High precision FPC plug laser cutting technology interpretation

- Sep 15, 2018-

The growth of the demand for portable products has promoted the development of the circuit board from one side to double-side, multi-layer, flexible and rigid flexible board, and the development of high-precision, high-density and high-reliability. 

The substrate of flexible circuit board (FPC board) is copper, which needs to be covered with a layer of covering film in the circuit. The covering film material is generally polyimide. Under high temperature, the hot glue tightly binds the covering film with the circuit board, and compacts to protect the circuit board surface.In the later stage of FPC board production, the shape needs to be processed. In the shape, there is a row of plugs for connecting with other electronic products.The reliability of circuit board connection is more strict and higher to laser cutting precision. 

At present, the method of batch processing of FPC shape is punching and cutting, and the small-batch FPC and FPC samples mainly use laser cutting.So far, already has multiple vendors at home and abroad to develop the UV laser cutting machine for making samples of FPC FPC board plug shape cutting methods: the cursor point identification method and the method of character recognition, no literature reports plug edge recognition method, and this method makes the FPC board laser cutting operation more convenient and more simple, cutting precision is also 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 to identify new plug edge with CCD, compensate the size of circuit board with large expansion and contraction deformation, and control the shape cutting within the accuracy requirement. 

FPC board production technology and shrinkage principle

FPC circuit board is mainly divided into single-side, double-side and multi-layer circuit board. Double-side circuit board is a product developed from single panel. The production process of single-side FPC board is as follows:

High precision FPC plug laser cutting technology interpretation

The materials of FPC board include flexible copper sheet, protective film and polyimide reinforcing film. 

  

FPC board production process of each working procedure will affect PCB shape increases, its reason is: by flexible copper clad, polyimide and polyimide reinforcing membrane composed of circuit board, laminated process needs to be material temperature rises to above 170 ℃, cooled by different copper and the higher shrinkage coefficient of polyimide and internal stress, destroyed the material balance, substrate contraction deformation, the base line graphic distortion, FPC PCB increases and inequality. 

FPC board is uneven, easy to cause shape processing precision is not up to the requirements.Shape of laser cutting technology is applied in this article, measure the circuit boards of different shrinkage and higher cutting deviation, mapped the increases in laser cutting precision curve, again through the increases - curve, precision for higher shrinkage of FPC board, the application of new CCD benchmark recognition technology, to distortion correction of FPC board, FPC board plug processing accuracy. 

Experimental materials and equipment

FPC board 10 sheets, ASIDA JG13 UV laser cutting machine, image projector (quadratic)

Experimental methods and data

Firstly, the cutting precision of laser equipment is measured, and the precision of design is judged.Then select several kinds of PCB for cutting, measure the cutting precision, and draw the curve of the rising shrinkage and cutting precision. 

Equipment accuracy test

The running state and cutting precision of the equipment are tested before cutting. 

Measurement method: measure the distance from the plate to the edge, and then subtract the corresponding theoretical value to get the deviation value.Three times were respectively cut on the circuit board. The measured data is as follows:

Table 1. Equipment cutting precision table

Table 1. Equipment cutting precision table

According to the data table, the cutting variance is 5 microns, the machining accuracy meets the requirements, and the equipment is in normal state. 

Cutting precision of different expansion and contraction plates

In the process of PCB production, the expansion and shrinkage deformation of the sample is caused by the combination, plating, lamination and high and low temperature difference.The laser equipment itself makes proper compensation for FPC board's expansion and contraction, but when the FPC board's expansion and contraction deformation is too large, the precision of cutting shape cannot be controlled within the range required by the customer. 

In order to measure the different shrinkage of FPC board cutting precision, respectively select 7 kinds of higher shrinkage rate 0.1 ‰, 0.2 ‰, 0.5 ‰, 0.8 ‰, 1.0 ‰, 2.0 ‰ and 3.0 ‰ of the circuit board material, after positioning, laser cutting shape, and then measured with two dimensional cutting size, and graphics, compared to the theoretical value calculated deviation, then the deviation from the mean value and variance of statistics. 

FIG. 1 FPC shrinkage - cutting precision curve

FIG. 1 FPC shrinkage - cutting precision curve

FPC board expansion rate and cutting precision curve showed that when the expansion rate was less than 0.8‰, the cutting accuracy fluctuated within the range of + / -0.05mm.With the increase of the expansion rate, the average cutting deviation value and the variance value both increase. When the expansion rate is greater than 0.8‰, the cutting accuracy is less than 0.05mm. 

The average deviation value of cutting was more than 0.020mm and the variance value was greater than 0.025mm.This indicated that the FPC board cutting precision could not meet the requirements of the accuracy of external + / -0.05mm after the expansion and contraction rate was more than 0.8‰. 

The cutting accuracy of FPC board with an expansion rate of more than 0.8‰ is controlled within the range of + / -0.05mm, which becomes a difficult problem for laser cutting.It has been reported in domestic literature that software algorithm is used to compensate the deformation of the circuit board to improve the cutting accuracy, but there is no report on calculating the cutting precision data. 

FPC board cutting technology with an expansion rate of more than 0.8‰

According to the literature and the quality requirements of the PCB manufacturer, the key dimensions of the FPC board plug are the plug size and the distance between the plug and the board edge.When the positioning system USES the plug edge as the reference point for deformation correction calculation, it can reduce the deviation of plug size and edge distance caused by overexpansion of the circuit board (as shown in figure 2), so as to guarantee the precision of cutting. 

Figure 2 FPC plug cutting shape

Figure 2 FPC plug cutting shape

The positioning system of the laser cutter used in the experiment has a resolution of plus or minus 3 clearance m, which clearly identifies the boundary between the plug and the normal flexible sheet, providing a precise reference point for the workpiece's deformity correction compensation.As verified by the PCB production site, the new laser cutting technology can control the contour and size accuracy of FPC board with high expansion rate. 

FIG. 3 high precision cutting effect of plug

FIG. 3 high precision cutting effect of plug

conclusion

In this paper, the size deviation of the circuit board with different expansion rates was calculated by the laser cutter, and the measured data was analyzed. It was concluded that when the expansion and contraction of FPC board was greater than 0.8‰, the cutting accuracy could not be controlled within the size tolerance range of + / -0.05mm.In order to solve the problem of cutting precision of the circuit board with large expansion and contraction deformation, the new CCD system is applied in this paper to identify the new positioning reference point of plug, compensate the distortion and control the contour accuracy of the finished board.