PCB Layout in KiCad — High-density wearable gadget (eMMC, BLE, STM32)

Technical specification for the design and routing of a printed circuit board (PCB Layout)

**Project:** SHIELD V1.1 — Tactical presence node

**Development environment:** KiCad (version 7 or 8)

**Task type:** High-Density Integrated Layout, mixed-signal PCB routing

### 1. Deep understanding of the task and the physics of processes

We are not creating yet another household jewelry or pedometer, but a somatic coprocessor for adaptive state control, operating within the open standard **ADN**. The main challenge of this project is extreme miniaturization combined with strict requirements for signal integrity. The board has a physical size of **50×40 mm** and is mounted in a hermetic polycarbonate case with a thickness of only **10 mm**.

The device must continuously register the microsomatics of the body without allowing data distortion. The physics of the process requires the engineer to understand how high-frequency digital lines, radio frequency paths, and ultra-sensitive analog sensors coexist. Any parasitic coupling, electromagnetic noise from the switching regulator, or vibration from the power section that reaches the measurement circuit turns the device into a useless noise generator.

### 2. Component base (BOM) and interfaces

The schematic is fully verified, and the netlist is ready. The following nodes must be placed on the 50×40 mm board:

* **Computing core:** STM32U575 (high-performance ultra-low-power microcontroller, LQFP100 / BGA package).

* **Data acquisition (Analog circuit):**

* Optical pulse wave and HRV sensor — MAX30102.

* 6-axis inertial module (accelerometer + gyroscope) — BMI270.

* **Memory (High-speed circuit):** Local storage of 8 GB eMMC for continuous event logging and telemetry.

* **Communication (Radio frequency circuit):** BLE 5.2 module/chip with built-in or precision routed on-board antenna (Trace Antenna).

* **Power section and tactile feedback:**

* Linear vibration motor LRA (10 mm) for generating tactile patterns.

* Power circuit: Li-Po battery with a capacity of 600–800 mAh, charging controller, battery protection circuits.

* **Input-output interfaces:**

* Magnetic charging and data transfer port USB-C.

* LED status indicator (RGB 0603).

### 3. Strict routing rules (Rules that eliminate defects)

To eliminate any questions during the work process and to fix technological boundaries, the routing must strictly adhere to the following criteria:

#### Isolation of the analog circuit (PPG and IMU)

* The MAX30102 sensor must be placed on a physically isolated island of the board. There should be no digital signal lines beneath or around it.

* The analog power supply for the sensors (VDD_Ana) must be completely filtered from digital using ferrite filters (Ferrite Beads) and bypass capacitors placed as close as possible to the power pins of the chips.

* The analog ground must connect to the digital ground strictly at one point (Star Ground).

#### eMMC routing (High-Speed Data)

* The eMMC bus lines must be routed following the length matching rule to eliminate phase shift of signals.

* The characteristic impedance of the eMMC digital lines must be strictly controlled. No right angles are allowed in routing — only curves or 45-degree angles.

* A continuous return current path (GND Plane) must be provided around the eMMC bus on the adjacent layer.

#### Radio circuit (BLE 5.2)

* The antenna output path must be designed and routed as a microstrip line with a characteristic impedance of exactly 50 Ohms.

* The area under the antenna and in close proximity to it on all layers of the board must be completely cleared of copper (Keep-out zone) in strict accordance with the chip manufacturer's datasheet.

#### Power lines and tactile feedback

* Current traces from the battery to the power controller and to the LRA vibration motor driver must have increased width (calculated based on the peak current consumption of the vibration motor during the start of the tactile pattern).

* The LRA motor driver and its power connections must be maximally distanced from the BMI270 sensor to minimize parasitic paramagnetic and mechanical interference during vibration response.

### 4. Mechanical and placement requirements

* The board size is strictly 50×40 mm. The contour tolerance is no more than 0.1 mm.

* There are 4 mounting holes for M2 screws at the corners of the board with isolated zones from metallization.

* The board thickness is 1.2 mm or 1.6 mm (to be agreed upon based on the results of the stack layer calculation for controlled impedance).

* The optical sensor MAX30102 must be positioned on the underside of the board exactly at the center of the through window of the case for perfect contact with the operator's body.

* The external magnetic clip is located close to the case. The engineer must consider the placement of permanent magnets and move the IMU sensor (BMI270) into the internal safe zone of the board, excluding saturation of the magnetometer/gyroscope.

### 5. Deliverables

Upon completion of the task, the contractor must provide a complete package of project source files:

1. Board topology files (.kicad_pcb) and schematic files (.kicad_sch), developed without the use of third-party proprietary auto-routing plugins.

2. A complete set of Gerber files (RS-274X or X2) along with drilling files (Excellon) for production.

3. Production files: bill of materials (BOM) and component center coordinates file (CPL / Pick and Place) for automatic SMD assembly.

4. A single, correctly assembled 3D model of the printed circuit board in STEP format for integration into the overall mechanics of the device case.

### 6. Candidate filter (Elimination of amateurs)

If you want to take on this project, please start your response with the word **"ADN-Protocol"**. This will confirm that you have read the technical specification carefully and completely.

In your response, answer three specific engineering questions:

1. What layer stack structure do you propose for this density of layout (4 or 6 layers) and why?

2. What method will you use to control the characteristic impedance (90 Ohms for differential pairs USB and 50 Ohms for BLE)?

3. Provide examples of high-density boards (Wearables / IoT) you have developed with eMMC or BCI/PPG sensors on board. Screenshots or links to your portfolio are mandatory.

Template responses copied by bots or neural networks without answers to the questions will be automatically rejected. We need a practicing engineer ready to deliver flawless industrial results from the first revision of the board.