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   性能  Characteristics                                               佳益科技

The device must be quite thick(high profile)since thelines of flux are parallel to its axis(i.e.inductor crosssection is width x thickness)and therefore,a thin partcannot be efficiently designed.Such high profile devicesmay actually skid across the board during high speedpick-and-place assembly.End terminations of the partare only partial.Furthermore,the wire conductor isdelicate,particularly at its point of contact with thetermination,leading to reliability concerns.

A New Design for SMD Inductors

An alternative approach which provides for a highfrequency inductor that is both small and rugged is toconstruct the device from layers on a substrate.Thisapproach,a common method of forming inductors onhybrids and which is also utilized in various wire bond-able inductor chips,has now been incorporated into atrue SMD device.

Figure 1 depicts the structure of the ACCU-L induc-tor.Multiple inductor layers are constructed on a non-magnetic substrate of alumina.Each layer consists of aconductor spiral which is coated with an insulatinglayer.The insulator includes a via connect to the over-laying spiral.The uppermost layer seals the device.Thetotal thickness of these layers is typically less than0.3mm and the device thickness(substrate plus inductorlayers)is only 0.9mm.

The manufacturing method used in building thisinductor is photolithography.This technique,which hasbeen extensively developed in the microelectronicsindustry,allows simultaneous formation of thousands ofdevices on a single substrate.The individual devicesmay then be separated from each other upon completionof all layers.

Bare inductor chips would,of course,be incompatiblewith surface mount assembly techniques.Figure 1shows that this inductor includes full end terminationwith lands.These terminations are formed of soldercoated nickel plate.It is this termination which convertsthe inductor chip into a true SMD inductor.

Summary

The ACCU-L SMD inductor meets both present andfuture needs of high frequency applications.It is small,rugged,includes full end terminations and exhibitsexcellent electrical performance.The multilayer con-struction provides a high level of control on the electri-cal and physical characteristics of the inductor,givingconsistent characteristics within a lot and on a lot-to-lotbasis.The ACCU-L inductor fills a void created by theinability of earlier technologies to satisfy continuingminiaturization and manufacturability requirements ofhigh frequency circuits.

SMD Inductor Guide

A Glossary of Important Inductor Parameters forHigh Frequency Applications

Inductance.High inductance values are not a factorin most high frequency applications.1.8nH to 39nH istypically the range of interest.The critical factors arethe stability and tolerance of the inductance at operat-ing frequency.Note that accurate measurement of theselow values is not trivial.Even at low frequencies(~10MHz),instruments available today exhibit measure-ment accuracy of no better than about±5%for an induc-tance of 10nH.At typical application frequencies(≥450MHz),measurement accuracy may be even furtherdegraded by parasitic capacitance of inadequately char-acterized test fixtures.

Parasitic Capacitance.All inductors include a certaincapacitance characteristic.This capacitance is derivedfrom the mutual proximity of the coil windings.Thecapacitance is further magnified by the inductor struc-tural materials,especially if these are of high dielectricconstant.It is critical for high frequency inductors thatthe parasitic capacitance be minimized since it deter-mines the device SRF.

SRF.Self resonant frequency of the inductor is thatfrequency at which parallel resonance is achievedbetween the device inductance and parasitic capaci-tance.Inductor Q drops to zero at SRF.It is quiteimportant,therefore,that inductor SRF be much higherthan the application frequency.

Resistance.In addition to inductance and parasiticcapacitance,the inductor also exhibits resistance.At lowfrequencies,the resistance of the conductor track is thedetermining factor in inductor Q.At very high frequen-cies,the resistivity of the conductor material is moreimportant than the resistance.(resistance=resistivity xconductor length)cross sectionThis is due to the skin effect whereby high frequencycurrent flow is largely restricted to the surface layer ofthe conductor.For high Q at high frequency,it is there-fore important that the coil conductor be constructed ofa metal with low resistivity.

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