What is the relationship between capacitors and ADAS?

Technological progress has greatly increased the electronic content in modern cars, and the development of electronic technology has made cars safer, smarter, more comfortable, more environmentally friendly and more energy-efficient. The development of the two complements each other and brings out the best in each other, providing great convenience for people's lives. However, at the same time, the reliability of electronic components has also become an important factor in ensuring car safety, especially products used in the increasingly popular advanced driver assistance systems (ADAS), which will have a profound impact on car safety.

At present, the penetration rate of new ADAS cars in my country is less than 4%, which is far behind that of developed countries. With the introduction of relevant policies, the domestic ADAS market will grow rapidly. According to forecasts, the scale of China's ADAS market will reach 96.3 billion yuan in 2020, with an average annual compound growth rate of 52%, far exceeding the international market.

Harsh environments challenge electronic components

Automobiles are harsh environments with wide temperature ranges (instant heating and cooling), mechanical vibration, noise, gas pollution, potential risks of transient voltage, high current, etc. These are the important factors that affect the functional stability, reliability and driving experience of modern automobiles.

Car heat is not only in the engine compartment

With the multifunctionalization of automobiles for the purpose of autonomous driving in recent years, such as the continuous development of ADAS, which is the focus of research and development at this stage, the mechatronics of various electronic control units (ECUs) has been continuously improved. More and more electronic devices are installed in automobiles, and the reliability of electronic components has an increasing impact on the overall reliability of automobiles. Especially in automotive applications with rapid temperature changes and high mechanical strength (stress), durable electronic components are needed to meet higher requirements. In addition, while achieving high performance, it is also necessary to meet the miniaturization requirements to solve space constraints.

Facing challenges head-on, capacitors show their talents

The development of electric vehicles has greatly increased the use of multilayer ceramic capacitors ( MLCCs ). Traditional cars use about 2,400 pieces, while new energy vehicles such as Tesla use around 8,000 pieces. The average ADAS usage per vehicle has reached 2,000-3,000 pieces.

In order to meet the higher level of reliability and robustness requirements of self-driving car components, capacitors are constantly improving in terms of advanced functionality, ease of use, and physical substitution. Traditional  MLCCs  have adopted some new technologies, while tantalum polymer capacitors are trying to replace them; supercapacitors are also making efforts in the ADAS application market.

MLCC technology innovation

ADAS is becoming more and more important in improving driving safety, and more and more ICs support this function. In order to suppress noise, more smooth decoupling MLCCs are needed. From the perspective of space-saving design, the demand for miniaturized, high-capacitance  MLCCs  will continue to increase.

MLCCs related to automotive safety devices ADAS/AD ECU

MLCC has the characteristics of small size, large specific capacity, long life, high reliability, and suitable for surface mounting, but it is inevitable that some failures will occur in cars with harsh environments. In order to meet the requirements of automotive grade, TDK developed a high-reliability product, resin electrode type (Soft Termination, also called flexible terminal electrode) MLCC, which can solve the problems of bending cracks and solder cracks and ensure the reliability of applications.

Generally, the main cause of component flex cracks is the bending stress of the substrate, including solder stress caused by the amount of solder, stress during substrate segmentation, and stress during manufacturing. When the component body cracks, "short-circuit mode" or "open-circuit mode" failure may occur.

The main causes and processes of bending

In automotive applications, substrate bending stress and thermal shock can cause expansion or contraction of the capacitor solder joint. The Cu (copper) base layer of the common MLCC terminal electrode is plated with Ni (nickel) and Sn (tin). The resin electrode product is a structure in which a conductive resin layer is added to the Cu and Ni plating layers. This structure has a certain degree of flexibility and can absorb the stress generated, which can improve the reliability of the connection compared to ordinary electrode products.

Differences in terminals between standard electrode MLCC and resin electrode products

The resin layer can absorb the stress caused by the expansion and contraction of the solder joint due to thermal shock and the substrate bending stress, thereby preventing cracks in the component body.

According to reports, TDK's MLCC resin electrode products have more than twice the bending resistance in the substrate bending resistance (limit bending) test compared to ordinary electrode products. The test showed that when cracks occurred in the ceramic component of ordinary electrode products, although the nickel plating layer of the conductive resin electrode product began to peel off from the conductive resin layer, no cracks occurred, thus confirming that the resin electrode product can suppress cracks.

Solder cracks in electronic components are caused by the soldering process during manufacturing and harsh usage conditions. The main reason is that in the environment of repeated temperature changes in automobiles, the difference in thermal expansion coefficients between the component electrode and the substrate causes thermal stress to be applied to the solder joint. At the same time, for environmental considerations, lead-free solder is used in automotive electronic components. From the perspective of temperature management and solder composition, the risk of solder cracks is higher than that of conventional eutectic solder.

The main cause of solder cracks

Generally, the bonding strength decreases after thermal shock. After 3000 thermal cycles (-55 to 125°C) shock test, the bonding strength of ordinary electrode MLCC decreased by about 90%, while the resin electrode product only decreased by about 50%. The excellent thermal shock resistance of resin electrode products was confirmed by thermal shock test data.

MLCC Thermal Shock Test Data

TDK believes that in places with large temperature changes such as automobile engine compartments or other heat source equipment, the electronic components used will cause solder cracks due to component body cracks and stress at the junction with the substrate, which may cause component detachment, short circuits, and open circuit failures. The use of resin electrode products can prevent cracks caused by substrate warping, bending, and thermal shock without changing the occupied space, and can also suppress external stress, thereby improving the connection reliability of the ADAS system.

In September, TDK began mass production of new automotive-grade CGA series resin electrode MLCC products, achieving larger capacitance, with the 2012 specification capacitance being 22μF and the 3216 specification capacitance being 47μF.

Tantalum polymer capacitors take over the market

As cars become increasingly populated with electronic components and increasingly compacted with space to house them, some companies have been exploring opportunities to shrink automotive-grade capacitors to save board space.

Wilmer Companioni, senior technical marketing manager at Kemet, believes that automotive applications are special, and the stability and capacitance accuracy of MLCCs correspond to the dielectric materials used. The main reason for their failure is the presence of various microscopic defects such as cracks, holes, and delamination on the outside or inside of the MLCC. These defects will directly affect the electrical performance and reliability of the MLCC, and ultimately affect the function of the ADAS system.

So, is there a reliable and compact product that can replace the MLCC on the market? Yes, tantalum polymer capacitors (Ta Polymer) provide automotive designers with a new potential solution that helps achieve board space saving, no piezoelectric noise and miniaturization solutions.

He said that since 2015, Kemet has launched a tantalum polymer SMD product portfolio, opening up opportunities for automotive component miniaturization and reducing overall solution costs, and challenging the MLCC supply chain.

At the beginning of 2018, the MLCC supply chain showed some loosening, and some customers were looking for alternative solutions to MLCC. In order to verify the feasible technology, Kemet conducted a lot of research. It was found that engineers can use tantalum polymer capacitors when the capacitance range is between 1uF and 1mF and below 75V.

Capacitance and voltage of different capacitor technologies

However, the conversion process from MLCC to tantalum polymer SMD components is not a “one-to-one” process and several considerations need to be followed. Mechanical and dimensional characteristics are the first parameters to be verified.

Parameters to consider when converting MLCC to Tantalum Polymer SMD

To determine whether MLCC and tantalum polymer capacitors can be directly replaced, a size comparison must be made: MLCC EIA codes 0805 and 1206 can be directly replaced using P-2012 and A-3216 sizes. Although the replacement is not direct, the larger MLCC EIA codes 1210 and 2220 can also be replaced by B3528 and D7343.

MLCC vs. Tantalum Polymer Size Comparison

In order to determine what solution is suitable, it is necessary to compare electrical characteristics. MLCC technology is characterized by capacitance drop effect, temperature drop effect and final life aging effect. On the other hand, MLCC technology has very low leakage current and insulation resistance ranges from 100 to 1000MΩ; the DCL (leakage current) indicator of tantalum polymer capacitors is defined as 0.1×C×V μA (C: rated capacitance in μF; V: rated voltage in V).