MLCC chip capacitor selection and application issues

MLCC (chip multilayer ceramic capacitor) has now become one of the most commonly used components in electronic circuits. MLCC seems very simple on the surface, but in many cases, design engineers or production and process personnel have insufficient understanding of MLCC. The following discusses some issues and precautions in the selection and application of MLCC.
Although MLCC is relatively simple, it is also a device with a relatively high failure rate. The high failure rate is due to the inherent reliability problem of the MLCC structure on the one hand, and the selection and application problems on the other.

Since capacitors are considered "simple" devices, some design engineers do not pay enough attention to them and do not understand the unique characteristics of MLCC. In an ideal situation, when selecting capacitors, it is enough to consider the two parameters of capacity and withstand voltage. However, for MLCC, it is far from enough to consider only these two parameters.

When using MLCC, you must understand the different materials of MLCC and the corresponding performance of these materials. There are many kinds of MLCC materials, and each material has its own unique performance characteristics. Without understanding these, the selected capacitor may not meet the circuit requirements. For example, common MLCC materials include C0G (also known as NP0), X7R, and Y5V. C0G has the best operating temperature range and temperature coefficient, with a temperature coefficient of 0 within the operating temperature range of -55°C to +125°C. ±30ppm/°C. X7R is second, and the capacity change is ±15% within the operating temperature range of -55°C to +125°C. The operating temperature of Y5V is only -30°C to +85°C, and its capacity change can reach -22% to +82% within this operating temperature range. Of course, the costs of C0G, X7R, and Y5V are also reduced in turn. When selecting, if the requirements for operating temperature and temperature coefficient are very low, you can consider using Y5V, but in general, you should use X7R, and when the requirements are higher, you must choose COG. In general, MLCC manufacturers design capacitors made of X7R and Y5V materials to have the largest capacity near room temperature, but as the temperature rises or falls, their capacity will decrease.

It is not enough to just know the above knowledge. Since the dielectric constants of C0G, X7R, and Y5V media decrease in sequence, the maximum capacity that can be produced under the same size and withstand voltage also decreases in sequence. Some inexperienced engineers think that they can have any capacity they want, and they will make mistakes when selecting and choose specifications that do not exist. For example, they want to use a capacitor of 0603/C0G/25V/3300pF, but the MLCC of 0603/C0G/25V is generally only 1000pF. In fact, as long as you read the manufacturer's selection manual carefully, you will not make such a mistake. In addition, for design engineers who have just started, it is not necessary to know the numerical sequence of component specifications (E12, E24, etc.). Even experienced engineers have no idea about the compression of specifications. For example, in the filter circuit, someone used a 3.3uF capacitor, and his circuit can also use a 3.3uF capacitor, but he might have chosen a 4.7uF or 2.2uF capacitor that no one has used. People who do not read the manufacturer's selection manual will also make the following mistakes, such as choosing a 0603/X7R/470pF/16V capacitor, when in fact most manufacturers only produce 0603/X7R/470pF capacitors with voltages of 50V and above, not 16V.

Also note that there are two ways to express the package of chip capacitors, one is the imperial system and the other is the metric system. American manufacturers use the imperial system, Japanese manufacturers basically use the metric system, and domestic manufacturers use both the imperial system and the metric system. The capacitor package used by a company can only be expressed in one standard. One engineer cannot use the imperial system and the other engineer cannot use the metric system. Otherwise, it will be confusing. In extreme cases, it will be wrong. For example, the imperial system has a 0603 package and the metric system also has a 0603 package, but the two are actually completely different sizes. The imperial 0603 package corresponds to the metric 1608, while the metric 0603 package corresponds to the imperial 0201! In fact, the imperial package number is approximately multiplied by 2.5 (the first 2 digits and the last 2 digits are multiplied separately) to become the metric package specification. Now it is popular to use the imperial package expression. For example, the 0402 package we often say is the imperial expression, and its corresponding metric package is 1005 (1.0*0.5mm).

In addition, in addition to understanding the temperature performance of MLCC, design engineers should also understand more performance. For example, although the capacity of Y5V dielectric capacitors is large, this ferroelectric ceramic has a disadvantage, that is, its static capacity decreases with the increase of its DC bias working voltage, and the maximum may even decrease by 70%. For example, a Y5V/50V/10uF capacitor may have a capacity of only 3uF at a DC voltage of 50V! Of course, the characteristics of different manufacturers are different, and some declines may not be so serious. If you must use Y5V capacitors, in addition to knowing the curve of its capacity changing with temperature, you must also ask the manufacturer for a curve of its capacity changing with DC bias voltage (even a comprehensive diagram of capacity temperature and DC bias). There must be sufficient voltage derating when using Y5V capacitors. The capacity of X7R also decreases with the increase of its DC bias working voltage, but it is not as obvious as that of Y5V. At the same time, the smaller the size of the MLCC, the more obvious this effect is.

Different materials have different frequency characteristics. Designers must understand the different frequency characteristics of different materials. For example, C0G (also known as high-frequency thermal compensation medium) has good high-frequency characteristics, X7R is second, and Y5V is the worst. When used for smoothing (power supply filtering), the capacity is required to be as large as possible, so Y5V capacitors can be used, that is, Y5V capacitors can replace electrolytic capacitors. When used for bypass purposes, such as the bypass capacitor next to the VCC pin of the IC, at least X7R capacitors must be used. The oscillation circuit must use C0G capacitors. Due to the poor performance of Y5V, I generally do not recommend its use, and require design engineers to consider using X7R capacitors (or X5R capacitors) as much as possible. If the capacitance-to-volume ratio is high, consider using tantalum capacitors and try to avoid using Y5V capacitors. Of course, if your company does not have high requirements, you can still consider Y5V capacitors, but be especially careful.

Generally speaking, the ESL (equivalent series inductance) and ESR (equivalent series resistance) of MLCC are small relative to electrolytic capacitors (including tantalum electrolytic capacitors). In fact, at high frequencies, the ESL and ESR of MLCC cannot be ignored. The resonance point of a general C0G capacitor can reach hundreds of MHz, the resonance point of a general X7R capacitor can reach tens of MHz, and the resonance point of a Y5V capacitor is only a few MHz or even less than 1MHz. The resonance point means that beyond this frequency, the capacitance is no longer a capacitance characteristic, but an inductance characteristic. If you want to use MLCC for higher frequencies, such as microwaves, then you must use MLCC made of special microwave materials and processes. Microwave capacitors require smaller ESL and ESR.

MLCC has been progressing in the direction of miniaturization. Now 0402 packaging is already a mainstream product. However, miniaturization may bring some other hazards. In fact, not all electronic products are so concerned about and welcome miniaturized MLCC. Electronic products that care about miniaturization, such as mobile phones, digital products, etc., have become the main driving force for MLCC miniaturization. For MLCC manufacturers, miniaturized MLCC accounts for the main shipment volume. However, from the perspective of the entire electronics industry, there are still many electronic devices that do not care so much about miniaturization. Performance and reliability are the key considerations. MLCC miniaturization brings hidden dangers to reliability. For example, communication equipment, medical equipment, industrial control equipment, power supplies, etc. These electronic devices have enough space and are not very interested in MLCC miniaturization; moreover, these electronic devices are not like personal consumer products that chase fashion and update quickly, but are more concerned about the reliability of long-term use, so the requirements for component margins are higher (in order to ensure reliability, the margin must be large, so larger MLCCs can meet the requirements. In addition, the larger size gives MLCC manufacturers more room to improve the reliability of capacitors). This is inconsistent with the direction of MLCC manufacturers pursuing miniaturization. This is a contradiction. The characteristics of these high-reliability electronic devices are that the quantity is not large, but the price is expensive (except for some types of power supplies), and the reliability requirements are also high. If it is a well-known electronic equipment factory, life will be a little better, because the MLCC factory will save some large-sized MLCC production for them. If it is not a well-known electronic equipment factory, there is no need to be so pessimistic. After all, there are still a few MLCC factories with different positioning, and they will continue to produce large-sized capacitors. Therefore, as a manufacturer of such electronic equipment, you should be good at finding larger-sized MLCC manufacturers that are positioned for high performance and high reliability. However, there is a note that the selected specifications cannot be exclusive specifications. At least two MLCC manufacturers that meet the requirements of their own company are producing this specification. In addition, when miniaturization does not affect the performance and reliability requirements, miniaturized MLCCs are still given priority.

Some companies also have some misunderstandings in the application of MLCC. Some people think that MLCC is a very simple component, so the process requirements are not high. In fact, MLCC is a very fragile component, and you must pay attention when applying it.

If the MLCC manufacturer has poor craftsmanship during the production process, there may be hidden dangers. For example, dielectric voids, sintering cracks, and delamination will bring hidden dangers. This can only be guaranteed by selecting excellent suppliers (the issue of supplier selection will be discussed later).

In addition, the inherent reliability of the thermal brittleness and mechanical stress brittleness of the ceramic itself leads to the failure of electronic equipment manufacturers when using MLCC improperly.

MLCC now has hundreds or even thousands of layers, and each layer is micron-level thick. So a slight deformation can easily cause cracks. In addition, for MLCCs of the same material, size, and withstand voltage, the higher the capacity, the more layers, and the thinner each layer, so it is easier to break. On the other hand, when the material, capacity, and withstand voltage are the same, small-sized capacitors require thinner dielectric layers, which makes them easier to break. The hazard of cracks is leakage, and in severe cases, it causes safety problems such as internal interlayer misalignment and short circuit. And one of the troublesome problems of cracks is that they are sometimes hidden and may not be found during the factory inspection of electronic equipment, and they are only officially exposed when they reach the client. Therefore, it is of great significance to prevent MLCC from cracking.

When MLCC is subjected to temperature shock, cracks are likely to form from the soldering end. In this regard, small-sized capacitors are relatively better than large-sized capacitors. The principle is that the heat conduction of large-sized capacitors cannot reach the entire capacitor so quickly, so the temperature difference at different points of the capacitor body is large, so the expansion size is different, thus generating stress. This is the same as when a thick glass is poured with boiling water, it is easier to break than a thin glass. In addition, during the cooling process after MLCC welding, the expansion coefficients of MLCC and PCB are different, thus generating stress and causing cracks. To avoid this problem, a good soldering temperature is required during reflow soldering. If wave soldering is used instead of reflow soldering, this failure rate will be greatly increased. MLCC should avoid manual soldering with a soldering iron. However, things are not always so ideal. Manual soldering with a soldering iron is sometimes unavoidable. For example, for electronic manufacturers that outsource PCB processing, some products are very small in quantity, and when the chip outsourcing manufacturers are unwilling to accept such orders, they can only solder manually; when samples are produced, they are generally soldered manually; when rework or repair soldering is performed in special circumstances, manual soldering is required; when repairmen repair capacitors, manual soldering is also performed. When manual soldering of MLCC is inevitable, great attention should be paid to the soldering process.
First of all, the process and production personnel must be informed of the problem of thermal failure of capacitors, so that they will attach great importance to this problem. Secondly, welding must be done by specialized skilled workers. Strict requirements must also be met on the welding process, such as using a constant temperature soldering iron, the soldering iron should not exceed 315°C (to prevent production workers from increasing the welding temperature in order to speed up), the welding time should not exceed 3 seconds, and the appropriate solder flux and solder paste should be selected. The pads must be cleaned first, the MLCC should not be subjected to large external forces, and attention should be paid to the welding quality, etc. The best manual welding is to tin the pad first, and then melt the tin on the pad with the soldering iron, and then put the capacitor on it. During the whole process, the soldering iron only touches the pad but not the capacitor (it can be moved closer), and then use a similar method (heat the tinned pad on the pad instead of directly heating the capacitor) to solder the other end.

Mechanical stress can also easily cause cracks in MLCC. Since the capacitor is rectangular (parallel to the PCB) and the short side is the solder end, it is natural that the long side is prone to problems when subjected to force. Therefore, the direction of force should be considered when arranging the board. For example, the relationship between the deformation direction during board separation and the direction of the capacitor. During the production process, try not to place capacitors in places where the PCB may produce large deformations. For example, PCB positioning riveting, mechanical contact of test points during single-board testing, etc. will all cause deformation. In addition, semi-finished PCB boards cannot be stacked directly. And so on.

Let's talk about the selection of MLCC manufacturers and sample certification.

MLCC is a relatively low-end component, and European and American manufacturers are generally reluctant to produce it or it has been acquired by Japanese brands. Only a very small number of manufacturers in the United States are still producing MLCC, but the market share is small and the delivery time is long. Therefore, the best MLCC is Japanese brands. Of course, there are also some well-known manufacturers in Taiwan and the mainland that have a good market share of MLCC. In terms of ordinary specifications, the technology and quality of famous MLCCs in Taiwan and the mainland are almost close to those of Japanese manufacturers. But if you want to use some MLCCs with better technology and quality, you may only be able to choose American and Japanese brands. In general, there are many MLCC manufacturers to choose from. Since the price of MLCC is not high, most companies are happy to choose well-known companies. If you choose products from first-class companies, then It is naturally worry-free to use. However, if you are not sure about the quality of a brand of MLCC, or it is not a first-class company, then you must certify their products when choosing a supplier. Capacitors are not expensive, and the number of samples can be more important. It is necessary to test the temperature characteristics, frequency characteristics, DC bias voltage characteristics, etc. of the samples. Some parameters cannot be measured by our own company, so we can go to the supplier for measurement, or ask them to issue a test report. In fact, the reliability of MLCC is the most critical. From the above content, we can see that the failure of MLCC is mainly thermal stress failure and mechanical stress failure. Then, the samples should also focus on measuring these properties: thermal shock resistance and bending resistance test. Note that when cracks occur, ordinary parameters such as the capacitance of the capacitor may still be good. At this time, it is mainly necessary to measure the leakage (especially when moisture enters) to measure it. Of course, if reliability is important.