An HDI PCB can help fit a smart ring’s electronics, but it cannot recover space already needed by the antenna, optical window, battery and assembly process. Freeze those constraints first, prove the package escape, then choose the least complex interconnect structure that meets the layout and reliability requirements.
This guide is an illustrative engineering workflow, not a teardown of a commercial ring. Nordic Semiconductor documents a J-Style smart ring using the nRF52840; that announcement does not establish its PCB layer count or microvia construction. The choices below must be checked against your actual components and enclosure.
Start with a three-dimensional space budget
Before assigning layers, obtain the enclosure model for every intended ring size. Mark the inner optical surface, antenna region, battery envelope, charging interface, adhesive zones and assembly insertion path. A flat component-placement drawing can look feasible while the assembled ring collides with a cell tab or cannot be inserted into its housing.
Create separate boundaries for component bodies, solder joints, electrical clearance and assembly clearance. Ask the mechanical designer which surfaces are fixed and which can move. Keep the battery manufacturer’s mechanical restrictions separate from the PCB outline. Include test contacts and their fixture approach direction now; a pad that becomes inaccessible after bonding needs a defined earlier test step.
The broader wearable HDI design guide covers shared enclosure constraints. A ring-specific review adds circumferential placement, size variants and the sequence used to form and install the electronics.
Choose the board architecture before adding build-up layers
| Candidate | When to investigate it | Decision that must be proved |
|---|---|---|
| Compact rigid board | The enclosure provides a usable rigid electronics pocket. | All components, test access and the antenna arrangement fit without relying on the board bending. |
| Flex with locally stiffened assembly areas | The electronics must follow a curved installation path. | The assembled construction can be formed with the required bend geometry and component support. |
| Rigid-flex with HDI in selected rigid areas | Dense package escape and separated component islands are both needed. | The rigid-to-flex transitions, fabrication sequence and total assembly cost are justified. |
HDI and rigid-flex describe different aspects of a board. HDI addresses interconnect density; rigid-flex addresses construction and mechanical connectivity. Neither term alone specifies the other. Treat a proposed 1+N+1 construction as a routing candidate, not a universal smart-ring recipe. First route the most difficult package using the fabricator’s approved pad, via and clearance rules.
Use the HDI stackup planner to organize candidate constructions, then obtain a region-by-region drawing from the manufacturer. A rigid-area stack does not specify the flex region’s copper, coverlay, adhesive or stiffener. Ask which layers actually continue through each bend.
Protect the optical measurement path
Reserve the sensor window and optical isolation geometry before squeezing other parts around the photodiode. Analog Devices’ opto-mechanical integration guidance explains how barriers between emitters and detectors reduce direct optical leakage. Its examples concern wrist devices: a ring needs its own geometry and contact validation rather than copied spacing dimensions.
Separate the optical problem from electrical pickup. For example, the ADPD7000 data sheet, Applications Information calls for short, ground-guarded photodiode input connections and separation from noisy signals. This is device-specific guidance, not a recommendation to use that part in every ring. Follow the chosen AFE’s pin-level requirements and reference layout.
In the placement review, identify the complete detector-to-AFE path, not just its top-layer segment. Check transitions and neighboring routes. Keep a proposed digital connector or charging route from silently consuming the quiet corridor. Prototype the optical assembly with the actual window and barrier materials before committing to a mechanically irreversible bonded build.
Reserve an antenna system, not only an antenna footprint
A radio’s feed line and its antenna clearance are different layout regions. TI’s CC256x QFN PCB guidelines illustrate a controlled-impedance RF feed referenced to solid ground and separate antenna implementation restrictions. Those dimensions and matching components apply to that reference design; do not shrink them into a ring and assume equivalent performance.
For your selected radio and antenna, obtain the current reference layout and permitted substrate conditions. Record which copper layers must be clear, where ground is required, and where matching components and measurement access will fit. Review the assembled battery, housing and finger-loading conditions with the antenna supplier. A routing improvement that moves metal into a reserved RF region needs another RF review.
Plan both matching measurements and an over-the-air performance check in the intended enclosure. A satisfactory feed match alone is not evidence that the finished product has sufficient useful radio performance.
Budget pulse current as well as average battery life
Build an operating-state table for sleep, sensing, computation, radio activity and charging. For a simple calculation, average current is the sum of each state’s current multiplied by its fraction of time. Account for overlapping functions rather than counting the same interval twice. Use measured state currents when hardware is available.
As an illustrative budget only, a 20 mAh cell with 80% of nominal capacity allocated to operation offers 16 mAh. At an assumed 0.10 mA average draw, the arithmetic gives 160 hours, about 6.7 days. At 0.20 mA it gives 80 hours, about 3.3 days. These assumptions are not a product specification and omit discharge-rate, temperature, aging and cutoff behavior unless those are included in the usable-capacity estimate.
Separately inspect peak-current paths during LED pulses and radio bursts. A narrow neck or shared supply path can cause a transient problem even when the average-current estimate is acceptable. Apply the HDI power-integrity review to the actual regulator, decoupling, vias and copper geometry. Confirm behavior at the device pins under the most demanding intended operating combination.
Specify forming, materials and manufacturing together
Tell the fabricator whether the flex is formed once during assembly or repeatedly moves during use. All Flex Solutions’ bend-radius design discussion emphasizes that a bend which is too tight can damage the circuit. Obtain a bend limit for the proposed construction; do not apply a single thickness multiplier to every material stack.
Request the exact flex laminate, copper construction, coverlay and adhesive system, plus the rigid-region dielectric where applicable. Review how these choices affect finished thickness, assembly exposure and the approved RF geometry. Ask the supplier to identify component, via and stiffener restrictions around each formed region. A material substitution requires a review of the complete construction, not just a matching nominal thickness.
For quotation, provide the board in both flat and installed form, package footprints, proposed via spans, fabrication quantity and inspection needs. Ask what adds cost: extra sequential laminations, via filling, panel utilization, assembly fixtures or inspection access. Compare total assembled cost before optimizing the bare-board price.
A release review the team can act on
- Mechanical: every ring size has a checked component envelope and assembly path.
- Optical: window, barrier and detector geometry have been tested as an assembly.
- RF: feed reference, antenna clearance and measurement access match the selected design guidance.
- Electrical: package escape works, and power paths survive the intended transient loads.
- Fabrication: the supplier has approved each rigid and flex region, via span and forming requirement.
- Production: programming, electrical test and functional checks have defined access before and after final assembly.
Explore the HDI PCB application library for related design contexts. To discuss a smart-ring PCB construction, start with the mechanical envelope, component pin maps, quantity and unresolved tradeoffs. Keep initial material and layer choices labeled as proposals until the fabrication review is complete.
Sources accessed 7 October 2026. Vendor examples support the specific principles cited; this article does not claim a validated ring design, a commercial product’s HDI construction or medical certification.