Technical report Approaches to Life Estimation of Electronic Circuits

Transcription

1 Technical report Approaches to Life Estimation of Electronic Circuits Akira MOTOYAMA Representative, M.A. Reliability Technology Office Abstract Concerning the short-life problem of electronic application products in these days, methods of life estimation for an electronics circuit is explained with examples. As a starter, steps for life estimation are illustrated. First, allowable cumulative failure probability against operating time that customers expect, i.e. target value of life, is set. Next, the electronic components that underperform the target values are picked up without experiment, among those mounted on the electronic circuit. Possessing acceleration formulas for each component are used for it. For the components picked up as have short life, accelerated life test is performed to estimate the life. This time, taking film capacitors as an example, life estimation of component is explained by the Eyring model formula. This formula is composed of ambient temperature, humidity and voltage. In the case of film capacitors, the capacitance decrease by anodic oxidation of the electrode by water due to moisture ingress and an applied voltage. Finally, as a result of life estimation of picked-up electronic components, solder joint parts and boards, the target value is cleared after changing carbon film resistors that do not meet the target value, to metal glaze resistors. 1. Introduction Many electronic components and mechanical components mounted on the circuit board in an electronic circuit. To ensure the target life of the electronic circuit, it is necessary to extract and change the components and joint parts to become a short life. In this paper, after showing a life estimation step of the electronic circuit, we introduce the method of it with an example. 2. Procedure for life estimation of electronic circuit Figure 1 shows a procedure for the life estimation. Set a target value of life and pick up operating conditions of electronic circuits and components. Estimate life of electronic components, then. Though life of every component and part has to be estimated, it is practically impossible because number of components of an electronic circuit is too large. So, pick up short life components in advance on a desktop, run accelerated tests of the picked up components only, and estimate the lives. Then estimate the life as an electronic circuit by combining estimated lives from accelerated test results of joints (other than components) and PCBs, with those estimated above. Take countermeasures for components and parts that do not meet the target value of life. Test Navi Report No.20 (87th) 1

2 Cum. Failure prob. ESPEC Technical Information 1. Set the target value of life (Set the life expectancy and the allowable cumulative failure probability.) 2. Pick up operating conditions 3. Pick up short-life components 4. Estimate life of short-life components 5. Estimate life of the circuit board assembly (1) Set ambient temperature/humidity of the products and the components in the field. ( (2) Set electrical stresses (voltage/current/frequency) ) applied to the components. (1) Prepare a list of stresses to the components mounted on the target circuit. ( (2) Pick up short-life components based on equations for ) life estimation obtained in advance. (1) Apply accelerated test to the picket up components per failure mode. ( (2) Estimate time to failure and cumulative failure ) probability of the picked up components. (1) Apply accelerated tests to joint areas other than the components. ( ) (2) Apply accelerated tests to the PCB. 6. Verify the target value of life Figure 1. Procedure for life estimation of electronic circuit 3. Example of life estimation of electronic circuit (1) Set the target value of life (1) Estimate the life of the electronic circuit from the accelerated test results. ( (2) Pick up components and parts that do not meet the ) target values of life. A target value of life is usually decided based on the consideration of life expectancy by customer and cost for fixing the trouble. Here, as an example, we set the target value of life as 10 years of operation (cumulative with slightly longer than a half day operation per day), and the allowable cumulative failure probability as 0.1%. By the way, JEITA sets 0.1% as the allowable cumulative failure probability of semiconductors at useful life. Define the cumulative failure probability as a ratio of a number of the products returned as a failure so far and a number of the products inputted to the field. The cumulative failure probability vs. operating time in the field is shown in Figure 2. Assuming the relation as the Weibull distribution, it can be expressed by linear regression each of the three periods: early failure, random failure, and wear-out failure period. Here the target value of life is defined as a time when failure mode classified as wear-out failure starts to occur. Life expectancy Random failure Wear-out failure Early failure Target value of life 0.1% Operating time (h) 50,000 Figure 2. Set the target value of life Test Navi Report No.20 (87th) 2

3 (2) Pick up operating conditions Assume that the ambient temperature, operating time of an electronic circuit and ambient temperature of the components mounted in the product per year are shown in Table 1, after investigating the ambient temperature/humidity of a product/component and operating time in the field. Table 1. Operating environment and time of a product and target components in a year Ambient temp. of the product 5 C 10 C 15 C 20 C 25 C 30 C Ambient temp. of components 54 C 61 C 69 C 76 C 82 C 86 C Operating time per year (h) Information on electrical stresses such as voltages, currents and frequencies applied to the components is also required as a part of operating conditions. (3) Pick up short-life components Understand which types of the components are mounted in the electronic circuit so that short life components can be picked up. Mainly for passive components (L, C, R) assumed to have relatively shorter lives, summarize the manufacturer, product no., rating, and operating environment (such as temp-humidity, electrical stress) of each component and create a list of operating stresses per component. Taking a film capacitor as an example, electrode structure, voltage (rating and operating), current (rating and operating), and temp-humidity in the field are summarized per circuit code in Table 2 List of operating stresses of each component. Table 2. List of operating stresses of each component Carbon film resistors Transformers and coils Al electrolytic capacitors Film capacitors Code Electrode structure (type) Voltage (V) Current (A) Rating Opr. Rating Opr. Temp Hmdty Metalized Metalized Metalized Foil Foil Next, pick up short-life components by using life estimation equations of electronic components/parts owned by analysis service companies such as M.A Reliability Technical Office, and by using reliability test data of the component manufacturer. As an example, let s estimate the life of a metalized film capacitor of circuit code C12 in Table 2, under the temp-humidity conditions in the field. Test Navi Report No.20 (87th) 3

4 In general, estimated life against temp-humidity can be given by the Eyring model formula (1) below. L = L 0 exp(a1 (1/( T) - 1/( T 0 )) + a2 (RH - RH 0 )) (1) where L is median life (h) at ambient temperature T ( C), relative humidity RH (%) in the field, L 0 is median life (h) at ambient temperature T 0 ( C), relative humidity RH 0 (%) in the accelerated test, a1 is a temperature acceleration constant and a2 is a humidity acceleration constant. Then the operating condition of C12 shown in Table 2, i.e. ambient temperature 75 C and relative humidity 9%, and the reliability test condition of electronic component manufacturer, i.e. ambient temperature 60 C and relative humidity 90%, are entered into equation (1) to obtain equation (2). L = L 0 exp(a1 (1/( ) - 1/( )) + a2 (9-90)) (2) For the two constant terms (a1, a2), use two C12 specific values at hand, and obtain C12 median life for the operating condition. Because this failure distribution is considered as a Weibull distribution, use the median life L obtained in the above and a form parameter m at hand in the equation (3) to get estimated life t = 52,800 hours at the cumulative failure probability 0.1%. ln(ln(1/(1-0.1%))) = m ln(t) + ( m ln(l)) (3) In this way, using life estimation equation of each component and data from electronic component manufacturers, estimate life against stresses (temperature, temp-humidity, and temp. cycle) corresponding to failure mode of each component. Here, seven components are picked up that may have short life based on the calculation result as an example. The result is shown in Table 3. As we have seen so far, without actual accelerated tests and by using proprietary data owned by data analysis service companies, life can be estimated in a short time on a desktop and short-life components can be picked up. Test Navi Report No.20 (87th) 4

5 Table 3. Pick up of short-life components by life estimation in advance Component name Al electrolytic capacitor Circ. Code C1 C2 Target life Operating time ON/OFF cycles - - Estimated lif e (Cum. failure prob.: 0.1%) Temp. 34,900h 57,300h Temp /Hmdy Temp. cycles No mode No mode C10 7,000 55,500h 54,600h Unknown Film capacitor C11 7, ,000h 57,700h Unknown C12 7, ,300h 52,800h 7,920 Trans. T1 Carbon film resistor R1 7,000-45,900h 34,700h No mode Fields of red number do not meet the target values and need the component evaluation without exception. Fields of blue number clear the target values but less than two times and need the component evaluation. According to Table 3, aluminum electrolytic capacitor C1 Trans. T1 and carbon film resistor R1 do not clear the target life of 50,000 hours, and need life estimation by accelerated tests. And other four components as well do not have enough margins and need life estimation by accelerated tests. As such, we need to have a life estimation equation in advance to pick up short-life components. In case the life estimation equation is not available, we may refer to the component manufacturer s test data (number of samples, characteristic values (such as capacitance change rate for a capacitor), failure criteria, etc.). Also we may pick up components operating at conditions closer to the component manufacturer s tolerances (temp-humidity, voltage, current, etc.), i.e. components with smaller derating, as short-life components. (4) Estimate life of short-life components Accelerated test per failure mode is required to have more precise life estimation of a short-life component. Test levels should be 3 or more here. Test items, test levels, failure criteria for short-life components picked up in the previous section are shown in Figure 4. For temp-humidity test, total 5 levels are required since 3 levels each are set for temperature and humidity. Test Navi Report No.20 (87th) 5

6 Table 4. Accelerated test items per component Component name Circ. code Test item Test level Failure criteria Al electrolytic capacitor C1 C2 Temperature 3 levels Ta: a C, b C, c C C/C <-a% Tanδ > spec x 2 C10 Temperature 3 levels Ta: a C, b C, c C C/C <-a% Tanδ > spec x 2 Film capacitor C10 C11 C12 Temp /Hmdty 5 levels Ta: a C, b C, c C RH: a%, b%, c% C/C <-a% Tanδ > spec x 2 C10 C12 Temp. cycles 3 levels T: a C, b C, c C Pulse tol. < aaop Tanδ > spec x 2 Trans. T1 Temperature 3 levels Ta: a C, b C, c C Layer short voltage < akv Carbon film resistor R1 Temp /Hmdy 5 levels Ta: a C, b C, c C RH: a%, b%, c% R/R >a% Obtain the Eyring model formula (1) in the above by the temp-humidity accelerated test, and obtain the acceleration constant a1 of the Arrhenius model formula (4) below, to estimate life in operating conditions. L = L 0 exp(a1 (1/( T) - 1/( T 0 ))) (4) where L is median life (h) at ambient temperature T ( C) in the field, L 0 is median life (h) at ambient temperature T 0 ( C) in the accelerated test, a1 is a temperature acceleration constant. That is, different from estimating life on a desktop, life is estimated by obtaining the constant (a1) of the Arrhenius model equation (4) and the constants (a1, a2) of the Eyring model formula (1). As an example, result of temp-humidity test of the film capacitor C12 based on the Eyring model is given below. Accelerated tests of 3 temperature levels (65, 75, and 85 C at humidity 85%) and 3 humidity levels (75, 80, and 85% at temperature 85 C), giving actual 5 levels, are conducted. From the test results, the constants (a1, a2) are obtained, giving life estimation equation (5) below. L = L 0 exp(10084 (1/( ) - 1/( )) (9-85)) (5) ln(ln(1/(1-0.1%))) = m ln(t) + ( m ln(l)) (6) By getting median life L from equation (5) and performing Weibull analysis with equation (6), we finally get 57,295 hours as the estimated value of life for the operating conditions in the field at the cumulative failure probability 0.1%. Test Navi Report No.20 (87th) 6

7 Estimated lives of all the short-life components obtained by this procedure are shown in Table 5. The result reveals that the carbon film resistor among the 7 components picked up by the desktop life estimation, is the only one that cannot clear the target value. Table 5. Life estimation by accelerated testing (5) Estimate life of a circuit board assembly To estimate life of a circuit board assembly, select and perform accelerated tests (corresponding to each failure mode) to solders connecting between components and the PCB and to the PCB itself, Using the result, estimate the time to failure against the target cumulative failure probability 0.1%. Based on this approach, life of solder joint part can be estimated by temperature cycle test and using Modified Coffin-Manson formula (7). N = N 0 (f/f 0 )^a1 ( T 0 / T)^a2 exp(a3*(1/( T) - 1/( T 0 ))) (7) where N is the median life (cycle) in actual operation, N 0 is the median life (cycle) in the accelerated test, f is the actual operating period (cycles/day), f 0 is the test period (cycles/day), T is the temperature difference in actual operation, T 0 is the temperature difference in the accelerated test, T is the upper limit temperature ( C) in actual operation, T 0 is the upper limit temperature ( C) in the accelerated test, a1 is the acceleration constant of the period, a2 is the acceleration constant of the temperature difference, and a3 is the acceleration constant of the temperature. For the PCB, perform temp-humidity test as an accelerated test for migration and estimate life using the Eyring model formula (1) above. Test Navi Report No.20 (87th) 7

8 Cum. failure prob. ESPEC Technical Information (6) Verify the target value of life As we have discussed so far, life of short-life component can be estimated by the accelerated tests in Section (4), and life of joint part other than components and of PCB can be estimated by the accelerated tests in Section (5). Here, we use the results of those accelerated tests to estimate life of the electronic circuit. We also verify if the target value of life of the electronic circuit (50,000 hours at cumulative failure probability 0.1%) can be cleared. If we plot test results of short-life components/parts and the target value of life on a same Weibull probability sheet, we get Figure 3. Short-lif e components/parts R1 C1 C2 C12 T1 Target value of life 0.1% 50,000 Operating time (h) Figure 3. Weibull analysis of short-life components/parts and verification of the target value of life On the other hand, life of an electronic circuit can be estimated by obtaining the cumulative failure probability F(t) from equation (8). F(t) = 1 (R1(t)) R2(t) Rn(t) (8) where R1(t) to Rn(t) are reliabilities of each component, part and board at time t. If the value F(50,000) can be plotted below the target value of life, we have verified that the target can be cleared. For the picked up components/parts so far that do not meet the target value of life, investigate collectively by referencing Figure 3 and others whether to change them or not, then decide. In case of this example, the target has been cleared by changing the resistor R1, which does not meet the target value, from carbon film resistor to metal glaze resistor, of different material. As is seen in Figure 3, slope of a line (form parameter) varies per component/part. Referencing to Figure 3 is effective not only to verify the target value of life but also to guess increasing trend of cumulative failure probability of an electronic circuit when its operating time is extended beyond the target value of life. Test Navi Report No.20 (87th) 8

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