Start with how the product is worn
A wrist tracker, ring, clip and head mounted device impose different restrictions. Describe the mounting position, enclosure materials, expected flexing and location of the skin facing sensors. Use a cross section through the enclosure to expose conflicts that a top view hides.
TI's wearable overview groups power management, wireless connectivity, user interaction and biosensing as important system functions. Translate those functions into placement zones before choosing the layer count; otherwise density improvements can consume the space needed for the actual measurement.
Budget energy and heat by use mode
List sleep, sensing, display, radio and charging modes with the blocks active in each. This is more useful to the layout discussion than one average power number. A short high activity interval may govern local decoupling and temperature even when daily energy use is low.
Review where a charger or processor releases heat relative to the sensor and enclosure surface. Preserve options for moving a heat source during prototype evaluation instead of filling every available gap with permanent copper and components.
Choose density where it buys usable volume
Compare a compact HDI board with a slightly larger conventional board in the actual enclosure. Include connector height and interconnect folding space. If the larger board forces a smaller battery, HDI may offer system value; if it merely creates an unused corner, extra buildup complexity may have little benefit.
- Keep antenna and matching reservations visible in mechanical CAD.
- Define the sensor window and optical or acoustic barriers.
- Check charging contacts for access and assembly tolerance.
- Provide a repeatable way to measure prototype current consumption.
Validate after final mechanical assembly
Repeat relevant measurements with the display, battery, enclosure and straps installed. Record the physical configuration with the results so an antenna or sensor issue can be reproduced. Where multiple enclosure materials are planned, treat each as a deliberate comparison rather than assuming that bare board performance transfers.
Maintain a list of geometric changes between prototype and production tooling, including adhesive thickness and shields. These can explain changes that are otherwise incorrectly blamed on PCB fabrication.
Adjacent-layer access, one connection at a time
Dielectric · geometry + material
Core or build-up dielectric
Illustrative only. Copper thickness, dielectric construction and via spans require a reviewed stackup.
Common questions
Is a rigid-flex board always preferable for a wearable?
No. Compare a rigid board, separate flex interconnect and rigid-flex construction against assembly, movement and service requirements. HDI and rigid-flex solve different packaging problems.
When should the enclosure enter PCB review?
At initial placement. Waiting until routing is complete makes antenna space, sensor alignment and battery conflicts much harder to resolve.
References & further reading
Method & assumptions · General design guidance. Validate the proposed construction and acceptance criteria for your project.