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Let us address the main issue here right away: the market faces successive waves of supply shortages, price hikes and lengthening lead times for MLCCs. The evolving needs of the auto, mobile, and industrial electronics industries have swiftly driven market demands, contributing to a shift in production focus from general-purpose MLCC products to MLCCs that are tailored to fit the needs of specialized applications. That as a trend is not happening for the first time but since brokers, stocks on the distribution side and even old MLCC components are running out you have to start to look for alternatives. All indications point towards one ugly truth you need to brace yourself for: nothing will change until 2020 (and even that is under debate).
Polymer Capacitors vs. MLCC
MLCC is by far the cheapest capacitor in the market and also the most commonly used one. And that for a good reason, since they work, are cheap and easy to manufacture. What they are not useful for is smoothing circuits, or places, where the capacitor needs to provide backup current, cause they cant hold that much capacitance, and they are prone to mechanical stress. When it comes to finding a suitable replacement though there is no such thing as a "one size fits all" solution. There are of course direct footprint considerations that allow the replacement part to fit on the board, but that is only the tip of the iceberg.
Panasonic is with more than 60% in volume the leading manufacturer of Solid Polymer Aluminium Capacitors and has paved the further development of this technology as a pioneer as well as a trendsetter. The polymer capacitor (as well as conventional aluminium electrolytic capacitors) features a 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 can 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 attributes by using solid polymer materials as an electrolyte. Polymer capacitors come in four main varieties with different characteristics, including the hybrid:
- OSCON: This is your go-to-choice if your application demands high ripple current, high voltage, and high capacity.
- SP-Caps: Similar to OSCON but a chip-type capacitor. Further main characteristics are a very low ESR what is already a substantial reason why you are looking at MLCC replacements because MLCC per-se has a very negligible ESR and exceptionally low ESL. The only downside you have to take into account – size options are limited.
- Hybrid: Since all Hybrid capacitors are automotive qualified this is your alternative to MLCCs when you address safety-critical applications.
- POSCAP: They can be a bit tricky, but they have a lot of size variations – starting from 2mm (A-case size) onwards.
Polymer Capacitors vs. MLCC - Specs & Facts
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 Capacitance: Figure 1 below 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: MLCCs cannot achieve the same high capacitance as a polymer for the same given footprint and volume. They also exhibit strong capacitance dependence on DC bias due to ferroelectric dielectric materials used for MLCCs.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). These advantages allow a significantly lower part count using SP-CAPs or POSCAPs instead of MLCCs, which not only saves space on the PCB but also is a cost factor by saving costs on parts and reducing the production steps.
Stability vs. Temperature: 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 ageing failure by exhibiting temperature dependency and require lower operating temperature. Ceramic capacitors are brittle and sensitive to mechanical shock, so precautions need to be taken to avoid cracking. 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). An 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.
- Robustness: 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. 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.