Using soft-termination capacitors with classic layer construction is the most future-proof method to be aligned with the failsafe strategy. Here, the available space is best utilized and simultaneous high mechanical robustness is assured. This makes it possible to manufacture resistant and high-capacitance components in small case sizes.

Construction of a standard class II automotive MLCC

Ceramic layers are usually printed with a nickel paste (electrode) and then stacked on top of each other. The capacitance is determined by the ceramic material, the electrode’s arrangement and the number of layers. Subsequently, a copper layer is added to the termination, which connects the electrodes and ensures good conductivity. Compared to other manufacturers, SEMCO (Samsung Electro-Mechanics Co.) adds an additional copper epoxy layer, commonly known as soft termination. It acts as a buffer for mechanical stress and protects the more sensitive ceramic against breaking. Finally, a nickel-tin plating is added as last outer layer to avoid oxidation and to ensure solderability.

Minimize security risk

Why is the additional protection in form of a soft termination needed? What happens if the ceramic cracks? There are generally two problems. On the one hand, the insulation resistance between the layers decreases, because the ceramic material is a better insulator as a cavity. On the other hand, in the worst case, a short circuit between two opposing layers can occur. Depending on the applied voltage, this can lead to an immense thermal power loss, which can destroy the component and make the entire circuit board inoperative. For this reason car makers create their own quality standards. For example VW Group defined the VW80808 standard, which includes the “MLCC failsafe strategy”. This strategy aims to minimize the mentioned security risk by setting a guideline of which products can be used in which applications.

If a short-circuit causes a power loss of >2.5W or significantly impair system function [1], the failsafe strategy must be used. This is particularly the case when battery voltage (terminal 15, 30) is applied directly to the component.

VW’s standard offers four options to keep the failsafe strategy. Out of these, all SEMCO product types use a soft termination layer and are qualified according to AEC-Q200. Option number one can be used in every electric system. The other options are limited to the 12V electric system.

I. Series connection of two mutually orthogonal capacitors

The first option that can be used independently of the electric system voltage is the orthogonal placement of two capacitors in series. Image 2 shows this setup in a simplified circuit diagram.

If one of the two capacitors is placed unfavorable to bending stress, the orthogonal arrangement ensures that a backup capacitor remains intact. Since both capacitors are connected in series, the isolation to ground potential is still given.

II.  Series Capacitor 

A Series or “Flexisafe” capacitor uses the previously demonstrated principle of a series connection of two capacitors in a single component. The opposing layers of different potential are not directly above each other. A “floating” layer connects them.

However, the internal structure limits the capacitance value extremely. Compared to the previous solution, not only the principle of series connection plays a role, but also the reduction of active area.

III. Open Mode Capacitor

If a crack would occur in the red marked “risky” area, it is less likely to end in a short circuit since only layers of the same potential would touch each other due to the shorter electrodes.

IV. Capacitor with Soft Termination

The advantages of using a capacitor with soft termination are the higher mechanical resistance and at the same time, the ideal utilization of the available component dimensions. If the layer structure is identical compared to a standard component, there is no performance gap.

As mentioned before, all SEMCO Automotive Class II capacitors have a soft termination. So, why is the standard component without this kind of termination not suitable for the use in failsafe applications? The reason is that the use of a component with soft termination is only permitted in VW Group, if it has been tested according to VW 80808-2 (Appendix A “Qualification of MLCC with soft termination”). The standard requires performing several thermal and mechanical stress tests in a row. The most critical tests are the 5mm bending (“Board Flex”) and the moisture resistance test.

Standard automotive components from SEMCO are also tested according to VW80808-2, but with a bending limit of 3mm. This is above the market standard of 2mm for comparable components. The use for the automotive sector is guaranteed, but not for fail-safe applications. When you talk about soft termination in the market, you usually associate it with 5mm bending strength and the failsafe strategy. Here a differentiated approach must be done for SEMCO. The separate product family “PJ” covers the required 5mm bending strength. The qualification according to VW 80808-2 was performed here with regular criteria.

For low and medium capacitance values, additional layers of same potential are placed on the top and bottom inside the component’s body. In case that the MLCC would crack, the risk of a short circuit is reduced extremely and at the same time, it results in an increased mechanical stability of the complete MLCC.

For high capacitance parts, an equivalent construction to the standard articles is used (see Image 1). Since a high capacitance value must be achieved, the space is filled anyway with layers, so that the stability is guaranteed.

The soft-termination tests according to VW 80808-2 were successfully completed for all actively promoted SEMCO Automotive components.

Downsizing

The trend of miniaturization (downsizing) supports the use of the soft termination option even more. The key to small and safe components is therefore on the one hand a robust termination and on the other hand fine ceramic powder, which enables a thinner layer structure.

Product Search

The shown images highlight the part number’s letters, which determine the product type (Standard, Open Mode…). To support you selecting the right component for your application, our „Product Search“-tool can be used:

http://www.samsungsem.com/global/support/product-search/index.jsp#

Datasheets and further technical information is available here. You have the chance to filter our portfolio in terms of application, electrical and mechanical specifications.

Literature:

[1] VW 80808-1: 2015-02, Page 9

[2] VW 80808-2, Appendix A „Qualifikation von MLCC mit Softterminierung“

Image 1: Standard SEMCO Class II Automotive MLCC               Samsung Electro-Mechanics (SEMCO)

Image 2: Orthogonal Setup of two Standard Automotive Capacitors         Samsung Electro-Mechanics (SEMCO)

Image 3: Series Capacitor Samsung Electro-Mechanics (SEMCO)

Image 4: Open Mode Capacitor        Samsung Electro-Mechanics (SEMCO)

Image 5: PJ Series – 5mm Bending Samsung Electro-Mechanics (SEMCO)

Wasilios Pitharas

Started his career after completing a degree in mechatronics in Würth Elektronik eiSos’ product management. Among other things, he expanded the product portfolio with the category “line filter”. Currently he is an Application Engineer at Samsung Electro-Mechanics and responsible for technical support and business development for passive components across Europe.

wasilios.pitharas@samsung.com