graphing the resistance change with time change during the accelerated life test; And
And recording a failure time of a sample of the planar heating element in which a failure occurs due to an abnormality of resistance during the accelerated life test.
The method of claim 1,
A method for evaluating the reliability of a planar heating element, which further generates a graph showing a correlation of unreliability vs. time as the failure time obtained as a result of the accelerated life test.
The method of claim 2,
The life of the planar heating element is

And defining an acceleration model, wherein a is a general constant, b is a material constant, L is a lifetime under acceleration conditions, and V is an acceleration voltage.
The method of claim 2,
Acceleration Factor (AF)

Further comprising the step of defining, V is the acceleration voltage and V ₀ is the rated voltage reliability evaluation method of the surface heating element.
The method of claim 2,
In the accelerated life test, using the implementation shape parameter and the Weibull distribution, the target life time (B ₁₀₎ and the confidence level (1-β) are determined by an extractive test method.

Reliability evaluation method of planar heating element calculated by.
The method of claim 5,
The lifetime guarantee time is taken into account the acceleration factor

B _10use is a method for evaluating the reliability of a planar heating element, which is the target life under actual use conditions.

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