2020. 3. 3. 20:28ㆍ카테고리 없음
Source: NASA Goddard Space Flight Centerreport prepared by Alexander Teverovsky, ASRC Federal Space and Defense. NASA recently released an extensive 70pages report on low voltage MLCC cracking issues published on nepp.nasa.gov. The report in detail describes manufacturing process of MLCC types, provide comparative analyses of MIL, NASA vs ESA vs JAXA specifications, explains qualification procedures and cracking risk issues including degradation and failure mechanisms.
The final section recommends methods for workmanship and failure detection.The excerpt is published herein at EPCI website under author’s permission.IntroductionCracking remains the major reason of failures in multilayer ceramic capacitors (MLCCs) used in space electronics. Due to a tight quality control of space-grade components, the probability that as manufactured capacitors have cracks is relatively low, and cracking is often occurs during assembly, handling and the following testing of the systems.Majority of capacitors with cracks are revealed during the integration and testing period, but although extremely rarely, defective parts remain undetected and result in failures during the mission. Manual soldering and rework that are often used during low volume production of circuit boards for space aggravate this situation.Although failures of MLCCs are often attributed to the post-manufacturing stresses, in many cases they are due to a combination of certain deviations in the manufacturing processes that result in hidden defects in the parts and excessive stresses during assembly and use.
This report gives an overview of design, manufacturing and testing processes of MLCCs focusing on elements related to cracking problems.The existing and new screening and qualification procedures and techniques are briefly described and assessed by their effectiveness in revealing cracks. The capability of different test methods to simulate stresses resulting in cracking, mechanisms of failures in capacitors with cracks, and possible methods of selecting capacitors the most robust to manual soldering stresses are discussed.MLCCs in hi-rel and space systemsCompared to commercial, hi-rel and especially space systems are using a more conservative approach for selection of components and a substantial proportion of MLCCs used in high-level space projects has size 1206 and larger. However, the trend for using smaller size, high volumetric efficiency capacitors exists, and the use of advanced technology capacitors in newly design space instruments and systems is increasing.Considering the amount of MLCCs used, the overall probability of their failures is extremely low. Still, because they are failing typically in a short circuit mode, failures might cause catastrophic consequences to the whole system, cease some system functions, or result in intermittent failures that can be misgauged as a software problem. Capacitors are typically responsible for up to 30% of the field failures in commercial systems, and until recently, approximately half of these failures were due to cracking in the parts.The proportion of MLCCs in space instruments is similar to commercial assemblies and varies from 10 to 20% of all electronic components. According to Paumanok Publications, the cost of capacitors procured by aerospace industry is 73% of the whole cost of passive components.
The major vendors that produce 57% of all capacitors procured by aerospace companies are AVX Corporation, Vishay Intertechnology, and Kemet Electronics. Japanese vendors who dominate commercial market (TDK Corporation, Murata Manufacturing, Matsushita, Rubycon) do not sell directly to the global defense markets; however, capacitors produced in Japan find their way into defense electronics through distribution.Cracks originCracks and delamination in MLCCs might originate from manufacturing processes or be introduced during assembly or the following handling and testing of the boards (flex cracks or thermal shock cracks). Examples of different types of cracks are shown in Figure below:Most cracks in MLCCs caused by deformation of PWBs during assembly, handling or testing are initiated at the surface close to the terminal areas of capacitors. These cracks might affect reliability of the parts if they are reaching active area of capacitors and cross opposite electrodes. For these reasons, parts with thicker cover plates and larger end margins are more resilient to cracking-related failures.Cracking Issues MitigationTo mitigate risks of cracking-related failures, special designs of MLCCs have been developed. For low capacitance values, KEMET, similar to other manufacturers, offers the Floating Electrode (FE-CAP) capacitors.
This design is also known as a Serial Cap design. For mid capacitance values, the Open Mode solution that has enlarged end margins that create safe zones on both ends of the capacitor.
Other manufacturers, e.g. TDK are using similar design approaches to reduce failures caused by cracking (see figure below).Soldering related cracks in MLCCs and schematics of a standard part (c), floating electrode capacitor (d), and Open Mode design (e) Yellow and blue lines in (c-e) represent electrodes with different polarity.Various designs of MLCCs that have been suggested by manufacturers to decrease the probability of flex cracking can be summarize as following:. Flexible termination.
Application of relatively soft and/or tear-away termination layers made of conductive polymers reduces the stress in ceramic and restricts flex cracks within a safe zone away from the body of the MLCC. Fail open design. End margins are widened, so if a crack occurs, it does not cross electrodes with opposite polarity, and thus prevents short-circuit failures. Floating electrodes. Two capacitors connected in series within an individual case size, so the probability of shorting cracks is reduced substantially. Clip-on lead frame.
Attachment of J-shaped leads mechanically decouples the MLCC from the PWB and allows for some stress relief.MLCC designs with flex crack countermeasures used by TDK.A combination of floating electrode and flex termination design (e.g. A TDK Dual-Fail-Safe, Mega Cap design 39) allows to reduce the risk of failures even further. This design has been proven highly reliable, and no cracks developed in the parts after 3000 thermal shock cycles.Obviously, the cost of improved reliability of MLCCs with increased margins or floating electrodes is a lesser volumetric efficiency. For this reason, flex termination is likely the most efficient cracking mitigating measure.Testing and quality assuranceApproaches for quality assurance of commercial and military capacitors are substantially different. Quality assurance of mass production commercial capacitors is based on the build-in-design approach and extensive use of the Statistical Process Control (SPC) system. Military specifications also require SPC system to be used, but it is not considered a major element of QA, and inability to make in-process improvements reduces substantially its efficiency. Instead, QA for military capacitors is based on extensive inspections and testing of the parts.
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As stated by Dan Friedlander “SPC targets failures prevention, burn-in (testing) targets failures detection”.Requirements for screening of low-voltage MLCCs by different standards.PDA = percent defectives allowable.HSSLV = humidity steady state low voltage test that is carried out at 1.3V for 240h at +85°C and 85%RH, typically using 12 samples.PRVT = Process Reliability Verification Test.Requirements for MLCC qualification testing by different standards. Failures of MLCCs with cracks are commonly explained by moisture sorption that increases conductivity along the surface of the crack or causes electrochemical migration (ECM) of electrode materials and formation of shorting metal dendrites. Silver that is used in PME capacitors is a metal most susceptible to ECM.Migration of silver can be observed at voltages as low as 0.4 V and relative humidity down to 40% RH, which is the reason of so-called low-voltage failure phenomena in capacitors.Much less is known about ECM in BME capacitors with internal electrodes made of nickel, which is often considered as a non-migrating metal. In the absence of moisture in environments, e.g.