【Introduction】Miniaturization has always been a hot trend in multilayer ceramic chip capacitor (MLCC) products. But scaling down is no easy task, especially given the many critical conditions that need to be taken into account. Although digital tools can provide users with a lot of assistance, if users rely entirely on these tools, some key technical issues are often overlooked.
Multilayer ceramic chip capacitors (MLCCs) are small in size and facilitate miniaturization. However, it is also important to consider factors such as ESD protection, EM interference and thermal management, as well as the typical characteristics and drift associated with these factors. While more and more developers are using digital tools to simplify the component selection process, all of the above aspects still need to be taken into account in order to quickly achieve design goals and avoid unnecessary redesign.
First of all, it is recommended that users do not simply follow the present value combination of MLCCs, especially in terms of capacitance (C value) and voltage, when downsizing, but make decisions based on the actual needs of the application and even the function of individual components. Ideally, the supplier’s preferred model should be considered. In addition to C value and voltage, other important values include impedance and equivalent series resistance (ESR).
Especially for high capacitance (hi-cap) devices, that is, MLCC products with C values in μF, the DC bias effect is also an important factor to consider. DC bias is the effect of reducing capacitance based on the applied DC voltage. At rated voltage, it is possible for the capacitance to drop to around 20% of its nominal value, depending on the component, so the absolute minimum C value must be observed during operation.
Figure 1 shows several examples of dc bias curves, showing that the dc bias ratio can be greatly improved using smaller components.
Figure 1: Smaller structure MLCCs have higher DC bias rates
Another factor that affects the DC bias performance is the operating temperature, as shown in the graph in Figure 2, for smaller construction MLCC capacitors with higher nominal values, the residual capacitance and temperature of the DC bias are much higher than the nominal value. Low larger structure MLCC capacitance.
Figure 2: Smaller structure capacitors have higher residual capacitance than larger structure capacitors.Source: Murata
When grading MLCCs for standard C-values, developers should choose based on basic guideline values (Tables 1 to 3), which means that ideally only preferred values with standard tolerances should be used. In fact, users no longer need to pay attention to Z5U and Y5V ceramic type capacitors, because these types of devices are gradually discontinued, and some have actually been discontinued.
Table 1: Capacitance Classification
Table 2: Tolerance Codes
Table 3: Preferred MLCC parameter combinations (capacitance >1μF: E3 series preferred)
In addition to DC bias issues, Class II ceramic capacitors such as X7R and X5R also need to consider temperature drift and aging.
Temperature drift can be easily determined using Table 4. For example, this table shows that the X5RMLCC has a predictable temperature drift of ±15% over the temperature range of –55°C to +85°C.
Table 4: Temperature Drift of Different MLCC Devices
Figure 3: The effects of aging can be reversed by exposing MLCCs to very high temperatures for a period of time.Source: Samsung
MLCCs – they age too
Aging phenomenon causes the capacitance value of MLCC to lose over time, the loss is about 1% to 6% at each logarithmic decade, which means that we can estimate 1 hour by this Capacitance value loss after 10 hours, 100 hours, and so on. Therefore, the higher the C value of the MLCC and the thinner the inner layer, the easier the MLCC is to age. That said, aging is essentially a negligible factor compared to the effects of DC bias and temperature drift, although it plays a key role in measuring C values for tolerance testing.
Unlike biological aging, the aging of MLCC devices is reversible. Proper heat treatment can reverse the aging effects. To achieve deaging, MLCC components are typically placed at +150°C for 1 hour and then left to stand for 24 hours. Welding operations can also de-aging.
Looking at the various C value drifts as a whole, it is clear that the use of Class II capacitors with a nominal ±10% tolerance range should be advocated rather than a standard tolerance range of ±5%, even though some suppliers still provide and deliver standard Capacitor products with a tolerance range of ±5%. This can lead to pointless debates about whether or not to respect the tolerance range. During the measurement process, users are often unable to meet the requirements regarding measurement equipment and measurement conditions. For example, the measurement voltage (usually defined as 1.0V rms) drops during the measurement process, causing the displayed capacitance value to be too low.
It is best to leave room for voltage requirements
The specified voltage is usually a DC voltage (even if not explicitly marked). If the value is AC voltage it will be indicated, eg “250V AC”. Vendors often provide additional details, such as those related to ripple current or peak-to-peak, in their detailed datasheets or specification/application information. It is important to note that MLCCs with the same C value but higher dielectric strength (regardless of predictability or error rate aspects) tend to have thicker inner layers, reducing the DC bias effect.
That said, some suppliers continue to offer lower voltage specifications for capacitors that currently support 50V. In both cases, the specification exceeds the voltage requirement is not a problem, for example, for a 16V voltage requirement, an MLCC capacitor with a specified specification voltage of 25 V or 50 V can be used.
In addition to the basic parameters considered here, there are many other aspects when selecting MLCC components, for example, depending on the desired level of quality or characteristics of the application and field of use. Such properties may include automotive grade requirements (usually AEC-Q200 qualified) or soft termination requirements (also known as flexiterm, flex crack resistance, resin external electrodes and polymer terminations, and other similar expressions) that prevent Clearly visible cracks were formed when the PCB was formed (Figure 4).
Figure 4: MLCCs with specific properties can meet special requirements
Power for miniaturization
The motivation behind miniaturization has other implications; while the industry has been pushing for modern electronics to offer more and more performance to meet requirements, which is increasingly limiting space on the PCB, however, the main driver of miniaturization today is more likely to be Availability and cost-effectiveness (Figure 5), especially among suppliers. For developers, this means that they increasingly need to adapt to the pace of their suppliers, and if they want to remain flexible and cost-effective, they must leave enough leeway to find alternatives when needed, and dual-source Procurement is key to making this happen. This is especially true during challenging market conditions, which are always present, even if only temporarily.
Figure 5: Cost-benefit comparison of MLCC capacitors of various frame sizes
(The author of this article is Jürgen Geier, technical support for Rutronik ceramic capacitors)