3C Electronics
ST-P Series Laser Displacement Sensor in FPC Warpage and Height Detection
For FPC warpage and local-height inspection after mounting, soldering, or assembly, this article describes datum creation, grid sampling, surface-validation, and production-line integration with an ST-P laser displacement sensor.
Background
This article introduces the application of ST-P series laser displacement sensors in FPC warpage and height detection, covering industry background, detection requirements, measurement challenges, sensor selection, implementation, and precautions, providing a high-precision non-contact measurement solution for 3C electronics manufacturing.
Pain Points
- • An FPC has low stiffness, so vacuum, clamping, and cable tension can change its shape and make the fixture part of the measurement error.
- • Polyimide, black coverlay, copper foil, and pads have very different colors and reflectance, causing abrupt signal changes across material boundaries.
- • A small number of points can miss local bubbles or edge lift, while a dense scan consumes cycle time and becomes more sensitive to motion-stage vibration.
- • Components, solder joints, and openings may share the same area, so substrate height must be separated from raised features before warpage is calculated.
Measurement Solution
Industry Background In 3C electronics manufacturing, FPC (Flexible Printed Circuit) is widely used in devices such as mobile phones, tablets, and laptops. FPC soft boards are prone to warpage during SMT, soldering, and assembly processes, affecting subsequent assembly and electrical performance. Meanwhile, dimensional accuracy of components, solder joints, and connector pins on PCB/PCBA directly impacts product yield. Therefore, online, high-speed, and high-precision warpage and height detection has become a key quality control link on production lines.
Detection Requirements FPC Warpage Measurement: Measure the height difference of the FPC surface relative to a reference plane to determine if warpage is within tolerance. Component Height Measurement: Measure the height of components (e.g., chips, capacitors, connectors) on PCB to ensure assembly requirements. Solder Joint Height Measurement: Detect solder joint height uniformity to avoid cold joints or shorts. Step Height and Gap Measurement: Such as step height between phone mid-frame and screen, frame flatness, camera module height, etc.
Measurement Challenges Material Characteristics: FPC surfaces have various colors (black, yellow, etc.) and may be transparent or translucent, causing large differences in laser reflectivity. Highly Reflective Surfaces: Metal solder joints and connector pins can cause specular reflection, affecting measurement stability. High-Speed Production Lines: Fast cycle times require high sampling frequency and fast response from sensors. Limited Installation Space: Detection stations are compact, requiring small sensors with flexible mounting. Recommended Sensor Solution ST-P series laser displacement sensors use laser triangulation non-contact measurement, suitable for FPC warpage and height detection. Based on detection distance and accuracy requirements, the following models are recommended: Model | Reference Distance | Measurement Range | Repeatability | Linearity Error | Application Scenario
ST-P25 | 25 mm | ±1 mm | 0.05 μm | <±0.6 μm | Micro height, precision step ST-P30 | 30 mm | ±5 mm | 0.15 μm | <±3 μm | FPC warpage, component height ST-P50 | 50 mm | ±10 mm | 0.25 μm | <±4 μm | PCB solder joint, connector pin ST-P80 | 80 mm | ±15 mm | 0.5 μm | <±6 μm | Camera module, screen lamination ST-P150 | 150 mm | ±40 mm | 1.2 μm | <±16 μm | Large warpage, flatness Implementation Installation: Sensor is fixed above the detection station, with laser beam perpendicular or at a small angle to the measured surface. For FPC warpage detection, multi-point array or scanning method can be used. Measurement Process: Sensor collects distance data in real time, transmits via Ethernet or RS485 to host computer or PLC, and software calculates warpage and height values.
Signal Output: Supports analog (4-20mA/0-10V) and IO signals for alarm or sorting. System Integration: Can be integrated with vision inspection equipment, dispensing machines, lamination machines, etc., for closed-loop control. Selection Considerations Measurement Range and Accuracy: Choose appropriate model based on FPC warpage amplitude and allowable error. For micro warpage, ST-P25; for larger warpage, ST-P80 or ST-P150. Surface Characteristics: For black FPC, transparent adhesive, or highly reflective metal, sample testing is required to confirm stable measurement. Adjust laser power or installation angle if necessary. Sampling Frequency: For high-speed lines, select models with max 160 kHz sampling frequency to avoid missing defects. Environmental Factors: Consider vibration and temperature changes; sensor should have anti-interference capability.
Application Value Yield Improvement: Real-time detection of warpage and height allows timely rejection of defective parts, reducing downstream assembly failures. Non-Contact Measurement: Avoids damage to FPC soft boards, suitable for inline inspection. High Speed and Precision: Meets 3C electronics production line cycle times, with repeatability up to 0.05 μm. Easy Integration: Multiple output options compatible with mainstream PLC and host computer systems.
Precautions For transparent or translucent FPC and adhesive, laser may penetrate or cause stray light; testing is needed to determine optimal measurement setup. Highly reflective surfaces (e.g., metal solder joints) may cause saturation; use diffuse reflective surfaces or adjust sensor angle. Avoid direct laser exposure to eyes; install protective covers. Parameters should be confirmed based on specific model; contact manufacturer for detailed technical data.
| Model | Reference Distance | Measuring Range | Repeatability | Linearity Error |
|---|---|---|---|---|
| ST-P25 | 25 mm | ±1 mm | 0.05 μm | <±0.6 μm |
| ST-P30 | 30 mm | ±5 mm | 0.15 μm | <±3 μm |
| ST-P50 | 50 mm | ±10 mm | 0.25 μm | <±4 μm |
| ST-P80 | 80 mm | ±15 mm | 0.5 μm | <±6 μm |
| ST-P150 | 150 mm | ±40 mm | 1.2 μm | <±16 μm |
Technical Advantages
- • Non-contact measurement avoids probe force on the flexible board and preserves its shape under the specified support or vacuum condition.
- • A defined sampling grid and fitted datum plane can provide maximum warpage, local peaks and valleys, and the position of out-of-tolerance areas.
- • A fixed multi-head layout can shorten cycle time, while a single scanning head expands coverage; both approaches can be validated with real FPCs and the production fixture.
- • High-speed sampling and multiple ST-P communication outputs support transfer of profiles, decisions, and lot data to a PLC or quality-traceability system.