New I.R. Thermography methodology for failure analysis on tantalum capacitors

Transcription

1 New I.R. Thermography methodology for failure analysis on tantalum capacitors F. Gonnet a *, J.C. Clément a, J. Perraud a, D. Carisetti a a Thales Research & Technology, Palaiseau, France Abstract Tantalum capacitors are widely used on electronic systems due to their high capacitance value in a small size and due to their long storage capability. They are particularly attractive for aerospace applications. From failure analysis perspective, tantalum capacitors are challenging. Their typical failure mechanisms are characterized by a local increase of the leakage current in the manganese dioxide (MnO 2 ) layer. If there is no external current limitation, the failure may result in a partial melt of the component, hiding the clues that could lead to understand the root failure cause. Nowadays, X-ray inspections and optical observations do not give the actual localization of the failure spot. This paper will show how the IR Thermography, with synchronous detection, gives an accurate localization. A new failure analysis methodology will be presented using the synchronous detection during the grinding steps in order to properly identify the defect even in the external MnO 2 layer. Case studies will show the very high accuracy achieved even with the dissipation down to some milliwatts. Thanks to this precise localization, the defect can be uncovered and examined in detail and related to the failure mechanisms theory in the MnO 2 layer. This allows finding out the root cause of the failure. Corresponding author. francoise.gonnet@thalesgroup.com Tel: +33 (0) ; Fax: +33 (0)

2 New I.R. Thermography methodology for failure analysis on tantalum capacitors F. Gonnet a, *, J.C. Clément a, J. Perraud a, D. Carisetti a 1. Introduction To carry out a failure analysis on tantalum capacitor, it is essential to know its technology including its weak points as well as the conditions encountered by the failed capacitor: voltages, currents, time to failure, storage, and so on. Solid electrolytic tantalum capacitors are electrolytic capacitors using tantalum metal for the anode, solid electrolyte manganese dioxide (MnO 2 ) for the cathode and pentoxide (Ta 2 O 5 ) for the dielectric. A typical tantalum solid electrolytic capacitor is shown in Fig.1. purposes, a layer of carbon from graphite dispersion is placed on the MnO 2 layer and coated with a silver metallic conductive coating. These layers constitute the cathode side where the external connection is attached [3] (See Fig.2). Fig.2: Schematic representation of the structure of a tantalum electrolytic capacitor with solid electrolyte and the cathode connecting layers [4]. 2. Theoretical failure mechanism in the MnO2 external layer Fig.1: Construction of a typical solid electrolytic tantalum capacitor [1]. The typical manufacturing process is as follows: an internal pellet, made of tantalum powder, is compressed and sintered. A tantalum wire inserted or welded to the pellet provides the anode lead. Then the pellet is anodized to provide a film of tantalum pentoxide (Ta 2 O 5 ) as a dielectric. The Ta 2 O 5 is then coated with several layers of manganese dioxide (MnO 2 ) making the cathode [2]. For connection Among various failure mechanisms of tantalum capacitors, a typical one is due to a reduced thickness in the MnO 2 layer as shown in Fig.3. Good Risky MnO 2 MnO 2 Fig.3: Thickness variations in MnO 2 layer [5]. * Corresponding author. francoise.gonnet@thalesgroup.com Tel: +33 (0) ; Fax: +33 (0)

3 The local reduction of MnO 2 layer induces an uneven leakage current distribution. More current will flow locally making a hot spot. If the external current is not limited, further heating occurs up to a catastrophic failure. (Fig.4). 3. Cases study Fig.4: uneven current distribution [5]. Two different cases of failure analysis on tantalum capacitors are present hereafter. The first one is a failure on a new part, not assembled on board. The second one is a part assembled on a board with a failure at the first use. The capacitors are different and are made by two manufacturers. 3.1 First Case study: Failure on a new tantalum capacitor After a field failure due to a solid tantalum capacitor, all the capacitors from the reel of the same batch are tested. An abnormal leakage current is detected on one of these capacitors Non-destructive analysis & electrical test The electrical test shows a faulty leakage current on this capacitor: insulating resistance of 600 Ω. The capacitance value is in accordance with the manufacturer data sheet. The external optical inspections reveal no visible defect on the component surface. X-Rays do not enable defect localization per se, but shows an irregular MnO 2 layer on the pellet periphery i.e a risky condition (see Fig.5). description Infrared Thermography is a well-known technique and recently developed with synchronous detection for failure analysis. Our system is based on the material emissivity at a given temperature in relation with Planck s law. This technique is currently used for temperature mapping and hot spot location on silicon and III-V integrated circuits down to 3µm spatial resolution [6]. The use of this synchronous detection can be extended to the hot spot detection on Tantalum capacitors for failure analysis. Leakage current between electrodes generates local hot spots that will be detected by Infrared Thermography synchronous detection. This can be used as a non-destructive technique and/or during further steps of the analysis Application to the case study Raw localization The capacitor is biased at 1.5V & 6 ma. The bias voltage is set as low as possible to avoid further degradation but high enough to be detected by the synchronous detection. The component was first analyzed from the top and after from the side in order to have a good spatial localization of the defect. The superposition with the X-ray picture highlights the area of defect on the pellet (see Fig.6). The localization is not perfect as the defect is deep inside the capacitor. The molded case spreads the heat as shown in Fig.6. Fig.6: Fault location: top on the left & side on the right : the hot spot is spread. This first observation shows the hot spot close to the bottom side. Fig.5 : X-Ray view of the periphery of the pellet IR Thermography with synchronous detection Fine localization To get a better localization, the bottom of the case is ground to reduce its thickness.

4 Then a new hot spot detection, from the back side, is done, giving a more precise localization as shown in Fig.7. this is not an artifact. Fig.9: Second observation of the cross section after the adjusted bias (4.4 V - 2mA) Detailed and in depth analysis Fig.7: Fault location, after grinding, from the back side. Dash-dot axis: Position of cross-section. With that more precise localization, we know where to make the cross section. To do so the part is molded in a resin, with two wires connected to the capacitor. This will allow grinding of the part under electrical and hot spot monitoring. The purpose of the grinding is to access to the interior of the sample to allow direct optical and scanning electronic microscope (SEM) observation. The grinding is performed with Silicon Carbide (SiC) grinding paper (500, 1200, 2400 & 4000 grit sizes). The grinding is mechanical and generates local degradation of the dielectric or of the MnO2 layer. That created short circuits between tantalum particles and manganese dioxyde in the pellet. These local short circuits in the tantalum pellet are also detected by the synchronous detection as shown on the thermal imaging in Fig.8. The synchronous detection, on the cross section, with more magnification, gives an extremely accurate localization of the defect. By superposing the thermal imaging (Fig.10) and an optical image it is possible to precisely locate the defect. The defect is localized in a thinner part of the MnO2 layer (Fig.11). Fig.10: Magnified thermal imaging of the hot spot on the cross section. Fig.11: Precise pinpointing of the fault location in a MnO2 void (blue area). By SEM, a fused area on the surface of the tantalum pellet is observed. This fused area takes place inside a void in the MnO2 layer (see Fig.12). This fused area is due to a breakdown in the dielectric and explains the measured leakage current. Fig.8: First observation of the cross section at low bias - Leakage current artifact shown by thermal imaging (1.5 V - 6mA). To avoid these artifacts, the bias must be carefully adjusted. By increasing the bias, the actual fault location becomes more visible than the artifact areas because the leakage current is more important. The result of the synchronous detection with the adjusted bias is shown in Fig.9. It is mandatory to compare the fault location with the initial localization to be sure that Fig.12: SEM view of the fused area.

5 3.2. Second Case study: Defect at the first use The defect appears on a component assembled on board at the first power-up Non-destructive analysis & electrical test The electrical tests reveal that the component is in short circuit: insulating resistance of 49 Ω, capacitance is not measurable. The external inspection indicates no defect. The X-Ray observations reveal an irregular thickness of the MnO 2 layer (see Fig.13), not observed on a new part from a different batch of capacitors (see Fig.14). Fig.15: Defect location from the top : the hot spot localization is precise. Dash-dot axis: Position of cross-section. Fig.13: X-ray view of fail capacitor. Irregular MnO 2 layer. Fig.16 : Pinpointing of the fault location. Side view : the hot spot localization is precise. As described previously, the component is molded in resin for cross-section with 2 wires connected to the capacitor to follow the resistance value during the grinding. The defect is located on the cross section, by thermal imaging (see Fig.17). Fig.14: X-ray view of reference capacitor. Note the regular MnO 2 layer IR thermography Synchronous detection We use the same methodology as in Section The capacitor is biased at 1 V & 22 ma and observed by synchronous detection. The component is first analyzed from the top and after from the side in order to have a good spatial localization of the defect. A thin hot spot is located. This is more precise because the capacitor size is smaller than in the first case. The superposition with the X-ray image highlights the defect area on the pellet (see Fig.15 & Fig.16). Fig.17 : picture of the hot spot on the cross section Detailed analysis The synchronous detection done on the cross section with a higher magnification, gives an accurate localization of the defect. By superposing both the

6 thermal (Fig.18) and the optical images, it is possible to detect the exact fault site. The defect is in a very reduced thickness of the MnO 2 layer (Fig.19) with silver conductive coating (Fig.20). MnO 2 Ag Fig.18 : Hot spot on the cross section. process. 4. Conclusion The IR thermography synchronous detection is a powerful tool for failure analysis on tantalum capacitors. This non-destructive technique allows the localization of the defect inside the component even if the resolution is limited because of the size of components. This new methodology of using synchronous detection associated with the tracking of the resistance during the grinding allowed a very accurate and reliable localization of the defect. On both case presented, this very precise localization allowed a detailed analysis of the defect. Thanks to that, the root cause of the failure can be uncovered and the actual observation related to the failure mechanism theory. MnO 2 Ag Acknowledgements The author thanks Mr C. Prévot for his assistance in the production of this paper. Fig.19 : Optical view of the fault site. Fig.20 : Detailed view of the cross section in failed area Ag is visible inside the pellet. (Ag is checked by Energy-dispersive X-ray spectroscopy). At that location the MnO 2 layer is both porous and much too thin. This induces an uneven current distribution as described earlier. Furthermore, in this area, a penetration of the silver is observed. This abnormal presence of the silver decreases the resistance, speeding up the catastrophic failure. This defect is due to a bad mastering of the MnO 2 layer References [1] ckemet Electronics Corporation,: SOLID TANTALUM CHIP CAPACITORS - T491 SERIES - Precision Molded Chip [2] Bobby L.Smith, Hughes Aircraft Company, Space and Communications Group Failure Mechanisms in Passive Devices Electronic Materials Handbook Volume 1 Packaging [3]. VISHAY INTERTECHNOLOGY, INC., Dc Leakage Failure Mode, Technical note VSD-TN , Revision 17-Mar-04 [4] John D. Prymak KEMET Electronics Corp: New Tantalum Capacitors in Power Supply Applications IEEE Industry Applications Society Annual Meeting [5] John Gill: Surge in Solid Tantalum Capacitors AVX Technical Information Ensuring an even manganese dioxide coat and Appendix 3 [6] Carisetti, Dominique, et al, Infrared Thermography Developments for III-V Transistors and MMICs, Proceeding from the 37th International Symposium for Testing and Failure Analysis (ISTFA), San Jose, CA, USA, November 13-17, 2011