Spiral Chip Inductors: What They Are and How to Choose Them for Microwave Circuits

Updated

Spiral chip inductors are planar coils built as thin-film metallization on a ceramic substrate, used as surface-mount or bare-die inductors in RF and microwave circuits. Choose them when you need a defined inductance with high Q and a known self-resonant frequency in a small, repeatable footprint. The main decision is which variant fits your circuit: a high-Q single-value part, a multi-tap tuning part, a flip-chip part, or a chip-scale gold stud bump part. US Microwaves lists all four families, with values as low as 1 nH to 10 nH in its LX3000Q20NH line.

How a spiral chip inductor works

A spiral inductor is a flat coil: current enters one end, spirals inward, and exits at the center or through an underpass. Because the coil is planar, inductance comes mainly from the number of turns, the coil diameter, and the trace width and spacing. Thin-film processing keeps those dimensions tight, so inductance is repeatable from part to part.

Three parameters drive most selection decisions:

  • Inductance (L): set by geometry; the LX3000Q20NH family covers 1 nH to 10 nH.
  • Q factor: the ratio of stored to lost energy. Higher Q means a sharper resonance and lower loss in matching and tuning networks. US Microwaves labels the LX5000Q20, LX3000Q20, and LX2500Q20 families as high-Q.
  • Self-resonant frequency (SRF): the frequency where the coil's own capacitance resonates with its inductance. Above SRF the part behaves capacitively, so the inductor is only useful below it. US Microwaves describes these parts as high Q and high self-resonance.

A practical example: in a 5 GHz matching network, an inductor whose SRF sits near 5 GHz will look capacitive at the design frequency. You need a part whose SRF is comfortably above the band you are matching, which usually means a smaller inductance or a geometry with lower parasitic capacitance.

Variant comparison

US Microwaves organizes its spiral chip inductors into families that differ in mounting and tuning approach. The table below compares them on the dimensions that matter for selection.

Family Key trait Mounting / connection Best suited for
LX5000Q20, LX3000Q20, LX2500Q20 High Q factor Surface-mount chip Fixed matching and tuned circuits where loss matters
LX3000Q20NH 1 nH to 10 nH range Surface-mount chip Small-value, high-Q needs
LXT5000Q20, LXT3000Q20, LXT2500Q20 Multi-tap, 1/4-turn tuning Surface-mount chip Circuits that need adjustment after assembly
LXT…Q20FC series Flip chip, multi-tap Flip-chip bumps Compact assemblies with short interconnect
LX…Q20CSP and LXT…Q20CSP Chip scale, gold stud bump Gold stud bump Dense, low-profile die-level integration

The naming pattern is consistent: the number (5000, 3000, 2500) identifies the geometry family, and the suffix identifies the mounting or tuning option. The LXT prefix marks multi-tap tuning parts; FC marks flip chip; CSP marks chip scale with gold stud bumps.

Matching the part to the circuit need

Start from what the circuit must do, then pick the family.

Fixed impedance matching or resonant tanks. Use the high-Q single-value families (LX5000Q20, LX3000Q20, LX2500Q20). Higher Q narrows the resonance and lowers loss, which matters in filters and narrowband matching. For values in the 1 nH to 10 nH range, the LX3000Q20NH line is the listed option.

Circuits that need trimming after assembly. Use the multi-tap 1/4-turn tuning families (LXT5000Q20, LXT3000Q20, LXT2500Q20). The taps let you select a turn count to adjust inductance on the board, which is useful when substrate tolerances or component parasitics shift the design away from simulation.

Space-constrained or die-level assemblies. Use flip-chip (LXT…Q20FC) or chip-scale gold stud bump (LX…Q20CSP, LXT…Q20CSP) parts. Flip-chip mounting shortens the interconnect, which reduces added parasitics; gold stud bumps suit direct die attach in dense layouts.

Very small inductance values. The LX3000Q20NH family is the explicit 1 nH to 10 nH high-Q option in the listing.

Selection and layout checklist

Before committing to a part, confirm these points:

  1. SRF margin. Check that the self-resonant frequency is well above your highest operating frequency. A part that is "high self-resonance" in general terms may still be too close for a millimeter-wave design.
  2. Q at your frequency. Q is frequency-dependent; a high-Q family is high-Q over its intended band, not everywhere.
  3. Mounting style. Match the part to your assembly process: surface-mount chip, flip chip, or gold stud bump. The suffix tells you which.
  4. Tuning need. If the design may need adjustment, the multi-tap 1/4-turn parts give you that option without a board respin.
  5. Value range. Confirm the inductance you need falls within the family's range; the LX3000Q20NH line is the listed small-value option.

Getting parts and pricing

US Microwaves provides contact, quote request, cross-reference, inventory check, user login, and online order entry on its site, and lists an online quotation path for custom thin-film circuits. The source material does not state prices, minimum order quantities, or whether ordering requires an account, so confirm those directly with the manufacturer before planning a build. For custom thin-film work, the site also documents layout design rules and application notes that are worth reviewing alongside the inductor datasheets.