When Life Gives You No MLCCs, Make Use of Polymer Capacitors

Taking into account price/cost, Tantalum capacitors had been avoided under many circumstances, while favoring Multi-Layer Ceramic Capacitors (MLCCs). Due to the massive MLCC shortage on the market it’s time to dismantle old prejudices

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Unless you have been living under a rock for the last 12 months, the following news will not be new to you - yet affecting your life tremendously, especially if you are working as an engineer or a supply chain manager:  Back at the beginning of 2017, major manufacturers of ceramic chip capacitors began to realise that the demand for MLCCs was stronger than expected. This demand was in particular articulated by the handset manufactures and automotive end markets, and lead in November 2017 to average lead times of min. 20 weeks. MLCC manufactures are leaving their low margin, high volume, consumer segment ad concentrating on supply of higher added value parts. According to the MLCC: FY 2018 Global market Outlook, this situation has even intensified due to a rising demand for high capacitance MLCC, as next generation products in multiple segments were viewing MLCC in the 100, 220 and 330 range as standard parts. And now the juicy part: the shortage will not improve until 2020 and beyond(Or in other words: It’s about time to look for an alternative).

MLCCs have been relied upon and used across multiple industries as a trustworthy workhorse. The most cost effective solution had been and probably would continue to be for the next ten years; the stacking of ceramic, but anywhere that other dielectrics can be employed, demand will grow as manufacturers have no choice but to distance themselves from such a tremendous reliance on Ceramics. Table 1 shows various kinds of capacitors including electrolytic capacitors, OSCON, SP-Cap, POS-Cap, film capacitor and MLCCs. The application typically dictates the best choice of capacitor type to use in the design. But before we get to that, generally speaking, the following differentiation can be made: whilst electrolytic capacitors provide the largest ESR these capacitors suffer significant degradation in capacitance and leakage current at higher temperatures and frequencies. Ceramic capacitors have a very low ESR and ESL that makes them great for transient performance but they have limitations on capacitance derating. Though ceramic capacitors can operate at very high ripple currents, they suffer the non-graceful aging failure and require lower operating electric fields. Polymer electrolytic capacitors are mainly used in power supplies of integrated electronic circuits as buffer, bypass and decoupling capacitors, especially in devices with flat or compact design. Thus they compete with MLCC capacitors, but offer higher capacitance values and display no microphonic effect.

Panasonic is a leading manufacturer of Solid Polymer Aluminum Capacitors: SP-Caps and OS-CON, Tantalum Polymer Capacitor – POS-CAP as well as Polymer Hybrid Aluminum Electrolytic Capacitor technologies and has the longest, design-in expertise and experience on the market. Conductive polymer aluminum solid capacitors, abbreviated to “polymer capacitors” in the following, have been extending in their application. The polymer capacitor (as well as conventional aluminum electrolytic capacitors) are featured by large capacitance and excellent bias characteristics which multilayer ceramic capacitors can never compete with. In addition to these advantages polymer capacitors have extremely low ESR characteristics. Regarding ESL, which is determined by inside structure and terminal configuration of the capacitors, by making structural improvements, the polymer capacitors have low ESL. Also concerning the dry-out of electrolyte in service life and the changes of characteristics at a range of low temperatures the polymer capacitors have realized very high reliability and superior low temperature characteristics by using solid polymer materials as an electrolyte.

Especially the SP-CAPs and POS-CAPs with their small size factor and cubical form are the go to products when it comes to the replacement of MLCCs. By looking closer at the characteristics of these different technologies once can see some major differences between them.

Stable vs. Frequency

Stable Capacitance: Figure 1 shows the change in capacitance over a wide frequency range for different technologies. It clearly shows that polymer capacitors exhibit very similar performance to multi-layer ceramic capacitors.

Capacitance Density / Stability vs. DC Bias

Looking at the results, figure 1 may rise the question why should somebody switch to polymer technology. The answer for this questions is:

MLCCs cannot achieve the same high capacitance as polymer for the same given footprint and volume. Secondly, the MLCC exhibits strong capacitance dependence on DC bias due to ferroelectric dielectric materials used for MLCCs. High-capacity, multilayer ceramic capacitors have a property often not well understood by electronic designers i.e. stability and reliability require extensive whetting. The capacitance of these devices varies with applied DC voltage which can lead to a capacity drop of more than 70% compared to the given specs on the data sheet. For polymer capacitors the capacitance does not vary significantly when the application voltage changes (compare figure 2). This advantages allow a significant lower part count using SP-CAPs or POSCAPs instead of MLCCs, which not only safes space on the PCB but also is a cost factor by saving costs on parts and reducing the production steps.

Stability vs. Temperature

Figure 3 shows typical temperature characteristics. The curve changes for MLCCs in various ways within the tolerance range of each product. For polymer capacitors, the capacitance is growing in parallel to the temperature rise. The temperature characteristics of MLCCs differ according to the dielectric type but all of them suffer aging failure by exhibiting temperature dependency and require lower operating temperature. Ceramic capacitors are brittle and sensitive to thermal shock, so precautions need to be taken to avoid cracking during mounting especially for high-capacitance large MLCCs. The typical temperature range for ceramic capacitors is -40°C to 85°C or 125°C, wherein their capacitance varies about from +5% to -40%; having the sweet spot around the low temperature of 5 to 25°C. Polymer capacitors have great development potential to achieve higher ratings on density, field stress and temperature (yet currently limited to 125°C) due to their working mechanism and dielectric materials advancement, yet higher dielectric constant polymers enable a high energy density.

Piezoelectric Effects Ceramic Chip Capacitor

Most dielectrics of ceramic capacitors exhibit a characteristic identified as piezoelectric effects, that can cause unexpected signals in certain circuits- In some cases, the piezoelectric effect may result in the appearance of electrical noise. When an electric potential or field is applied on the surface of an MLCC causes a deformation at a frequency range from 20Hz-20kHz, it could be audible to humans. This is then called the MLCC acoustic noise or singing noise (compare figure 4). A MLCC alone is in most cases not sufficient to generate problematic or disruptive Sound Pressure Level (SPL). But soldered on a PCB board the MLCC generates a spring mass system, which increases or dampens the oscillations depending on the frequencies.

Robust

Cracks in ceramic surface mount technology (SMT) components limit assembly reliability and yields. These cracks manifest themselves as electrical defects: intermittent contact, variable resistance, loss of capacitance and excessive leakage currents. That is why MLCCs are exposed to different reliability tests including thermal shock, board flex (bending), and biased humidity tests, etc., depending on the targeted applications. MLCCs are exposed to more than ten reliability test including thermal shock, board flex (bending) and biased humidity tests, etc., depending on the targeted applications. Among the reliability tests, the board flex test evaluates the mechanical resistance to cracking when MLCCs are subjected to bending stress on the printed circuit board (PCB) that the MLCC is soldered on. The bending of PCB can occur frequently during/between manufacturing steps and during operation under temperature variations. Flex cracking is due to excessive circuit board flexure.

As for the causes of board flexure, there are various causes including problems during the manufacturing process - such as solder stress due to an inappropriate amount of solder, stress applied at the time of depanelling or screw fastening or board flexure at the time of final assembly, in addition to drops, vibration, or thermal expansion during use. Ceramics are strong in compression but weak in tension. It is for this reason MLCC suppliers provide guidelines for buying out MLCC on PCB boards, often directing the MLCC caps not to be laid out at the edges of the board or control the direction of MLCC chip along the length rather than the width, when tension is expected at a certain point of the PCB. This limits and may require changes on the part of the design engineer before closing a PCB layout; thus, when a soldered MLCC experiences excessive board flex, a crack is easily generated in the element (compare figure 5). A flex crack can cause an electrical conduction between opposing internal electrodes. It is also possible that an fail open can progress to a fail short with continue product usage. If a crack on a capacitor element progresses to a short circuit failure, it may cause problems such as heat generation, smoking, or ignition; therefore, it is indispensable to take measures against them, particularly in equipment where reliability is essential.

Safety

Most ceramic capacitors have a fairly high voltage rating. If the capacitor experiences a voltage between its terminals higher than its rated voltage, the dielectric may break down and electrons will flow between the thin metal layers inside of the capacitor, creating a short. Luckily, most ceramic capacitors are built with a hefty safety margin and do not experience any sort of catastrophic failure (such as exploding). However, the rule of thumb dictates that you should derate ceramic capacitors by 50%, which means that if you are expecting to have a maximum of 5V between the capacitor’s leads, then you should use a capacitor rated for 10V or more. In order to understand the lifetime properties of polymer vs. MLCCs you have to take the construction and properties of each one into account (Please also compare figure 6 on the next page). Polymer capacitors are coming as chip type or wound type products. Since solid polymer capacitors are not regarded as components which are likely to be replaced in a device, they are often made in SMT (Surface-Mount-Technology). This allows them to take up less space on the PCB at the expense of being harder to unsolder if replacements are necessary. However, adopting SMT technology overall for a PCB also allows automated mounting, reducing costs and minimal human interference. The use of solid electrolyte is a major advantage over electrolytic capacitors. In a wet electrolytic capacitor overheating can cause the electrolyte to evaporate. As it evaporates, pressure builds up within the capacitor and it may burst or even explode. Solid polymer capacitors don’t have such risks– the capacitor either shorts or starts acting like an open circuit towards the end of its lifetime. Generally speaking, the reliability of polymer capacitors is much better than the reliability of electrolytic capacitors and particularly MLCCs.

USER CASE

The comparison of the characteristics outlined above is only partial in nature. Each type of capacitor is well suited to some fields and poorly suited to others. When actually designing circuits it is necessary to consider a number of factors on addition to the characteristics described above, such as cost and size, on order to select the best capacitor for the job if space is limited, all-purpose MLCCs are the best. MLCCs are also suitable for applications where the ability to deal with very high voltages is needed because of their high withstand voltage performance and ability to withstand reverse voltage. Typical polymer capacitors are a good choice when both, higher capacitance and low ESR are important. Especially the potential cost and space saving when comparing single polymer capacitors to an equivalent of multiple MLCCs can make a big different in designing PCBs. Taking these considerations into account we define following target specs for MLCC replacements with polymer capacitors:

>> Voltage Lines: 16 to 35V

>> Capacitance: 47μF to 560μF

(and since large capacitance is THE strength of polymer capacitors – the higher the better)

>> Preferably B & D case sizes

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