A comparison of thin-film and wire-wound inductors

It is important to understand the specific performances between thin-film and wire-wound inductors, particularly in terms of specific performance benefits in specific applications. In many cases, particularly in the wireless telecommunications industry, thinfilm inductors may prove the better choice.



In its simplest form, an inductor is a piece of wire coiled in such a way that the magnetic flux linkage is maximized beyond that of a straight wire. The number of turns, the dimensions of the turns/core and the core material itself determine the inductor’s value.



The performance of a simple wire-wound inductor is affected by the resistance of the wire used in the winding as well as the distributed parasitic capacitance along the length of the adjacent coiled wires. Prior to the emergence of thin-film inductors and commercially wound SMT inductors, RF engineers had to wind their own coils on forms to create inductors needed for their designs. Those engineers would constantly battle the desire to make small, light coils by using tightly wound, small AWG wire at the risk of hurting inductor performance. Tightly wound thin wire would create low Q inductors due to high amounts of distributed parasitic capacitance and high levels ESR from the winding wire. Ultimately, companies were founded that would do nothing but wind families of coils in the most optimal manner. Each different inductor family type had concessions made relative to inductor performance, size and frequency response. The different families were targeted at specific applications that could accept the performance of the coil type. It wasn’t until recently that other manufacturing technologies were developed capable of producing high performance, low inductance value, and miniature inductors.



Thin-film technology is commonly used in producing semiconductor devices. In the last few decades, this technology has evolved tremendously in performance, process control and cost. Applying this technology to the manufacture of miniature, low value inductors has created a family of components that can offer both electrical and physical advantages over existing inductor choices.

PICK, PLACE AND PROCESS

Compared to wire-wound inductors, thin-film inductors are physically easier to pick, place and process. AVX thin-film inductors have a nickel barrier termination that allows them to be processed in the standard IR, vapor phase and wave solder processes typically used in assembly. Alternatively, manual soldering can be used with a PCB preheated to 100C, and a 260C temperature-controlled soldering iron of 30W.



Thin-film inductor chips can be built in miniature SMT case sizes of 0402, 0603 and 0805. Termination styles of land grid array are used on 0402 and 0603 devices and standard termination are used on 0603 and 0805 sizes. Land grid array technology reduces parasitics and results in a higher Q than standard termination-style thin-film inductors. In addition, LGA termination technology helps miniature SMT inductors self-align during the assembly process.



Examples of LGA termination are shown in Figure 1.



Thin film technology allows for the deposition of low ESR line width structures to the micron and below level. The result of such small structures yields extremely tight tolerance inductors in virtually any value imaginable. Typically, inductor tolerances as tight as 0.05- nH are available in low value 0402 and 0603 case sizes. The 0805 case size is available in values as low as 1.2nH, and tolerances as tight as 0.1nH.



Though the maximum value of thin-film inductors is less than airwound inductors’ maximum values of 6.8nh, and 22nh are available in 0402 and 0603/0805 sizes respectively. As RF frequencies increase, lower-value inductors become the more necessary in circuit design.



The tight tolerance and electrical characteristics of thinfilm inductors is stable pre- and post-processing and have outstanding stability in application. Sometimes the processing of wirewound devices can interfere with their specific inductance values and make adjustments necessary. Further, the low height of thin-film devices makes them able to withstand high G forces and vibration while maintaining a high degree of electrical stability.



Thin-film inductors are also highly stable in environmental extremes of temperature, humidity, moisture and time.



THIN-FILM INDUCTOR APPLICATION ADVANTAGES

Thin-film inductors offer significant advantages over wirewounds for an ever-increasing number of applications (even though wire wound inductors may have a larger value range, or a higher Q and even higher current capacity). In fact the standard terminated thin-film device can have a current capability of 1000mA. Depending on the application, the thin-film inductors’ lower Q can be an advantage. One example of this is in frequency compensation on broadband amplifiers. Previously a resistor/inductor combination was used. A thin-film inductor can now replace these two components in a single component solution, thereby improving system reliability while saving circuit size and weight.



Thin-film inductors’ maximum value is less than that of wirewounds’ inductance values. However wire-wounds may not be available in low inductance values and this is another problem. At some point, when designing a circuit (like a multi-GHz oscillator, for example) it is just not possible to obtain an extremely low value wirewound inductor.



Manufacturing techniques do not exist to build such a low value wire wound part in a cost-effective manner.



At that point, the designer is left with the choice of creating a low value inductor with a meandering PC board trace design or choosing a thin-film inductor chip. In this case though the PC board trace inductor could be considered free, it consumes a lot of board space and will have many disadvantages such as having parasitic effects associated with it. One interesting case study even showed the cost of the PCB actually increasing due to the fine width demands of the printed inductor. In many cases it is much more efficient to choose the thin-film inductor.



It is also important to note that thin-film inductors offer designers a miniature intermediate-to-high Q inductor that is electrically repeatable on a lot-to-lot basis and, most importantly, thin-film inductors offer extremely tight tolerance options for designers. This is significant and solves many problems for designers.



Thin-film inductors usage is dramatically increasing due to frequency spectrum crowding, narrow band circuit needs and maximum useable frequency band increases. Further, significant progress is being made with increasing the Q factor and maximum value range of the part, making it more and more often the necessary solution for design engineers in the wireless field.