FingerScore Hardware 6 — PCB Design and Miniaturization (Building It Into a Ring)

• Introduction
• 1. From Schematic to PCB — the Whole Flow
• 2. Component Placement and Routing Basics
  • Placement principles
  • Routing basics
• 3. Ring Form-Factor Constraints — the Biggest Challenge
  • Curvature and size
  • Using both sides and component height
• 4. Antenna Layout — the Fussiness of 2.4GHz
  • Antenna types
  • Keep-out and matching
• 5. Ground/Power Planes and Layer Count
  • What is a plane
  • 2-layer vs 4-layer
• 6. Component Packages and Soldering Difficulty
  • Tradeoffs
• 7. DFM, Gerber, and Ordering
  • DFM (design for manufacturing)
  • Gerber output
  • Ordering
• 8. Assembly — Stencil/Reflow vs Hand-Soldering
  • Reflow flow
  • BOM and Cost — the Parts List Is the Cost
  • Prototype Iteration — It Is Not Finished in One Go
• 9. Mechanical Design — Waterproofing and Comfort
• 10. Mass Production Considerations — Testing and Certification
  • Production testing
  • Certification (FCC/KC, etc.)
• 11. Common Pitfalls
• 12. Closing — Series Wrap-Up
  • Series retrospective
• References

Introduction

In Part 4 we communicated smartly over BLE, and in Part 5 we designed power that stays on for a long time. Now the final step: we must actually place all that circuitry onto a ring-sized board. This is PCB (Printed Circuit Board) design.

PCB design is the crown of hardware making and its hardest part. The schematic deals with the logic of "what connects to what," but the PCB deals with the reality of "how to physically arrange those connections." Especially on a device as extremely small, curved, and skin-touching-antenna as a ring, the constraints pile up like a mountain.

This Part 6 covers everything that turns the circuit into a real object: from the flow of moving a schematic to a PCB in a tool like KiCad, to component placement and routing, the special constraints of the ring form factor, 2.4GHz antenna layout, design for manufacturing (DFM) and Gerber output, ordering and assembly, and mass production and certification (FCC/KC). With this, we wrap up the FingerScore hardware series.

1. From Schematic to PCB — the Whole Flow

PCB design usually proceeds in this order. We describe it with KiCad (free, open source), but Altium, EAGLE, and others follow the same flow.

PCB design flow

1. draw schematic      : logically draw parts and connections
2. assign footprints   : specify physical pad shape for each part
3. generate netlist    : list of "what connects to what"
4. import to PCB       : parts float on the board
5. define board outline: draw the ring-shaped boundary
6. place components    : decide where to put what (most important)
7. route               : connect pin to pin with copper traces
8. planes/stitching    : ground/power planes, via placement
9. DRC check           : verify design-rule violations
10. export Gerber      : generate manufacturing files

The steps that take the most time and reveal skill are 6 (placement) and 7 (routing). Good placement makes routing easy; bad placement makes traces tangle no matter how you route. There is a saying: "placement is 80%, routing is 20%."

2. Component Placement and Routing Basics

Placement principles

A few principles when placing parts on the board:

• Follow the signal flow: laying parts in a line in input -> process -> output order shortens traces. e.g., antenna -> BLE SoC -> sensor.
• Decoupling capacitors right next to chip pins: as stressed in Part 5, as close to the power pin as possible.
• Crystal/oscillator near the chip, short traces: clock signals are noise-sensitive.
• Keep hot parts apart: place the charging IC and LDO away from each other and from sensitive parts.
• Connectors/buttons on the edge: parts a human approaches go on the outline.

Routing basics

Routing connects part pins with copper traces.

• Trace width: make high-current power traces wide (e.g., 0.3mm+) and signal traces thin (0.15mm). Too thin and they break or overheat.
• Avoid right angles: do not bend traces at 90 degrees; use 45 degrees or curves. Right angles are unfavorable for manufacturing and signal.
• Vias: small holes for moving between layers. Too many hurt reliability and area.
• Differential pairs: signals that travel in pairs (like USB data) should be length-matched and run side by side.

trace routing (good vs bad)

bad: right-angle bend       good: 45-degree bend
   ───┐                        ───╮
      │                           ╲
      │                            ╲───
      └───                  

avoid right angles, bend smoothly.

3. Ring Form-Factor Constraints — the Biggest Challenge

A normal PCB is a flat rectangle, but a ring is different. This is the core challenge of the FingerScore PCB.

Curvature and size

To wrap around a finger, the PCB must bend. There are two approaches.

If FingerScore does not have many parts, starting with a small rigid PCB and placing the curved LiPo on another face of the ring is a realistic hybrid. In full production, a flex PCB can wrap the whole thing to make it slimmer.

Using both sides and component height

With no space, you place parts on both sides of the PCB. You must also mind component height (the Z axis). Tall parts (connectors, large capacitors) increase the ring's thickness. So choose low-profile parts wherever possible.

ring cross-section (conceptual)

   outer (air, antenna direction)
  ┌─────────────────────┐
  │   PCB (parts both sides) │
  │  [BLE SoC] [sensor]  │
  └─────────────────────┘
  ┌─────────────────────┐
  │   curved LiPo battery │
  └─────────────────────┘
   inner (touches finger flesh)

4. Antenna Layout — the Fussiness of 2.4GHz

In Part 4 we noted that the antenna being near skin reduces range. In PCB design, the antenna is the fussiest part. Get it wrong and BLE will not reach even one meter.

Antenna types

Small devices use a chip antenna or a PCB trace antenna. Both need precise design.

Keep-out and matching

For an antenna to radiate well, two things matter.

• Ground keep-out: clear the copper (ground) around and beneath the antenna. With ground nearby, the antenna is blocked and will not radiate.
• Impedance matching: an antenna usually needs to be tuned to 50 ohms. Insert a matching circuit (a small inductor/capacitor combination, a pi-network) between the SoC and the antenna to adjust. Usually you leave the part footprints empty (DNP) and decide values after measurement.

antenna area layout (top view)

┌──────────────────────────────┐
│  [ground-filled region]       │
│  [BLE SoC]──[matching]──┐     │
│                         │     │
│ ┌───────────────────────┴───┐ │
│ │   antenna (ground keep-out)│ │  <- no copper in this region
│ │   ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓     │ │
│ └───────────────────────────┘ │
└──────────────────────────────┘
       placed toward the outer (air) direction

Antennas are hard to design yourself, so copying the SoC vendor's reference layout verbatim is the safest path. nRF52, ESP32, and others publish validated antenna layouts.

5. Ground/Power Planes and Layer Count

What is a plane

Filling an entire PCB layer with ground or power is called a plane. Planes serve:

• They keep the signal return path (the path current flows back on) clean.
• They reduce noise and suppress EMI (electromagnetic interference).
• They spread heat.

In particular, high-frequency (2.4GHz BLE) signals need a clean ground plane directly beneath them to propagate stably. Break that and the signal scatters and antenna performance collapses.

2-layer vs 4-layer

A small, RF-critical device like FingerScore makes a 4-layer PCB nearly essential. Dedicating the two inner layers entirely to ground and power puts a clean ground right beneath the RF signal for stability, while the two outer layers focus on signal routing. The cost goes up, but for a small RF device it is hard to compromise.

6. Component Packages and Soldering Difficulty

The same function comes in various sizes and shapes (packages). Smaller saves space but is harder to solder.

Tradeoffs

• For prototypes, start at around 0603 so you can hand-solder.
• For the ring product needing miniaturization, 0402 is the balance point.
• BLE SoCs are usually QFN, with pins hidden beneath the chip, making hand-soldering hard. A hot-air gun or reflow (explained below) is needed.

QFN package (pins on the underside)

   top view:       side view:
   ┌────────┐     ┌────────┐
   │  chip  │     │  chip  │
   └────────┘    ─┴────────┴─  <- pads hidden under the chip
   (pins unseen)   (no iron access by hand)

7. DFM, Gerber, and Ordering

When the design is done, you must export it in a form the factory can build.

DFM (design for manufacturing)

DFM is verifying "can the factory actually build this?" Every manufacturer has capability limits: minimum trace width, minimum spacing, minimum via size. If the design exceeds these, it cannot be made or the defect rate rises.

• Check minimum trace width/spacing (e.g., 0.15mm/0.15mm)
• Check minimum via drill/pad sizes
• Clearance between the board outline and parts
• Solder mask / silkscreen rules

Gerber output

Gerber is the standard file format for PCB manufacturing. It is a bundle of files holding each layer (copper, solder mask, silkscreen) and drill info.

Gerber file bundle (example)

board.GTL   - top copper (Top Layer)
board.GBL   - bottom copper (Bottom Layer)
board.GTO   - top silkscreen (text/marks)
board.GTS   - top solder mask
board.GBS   - bottom solder mask
board.drl   - drill file (hole positions)
+ inner layers (for 4-layer, .G2L, .G3L)

Ordering

Upload the Gerbers to a PCB manufacturing service (JLCPCB, PCBWay, etc.) and boards arrive in a few days. There are PCBA services that also handle part assembly. It is a good habit to eyeball the Gerbers once more in a viewer before ordering.

8. Assembly — Stencil/Reflow vs Hand-Soldering

Even when the board arrives, it does nothing until parts are attached. There are two approaches.

Reflow flow

reflow assembly steps

1. align stencil   : fit a metal stencil to the PCB
2. apply paste     : spread solder paste onto the pads
3. place parts     : place parts with tweezers / pick-and-place
4. reflow heat     : heat in oven/hotplate -> solder melts and bonds
5. inspect         : check joints by eye/microscope/X-ray

Parts with pins hidden underneath, like QFN, are nearly impossible to hand-solder, so reflow is effectively essential. Even small prototype runs can be done with solder paste and a hotplate (or a cheap reflow oven).

BOM and Cost — the Parts List Is the Cost

When the design is done, you compile a BOM (Bill of Materials). The BOM lists "what parts and how many you need to build this board." Each part is both a cost and an inventory/supply risk.

FingerScore BOM (brief example)

part           qty  unit price(low qty)  notes
-----------------------------------------------
BLE SoC        1    high                 QFN, core part
accelerometer  1    medium               gesture detection
LiPo battery   1    medium               curved
charging IC    1    low                  TP4056 family
LDO            1    low                  ultra-low Iq
protection IC  1    low                  over-charge/discharge
passives(R/C)  ~20  very low             decoupling, pull-up, etc.
connector/contact 1 low                  for charging
-----------------------------------------------

Common BOM mistakes:

• Picking EOL (end-of-life) parts: if a part is discontinued by production time, you must redesign. Choose parts with stable supply.
• Deciding on low-quantity price only: low-volume and high-volume unit prices differ greatly. Check production-scale pricing in advance.
• No alternate parts ready: defining a 2nd source (alternate supplier) per key part reduces supply-chain risk.

Prototype Iteration — It Is Not Finished in One Go

Hardware cannot be fixed instantly like software. Re-ordering a board takes days. So you finish it gradually through revisions (rev).

typical PCB revision flow

rev A : first board. almost always has mistakes (no power, swapped pins, etc.)
        -> verify with temporary wire jumpers/rework
rev B : fix rev A issues + finalize antenna matching values
        -> mostly works, finds minor improvements
rev C : production-ready. reflects DFM / test points / certification

lesson: do not expect perfection in rev A. leaving debugging room
        (test points, jumper spots) makes fixes easier.

Deliberately leaving spare part footprints (DNP), test points, and jumper options on the first board is a veteran's habit. Rather than aiming for perfect miniaturization from the start, it is safer to verify operation on a slightly larger validation board first and shrink gradually.

9. Mechanical Design — Waterproofing and Comfort

Even with a finished PCB, you need a case (housing) to wrap it. A ring touches the body, so mechanical design is especially important.

• Comfort: the ring's inner diameter, weight, and thickness must be comfortable. Too heavy or with sharp edges and it cannot be worn.
• Waterproofing: exposed to sweat and water, so aim for an IP rating (e.g., IPX5). Seal (gaskets, potting) so water does not enter through charge contacts or button gaps.
• Material: since it touches skin, use low-irritation materials (e.g., medical-grade silicone, certain plastics).
• Buttons/gestures: minimizing physical buttons and replacing them with gestures (accelerometer) favors waterproofing.

waterproofing points (where water can enter)

- charge contact gap  -> seal with gasket/potting
- button hole         -> replace with a gesture if possible
- case seam           -> ultrasonic weld or bond
- mic/hole            -> waterproof membrane (if any)

10. Mass Production Considerations — Testing and Certification

Building one prototype and building thousands reliably are different worlds.

Production testing

• ICT/functional test jig: a fixture that automatically checks each board on the production line. You must pre-place test points (pads) on the PCB.
• Programming point: a point to automatically flash firmware in production.
• Yield management: track the defect rate and analyze causes.

Certification (FCC/KC, etc.)

Selling a wireless device requires radio certification.

Fortunately, using a BLE module that is a pre-certified module greatly reduces the certification burden. Designing your own chip and antenna requires full certification, which is costly and time-consuming. Early startups usually begin with a pre-certified module.

certification strategy

self-designed chip+antenna -> full RF certification needed (costly, months)
pre-certified BLE module   -> certification greatly simplified (recommended start)

tradeoff: a module is slightly larger and pricier, but greatly reduces
          certification and development risk.

11. Common Pitfalls

• Ground beneath the antenna: laying ground without respecting the antenna keep-out makes BLE nearly fail. The most common RF failure.
• Decoupling placement: capacitors far from the chip cause supply sag and intermittent misbehavior.
• Ignoring DFM: designing thin traces or small vias beyond the manufacturer's limit mass-produces defects.
• Missing test points: without production test pads, the line cannot inspect.
• Overlooking part height: a single tall part increases the ring's thickness and ruins comfort.
• Considering certification late: thinking of certification after building everything may force a redesign. Consider modules/certification from the start.

12. Closing — Series Wrap-Up

In this Part 6 we covered the PCB design that turns the FingerScore circuit into a real, touchable ring. We walked through the schematic-to-PCB flow, placement and routing, ring form-factor constraints, 2.4GHz antenna layout, 4-layer planes, component packages and soldering, DFM/Gerber/ordering, assembly, mechanical design, and mass production and certification — the full process of turning a circuit into a thing you can hold.

Series retrospective

Across the FingerScore hardware series, we walked the journey from idea to product together.

FingerScore hardware series map

Part 1: MCU choice and dev environment    -> pick the brain
Part 2: sensing and gesture detection     -> read the finger motion
Part 3: firmware basics                   -> plant thought in the brain
Part 4: BLE wireless comms  (prev)        -> send scores wirelessly
Part 5: power/battery/circuit (prev)      -> keep it on for a long time
Part 6: PCB design and miniaturization (this) -> build it into a ring

For someone meeting embedded for the first time, hardware looks daunting. Unlike software, there is no "undo," you handle invisible radio and current, and a mistake can burn a part. But broken into steps, each one stands on a clear principle you can absolutely learn.

I hope this series is your first step toward building a small device yourself. The little magic of a score ticking up with a tap on your finger is the result of all these steps stacking up neatly. Build it yourself. After all the frustration of dropped connections, dead batteries, and antennas that will not catch, the joy of the moment the score first appears on the phone is irreplaceable.

Thank you for reading. Happy building.

References

• KiCad official docs — https://docs.kicad.org/
• JLCPCB capabilities guide — https://jlcpcb.com/capabilities/pcb-capabilities
• PCBWay PCB design resources — https://www.pcbway.com/pcb_prototype/
• Nordic nRF52 hardware design guide — https://docs.nordicsemi.com/bundle/ug_nrf52840_dk/page/UG/dk/intro.html
• 2.4GHz antenna layout guide (Texas Instruments) — https://www.ti.com/lit/an/swra351a/swra351a.pdf
• FCC equipment authorization overview — https://www.fcc.gov/engineering-technology/laboratory-division/general/equipment-authorization
• Korea radio certification (KC, RRA) — https://www.rra.go.kr/en/index.do
• Sparkfun PCB design intro — https://learn.sparkfun.com/tutorials/pcb-basics