Design choices for MLCC multilayer ceramic capacitors

Design choices for MLCC multilayer ceramic capacitors

Feature articles |
By Nick Flaherty

Multilayer ceramic capacitors, or MLCCs, come in the form of blocks with a specific amount of stacked ceramic layers. And while this structure seems to be simple, it requires a row of complex operations, as well as advanced techniques and materials, to meet the increasingly demanding market for quality and performance. In this first part, we’ll focus on the essential phases of the MLCC manufacturing cycle.

Launching the Batch

The 1st phase, called the batch, consists in preparing the raw material in order to manufacture the components. Slurry is mixed with ceramic powder, binder and solvent. At this stage, the ceramics are manufactured in-house by Samsung. This allows Samsung full control of the production process and quality. It also allows Samsung to obtain high-value capacities. The slurry is then coated on a film to obtain a thin and uniform ceramic sheet. Inner electrodes in the shape of metallic nickel paste are printed on this ceramic sheet. Nickel strips are then stacked in such a way as to obtain the multilayer component. The electrodes are deposited by the number of superimposed layers, and the type of dielectric used defines the value of the capacitance of the manufactured component. The stacked layers are then cut in order to obtain the desired chip form. After several additional steps (laminating, firing, tumbling), sintering is applied to ensure good conductivity between the internal and external electrodes. Finally, each terminal of the component is dipped in copper to form the external electrodes.

Next: Avoiding MLCC fractures

Until then, the MLCC manufacturing cycle is described as more or less a standard process. The difference in processes between Samsung’s and conventional methods, is that Samsung provides more safety and reliability to the component by protecting the fragile ceramic from the risk of fracturing. In fact, all the components are dipped a second time in epoxy and copper resin.

By applying this phase, Samsung is preventing potential risks such as fractures caused by mechanical stress and thermal shock on the printed circuit board. Finally, to protect the component from oxidation and to enhance its weldability, a nickel and tin plating is coated on the capacitor.

Multilayer ceramic capacitors can be susceptible to cracking when subject to excessive flexing of the printed circuit board (PCB), when subjected to thermal shock or during mishandling. To deal with this excessive bending, SEMCO applies an epoxy and copper metal resin to the external terminations of all of its Class II automotive grade MLCCs. This flexible termination prevents the transfer of mechanical stresses from the board to the ceramic component, mitigating flex cracking. Thus, SEMCO guarantees a bending strength of up to 3 mm for its standard PN series MLCCs.

Due to the large amount of energy stored by capacitors, an internal short-circuit can cause large temperature increases which can lead to explosions. This can not only destroy the component and erase any source of evidence, but can also damage surrounding components, the circuit board, adjacent circuit board assemblies, and in extreme cases, cause fires [1]. How well can the flexible termination prevent cracking and therefore short-circuits from occurring? Are there any applications in the vehicle that are more prone to risk than others? To answer these questions, car manufacturers have created their own quality and safety standards. Among the standards developed by automobile manufacturers, such as the Volkswagen group, is an integrated security strategy, also called “Failsafe Strategy” by creating the VW 80808 standard.

Failsafe MLCC

Unlike ordinary MLCCs, soft-terminated MLCCs prevent short circuits inside the chip by reducing internal mechanical stresses that result in severe deformation of the circuit board. However, are standard automotive grade MLCCs, able to guarantee the required safety?

The bending strength guaranteed by SEMCO (3 mm), certainly meets the requirements of automotive applications and is even higher than the standard applied on the market (2 mm for comparable products); however, it is not sufficient to be integrated in applications requiring stricter safety. That is why the VW 80808 standard requires stricter testing than the ones required by the AEC-Q200 standard. VW standard also classifies applications by degrees of required security.

According to the VW 80808 standard, any short-circuit potential that could cause a power loss of more than 2.5 W, in particular for MLCCs placed directly at the battery voltage (terminal 15, 30) or for MLCCs that can significantly alter the operation of the system in case of a breakdown, the security policy must be applied.

The first option is to place the capacitors in series and in an orthogonal way. This solution can be implemented regardless to the value of the voltage of the electrical system. This series of capacitors resists short-circuiting by placing, in series, two capacitors in one component. As a result, an MLCC acts as a backup capacitor in case one of the two components is damaged.

As for a voltage of 12 V, other solutions are proposed. These solutions are only applicable with the flexible metal termination.

Fail-Safe Function with a Series Structure Design

Following the same principle as the previous solution, this concept of double-security multilayer series capacitor is also called the concept of floating or flexisafe electrodes. The electrodes inside the component are shortened and the counter electrodes on the layer above become floating electrodes with no conductive connection at the termination. In case of a cut in the ceramic connection area, no electrical connection to the counter electrode can be established. Therefore, a cut does not lead to a short circuit of the capacitor — it instead causes a change in the capacitance value. This value remains, in any case limited, due to the reduced active surface.

Open Mode Design

The WP series is designed in such a way that the internal electrodes are shortened and embedded in the component. In case of a crack, only electrodes of the same potential are connected in the risk zone, thus minimizing the probability of a short circuit. But as the active area is reduced, the available capacity is relatively restricted.

Softterm MLCC

For applications requiring even more stringent safety and where the risk is higher, the soft-terminated MLCCs of the PJ series, guarantee a bending strength of up to 5 mm.

The term “soft termination” is generally associated with a bending force equivalent to 5 mm. SEMCO, on the other hand, distinguishes between two different series, i.e. PN series (3mm) and PJ series (5mm).

MLCCs from the PJ series must pass the extreme thermal and mechanical tests of VW 80808.

In addition to a better mechanical resilience, and the capacity to be used in high risk applications, the PJ series also has other advantages over the PN series. For small and medium capacity values, additional layers of the same potential are placed at the top and at bottom. This decreases the risk of short circuit and provides better mechanical stability for the whole structure of the component.

The mobility of the future is electrified, autonomous, shared, connected and regularly updated [3]. This requires a very large number of capacitors and at the same time demands high performance capacitors. In order to satisfy all these applications, an ever-increasing number of electronic control units (ECUs) are particularly required. Car makers must therefore place an increasing number of components for the same space in the vehicle. Accordingly, the dominant trend in the automotive industry is the requirement for small capacitors with high-capacitance values. This requires precise know-how in the manufacturing technique, including the production of a fine ceramic powder required for this type of capacitors. Samsung is one of the few manufacturers on the market capable of performing this technique.

The miniaturization of capacitors not only counters space problems but also fits with the flexible termination structure. Achieving a thinner layer structure becomes less difficult with the fine ceramic powder.

Mokhtar Marzouk: Application Engineer at Samsung ELECTRO-MECHANICS since June 2019. Currently, he is technical and commercial development manager of passive components in the EMEA region. He also leads the approval activities at major Automotive Tier1 customers in EMEA. He holds a Master’s degree in Electrical Engineering, Electronics and Information Technology from the University of Erlangen-Nurnberg in Germany.

Samsung ELECTRO-MECHANICS (SEMCO) offers a wide range of Multilayer Ceramic Capacitors (MLCC). In addition to its traditional telecom, industrial, and consumer electronics market, Samsung established itself in the automotive industry where zero defect is required. SEMCO provides AECQ-200 qualified MLCCs, which have successfully undergone stringent testing and can integrate into all parts of a vehicle (infotainment, airbag, braking system, transmission system, battery, body & chassis etc.)



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