PCB Tech - How to solve the problem of FPC board cutting deviation?

The increase in demand for portable products has promoted the continuous development of circuit boards from single-sided to double-sided, multi-layer, flexible and rigid-flex boards, and continues to develop in the direction of high precision, high density and high reliability.

The flexible circuit board (FPC board) base material is copper, which needs to be covered with a layer of covering film on the line. The covering film material is generally polyimide. The surface of the circuit board plays a protective role. The FPC board needs to be processed in the later stage of production, and there is a row of plugs for connecting with other electronic products in the shape. The reliability of the circuit board connection is stricter and higher for the laser cutting accuracy.

The current method for batch processing of FPC shapes is punching, and small batches of FPC and FPC samples are mainly processed by laser cutting. So far, many manufacturers at home and abroad have developed UV laser cutting machines for making FPC samples, and the commonly used cutting methods for the shape of FPC board plugs: cursor point recognition method and character recognition method. There is no literature report on the plug edge recognition method. And this method makes the operation of FPC board laser cutting more convenient and simple, and the cutting precision is higher.

This article introduces the FPC board production process and the principle of expansion and contraction, in order to solve the problem of FPC board cutting deviation caused by expansion and contraction, using existing laser processing equipment, using CCD to identify the new plug edge method to compensate for the large expansion and contraction deformation of the circuit board The size and control of the shape cutting are within the accuracy requirements.

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

FPC circuit boards are mainly divided into single-sided, double-sided and multilayer circuit boards. Double-sided circuit boards are products developed from single-sided boards. The production process of single-sided FPC boards is as follows:

FPC circuit boards are mainly divided into single-sided, double-sided and multilayer circuit boards. Double-sided circuit boards are products developed from single-sided boards. The production process of single-sided FPC boards is as follows

The main materials of FPC board are: flexible copper clad laminate, protective film and polyimide reinforcing film.

Each process of the FPC board production process will affect the appearance of the circuit board. The reason is: the circuit board composed of flexible copper clad laminate, polyimide and polyimide reinforcement film, etc., the lamination process requires the material The temperature rises above 170 degree Celsius. After cooling, internal stress will appear due to the difference between the expansion and contraction coefficients of copper and polyimide, which destroys the material balance, the substrate shrinks and deforms, and the substrate circuit pattern is distorted, causing the FPC circuit board to expand. Uneven shrinkage.

The uneven expansion and contraction of the FPC board can easily cause the shape processing accuracy to fail to meet the requirements. In this paper, the contour laser cutting technology is used to measure the cutting deviation values of different expansion and contraction rates of the circuit board, draw the expansion and contraction-precision curve of laser cutting, and then use the expansion and contraction-precision curve to apply new The CCD reference point recognition technology can correct the distortion of the FPC board to achieve the purpose of improving the processing accuracy of the FPC board plug.

2. Experimental materials and equipment

10 FPC boards, ASIDA JG13 UV laser cutting machine, image projector (two-dimensional)

1. Experimental methods and data

First, measure the cutting accuracy of the laser equipment to determine whether the equipment meets the accuracy requirements of the design. Then select and cut several kinds of circuit boards with expansion and contraction rates, measure their cutting accuracy, and draw a curve of expansion and contraction rate and cutting accuracy.

2. Equipment accuracy test

Before cutting, test the running state and cutting accuracy of the equipment.

Measurement method: measure the distance from the board to the edge, and then subtract the corresponding theoretical value to get the deviation value. Cut the circuit board three times, and the measured data.

3. The cutting accuracy of different expansion and shrinkage templates

In the PCB production process, due to the splicing, electroplating, lamination, and high and low temperature differences, the model will shrink and deform. The laser equipment itself properly compensates for the expansion and contraction of the FPC board, but when the expansion and contraction deformation of the FPC board is too large, it is impossible to control the cutting shape accuracy within the customer's requirements.

In order to measure the cutting accuracy of FPC boards with different expansion and contraction rates, 7 kinds 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, laser cutting Shape, and then measure the cutting size with the second element, calculate the deviation value compared with the theoretical value of the figure, and then calculate the average deviation value and variance.

The graph of FPC board shrinkage rate and cutting accuracy shows that when the shrinkage rate is less than 0.8‰, the cutting accuracy fluctuates within ±0.05mm. As the expansion and contraction rate increases, the average cutting deviation and variance values both increase. When the expansion and contraction rate is greater than 0.8‰, the cutting accuracy cannot meet the customer's requirements of ±0.05mm.

The expansion and contraction rate is greater than 0.8‰, the average cutting deviation value is greater than 0.020mm and the variance value is greater than 0.025mm. This shows that after the shrinkage rate exceeds 0.8‰, the cutting accuracy of the FPC board cannot meet the accuracy requirements of the shape ±0.05mm.

Controlling the cutting accuracy of FPC boards with a shrinkage rate greater than 0.8‰ within ±0.05mm has become a difficult problem for laser cutting. There are reports in the domestic literature that use software algorithm theory to compensate the deformation of the circuit board to improve the cutting accuracy, but there is no report on the calculated cutting accuracy data.

4. FPC board cutting technology with a shrinkage rate greater than 0.8‰

According to literature reports and the quality requirements of circuit board manufacturers, the key dimensions of FPC board plugs are the size of the plug and the distance between the plug and the board edge. When the positioning system takes the edge of the plug as the reference point for the calculation of the distortion correction, it can reduce the deviation of the plug inspection size and the margin caused by the excessive expansion and contraction of the circuit board, thereby ensuring the accuracy of the cutting.

When the positioning system takes the side of the plug as the reference point for the calculation of the distortion correction, it can reduce the deviation of the plug inspection size and the margin caused by the excessive expansion and contraction of the circuit board

The laser cutting machine positioning system used in the experiment has a resolution of ±3μm, which clearly distinguishes the boundary between the plug and the ordinary flexible plate, and provides an accurate reference point for the distortion correction and compensation of the workpiece. After verification at the circuit board production site, the new laser cutting technology can control the dimensional accuracy of the FPC board with a large expansion and contraction rate. Figure 3 shows an application example that meets the plug cutting deviation ±0.05mm.

3. Summary of solving the problem of FPC board cutting deviation

This article counts the PCB size deviations with different expansion and shrinkage rates of laser cutting machines, analyzes the measurement data, and concludes that when the expansion and shrinkage of the FPC board is greater than 0.8‰, the cutting accuracy cannot be controlled within the range of dimensional tolerance ±0.05mm. In order to solve the problem of cutting accuracy of circuit boards with large expansion and contraction deformation, this paper uses the new CCD system to identify the new positioning reference point of the plug, compensate for the distortion, and control the shape accuracy of the finished board.