study on the effects of small swing of junction ......study on the effects of small swing of...

Study on the Effects of Small Swing of Junction Temperature Cycles on Solder Layer in an IGBT

Module

Yigao Chen, Minyou Chen, Wei Lai, Li Ran, Shengyou Xu, Nan Jiang School of Electrical Engineering

Chongqing University Chongqing, China

Email: [email protected]

Olayiwola Alatise and Phil Mawby School of Engineering

The University of Warwick Coventry, The UK

Abstract—This paper proposes a method to obtain the effect of junction temperature swing with low amplitudes on the power modules, so that the knowledge about the reliability characteristics of the modules and the lifetime model could be improved. Power cycling tests and finite-element analysis (FEA) models, for non-aged and aged IGBT modules, are designed to illustrate the failure mechanisms of the power modules and the stress distribution. IGBT modules in actual converters such as those in wind turbine systems are usually operated in a junction temperature swing range up to 40⁰C, therefore small swing junction temperature tests are designed to obtain the effects of small swings of junction temperature on the lifetime of IGBT modules. It is found that such relatively minor stress cycles, which happen frequently during normal operation, may not be able to directly initiate a crack but can contribute to the development of damage in die attach solder layer due to the stress concentration. The experimental results show that the effects of small swing of junction temperature are affected by the ageing state, stress level and load sequence.

Keywords—Solder failure; small swing of junction temperature cycles; failure mechanism; power cycling test; stress concentration.

I. INTRODUCTION

Power semiconductor devices are the core components in all power electronic systems to fulfil the functionalities of power conversion and control, and the failures of power modules are responsible for a significant proportion of overall system faults [1]. The reliability of these power devices is crucial to the overall performance and life-cycle cost of a power electronic system and of the power system as a whole. Therefore a detailed understanding of the factors influencing the reliability of power semiconductor devices is becoming an increasingly important topic for power module design, converter design as well as operational management. In the actual converter operation, the junction temperature Tj would fluctuate as the load condition varies. Owing to temperature cycling, coefficient of thermal expansion (CTE) mismatch between adjacent layers and internal temperature gradients, a module can be subject to cyclic shear stresses leading to fatigue damage. Previous studies of power cycling test focused on the large ΔTj (≥80⁰C) [2-4] because the small ΔTj ageing test would take a unrealistically long time to complete, but in

the actual inverter ΔTj is likely to be just about 40⁰C or lower. For instance, using the model given in [5] a narrow power cycling tests with ΔTj=30⁰C would take 30 to 100 years as shown in Fig. 1. Therefore it is unrealistic to test to the end of lifetime under such stress conditions.

It is well known that stress concentration can be very high at the crack tip and decreases gradually away from the tip. A stress concentration is a location in an object, structure and materials where stress is concentrated. An object is strongest when the stress is evenly distributed over its area, so a reduction in area, e.g., caused by a crack, results in a localized increase in stress. A material can degrade, via a propagating crack, when a concentrated stress exceeds the material’s theoretical cohesive strength. The real fracture strength of a material is always lower than the theoretical value because most materials contain small cracks, defects or contaminants that concentrate stresses. Fatigue cracks always start at stress raisers. It means that once there is the initial crack in the die-attach solder which is found out to be the dominant failure mode of IGBT modules, the effects of the small ΔTj may not be negligible. This paper mainly examines the effects and the characteristics of the small ΔTj swings on the aged and un-aged modules by power cycling test.

Fig. 1. Power cycling lifetime as a function of ΔTj and Tjm [5].

II. SIMULATIONS OF STRESS CONCENTRATION

Thermo mechanical breakdown of device packaging is a

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2016 IEEE 8th International Power Electronics and Motion Control Conference (IPEMC-ECCE Asia)

978-1-5090-1210-7/16/$31.00 ©2016 IEEE

major source of reliability concern in power semiconductor modules due to the cyclic fatigue stresses. The packaging techniques for integrating power semiconductor devices has made impressive progress from single-chip discrete devices to multi-chip power modules to match the growing demand of high current handling capability, increasing power density and effective thermal management. However, the packaging of power modules adversely gives rise to the corresponding reliability issues. Therefore a sound understanding of the physics underlying the potential failures and stress distribution within power electronic modules is a prerequisite for their reliable applications and this study attempts to achieve such an understanding by using electro-thermal-mechanical models.

Finite element analysis (FEA) models with and without a crack are established using ANSYS to understand the stress concentration effect under low ΔTj profiles. A crack as shown in Fig. 2 with a width and length of 0.013mm and 0.488mm respectively is selected and this corresponds to an increase of the thermal resistance of the solder layer by about 10% as confirmed by the simulation results.

Fig. 2. Sizes of crack. Tested parameters of a commercially available single-chip

IGBT power module (SKM50GB12T4, 50A/1200V), materials and junction temperature profiles are used in the FEA models. The results are summarized in Figure 3.

The junction temperature (green lines) is customized to follow that in the power cycling tests as shown in Fig. 3(a) and Fig. 3(b). Fig. 3(c) shows that the lines of force are uniform and parallel away from the corners of solder, while the stresses at the corners are concentrated, complying with the definition of stress concentration. The Fig. 3(e) is the crack enlargement of the Fig. 3(d). Fig. 3(d), Fig. 3(e) and Fig. 3(f) show that the stress is the largest at the crack tip and the stress gradually drops away from the crack tip. Fig. 3(f) shows that with concentration, the maximum stress is almost five times as large as the mean stress. Therefore it may cause the crack to propagate along the crack tip even under ΔTj=40⁰C cycles which are simulated. The maximum stress when Tj=95⁰C with crack is more than doubled as compared to the case without the crack as shown in Fig. 3(a). The stress in the crack tip is greater than the case without the crack but Tj=140⁰C, by comparing Fig. 3(a) with Fig. 3(b). The stress at the crack tip is much higher than the mean tensile stress, and the stress gradually drops away from the crack tip and equals to mean tensile stress eventually. Moreover the larger crack length is, the greater impact of stress concentration is.

(a)

(b)

(c)

(d)

(e)


(f)

Fig. 3. Results of ANSYS simulation. (a) junction temperature and stress under ΔTj=40⁰C; (b) junction temperature and stress without preset initial crack under ΔTj=110⁰C; (c) stress distribution without crack under Tj=95⁰C; (d) stress distribution with crack under Tj=95⁰C; (e) enlargement of the crack tip for solder layer under Tj=95⁰C; (f) stress away from crack tip.

III. POWER CYCLING TESTS UNDER SMALL SWINGS OF

JUNCTION TEMPERATURE

Power modules in wind turbine converters are often used with ΔTj up to 40⁰C according to the converter simulation under actual wind profiles [6]. Therefore it is important to understand the effect of the low ΔTj stress cycles in lifetime modeling. Given the difficulty to wear out a new module, a staged test scheme is proposed for this purpose. Some new power modules are first subject to large ΔTj cycles to cause initial solder cracks. Usually, once the thermal resistance from the junction to the case shows a noticeable increase, the test condition is changed so that ΔTj is reduced to about 40⁰C. Further change of the thermal resistance Rth is monitored as the number of stress cycles increases; Rth is normalized with respect to its initial value for the new power module: Rth0. The DUT’s (device under test) normalized collector-emitter voltage under the heating current in the power cycling test, i.e. the IGBT’s on-stage voltage drop Vce with temperature compensation is also included in the plot to make sure that a bond wire failure has not occurred, hence the main ageing-to-failure mechanism under investigation is indeed the die-attach solder fatigue. Fig. 4 shows the test results.

Fig. 4(a) shows that the condition of a non-aged module, measured by the junction-to-case thermal resistance Rth, hardly changes. In Fig. 4(b), a brand new module is first subject to a sufficient number of stress cycles with ΔTj=118⁰C and mean junction temperature Tjm=85⁰C to cause a 17% increase of Rth. The thermal resistance then continues to increase under cycles with ΔTj=40⁰C and Tjm=74⁰C, which had no effect in Fig. 4(a). This is because once there is a crack in the solder layer, the stress close to the crack tip becomes greater than the overall average due to stress concentration meanwhile a plastic deformation zone is formed around the tip, leading to progressive further degradation. Therefore it may be the case that low ΔTj stress cycles should not be ignored for aged modules. Fig. 4(c) shows that the effect of further low ΔTj stress cycles is again observed but is reduced when compared

to the previous results for the same ΔTj and Tjm on a more aged module. The ageing effect is still present when ΔTj is reduced close to 30⁰C. From test results, the dependency of the Rth’s rate of subsequent change on the severity of the stress cycles applied and the present health condition of the module is summarized in Table I.

(a)

(b)

(c)

Fig. 4 Results of power cycling test. (a) normalized Rth in an un-aged module; (b) normalized Rth for a new module under large ΔTj and then small ΔTj; (c) normalized Rth in an aged module under small ΔTj.

Next, another new power module is tested under different

load sequences. Firstly, in the test the module should degrade


to the same degree (Rth=1.17Rth0) using large △Tj stress cycles. Secondly the test conditions are changed to ΔTj=32.8⁰C and Tjm=56⁰C and last the same stress of cycles as previous tested module. After that conditions are changed to about ΔTj=40⁰C and Tjm=74⁰C as shown in Fig. 5 and Table II. The results indicate that module 1 are degraded faster in stage ③ than module 2 in stage ② and module 2 are degraded faster in stage ③ than module 2 in stage ②. The reason is that the larger the crack length is, the faster the crack propagates under

the same stress cycles. It is shown that once the junction-to-case thermal resistance Rth starts to increase, the further increase of the thermal resistance has nonlinear relationship with the number of the stress cycles even though the stress level is the same. The ageing process generally develops progressively faster. This means that the sequence for different stress cycles to occur would affect their ageing consequence.

More details about some of the findings in this study will be reported in reference [7].

Table I Rth average increase rate in different stages (* test results unavailable)

Power Cycling Test

Stage ① Stage ② Stage ③ Stage ④ Test Conditions and Rth Increase Rate (per 10000 Cycles)

Test 1 ΔTj=113⁰C Tjm=84⁰C

ΔTj=40⁰C Tjm=74⁰C

Rth=1.06Rth0 8.95%


Rth=1.20Rth0 1.91%


Rth=1.24Rth0 7.03%



Rth=1.18Rth0 11.7%

ΔTj=32.5⁰C Tjm=55.8⁰C Rth=1.40Rth0

5.44% * *


ΔTj=40.7⁰C Tjm=74⁰C

Rth=1.41Rth0 29%

ΔTj=41⁰C Tjm=60.5⁰C Rth=1.78Rth0

8% * *

Table II Rth average increase rate in different stages of two modules

Module number Stage ① Stage ② Stage ③

Test Conditions and Rth Increase Rate (per 10000 Cycles)

Module 1 ΔTj=118⁰C Tjm=85⁰C

ΔTj=40⁰C Tjm=74⁰C 11.7% ΔTj=32.8⁰C Tjm=56⁰C 5.44%

Module 2 ΔTj=118⁰C Tjm=85⁰C

ΔTj=32.8⁰C Tjm=56⁰C 3.54% ΔTj=40⁰C Tjm=74.5⁰C 12.4%

Fig. 5 Results of power cycling test under two test conditions.

IV. CONCLUSIONS

This study provides a new perspective which indicates that the fatigue life of a power module is not only related to the magnitude of temperature change in the stress cycle but is also dependent on the pre-loading history leading to the present condition of health. ANSYS FEA models with and without the initial crack are used to verify the stress concentration. It indicates that the stresses near the crack tip are significantly larger than the stresses in the case without the initial defect layer. A series of experiments have been

carried out to verify that the small temperature swing can also accelerate the module failure once defects are pre-generated in the IGBT modules. It demonstrates the effect of the crack length (inferred through the thermal resistance increase) on the susceptibility to the small ΔTj swing cycles and the damage depends on the sequence of temperature cycling conditions which agrees with the previous observations reviewed in reference [8]. Moreover, the study seems to indicate that the degradation rate of thermal resistance is only impacted by the current Rth and load stress levels; while the history of stress levels does not influence further aging progress. The conclusion also implies that the traditional stress life (S-N) curve (without the information of pre-loading history) and linear accumulation lifetime model may need to be improved to accurately predict the fatigue life in real complex loading conditions.

ACKNOWLEDGMENT

This collaborative work was supported by the National Natural Science Foundation of China (grant numbers 51477019, 51577074 and 51137006), in part by UK EPSRC funding through underpinning power electronics devices and components themes (EP/L007010/1 and EP/K034804/1).

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[2] V. Smet, F. Forest, J. Huselstein, A. Rashed and F. Richardeau, “Evaluation of Vce Monitoring as a Real-Time Method to Estimate Aging of Bond Wire-IGBT Modules Stressed by Power Cycling,” Industrial Electronics, IEEE Transactions on, vol. 60, no. 7, pp. 2760-2770, July 2013

[3] V. Smet, F. Forest, J. Huselstein, F. Richardeau, Z. Khatir, S. Lefebvre and M. Berkani, “Ageing and Failure Modes of IGBT Modules in High-Temperature Power Cycling,” Industrial Electronics, IEEE Transactions on, vol. 58, no. 10, pp. 4931-4941, Oct. 2011

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[7] Lai W, Chen M, Ran L, Xu S, Jiang N, Wang X, Alatise O and Mawby PA, Experimental Investigations on the Effects of Narrow Junction Temperature Cycles on Die-attach Solder Layer in an IGBT Module, Power Electronics, IEEE Transactions on, (accepted, March 2016)

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study on the effects of small swing of junction ......study on the effects of small swing of...

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