Robust HVIGBT module design against high humidity

Transcription

1 PCIM Europe 205, 9 2 May 205, Nuremberg, Germany Robust HVIGBT module design against high humidity Nobuhiko Tanaka*, Kenji Ota*, Shinichi Iura*, Yasutaka Kusakabe*, Keiichi Nakamura*, Eugen Wiesner**, Eckhard Thal** *Mitsubishi Electric Corporation, -- Imajukuhigashi, Nishi-Ku, Fukuoka, Japan **Mitsubishi Electric Europe B.V., Germany Tanaka.Nobuhiko@aj.MitsubishiElectric.co.jp Abstract This paper describes how HV (High Voltage) IGBT module components influence the resistance to humidity. To evaluate HVIGBT module components for humidity resistance, we conducted a dew condensation test with two kinds of 6.5kV HVIGBT modules, and from the result we considered the failure mechanism. Consequently, based on our analysis of the failure mechanism, we found suitable HVIGBT module components that provide resistance against humidity.. Introduction HVIGBT modules (HVIGBT) are frequently used for applications such as railways and power systems under the conditions which temperature and humidity are not controlled. Therefore, HVIGBT with high reliability under severe environmental conditions is needed. Although there heve been some reports on the humidity resistance performance of HVIGBT [][2], it is not clear which HVIGBT components most significantly influence this performance. In this paper, we investigated leakage current (I CES ) behavior of 6.5kV HVIGBT under high-humidity conditions and verified by experiment that the humidity resistance of HVIGBT was improved. 2. Influence of High Humidity In order to clarify the influences that HVIGBT receives from high humidity, a dew condensation test (I CES measurement) was carried out on two kinds of 6.5kV HVIGBT module samples, called Type-A and Type-B in this paper. As depicted in Table, gel appearance becomes foggy after cooling (STEP2) as a result of dew condensation. Three different main materials and processes were evaluated in the Type-A and the Type-B module sample: a) Silicone gel b) Passivation material (on guard ring) c) IGBT and diode chip structure Figure shows the dew condensation test results. After applying high voltage between the collector and emitter under the presence of dew condensation, the I CES of Type-A samples increased after a few minutes. However, the I CES of Type-B samples did not increase. Therefore, the differences in module materials and processes (Table 2) had an influence on the humidity-resistance performance of HVIGBT. 368

2 PCIM Europe 205, 9 2 May 205, Nuremberg, Germany Table. Dew condensation test flow STEP Item Ta RH Time Gel appearance STEP0 I CES V CE =5200V (DC) 25 C 50% 3 min STEP Storage 85 C 85% 36 hr STEP2 Cooling 25 C 50% 4 hr STEP3 Storage 25 C 50% 2 hr Clear STEP4 I CES measurement 25 C 50% 3 V CE =5200V (DC) Foggy Table 2. Differences between Type-A and Type-B modules Materials and processes Type-A Type-B Silicone gel Gel A Gel B Passivation material (on guard ring) Passivation A Passivation B IGBT and diode chip structure Structure A Structure B I CES measurement result (STEP0) I CES measurement result (STEP4) Type-A Type-B Leakage current[ma] time[sec] Leakage current[ma] time[sec] Figure. Dew condensation test results. Horizontal axis : measurement time; vertical axis : leakage current (I CES ). I CES was measured at V CE =5200V (DC), V GE =0V, Ta=25 C. 369

3 PCIM Europe 205, 9 2 May 205, Nuremberg, Germany 3. Failure Mechanism Hypothesis Generally, the avalanche voltage decreases if surface charges () accumulate on the guard ring of a chip. For one instance, avalanche voltage and electric field simulation results of a 6.5kV HVIGBT chip are shown in Fig. 2 and Fig. 3, respectively. Figure 2 indicates that the increase in causes a decrease in avalanche voltage. In addition, from Fig. 3, the electric field on the guard ring surface becomes stronger by an increase in..e Jc [A/cm2] 5.E-05 0.E Collector-Emitter voltage [V] Figure 2. Avalanche voltage simulation results of 6.5kV HVIGBT chip for different at 25 C. Horizontal axis : collector-emitter voltage; vertical axis: current density. Figure 3. Electric field simulation results of 6.5kV HVIGBT chip for different. Left side shows case of normal quantity, and right side shows case of large quantity. We analyzed the following failure mechanism under high-humidity conditions from the simulation results described above and different materials and processes between the Type-A and Type-B module samples. ) The silicone gel on the guard ring of a chip is polarized by applying high voltage between collector and emitter. 2) accumulate on the guard ring of a chip due to silicone gel polarization. 3) Absorbed moisture in the silicone gel accelerates accumulation. 4) The avalanche voltage decreases if increases (Fig.2). 5) I CES starts to increase when the applied voltage between collector and emitter reaches the avalanche voltage. Therefore, it is assumed that the humidity resistance of HVIGBT can be improved by using a silicone gel that does not absorb moisture and by applying a passivation that does not strengthen the electric field (i.e., avoiding accumulation of ). In order to confirm that the cause of increased I CES under the condition of dew condensation condition is not a kind of silicone gel pollution, the silicone gel in Type-A was analyzed by the FT-IR (Fourier Transform Infrared Spectroscopy) method. The analysis results of the silicone gel on a chip with increased I CES are shown in Fig. 4. Because both spectrums before and 370

4 PCIM Europe 205, 9 2 May 205, Nuremberg, Germany after the dew condensation test show the same shape, it is confirmed that the increased I CES under the dew condensation condition is not related to silicone gel pollution. Silicone gel on a IGBT chip in the Type-A module ( after dew condensation) Silicone gel (initial) Figure 4. Comparison of spectrum analysis results for silicone gel by FT-IR method. Horizontal axis: spectral period; vertical axis: spectral intensity. Upper line: Initial condition; lower line: dew condensation condition. 4. Verification Experiment Results with Improved Modules 4. Verification modules In order to confirm which material and process can improve humidity degradation based on the estimated failure mechanism, we attempted experiments with two different verification module samples using a dew condensation test. The materials and processes of the verification modules are shown in Table 3. A-) Type-A + Gel B (used in Type-B) A-2) Type-A + Gel B + Passivation B (used in Type-B) Table 3. Materials and processes of Type-A- and Type-A-2 Materials and processes Type-A- Type-A-2 Silicone gel Gel B Gel B Passivation material (on guard ring) Passivation A Passivation B IGBT and diode chip structure Structure A Structure A 4.2 Dew condensation test results Dew condensation tests of the Type-A- and Type-A-2 verification modules were performed. Measurement results are shown in Fig. 5. By applying high voltage between the collector and emitter in the presence of dew condensation, the I CES of Type-A- increased after a few minutes, as well as the I CES of Type-A. Because the I CES of Type-A- increased more slowly than did that of Type-A, it is assumed that the polarization characteristics of the silicone gel differ between Type-A and Type-B. While the I CES of Type-A-2 did not increase, the humidityresistance performance was more greatly influenced by the passivation materials and processes used than by the silicone gel. Furthermore, the test results confirmed that chip structure variation had no influence on the high-humidity I CES phenomenon. 37

5 PCIM Europe 205, 9 2 May 205, Nuremberg, Germany I CES measurement result (STEP0) I CES measurement result (STEP4) Type-A Type-A Figure 5. Dew condensation test results for verification modules. Horizontal axis: measument time; vertical axis: leakage current (I CES ). I CES was measured at V CE =5200V (DC), V GE =0V, Ta=25 C. 4.3 Temperature humidity bias test results The accelerated temperature humidity bias (THB) test of the Type-A and Type-A-2 modules were performed for verification of long-term reliability against high humidity at Ta=85 C, 95%RH, V CE =4000V (DC), and V GE =0V. The accelerated THB test results are shown in Table 4 and plotted on a Weibull chart in Fig. 6. It is clear that the failure time of Type-A-2 is over four times more than that of Type-A in the case of a comparison at F(t)=50%. Figure 7 shows the appearance of a failure chip. A failure point was found on the edge termination corner, which had the strongest electric field from applying voltage between the collector and emitter. Moreover, the electric field at the edge termination corner is most influenced by the increase in by moisture absorption. Therefore, the product lifetime of HVIGBT under highhumidity conditions can be improved by optimizing the silicone gel, passivation materials and processes. Table 4. Accelerated THB test results for two kinds of 6.5kV HVIGBT Type-A (n=5) Type-A-2 (n=4) DUT 8.7 hr hr DUT hr hr DUT3 86. hr hr DUT4 5 hr No failure (under examination) DUT hr F(t)=50% 43.4 hr Over 580 hr 372

6 PCIM Europe 205, 9 2 May 205, Nuremberg, Germany cumulative probability Type-A Type-A-2 Type-A F(t)=50% Type-A-2 F(t)=50% Time[hr] Figure 7. Typical appearance of a failure chip. Figure 6. A Weibull plot of failures in accelerated THB test. 5. Conclusion In this paper, we investigated I CES behavior of 6.5kV HVIGBT modules under dew condensation conditions and verified, through an accelerated THB test, improved humidity resistance performance in the proposed 6.5kV HVIGBT. It was confirmed that I CES behaviors are different among HVIGBT samples having different components. Consequently, the humidity resistance performance of HVIGBT can be improved by optimizing silicone gel, passivation materials, and processes. 6. References [] C. Zorn and N. Kaminski, Temperature Humidity Bias (THB) Testing on IGBT Modules at High Bias Levels, CIPS 204 pp [2] B. Luts nad J. Kindersberger, Influence of Relative Humidity on Surface Charge Decay on Epoxy Resin Insulators, ICPADM 2009 pp