Advances in Class-I C0G MLCC and SMD Film Capacitors Xilin Xu, Matti Niskala*, Abhijit Gurav, Mark Laps, Kimmo Saarinen*, Aziz Tajuddin, Davide Montanari**, Francesco Bergamaschi**, and Evangelista Boni** KEMET Electronics Corporation, 2835 KEMET Way, Simpsonville, SC 29681 Tel: +01-864-963-6307, Fax: +01-864-963-6492, e-mail: [email protected] * SMD products, Evox Rifa Group Oyj, a Kemet Company Lars Sonckin kaari 16, 02600 Espoo, Finland Tel: + 358 50 3873205, Fax: + 358 50 83873205, e-mail: [email protected] ** SMD Products, Arcotronics Group, a Kemet Company via San Lorenzo 19, 40037 Sasso Marconi (Bologna), Italy Tel: +39 51 939 220, Fax: +39 51 939 324, e-mail: [email protected] ABSTRACT For applications requiring low dielectric losses (or low DF), low acoustic noise (no piezoelectric effect) and good temperature stability of capacitance, the top two choices are Class-I C0G MLCC and SMD film capacitors. There have been recent advances in both C0G MLCCs and SMD film capacitors. The C0G MLCCs have benefited from base metal electrodes (BME) in combination with an improved ability to stack well over 400 layers in the MLCC, and have resulted in cost effective and volumetrically efficient ratings up to 1 μF. The SMD film capacitors have seen significant advances in capacitance and voltage extensions, as well as heat resistance under lead-free soldering conditions. This paper will discuss the technical basis for advances in each of these technologies and give some guidance on the optimum areas (capacitance, size, voltage) for the application of each technology. INTRODUCTION In applications where capacitance needs to be precisely controlled over a wide temperature range with low dielectric losses or low acoustic noise, thru-hole film capacitors have been the optimum choice. These applications include digital tuning and high fidelity audio devices. Furthermore, the new Microsoft operating system - Windows Vista has strict audio-performance requirements to enhance the desktop and laptop PC audio quality and fidelity. Thus, hardware manufactures have to meet these requirements in circuit design and component selections in order to license the Windows Vista logo [1]. More recently, higher capacitance ceramic capacitors with a C0G dielectric and surface mount (SMD) film capacitors have become available. This paper describes the technological advances that have produced those solutions and gives some guidance on which solution should be selected. The Electronic Industry Association (EIA) specification for C0G dielectric, also known as NP0 (“negative-positive- zero”), is that the capacitance variation from room temperature (25°C) should be within 0 ± 30 ppm/°C (or ΔCMax/C ≤ 0.3%) over the temperature range of -55°C to 125°C. The C0G dielectrics are usually non-ferroelectric materials, and exhibit linear response to voltage and temperature. Compared with Class-II dielectrics, typically X7R/X5R materials, C0G dielectrics have the advantages of high stability of capacitance over temperature and voltage, no aging, no microphonics, as well as a low dielectric loss. However, in the past, maximum capacitance values have been limited due to thick dielectric layers, and costs have been relatively high due to the use of precious metal electrodes (PME). This paper describes the advances that have allowed the use of thinner dielectric layers and the replacement of PME with base metal electrodes (BME) that allow the ceramic C0G dielectric capacitors to have competitive capacitances with film capacitors over a range of case sizes and voltages. ©2008 ECA (Electronics Components, Assemblies & Materials Association), Arlington, VA Proceedings CARTS USA 2008, 28th Symposium for Passive Electronics, March, Newport Beach, CA Typical plastic dielectrics for film capacitors include polypropylene (PP), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyphenylene sulphide (PPS), and more rarely polytetrafluoroethylene (PTFE) and polyacrylic. Thermal stability requirements for surface mounting are more stringent than for thru-hole, and PP dielectric cannot meet the requirements. While PTFE meets the thermal stability requirements, it is expensive and not easily metallized, so it is only used in specialized applications. The typical plastic materials for SMD film capacitors are PET, PEN, and PPS. This paper describes the advances that have been made in order to make film capacitors based on these dielectrics resistant not only to surface mount reflow conditions, but to the very demanding reflow conditions required for Pb-free surface mounting. EXPERIMENTAL The capacitance was measured on a HP-4284A Precision LCR meter at 1 kHz and 1 Vrms. Insulation resistance (IR), temperature coefficient of capacitance (TCC), and voltage coefficient of capacitance (VCC) were measured in a Saunders & Associates 4220A temperature test chamber. The reliability of MLCCs was characterized by highly accelerated life test (HALT), in which the acceleration is accomplished by temperature and voltage stresses. The HALT test was performed at 175°C and 400 Volts for 46 hours. A sample size of 40 pieces was used for each sample. HALT time-to-failure (TTF) was recorded when IR at test temperature dropped below 3.78 MΩ. In order to evaluate the electrical noise generated from a capacitor, a test setup was required to generate a constant mechanical shock to the capacitor while monitoring the electrical noise generated from the shock. To generate the mechanical shock, the capacitor was mounted to an FR4 PCB measuring 1”x1”x0.03” and then mounted to a fixture to hold the board in place. A rod was fabricated and dropped on the PCB directly adjacent to the capacitor from a constant height. Dropping the rod on the PCB generated sufficient energy to the board to create electrical noise in the capacitor. To monitor the amount of shock delivered to the capacitor, an accelerometer was mounted on the board directly below the capacitor. The accelerometer readings were registered on a multi-channel oscilloscope. Alone, an electrically noisy capacitor will not generate enough noise to be detectable on an oscilloscope. When these capacitors are used in the input stage of an amplifier, their induced electrical noise can be amplified and delivered to the output of the amplifier. It is therefore necessary to replicate this scenario by connecting the capacitor samples to the input stage of an amplifier and monitoring its output on an oscilloscope. For this setup, two wires connected the FR4 PCB to the footprint where the capacitor would be mounted in the amplifier. When the test sample is given a mechanical shock, the electrical noise will run through the input stage and then will be amplified to the output. The signal is then put through a differential amplifier with a 20dB gain. The output from the differential amplifier is analyzed on the oscilloscope. A schematic of this measurement setup is shown in Fig. 1. Capacitor samples made from BME C0G, standard X7R, PET, PEN, and PPS material with the same capacitance of 0.22 μF were tested. Oscilloscope Differential Amplifier Signal Conditioner DUT Shock Point Amplifier Accelerometer Fig. 1. Setup for measuring electrical noise performance. ©2008 ECA (Electronics Components, Assemblies & Materials Association), Arlington, VA Proceedings CARTS USA 2008, 28th Symposium for Passive Electronics, March, Newport Beach, CA DISCUSSION (1) Advances in C0G MLCCs Traditional C0G dielectric materials are mainly based on the barium neodymium titanate (BNT) and compatible with PME, such as Pd or Ag/Pd. To provide a more cost effective solution, the MLCC manufacturers have mostly converted from PME to BME (mainly Ni). The BME C0G dielectrics are mainly CaZrO3-based materials. BNT has a dielectric constant (k) of around 70, while the k of CaZrO3-based materials is around 30. Intuitively, we would expect that less capacitance in a given case size would result from the switch from BNT to CaZrO3-based materials. However, we are able to use much thinner layers of CaZrO3-based materials compared to BNT, and remarkably still obtain higher insulation resistance (IR) and higher reliability at a given voltage. This breakthrough has led to higher capacitance, lower cost, and robust MLCCs with C0G dielectrics [2, 3]. A comparison of the maximum capacitance offering for BME C0G with PME C0G by case size is shown in Fig. 2. Three voltage ratings, 25V, 50V, and 100V are compared. It is clear to see that the BME C0G can offer at least an order of magnitude higher capacitance than PME C0G in the same case size and same voltage rating. For example, for 25Vdc rated 1206 case size, the maximum capacitance offering of BME C0G is 100 nF, while PME C0G can only offer 10 nF capacitance. This plot also indicates that at the same capacitance and same voltage rating, the BME C0G MLCC has a much smaller case size than PME C0G MLCC, or much higher volumetric efficiency. 1000 PME 25V C0G BME 25V C0G PME 50V C0G BME 50V C0G 100 PME 100V C0G BME 100V C0G 10 Max. CAP (nF) Max. CAP 1 0.1 0402 0603 0805 1206 1210 Case Size (EIA) Fig. 2. PME C0G vs. BME C0G: maximum capacitance offering. The reason why at the same case size and same voltage rating, BME C0G can offer much higher capacitance than PME is because of its thin, but high reliability dielectric layers. For example, the HALT reliability of two 1206 case size 103 (10nF) capacitance samples (one is PME C0G and the other is BME C0G) is shown in Fig. 3. These two samples both passed the required QA life test, which was performed at 125°C and twice rated voltage for 1000 hours. In order to make these parts fail, the HALT test was conducted at an extreme severe environment, which was at 175°C and 400V. Figure 3 shows that the BME1206-103 sample exhibits markedly longer time-to-failure (TTF) values compared to the PME1206-103. The median time-to-failure (MTTF) at HALT for PME1206-103 was 62.6 minutes, while for BME1206-103 was 869.6 minutes, more than an order of magnitude improvement in MTTF.