March 2011

High Q multilayer chip inductors

Keeping high-frequency circuits noise-free

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Thanks to its advanced high-precision multilayer technology TDK-EPC has developed extremely compact 0402 and 0603 series of multilayer chip inductors with high Q values.

The new MLG0402Q and MLG0603P series from TDK are designed for use in the high-frequency circuits and modules of mobile phones.

As mobile handsets continue to shrink in size they are simultaneously offering an increasingly complex array of functions. This raises the requirements both for miniaturization and performance on all components used, including inductors. In particular, the multilayer chip inductors therefore also need to be made even smaller in size while providing high Q ratings. TDK-EPC was able to meet these demands by further refining the process technology for low temperature co-fired ceramic (LTCC) multilayer substrates.

The manufacturing process for the newly designed internal electrodes of the chip now features even more accurate position control. The result is the MLG0402Q and MLG0603P series of multilayer chip inductors in 0402 and 0603 packages with high Q values. Multilayer chip inductors are manufactured using thin sheets made of ferrite or other materials on which coil patterns are printed with metallic paste (normally silver). By arranging these sheets in multiple layers, a spiral-shaped internal electrode pattern is created. The multilayer technique developed by TDK allows the coil to be formed without the need to wind wire on a core, facilitating both miniaturization and mass production.

Low loss and high Q values a must for high-frequency applications

Multilayer chip inductors for high-frequency circuit applications use sheets made of dielectric ceramics instead of ferrite. This is because ferrite has higher losses in the frequency range of several hundred MHz and higher, making it difficult to achieve high Q values (Figure 1). Coils easily pass direct current but act as a resistor to alternating current. This behavior is called inductive reactance. The higher the frequency of the alternating current, the higher the inductive reactance. However, although the coil is a conductor, the wire winding has a certain DC resistance (R). The ratio between the DC resistance and the frequency-dependent inductance (R/2πfL) is called the loss factor, and its inverse number is the Q value. Because f is the frequency of the current flowing through the coil, the Q value will vary according to the frequency. In simple terms, a higher Q value means lower losses and better suitability for use as a high-frequency inductor. Because the increasing number of functions incorporated in mobile phones is linked to higher battery power consumption, multilayer chip inductors used in the high-frequency circuitry must have low loss and high Q values.

Figure 1: Q value and frequency response of inductors with different substrate materials
The Q value changes depending on frequency and substrate material. Ferrite substrates cannot be used in the frequency range of several hundred MHz and above. Dielectric ceramics are used in this range instead.

Overcoming distributed capacitance effects

Inductors to be used in the high-frequency circuitry of mobile phones should have high Q values and small dimensions. Unfortunately, however, if the coil is reduced in size to allow for more compact dimensions, its DC resistance rises, which leads to lower Q values. In addition, at higher frequencies the influence of the distributed parasitic capacitance (a capacitor component that does not appear in the circuit diagram) of the internal electrodes and other parts on Q becomes more significant. The inductive reactance (X) of the coil is directly proportional to the frequency and coil inductance, and is defined by the equation X=2πfL. In the ideal inductor, if the inductance is constant, the reactance is proportional to the frequency. Plotting frequency vs. reactance therefore should produce a straight line, but in reality, the reactance drops at higher frequencies. This is due to the distributed capacitance of the coil. In multilayer chip inductors, the coil patterns act like capacitor electrodes, resulting in distributed capacitance (Figure 2). Similarly, distributed capacitance also occurs between the terminal electrodes and the coil patterns.

Figure 2: Distributed capacitance between electrodes of a TDK multilayer chip inductor

The fact that multilayer chip inductors have distributed capacitance means that the equivalent of a parallel LC circuit is formed at high frequencies. As opposed to an inductor, a capacitor blocks DC current while acting more like a conductor for AC current the higher the frequency becomes. Similar to a parallel LC element being used as a resonance circuit, the multilayer chip inductor with distributed capacitance has a resonance frequency, the self-resonant frequency (SRF). At frequencies above the SRF the chip no longer acts as an inductor, and the Q value also drops drastically to zero at the SRF. Therefore, it is not sufficient to simply consider the required inductance when selecting multilayer chip inductors for use in high-frequency circuits and high-frequency modules. The self-resonant frequency must also be sufficiently higher than the usage frequency. In addition, inductors for high-frequency applications must also take the so-called skin effect into consideration. This significantly increases electrical resistance and therefore causes a drop in inductance.

Revolutionary internal electrode design of the new MLG series

As an extremely large number of electronic components must be tightly packed on the circuit boards of mobile phones, the multilayer chip inductors need to become even smaller and feature a low profile. While thin-film chip inductors where the coil is formed using thin-film process technology can be made very small with a low profile while maintaining high accuracy, it is still difficult to realize high Q values.

To overcome this dilemma high-precision manufacturing techniques are required that help to minimize distributed capacitance and the skin effect while enabling optimized design of internal electrodes which is the key to realizing compact dimensions.

TDK-EPC’s advanced LTCC process technology enables stable mass production of multilayer inductors with extremely precise internal spiral conductors. The shape, layer width, and layout of the internal conductors are designed so as to keep distributed capacitance at negligible levels while achieving high Q values.

This was made possible by redesigning coil patterns and layouts for the MLG0402Q and MLG0603P series from the ground up, TDK engineers succeeded in thus minimizing the distributed capacitance between terminal electrodes while largely maintaining the coil surface area, and achieving excellent Q value characteristics (Figure 3).

Figure 3: Q vs. frequency characteristics for the TDK MLG0402Q and MLG0603P series

Even a slight shift in coil pattern will cause a drop in Q value. To prevent this, high-accuracy positioning control technology and other advanced measures were applied when developing the MLG0402Q series. By further refining and perfecting these techniques, the MLG0603P series with newly designed internal electrodes was added to the product lineup of TDK-EPC. The new series features significantly higher Q, especially at frequencies of 800 MHz and higher. The new MLG0402Q and MLG0603P series are designed for the high-frequency circuitry of mobile phones, for example, for impedance matching in SAW filters and VCO circuits and as chokes. Further applications also include Bluetooth, WLAN, UWB, digital TV tuners and other high-frequency circuits and modules, where these multilayer chip inductors provide optimum performance.

Key technical data for the MLG0402Q and MLG0603P series of multilayer chip inductors from TDK

Parameter/ TypeMLG0402Q seriesMLG0603P series
Inductance [nH]1 to 150.6 to 120
Operating temperature range [°C]−40 to +85−40 to +85
DC resistance (max.) [Ω]0.4 to 2.60.06 to 5
Rated current [mA]100 to 25080 to 1000
Dimensions [mm³]0.4 × 0.2 × 0.20.6 × 0.3 × 0.3



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