Smart ring manufacturing: miniaturization, sizing, encapsulation and sensor accuracy

What makes a smart ring hard to manufacture

A typical smart ring has a metal outer shell, usually titanium, stainless steel or ceramic; a flexible or rigid-flex circuit curved inside it; a small curved lithium-polymer battery; a Bluetooth Low Energy chip with memory for data collected offline; an optical heart-rate (PPG) sensor with LEDs and photodiodes facing the skin; a temperature sensor and an accelerometer. In many current designs the electronics are encapsulated in a resin inner layer, often clear, and the ring charges on a dock. There is almost no room for error. Every process, from fine-pitch assembly on flex to void-free encapsulation and shell finishing, runs close to its limits, and the product ships in many sizes.

The supplier pool is smaller than for wrist wearables. Fewer factories have production experience with ring encapsulation, and few battery makers produce curved cells this small. The practical routes are an OEM/ODM with a ring production history, or a contract manufacturer working with a specialist curved-cell maker, with more EVT and DVT iterations budgeted than a wristband would need. Smart ring designs have also been the subject of patent disputes in the US, including exclusion orders at the International Trade Commission, so a freedom-to-operate review belongs in the early budget.

Architecture and key components

Component	What matters	Sourcing considerations
Shell	Titanium, stainless steel or ceramic; polished, brushed, PVD or DLC finish	Machining capacity and scrap rate; coating abrasion data; nickel release for steel
Circuit	Flex or rigid-flex with chip-scale parts and stiffeners	Fine-pitch flex assembly experience; bend reliability
Battery	Curved lithium-polymer cell, with capacity limited by ring size	Few qualified makers; minimum orders per size; swelling behavior; a second source is hard to qualify
Sensors	PPG with green, red and infrared LEDs; skin temperature; accelerometer	Optical performance depends on window design and placement as much as on the part
Radio	BLE SoC with low sleep current and flash for stored data	The metal shell blocks RF, so the antenna radiates through the inner side
Charging	Contact pins or inductive charging in a dock	The dock is a second product with its own certification
Inner layer	Clear resin over the electronics	Optical clarity, hardness, adhesion to metal and skin compatibility

Optical design inside clear resin

A clear inner layer helps appearance and sealing, but it creates a problem for the sensors. Light from the PPG LEDs can travel through the resin straight to the photodiodes without passing through the finger, which adds a constant signal that swamps the small pulse the sensor is trying to measure. Designs prevent this with opaque barriers between LEDs and photodiodes, dark or masked zones in the resin, and windows positioned to sit against the skin. These features have to survive encapsulation in the right place, so their position becomes a process control and a test item, not just a drawing detail. The same applies to the skin temperature sensor, whose reading depends on how thermally coupled it is to the skin through the resin.

Sizing: the hidden SKU problem

Rings do not adjust, so brands commonly sell eight or more sizes, and some add widths or finishes on top. Each size can need its own shell, battery capacity, flex shape, fixtures and packaging. Demand by size is hard to forecast, and slow-selling sizes tie up inventory. Most brands send a sizing kit before the ring itself, which adds a logistics step and a cost to every sale.

The decision rule: minimize the number of distinct internal assemblies. Some designs share one electronics assembly across several sizes and change only the shell and the battery. Agree the size architecture before tooling, because it decides the number of fixtures and test stations and the minimum orders for batteries.

Core manufacturing processes

1. Flex PCBA. SMT on a carrier fixture, often with underfill under chip-scale packages for bend and drop reliability, then AOI and X-ray.
2. Pre-encapsulation test. Programming and a full functional test before encapsulation, because this is the last point where rework is practical.
3. Forming and battery attach. The assembly is shaped to the ring’s curvature and connected to the battery.
4. Shell production. Machining or metal injection molding, then polishing, coating and laser marking of the size and serial number.
5. Encapsulation. The assembly is placed in the shell and resin is dispensed, usually under vacuum to remove bubbles, then cured with a profile the battery can tolerate. The inner surface is finished and the sensor windows are checked for clarity.
6. Final test. Charging, radio, PPG signal on an optical test fixture, temperature sensor offset, accelerometer, current draw, and cosmetic inspection for bubbles, inclusions and marks.
7. Pack-out with the charging dock.

After encapsulation, a failed ring is scrap, including a machined shell and a battery. That makes first-pass yield at encapsulation the number that drives unit cost.

Testing and validation from EVT to PVT

EVT proves the sensing and electronics in the real form factor: PPG signal quality across skin tones, finger sizes and motion; battery life on real firmware; Bluetooth range with the ring on a hand; temperature during charging.

DVT proves the construction. It covers thermal cycling to find resin cracks or separation from the metal, since the materials expand at different rates; water pressure testing to the rating you will claim, before and after aging; artificial sweat, sunscreen, hand sanitizer and cleaning chemicals; scratch and impact tests on the finish; drop and crush; battery safety testing; biocompatibility testing on the final skin-contact materials; and nickel release testing for metal parts. Accuracy checks against reference devices, such as a chest strap for heart rate, belong here too, with the depth of the study set by your claims.

PVT confirms the line: void rate at encapsulation, PPG station correlation against a golden unit, yield by size and battery lot traceability.

Certifications and claims

• Radio: FCC, ISED, CE under the Radio Equipment Directive (including its cybersecurity requirements), UKCA with the PSTI security rules, and MIC, KC or RCM where sold; an RF exposure assessment, usually by calculation at Bluetooth power levels; Bluetooth SIG listing.
• Battery: IEC 62133-2, and UN 38.3 for the cell and for shipping. The EU Battery Regulation adds user-removability requirements for portable batteries from 2027, with exemptions that include some products designed for use in wet environments; check how they apply to a sealed ring.
• Materials: RoHS; REACH, including its limit on nickel release from items in prolonged skin contact; California Proposition 65 where relevant.
• Skin contact: biocompatibility testing, commonly to the relevant parts of ISO 10993 (cytotoxicity, sensitization and irritation).
• Health claims: wellness claims and medical claims fall under different rules. Claims about detecting or monitoring a condition can make the ring a medical device, which changes the evidence, the documentation and the factory’s quality-system requirements. Decide the claims before DVT.

Main cost drivers

• Curved batteries: custom cells, per-size variants and minimum orders.
• Shell material and finishing: titanium machining, coating and cosmetic scrap.
• Encapsulation yield, because every failure scraps the whole assembly.
• Size count: fixtures, inventory and forecasting risk.
• The charging dock and packaging.
• Sizing kits and their logistics.
• Validation: biocompatibility, water resistance and, for any health claim, accuracy studies.

What to ask a prospective OEM/ODM

• How many smart ring programs have you taken into mass production, and at what volumes?
• Who makes the curved battery, how many sizes do they supply, and what are the minimum orders?
• What do you test before encapsulation, and what is your encapsulation yield by size?
• How do you prevent and detect voids in the resin?
• Which finish and coating abrasion tests do you run, and to what limits?
• How is the PPG optical test fixture calibrated?
• What biocompatibility and nickel release data do you hold for the materials?
• Has the design been reviewed for patent risk in our markets?

Use the answers as evidence in supplier qualification, not as a substitute for seeing the encapsulation line and its yield records.

Common mistakes

• Underestimating the size matrix and its effect on fixtures, batteries and inventory.
• Encapsulating before a full functional test.
• Validating PPG only on the engineering team’s own fingers.
• Choosing a finish from a sample photo rather than abrasion data.
• Writing health claims in marketing copy before deciding the regulatory path.
• Relying on one battery maker with no plan for a second.

Where Zefinity helps

We help wearable teams settle the size architecture and the claims early, prepare the design for manufacturing and find OEM/ODM partners with real ring encapsulation and curved-battery experience. For wrist-worn products, see the fitness tracker guide. See how we approach wearables.

Frequently asked questions

Why do smart rings come in so many sizes?

A ring has to fit closely, both for comfort and for the optical sensors to read well, and unlike a band it cannot adjust. Brands commonly offer eight or more sizes, often with a sizing kit sent before purchase. Each size can mean its own shell, battery capacity, fixtures and inventory, which makes sizing a cost that is easy to underestimate.

Is a smart ring a medical device?

It depends on the claims. A ring marketed for general wellness, such as sleep or activity insights, is usually regulated as a consumer product. Claims to detect, diagnose or monitor a medical condition can make it a medical device under FDA rules, the EU MDR and similar regulations, with clinical evidence and quality-system requirements. Decide the claims before DVT.

How waterproof are smart rings?

Many are sold as suitable for swimming, with ratings expressed as water resistance to a depth or pressure. Many rings are sealed by encapsulating the electronics in resin rather than with gaskets, so the rating depends on the encapsulation process and its bond to the shell. Verify it with pressure testing after thermal cycling and chemical exposure, not only on new units.