A new type of inductor provides design engineers with a wider range of inductance
BY RON DEMCKO
AVX Fellow
AVX
www.avx.com
To operate at maximum efficiency, RF inductors should exhibit low parasitics and low resistive loss at the frequency of use. Consequently, air core inductors have long been the traditional choice for this type of circuit design.
Air core inductors exhibit high Q, maintain a constant inductance value regardless of current, and can be tuned to specific inductance values by manipulating the distance between coil windings. They are also limited to a fairly low inductance range, which, for many RF power applications, is a non-issue. But, air core inductors are not an ideal design solution for RF power applications that need higher inductance values, as they become to large.
Expanding design options
Providing an alternative to scaled-up air core inductors for high-inductance RF power applications, multilayer organic inductors provide design engineers with several attractive features in a low profile and lightweight package. Multilayer organic inductors feature thermal coefficient of expansion (CTE 16-18) matching to FR-4 PCB and a comparatively high Q that is stable across frequency. They also provide an extremely high level of consistency, maintaining their inductance value through processing, handling, assembly, and other high-shock scenarios. They can also withstand processing of 260*C 3x reflow, and are RoHS capable and MSL 1 rated. Multilayer organic 0402 inductors offer an inductance range of 1.0 to 32 nH in tolerances as tight as ±0.05 nH or as wide as ±10%.
Fig. 1: 0402 multilayer organic inductor chips.
Multilayer organic inductors are produced through a process of laminating low-loss polymers and solid copper traces and then plating castellated via holes, which act as thermal piping for heat conduction, for layer interconnects. Currently, multilayer organic inductors are offered with castellated terminations, but land-grid array and high-density interconnect configurations are possible. Figure 1 shows 0402 multilayer organic inductors, and Fig. 2 illustrates the various lamination layers.
Fig. 2: Multilayer organic inductor – internal view.
As shown in the multilayer lamination view in Fig. 2, MLO inductors utilize a copper trace for metallization. The MLO’s Q is optimized by geometric design rules involving metallization dimensions and routing. An example of a 1-nH 0402 MLO type device from AVX shows its Q and inductance versus frequency in Fig. 3 . The markers 1, 2, and 3 on this graph correspond to resonant point, maximum Q, and initial inductance value.
Fig. 3: A 1- nHh multilayer organic inductor Q and inductance vs. frequency.
Applications
Frequency compensation on broadband multi-gigahertz oscillators is one fitting example of a multilayer organic inductor design. At high frequencies, wirewound inductors may not be available due to the lack of cost-effective manufacturing techniques capable of building such low-values. As such, designers must choose whether to create a low-value inductor with serpentine PC board trace, or a miniature SMT multilayer organic inductor. Although the PCB-based solution may be considered low cost, it takes up a lot of valuable board space and may have to be adjusted for different PCB manufacturers. Conversely, the multilayer organic solution will provide consistent performance on a lot-to-lot basis within a small footprint.
For example, AVX has developed a multilayer organic RF inductor in a 0402 case size that offers high current and high self-resonance. The HLC02 MLO RoHS-compliant devices feature inductances from 0.8 to 32 nH, a Q of 42 to 27 at 900 MHz, and maximum current of 875 to 175 mA. Self-resonate frequencies range from greater than 20 GHz to 2 GHz. ■
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