The background,

With the development of electronic devices towards miniaturization and portability, the assembly of electronic devices is becoming more and more intensive. Flexible circuit board, because of its advantages of small size, light weight and high circuit density, has gradually replaced the role of traditional wire in electronic device assembly. In recent years, the market share of flexible circuit board (FPC) in printed circuit board (PCB) has increased from less than 10% to more than 20%, which also proves the development of FPC market demand.

As the connecting line in electronic devices, FPC mainly plays the role of conducting current and transmitting signals. When signal transmission lines are distributed in the outermost layer of the FPC, in order to avoid signal distortion caused by electromagnetic interference in the signal transmission process, the FPC presses an integrated conductive layer (electromagnetic shielding film) after the covering film is pressed to shield the external electromagnetic interference. One of the most common is the DIGITAL camera as an image signal transmission FPC. As a transmission line, FPC usually has special impedance requirements, but the structure of FPC after pressing electromagnetic shielding film changes, and its impedance calculation method also needs to be modified. Therefore, this paper revises the impedance calculation method by studying the impedance change of FPC pressed electromagnetic shielding film, and provides reference for the engineering design of FPC pressed electromagnetic shielding.

2. Experimental design

1. Test materials

Pi-based non-adhesive sheet: PI thickness 2 mil, copper thickness 0.5/0.5 OZ;

Covering film: PI thickness 0.5~2 mil, adhesive thickness 25~35 μm;

Electromagnetic shielding film: conductive adhesive thickness of 10 um, PI thickness of 6-10 μm.

Testing and testing equipment and conditions

Quick press, network analyzer.

2. Test plate lamination

3. Test parameters

4. Results and discussion

Change in impedance of flexible plate

The flexible plate pressing the electromagnetic shielding film is usually double panel, the signal transmission line is distributed on the top layer, and the ground layer is the bottom layer. Double-sided flexible plate structure was used in the experiment. Based on the line impedance after etching, 25-50 Ohm single-ended line and 50-100 Ohm differential line were designed respectively. The impedance values of the line were tested successively after etching, CVL pressing and shielding film pressing, and the results were shown in Table 1.

After etching, the measured impedance value of the impedance line is only 0.5~3.0 Ohm different from the design value. In terms of percentage deviation, the impedance values of all impedance lines deviate from the design values by less than 5 %. For the double-sided flexible plate, the thickness of the base material and surface copper is relatively stable. Therefore, it can be said that the line width can be controlled through the etching process to achieve accurate control of the line impedance.

From the comparison of line impedance after different processes, it can be seen that the impedance values of CVL and electromagnetic shielding film on the etched line successively decrease, as shown in Table 2.

Compared with the impedance changes of double-sided flexible plates after CVL pressing and electromagnetic shielding film pressing, the impedance reduction after CVL pressing is much smaller than that after electromagnetic shielding film pressing. It can be seen that the impedance calculation mode after pressing electromagnetic shielding film is different from that after pressing CVL. Therefore, it is necessary to confirm the impedance calculation method of flexible plate after pressing electromagnetic shielding film.

4. Comparison of different impedance calculation models

At present, the impedance calculation of CVL pressed by flexible plates is mainly carried out by mode A in FIG. 1, and CVL is taken as the medium layer on the line. Based on previous research experience, the dielectric constant of pure rubber layer is 2.0 and that of PI layer is 2.7. The theoretical impedance after CVL pressing of double-sided flexible plate can be calculated through the line width/line distance measured in practice, and compared with the impedance measured in practice. The results are shown in Table 3.

The theoretical impedance of the flexible plate after CVL compression is very close to the actual measured value, and the deviation of both is within 1%. Thus, mode A in FIG. 1 can be used to simulate the theoretical impedance of flexible plate compression of CVL.

At present, there are two ways to calculate the theoretical impedance after the flexible plate presses the electromagnetic shielding film. One is to take the electromagnetic shielding film as A dielectric layer, that is, to use mode A in Figure 1 for calculation. In this mode, the circuit of the double-sided flexible plate is regarded as the structure of the outer microstrip line. Second, the conductive layer of the electromagnetic shielding film is regarded as a layer of copper skin, that is, mode B in FIG. 1 is used for calculation. In this mode, the circuit of the double-sided flexible plate is regarded as the structure of the inner ribbon line. The dielectric constant of the electromagnetic shielding film provided by the supplier is 68.3, the dielectric constant of the pure rubber layer is 2.0, and the dielectric constant of the PI layer is 2.7. Mode A and mode B in Figure 1 are adopted respectively, and the theoretical impedance is calculated according to the measured line width and line distance. The results are shown in Table 4.

When the mode A structure of outer microstrip line is used to calculate the theoretical impedance of the double-sided flexible plate pressed with electromagnetic shielding film, there is A difference of 8.81~36.10 % between the theoretical impedance and the measured impedance. However, the deviation between theoretical impedance and measured impedance is within 4% when the theoretical impedance is calculated by using mode B structure with inner band line. Compared with mode A, mode B is more accurate to calculate the theoretical impedance of double-sided flexible plate after pressing electromagnetic shielding. It can be seen that the theoretical impedance of the electromagnetic shield pressed by the double-sided flexible plate should adopt the structure of inner ribbon line, and the electromagnetic shield film should be regarded as a layer of copper foil.

5. Influence of PI/ thickness of covering film on impedance

When the inner band line structure is used for theoretical impedance calculation, the covering film becomes the dielectric layer on the circuit of the double-sided flexible plate. As can be seen from previous research experience, in order to make the theoretical impedance value of the double-sided flexible plate closer to the actual measured impedance value, PI and pure glue of the covering film are distinguished as different media respectively. Therefore, the change of PI thickness or pure adhesive thickness will also affect the impedance change of flexible plate. In CVL, PI thickness of 0.5~2.0mil and pure adhesive thickness of 15~35μm were compared. The results are shown in Table 5-6.

The theoretical impedance value 1 was uniformly calculated with PI dielectric constant of 2.7, and the deviation between the theoretical value and the measured impedance value of 0.5mil and 2mil PI was more than 5%. The dielectric constant of PI was adjusted according to the thickness of PI, and the dielectric constant of 0.5mil PI was 2.5, and that of 2.0mil PI was 3.1, respectively. The deviation between the calculated theoretical value 2 and the measured value was less than 3%. Therefore, the permittivity needs to be adjusted according to the PI thickness of different CVL.

As can be seen from the comparison of different pure adhesive thicknesses of CVL in Table 6, the deviation between the theoretical impedance value calculated by the dielectric constant 2.0 and the measured value of pure adhesive is less than 2.5%. In other words, CVL pure adhesive can be calculated according to a uniform dielectric constant.

In order to verify the correctness of the above analysis process, PI thickness and pure adhesive thickness of different CVL were used in the experiment, and the actual impedance value and theoretical impedance value were tested respectively. The results are shown in Table 7. It can be seen from Table 7 that the deviation between measured impedance and theoretical impedance of the flexible plate pressed with electromagnetic shielding film is less than 5%, that is, the theoretical impedance of the flexible plate pressed with electromagnetic shielding film can be correctly predicted through the above analysis.

Third, the conclusion

The following conclusions can be drawn from the above analysis:

For the laminated electromagnetic shielding film of flexible plate, the calculation of impedance value can be carried out by inner layer banded line structure.

The PI dielectric constant of CVL needs to be adjusted according to the thickness.