SIG OMM Technical Application for Precision Geometric Dimension Measurement of Irregular FPC Flexible Boards

Introduction

Flexible printed circuit boards (FPC), with their light weight, thin profile, bendability and 3D wiring characteristics, have become indispensable core components in foldable-screen smartphones, smart wearable devices, automotive electronics, medical equipment and other fields. FPC consists of a conductive layer (high-purity electrolytic/rolled copper foil, purity ≥99.7%) and a substrate layer (polyimide PI or polyester film PET). Irregular FPC flexible boards — flexible circuit boards with irregular outer contours and non-standard geometric shapes — face even more complex challenges in manufacturing and inspection. The SIG optical measuring machine (OMM) provides a comprehensive solution for irregular FPC flexible boards, from contour acquisition to precision geometric dimension measurement, through non-contact, high-efficiency and high-precision optical measurement technology.

1. Inspection Challenges of Irregular FPC Flexible Boards

Irregular FPC flexible board inspection faces three core challenges:

First, FPC substrates are soft and easily deformed. Traditional contact measurement tends to cause workpiece deformation or surface scratching when the probe makes contact, resulting in data distortion. FPC substrate thickness typically ranges from 25 μm to 125 μm; the extremely thin material characteristics make deformation almost inevitable during contact measurement, making it difficult for measurement results to truly reflect the actual dimensions of the workpiece.

Second, irregular contours are complex and diverse. Irregular contours contain multiple types of geometric features such as curves, arcs and irregular windows. Traditional projectors and microscopes rely on manual point-by-point measurement, which is inefficient and prone to reading errors. The outer shape of irregular FPC is often not a standard rectangle or circle but an irregular shape customized according to product assembly space, placing higher demands on the contour recognition and complex geometric measurement capabilities of measuring equipment.

Third, bending deformation during FPC handling leads to dimensional misjudgment. The flexible characteristics of FPC make positioning and clamping extremely difficult during inspection; even slight bending can cause misjudgment such as hole position offset and trace width measurement deviation. Studies indicate that bending deformation during FPC handling can lead to AOI dimensional misjudgment rates as high as 18%, with trace width misjudgment accounting for 60% of cases.

2. Precision Acquisition and Measurement of Irregular Contours

The SIG OMM is equipped with a high-resolution industrial camera and telecentric optical system, combined with intelligent image processing algorithms, to quickly capture irregular contours, curved boundaries and irregular window features of FPC. The device supports measurement of complex contours such as arcs, curves and slots, and clearly captures irregular edge details through multi-angle light source adjustment. For local features such as gold fingers and stiffeners on irregular FPC, the device uses the eagle-eye zoom function to locate local areas for precision measurement.

In irregular contour measurement, the device's intelligent software supports automatic edge extraction and complex geometric relationship construction. Operators do not need to manually collect points one by one; the system automatically identifies contour boundaries and fits complete geometric features, significantly improving measurement efficiency and data consistency. For FPC with irregular windows, curved cutouts and irregular slots, the device completes measurement and data analysis of all contour features within seconds.

3. Precision Measurement of Key Dimensional Parameters

The SIG OMM performs precision measurement of the following core parameters on irregular FPC flexible boards:

Trace width and spacing: For precision FPC products, trace width/spacing can reach the 25–40 μm level, with inspection tolerances controlled within ±5 μm. Equipped with a high-resolution color industrial camera and telecentric optical system, combined with intelligent edge-detection algorithms, the device quickly captures trace contours and efficiently identifies subtle defects such as opens (broken traces), shorts (trace connections), notches and pinholes.

Pads and gold fingers: Measures pad diameter, position and gold finger width, ensuring accurate cover film alignment without offset-induced pad exposure. For application scenarios such as consumer electronics foldable-screen smartphones and smart watches, the dimensional accuracy of FPC gold fingers directly affects the reliability of signal transmission.

Vias and hole positions: High-precision measurement of via diameter, hole pitch and interlayer alignment deviation of multi-layer FPC. For multi-layer FPC rigid-flex boards, the SIG composite OMM uses coaxial light illumination to penetrate the semi-transparent solder mask, clearly capturing bottom-layer trace contours and controlling interlayer deviation ≤10 μm.

Irregular contour dimensions: Measures the outer contour, window size, arc radius and irregular boundaries of irregular FPC. The device supports boundary fitting and dimension annotation of arbitrarily shaped contours, meeting the diverse measurement needs of irregular FPC.

4. Core Value of Non-Contact Measurement

The SIG OMM uses non-contact optical measurement technology, avoiding deformation and damage to soft FPC substrates caused by physical contact. The device's programmable multi-angle LED lighting system intelligently adjusts illumination parameters to enhance the edge contrast of irregular FPC, ensuring clear imaging and stable edge detection for FPC of various materials such as polyimide (PI) and polyester film (PET). Repeat measurement accuracy reaches ±1.5 μm, meeting the stringent micron-level tolerance requirements of irregular FPC.

The device is equipped with an imported 0.1 μm resolution grating ruler and H-grade linear guides, combined with a granite base, meeting ISO international standards. Configured with a continuous auto-zoom lens and high-definition industrial camera, supporting 256-level brightness-adjustable LED light sources, it clearly captures micron-level trace features. Fully automatic CNC batch inspection pre-sets measurement paths through intelligent software, achieving unattended batch scanning and shortening single-piece inspection time to the second level. Measurement results are automatically uploaded and generate SPC statistical analysis reports for real-time monitoring of production-line process capability and early warning of batch defect risks.

5. Shrinkage Deformation and Flatness Assessment

Irregular FPC is prone to deformation during production due to material shrinkage. The lamination process in FPC production (temperature rising above 170°C) generates internal stress due to differences in the expansion and contraction coefficients of copper and polyimide, leading to substrate shrinkage deformation and circuit pattern distortion. The SIG OMM can be optionally equipped with a laser sensor to automatically scan multi-point height differences, calculate shrinkage values and output color deviation heat maps, intuitively presenting flatness and warpage distribution and effectively preventing hole position offset and machining accuracy loss caused by deformation.

When optionally equipped with a contact probe or laser probe, the device achieves 2.5D/3D data fusion, high-precision analysis of FPC flatness, warpage and step height, solving the dilemma that traditional image measurement methods cannot balance efficiency and 3D accuracy. This capability is critical for ensuring assembly accuracy of irregular FPC in subsequent processes such as SMT placement and thermal compression bonding.

6. Application Scenarios

The SIG OMM is suitable for full-process scenarios such as irregular FPC flexible board incoming inspection, pre-SMT pad inspection and final shipment inspection. In the consumer electronics field, it covers foldable-screen smartphone flex cables, smart watches/earphones, camera modules and other applications, with key inspection items including trace width/spacing, via diameter and gold finger position. In the automotive electronics field, it covers in-vehicle central control screens, sensors, BMS battery management systems and other applications, with key inspection items including hole pitch, pad position, shrinkage value and flatness. In the medical equipment field, FPC flexible boards also play an important role, and their high reliability requirements impose higher standards on the precision and consistency of dimensional inspection.

The SIG OMM, with its non-contact, high-efficiency and high-precision optical geometric dimension measurement technology, provides full-process quality control solutions from R&D prototyping to batch production for irregular FPC flexible board manufacturers, helping the flexible circuit board industry achieve high-quality manufacturing and continuous innovation.