Introduction
Why Rigid-Flex? The Engineering Case
Step 1 — Stackup Design
Step 2 — Flex Zone Layout Rules
Step 3 — DFM Review Findings
Step 4 — Fabrication
Step 5 — SMT Assembly
- Reflow profile: Lead-free SAC305, peak at 245°C (PI stable to 260°C, we keep 15°C margin
Results

Published April 2, 2026 © CC BY

Rigid-Flex PCB: IoT Sensor Node Design to Assembly

How we designed and assembled a rigid-flex PCB for a cylindrical IoT sensor node — stackup, DFM, and turnkey PCBA in Tianjin, China.

AdvancedFull instructions providedOver 1 day7

Rigid-Flex PCB: IoT Sensor Node Design to Assembly

Things used in this project

Hardware components

Espressif ESP32

Seeed Studio Grove - Temperature, Humidity, Pressure and Gas Sensor (BME680)

Tibbo Technology Barebone Size 3 Tibbo Project System, Gen. 2 (TPS3G2)

Software apps and online services

KiCad

Arduino IDE

Story

Introduction

Most electronics projects have the luxury of mounting everything on a flat, rigid board. But what happens when your product lives inside a cylindrical enclosure, wraps around a structural frame, or needs to survive repeated bend cycles in a compact device?

That's exactly the challenge we faced: a compact IoT environmental sensor node designed to fit inside a 32mm diameter cylindrical housing. The main MCU and wireless module needed to sit on a rigid section, while a thin flexible tail connected to a secondary sensor array positioned at a 90-degree angle inside the enclosure. A standard 2-board connector approach would have been unreliable and eaten up too much Z-height. Rigid-flex was the right answer.

In this write-up, we walk through the entire process: stackup design, flex zone layout rules, DFM review, fabrication, and SMT assembly at our facility in Tianjin, China.

Why Rigid-Flex? The Engineering Case

Rigid-flex PCBs combine rigid laminate sections (where components are mounted) with flexible polyimide sections (which bend and route between the rigid zones). For this project, the key drivers were:

Space constraint: The cylindrical enclosure left no room for a wire harness or board-to-board connector stack
Reliability: Flexible connections outperform ribbon cables under repeated vibration and thermal cycling
Assembly simplification: One rigid-flex board replaces two separate PCBs plus a connector, reducing BOM and assembly labor
Signal integrity: Short flex traces between sensors and the MCU reduce noise pickup compared to flying wires

Rigid-flex is typically more expensive than standard FR4, but when you factor in eliminated connectors, reduced assembly labor, and improved MTBF, the total cost of ownership often comes out ahead for volume production.

Step 1 — Stackup Design

Rigid Zone (4-Layer):

Layer 1 (Top): Signal + Component pads | 35μm Cu

Layer 2: Ground plane (solid) | 35μm Cu

Layer 3: Power plane (3.3V / 1.8V split) | 35μm Cu

Layer 4 (Bottom): Signal + fanout | 35μm Cu

Flex Zone (2-Layer):

The flex section is a 2-layer polyimide construction. Key material choice: Rolled Annealed (RA) copper is specified instead of electrodeposited (ED) copper. RA copper has a grain structure aligned with the rolling direction, giving it far better flex fatigue resistance.

Step 2 — Flex Zone Layout Rules

All traces run perpendicular to the bend axis. Traces parallel to the bend axis concentrate stress and crack during flexing.

No right-angle bends in the flex zone — all routing uses 45-degree or curved segments.

Traces are fanned out across the full width of the flex zone to distribute mechanical stress evenly.

Minimum bend radius maintained at 3mm for static installation (10x flex thickness per IPC-2223).

No vias in the flex zone. All vias are placed in the rigid sections at least 1mm from the rigid-flex transition boundary.

IPC-2223 Compliance:

Min bend radius (static): ≥10x flex thickness → Our design: 3mm (20x ✓)
Conductor spacing in flex: ≥0.1mm → Our design: 0.15mm ✓
Via-to-flex-boundary clearance: ≥1mm → Our design: 1.2mm ✓
Coverlay overlap on rigid: ≥0.3mm → Our design: 0.5mm ✓

Step 3 — DFM Review Findings

Three issues were flagged before fabrication:

Issue 1 — Via Too Close to Flex Boundary: A bypass capacitor via was placed 0.7mm from the rigid-flex transition edge, below our 1mm minimum. Relocated 1.5mm inward.

Issue 2 — Trace Width in Flex Zone: Two power traces (3.3V, 150mA) were routed at 0.1mm through the flex zone. Given the thinner 18μm RA copper on flex layers vs. 35μm on rigid, current-carrying capacity is lower. Widened to 0.2mm per IPC-2152.

Issue 3 — Component Too Close to Flex Boundary: A 0402 decoupling capacitor was placed 0.3mm from the transition zone edge. Components this close experience mechanical stress from flex movement. Moved 1.5mm inward.

Step 4 — Fabrication

Rigid-flex fabrication differs from standard FR4 in three key areas:

Layup Sequence: The flex core is fabricated first as a separate sub-assembly, then rigid sections are built up around it using precise registration. Any mis-registration causes trace continuity failures at the transition zone.

Controlled Depth Routing: Rigid sections are routed away from the flex zone using a controlled-depth router — cutting through rigid material only, stopping at the PI layer.

Electrical Testing: All boards received 100% flying probe testing to verify continuity across rigid-to-flex boundaries before shipment.

Lead time: 14 working days for prototype quantity (10 panels).

Step 5 — SMT Assembly

Assembly of rigid-flex boards requires special fixturing. The flex zone cannot run flat through a standard reflow conveyor without risking deformation.

Custom FR4 carrier fixture fabricated to hold both rigid sections in correct relative orientation
Stencil thickness: 0.12mm (thinner than standard 0.15mm) to control paste volume on 0201 passives

- Reflow profile: Lead-free SAC305, peak at 245°C (PI stable to 260°C, we keep 15°C margin)

AOI after reflow on both rigid sections independently
First-pass yield: 97.2%

Results

Wi-Fi connectivity: -78 dBm sensitivity at 10m through one concrete wall
BME688 readings: Within ±0.3°C and ±1.5% RH vs. calibrated reference
IMU noise floor: 0.003 dps/√Hz at 25°C
Rigid-flex continuity after 500 bend cycles (3mm radius): 100% pass
Board fits within 32mm diameter cylindrical housing with 0.4mm clearance

The rigid-flex design eliminated two board-to-board connectors, reduced total BOM by 8 line items, and enabled a 31% reduction in housing diameter compared to the original 2-board design.

About TJHXPCB

TJHXPCB is a PCB design and assembly manufacturer based in Tianjin, China, providing one-stop services from schematic design to box-build assembly.

If you are designing a rigid-flex board or need a manufacturing partner for a tight-enclosure project, feel free to reach out:

Website: https://tjhxpcb.com/rigid-flex-pcb-design/

Email: contact@tjhxpcb.com

WhatsApp: +86 150 2256 3520

Code

code

#include <Arduino.h>
#include <Wire.h>
#include <Adafruit_Sensor.h>
#include <Adafruit_BME680.h>

// ====================  ====================
// ESP32-S3  I2C 
#define I2C_SDA  8
#define I2C_SCL  9

//  hPa
#define SEALEVELPRESSURE_HPA (1013.25)

// I2C
Adafruit_BME680 bme;

void setup() {
  Serial.begin(115200);
  delay(1000);

  Serial.println("ESP32-S3 + BME688 ");
  Serial.println("=====================================");

  //  I2C
  Wire.begin(I2C_SDA, I2C_SCL);

  //  BME688 
  if (!bme.begin()) {
    Serial.println("  BME688 ");
    Serial.println("    I2C 0x76  0x77");
    while (1) delay(10);  // 
  }

  Serial.println(" BME688 ");

  // 
  bme.setTemperatureOversampling(BME680_OS_8X);   // 
  bme.setHumidityOversampling(BME680_OS_2X);      // 
  bme.setPressureOversampling(BME680_OS_4X);      // 
  bme.setIIRFilterSize(BME680_FILTER_SIZE_3);     // 
  bme.setGasHeater(320, 150);                     // 320C  150msBME688 
}

void loop() {
  // 
  if (!bme.performReading()) {
    Serial.println("  BME688 ");
    delay(2000);
    return;
  }

  // 
  Serial.println("\n--- BME688  ---");
  Serial.printf("       : %.2f C\n", bme.temperature);
  Serial.printf("      : %.2f %%\n", bme.humidity);
  Serial.printf("      : %.2f hPa\n", bme.pressure / 100.0);
  Serial.printf("  : %.2f K\n", bme.gas_resistance / 1000.0);
  Serial.printf("   : %.1f m\n", bme.readAltitude(SEALEVELPRESSURE_HPA));

  delay(3000);   //  3 
}

Credits

TJHXPCB

2 projects • 0 followers

PCB design and assembly manufacturer based in Tianjin, China. One-stop services: PCB assembly, electronic enclosure,, motor controllers.

Rigid-Flex PCB: IoT Sensor Node Design to Assembly