The growing demand for portable products has driven the continuous evolution of circuit boards from single-sided to double-sided, multi-layer, flexible, and rigid-flex designs, with ongoing advancements toward higher precision, density, and reliability.

The flexible circuit board (FPC) base material consists of copper, which requires a covering film layer over the traces. This covering material is typically polyimide, serving a protective role for the board’s surface. After production, the FPC board undergoes additional processing, featuring a series of plugs designed for connections with other electronic devices. The reliability of these connections demands stringent laser cutting accuracy.

Currently, the predominant method for batch processing FPC shapes is punching, while small batches and samples are primarily handled through laser cutting. Numerous manufacturers, both domestic and international, have developed UV laser cutting machines specifically for creating FPC samples. Common cutting techniques for the shapes of FPC board plugs include cursor point recognition and character recognition methods. However, literature lacks reports on plug edge recognition methods. This method enhances the convenience and simplicity of laser cutting for FPC boards while achieving superior cutting precision.

This article outlines the FPC board production process and the principles of expansion and contraction. It addresses the issue of cutting deviations caused by these factors by utilizing existing laser processing equipment and employing a CCD-based method for plug edge recognition, thereby compensating for significant expansion and contraction deformations of the circuit board. This approach ensures that the size and shape cutting remain within the required accuracy standards.

1. FPC board production process and principles of expansion and contraction


1. FPC circuit boards are primarily categorized into single-sided, double-sided, and multilayer designs. Double-sided circuit boards are advancements derived from single-sided boards. The production process for single-sided FPC boards is outlined as follows:

2. The main materials used in FPC boards include flexible copper clad laminate, protective film, and polyimide reinforcement film. Each step in the production process can significantly impact the appearance of the circuit board. This is due to the materials involved, such as flexible copper clad laminate and polyimide. During the lamination process, temperatures must exceed 170 degrees Celsius. Upon cooling, internal stress arises from the differing expansion and contraction coefficients of copper and polyimide, disrupting material balance, leading to substrate shrinkage and deformation, and ultimately distorting the substrate’s circuit pattern, resulting in uneven expansion of the FPC circuit board.

3. The irregular expansion and contraction of the FPC board can easily result in inaccuracies in shape processing. This paper employs contour laser cutting technology to measure the cutting deviation values associated with various expansion and contraction rates of the circuit board, drawing a curve to illustrate the relationship between expansion/contraction and cutting precision. Utilizing this curve, new CCD reference point recognition technology is applied to correct the distortion in the FPC board, thereby enhancing the processing accuracy of FPC board plugs.

4. **Experimental Materials and Equipment**: Ten FPC boards, ASIDA JG13 UV laser cutting machine, and an image projector (2D).

5. **Experimental Methods and Data**: First, the cutting accuracy of the laser equipment is measured to ensure it meets the design specifications. Various circuit boards with different expansion and contraction rates are then selected and cut, followed by measurements of their cutting accuracy and the generation of a curve illustrating the relationship between expansion/contraction rates and cutting precision.

6. **Equipment Accuracy Testing**: Before cutting, the operational state and cutting accuracy of the equipment are assessed. The measurement method involves gauging the distance from the board to the edge, then subtracting the corresponding theoretical value to determine the deviation. The circuit board is cut three times, and the collected data is analyzed.

7. **Cutting Accuracy of Different Expansion and Contraction Templates**: During PCB production, factors such as splicing, electroplating, lamination, and temperature fluctuations can cause models to shrink and deform. Although laser equipment can adequately compensate for the expansion and contraction of FPC boards, excessive deformation can hinder the ability to maintain cutting shape accuracy within customer specifications.

8. To assess the cutting accuracy of FPC boards with varying expansion and contraction rates, seven types of circuit board materials with expansion and contraction rates of 0.1‰, 0.2‰, 0.5‰, 0.8‰, 1.0‰, 2.0‰, and 3.0‰ were selected. After positioning and laser cutting, the dimensions were measured using a second element, and deviation values were calculated against theoretical values to derive average deviation and variance.

9. The graph illustrating the relationship between FPC board shrinkage rate and cutting accuracy indicates that when the shrinkage rate is below 0.8‰, cutting accuracy remains within ±0.05mm. As the expansion and contraction rate increases, both the average cutting deviation and variance values also rise. When the expansion and contraction rate exceeds 0.8‰, cutting accuracy fails to meet the customer’s requirements of ±0.05mm.

10. For rates exceeding 0.8‰, the average cutting deviation exceeds 0.020mm, and the variance exceeds 0.025mm, indicating that beyond this threshold, the cutting accuracy of the FPC board does not satisfy the ±0.05mm specification. Controlling the cutting accuracy of FPC boards with a shrinkage rate greater than 0.8‰ within ±0.05mm poses a significant challenge for laser cutting. Although domestic literature discusses software algorithms to compensate for circuit board deformation, reports detailing the calculated cutting accuracy data are lacking.

11. **FPC Board Cutting Technology for Shrinkage Rates Greater than 0.8‰**: Based on literature and manufacturer quality requirements, key dimensions for FPC board plugs include the plug size and the distance from the plug to the board edge. By using the edge of the plug as a reference point in the positioning system for distortion correction calculations, deviations in plug inspection sizes caused by excessive expansion and contraction can be minimized, ensuring cutting accuracy.

12. The positioning system of the laser cutting machine utilized in this experiment features a resolution of ±3μm, which effectively differentiates the boundary between the plug and the regular flexible plate, providing a precise reference for distortion correction and compensation of the workpiece. After validation in the circuit board production environment, the new laser cutting technology can maintain dimensional accuracy for FPC boards with significant expansion and contraction rates. Figure 3 presents an application example where plug cutting deviation meets the ±0.05mm requirement.

13. **Summary of Addressing FPC Board Cutting Deviations**: This article compiles PCB size deviations at different expansion and shrinkage rates from laser cutting machines, analyzes the measurement data, and concludes that when expansion and shrinkage rates of the FPC board exceed 0.8‰, cutting accuracy cannot be maintained within the dimensional tolerance of ±0.05mm. To address the cutting accuracy challenges of circuit boards with significant expansion and contraction, this paper employs a new CCD system to identify a fresh positioning reference point for the plug, compensating for distortion and controlling the shape accuracy of the finished board.