With the development of communication technology, electronic equipment is developing towards the direction of wearable and flexible. Flexible Printed Circuit Board ( FPC ), as the carrier of its electrical interconnection, is playing an increasingly important role. Copper foil is widely used in the current research of electronic field due to its excellent properties such as good conductivity and ductility. As an advanced polymer material, liquid crystalline polymer ( LCP ) is widely used in the research of flexible printed circuit board substrates. At present, the most commonly used flexible films in FPC include polyethylene terephthalate ( PET ), polyimide ( PI ) and liquid crystal polymer ( LCP ). Compared with PET and PI, LCP has better heat resistance, lower water absorption, lower dielectric constant and dielectric loss factor. However, the use of LCP in FPC is hindered by its relatively serious chemical inertia, which leads to many challenges in the process of direct bonding with copper foil without glue. Traditional chemical etching is difficult to use due to its chemical inertia. The plasma consists of ions, electrons, neutrons and free radicals, which react with LCP to change its surface morphology and group composition, making it easy to combine with copper foil.

In this paper, the effect of oxygen plasma treatment power on the final peel strength in the combination process of high-frequency LCP and copper lamination was investigated. Figure 1 is a schematic diagram of the experimental process in this paper.

Fig.1 Experimental process diagram

The experiment is divided into three parts. The purpose of the first part of the experiment is to increase the holes on the LCP surface and improve its hydrophilicity. The purpose of the second part of the experiment is to oxidize the surface of the copper foil and then coat the silane coupling agent. One side of the silane coupling agent group reacts with the oxygen-containing group on the metal surface, and the other side of the organic group reacts with the LCP surface treated in the previous part. The third part of the experiment is baking hot pressing.

Experimental Results and Discussion

Fig.2 is the final product peel strength test result diagram under different plasma power treatment. It can be seen from the figure that compared with the samples without plasma treatment, all the groups with plasma treatment produced peel strength, while the LCP film without oxygen plasma treatment could not produce peel strength even if it was hot pressed with the silane coupling agent coated in the subsequent steps, which proved that the initial single-panel LCP surface was relatively stable and difficult to combine with other substances.

Fig.2 Effect of different oxygen plasma treatment conditions on the peel strength of the final multi-layer plate

Among the 7 groups of experimental samples treated with different plasma power, the peel strength of the 5kW and 6kW power groups reached 8.26 / cm and 8.92 / cm, respectively, which exceeded the 8N / cm in the industrial application standard and achieved the effect of practical application. In addition, the peel strength of the sample group treated with 4kW plasma power also reached 6.88 / cm, which was close to the industrial standard. In the process of plasma treatment power increasing from 2kW to 8kW, the peel strength of the corresponding products generally increases first and then decreases. Fig.3 shows the photos of the multi-layer plates before and after the peel strength test. It can be seen that some of the fractures occurred in the LCP layer during the peel strength test. It can be seen that the bonding force between the LCP and the copper layer has reached a fairly strong degree.

Fig.3 Effect of different oxygen plasma treatment conditions on the peeling strength of the final multi-layer plate 

( a ) Before the peeling strength test ;  ( b ) After peel strength test

Fig.4 is the comparison of SEM images of LCP surface morphology under 4kW, 6kW, 8kW plasma treatment power and untreated initial material. It can be seen from the figure that Figure ( a ) is the untreated initial material, the surface is relatively flat, and there is no obvious hole. Compared with the other three treated figures, the LCP material without plasma treatment is relatively smooth. By observing the difference between Fig ( b ), Fig ( c ) and Fig ( d ), it can be seen that the surface of LCP treated by plasma has holes of different sizes and densities. Among them, the 8kW treatment group ( Fig ( d ) ) is the most obvious, with very dense holes, while the 4kW ( Fig ( b ) ) and 6kW ( Fig ( c ) ) groups have a significant increase in the number of holes on the surface of LCP without treatment. It can be seen that by treating the LCP surface with different powers of pure oxygen plasma, the voids on the surface of the material can be increased, which in turn has a significant effect on the peel strength of the multilayer composite product after hot pressing.

Fig.4 Surface morphology of LCPSEM under different oxygen plasma treatment conditions 

( a ) Raw materials ; ( b ) 4kW plasma treatment ; ( c ) 6kW plasma treatment ; ( d ) 8kW plasma treatment

Figure 5 is the experimental results of the water contact angle of the LCP surface with the untreated initial material under the plasma treatment power of 2kW, 4kW, 6kW, and 8kW. From the experimental results, it can be seen that the surface water contact angle of the LCP initial material without plasma treatment is 81.0 °, and its hydrophilicity is poor. When the plasma power was gradually increased to 2kW, 4kW, 6kW and 8kW, the water contact angles of the treated LCP surface were 68.1 °, 60.7 °, 56.4 ° and 39.3 °, respectively. The higher the power of oxygen plasma treatment, the greater the improvement of the wettability of LCP surface. This is because in the process of plasma treatment, the excited oxygen plasma increases the number of hydrophilic polar groups on the surface of the material. In the subsequent hot pressing step, the polar groups produced by the oxygen plasma are easy to produce hydrogen bonds in a similar way with the-NH2 groups of the silane coupling agent, so that the LCP surface and the oxidized copper surface produce a strong peeling strength.

Fig.5 Water contact angle test results of LCP surface under different oxygen plasma treatment conditions 

( a ) raw materials ; ( b ) 2 kW equidistant; sub-treatment ; ( c ) 4kW plasma treatment ; ( d ) 6kW plasma treatment ; ( e ) 8kW plasma treatment