The growing 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 has continued to develop in the direction of high accuracy, high density and high reliability.
The base material of the flexible circuit board (FPC board) is copper, and the circuit needs to be covered with a cover film. The cover film material is generally polyimide. The thermosetting adhesive tightly combines the cover film with the circuit board at high temperature, The surface of the circuit board plays a protective role. In the later stage of FPC board production, the shape needs to be processed. There is a row of plugs at the shape for connection with other electronic products. The reliability of circuit board connection is more stringent and higher for laser cutting accuracy.
At present, the method of batch processing the shape of FPC is punching. The small batch 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 common cutting methods for the shape of FPC board plugs: cursor point recognition and character recognition. There are no reports of plug edge recognition in the literature. And this method makes the laser cutting operation of the FPC board more convenient and simple, and the cutting accuracy is higher. In this paper, through the production process of FPC board, in order to solve the problem of cutting deviation of FPC board caused by expansion and contraction, the existing laser processing equipment is used, and the method of identifying the new plug edge by CCD is used to compensate the size of the circuit board with large expansion and contraction deformation and control the shape cutting Within accuracy requirements.
2.FPC board production process and expansion principle
FPC circuit boards are mainly divided into single-sided and 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 materials of FPC board are: flexible copper clad board, protective film and polyimide reinforcing film.
Each step of the FPC board production process will affect the shape of the circuit board. The reason is that the circuit board consists of flexible copper-clad board, polyimide and polyimide reinforcement film, etc. After the temperature rises above 170 ° C, internal stress occurs due to the difference between the expansion and contraction coefficients of copper and polyimide after cooling, which destroys the material balance force, shrinks and deforms the substrate, and distorts the circuit pattern of the substrate, causing the expansion of the FPC circuit board. Shrink uneven.
The uneven expansion and contraction of the FPC board may easily cause the shape processing accuracy to fall short of requirements. In this paper, the shape laser cutting technology is used to measure the cutting deviation values of different expansion and contraction rates of circuit boards, and the expansion and contraction-accuracy curves of laser cutting are drawn. The CCD reference point recognition technology corrects the distortion of the FPC board to achieve the purpose of improving the processing accuracy of the FPC board plug.
3. Experimental materials and equipment
4.Experimental methods and data
First, measure the cutting accuracy of the laser equipment to determine whether the equipment meets the design accuracy requirements. Then select the circuit board that cuts several expansion and contraction rates, measure its cutting accuracy, and draw the curve of expansion and contraction rate and cutting accuracy.
4.1 Equipment accuracy test
Test the running status and cutting accuracy of the equipment before cutting.
Measurement method: Measure the distance from the board to the edge and subtract the corresponding theoretical value to get the deviation value. Three cuts on the circuit board respectively, the measured data are as follows:
Table 1. Equipment cutting accuracy table
From the data sheet, the cutting variance is 5 microns, the processing accuracy meets the requirements, and the equipment is in a normal state.
4.2 Cutting accuracy of different expansion and contraction templates
In the production process of circuit boards, due to the reasons of jigsaw, electroplating, lamination, and high and low temperature differences, the sample is deformed. The laser equipment itself properly compensates for the expansion and contraction of the FPC board, but when the expansion and contraction of the FPC board is too large, it is impossible to control the accuracy of the cutting profile within the customer's requirements.
In order to measure the cutting accuracy of FPC boards with different 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, laser cutting was performed. The shape is then measured with a quadratic element, compared with the theoretical value of the graph, the deviation value is calculated, and then the average deviation value and variance are calculated.
FPC board expansion and contraction rate and cutting accuracy curve shows that when the expansion and contraction rate is less than 0.8 ‰, the cutting accuracy fluctuates within ± 0.05mm. With the increase of the expansion and contraction rate, the average cutting deviation value and variance value increase. When the expansion and contraction rate is greater than 0.8 ‰, the cutting accuracy can not meet the customer requirements of ± 0.05mm.
The expansion and contraction rate is greater than 0.8 ‰, the average cutting deviation is greater than 0.020mm, and the variance value is greater than 0.025mm. This shows that after the expansion and contraction 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 the FPC board whose expansion and contraction rate is greater than 0.8 ‰ within the range of ± 0.05mm has become a problem in laser cutting processing. There are reports in the country that software algorithm theory is used to compensate the deformation of the circuit board to improve the cutting accuracy, but no data has been reported on the accuracy of the cutting process.
4.3 FPC board cutting technology with an expansion rate greater than 0.8 ‰
According to literature reports and quality requirements of circuit board manufacturers, the key dimensions of FPC board plugs are the plug size and the distance from the plug to the board edge. When the positioning system uses the plug edge as a reference point to perform distortion correction calculation, the deviation of the plug inspection size and margin caused by the excessive expansion and contraction of the circuit board can be reduced (as shown in Figure 2), thereby ensuring the cutting accuracy.
Figure 2 FPC plug cut shape
The resolution of the laser cutting machine positioning system used in the experiment is ± 3 μm, which clearly distinguishes the boundary between the plug and the ordinary flexible plate, and provides an accurate reference point for the compensation of the distortion correction of the workpiece. After the circuit board production site verification, the new laser cutting technology can control the accuracy of the FPC board with large expansion and contraction. Figure 3 is an application example that meets the plug cutting deviation of ± 0.05mm.
Figure 3 High-precision cutting effect of plug
This paper analyzes the dimensional deviation of laser cutting machine cutting different expansion and contraction circuit boards, analyzes the measurement data, and concludes that when the expansion and contraction of 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 cutting accuracy of the circuit board with a large amount of expansion and contraction deformation, a new CCD system is used to identify the new positioning reference point of the plug, compensate for the distortion, and control the accuracy of the finished board.