With the advent of the 5G era, the demand for high-frequency and high-speed communication equipment is becoming increasingly prominent. As a printed circuit board ( PCB ) substrate material for lapped semiconductor devices, its high-frequency and high-speed development is particularly important. The skin effect of high-frequency and high-speed signal transmission will be more serious, that is, the signal will be more concentrated on the surface layer of the copper conductor, which means that the influence of the surface roughness of the electronic copper foil on the signal loss will increase exponentially. Therefore, compared with conventional electronic copper foils, high-frequency and high-speed electronic copper foils need to develop ( ultra ) low-roughness surfaces to reduce signal loss. However, the decrease of roughness will inevitably lead to the decrease of the bonding force of the copper foil / resin interface in the PCB substrate, which is difficult to meet the requirements of industrial applications. Therefore, how to reduce the surface roughness of copper foil while ensuring good interface adhesion is one of the key issues in the research of high frequency and high speed PCB substrate.

The bonding force of copper foil / resin interface in PCB substrate mainly comes from two aspects : one is the mechanical anchor provided by the rough outline of copper foil surface, and the other is the chemical bonding between copper foil and resin. Usually, the silane coupling agent that can connect the inorganic / organic interface is used to achieve effective chemical bonding force. As far as the conventional PCB substrate is concerned, the surface roughness of the copper foil is large, the interface bonding force is mainly provided by the mechanical anchor cooperation, and the chemical bonding plays an auxiliary role. For high-frequency and high-speed PCB substrates, the roughness of copper foil is greatly reduced, and the mechanical anchor cooperation is reduced. In order to meet the requirements of interface bonding force, measures must be taken to increase the chemical bond cooperation and reduce the dependence on roughness. The measures to improve the chemical bonding are generally divided into two aspects, including the development of new interfacial binders, such as modified silane coupling agents ; the other is to directly modify the substrate ( usually resin ) to increase its surface chemical activity. Compared with the development of new adhesives, this method of directly modifying the substrate by physical or chemical methods is simpler and has lower industrialization cost. The methods of direct modification of resin surface to improve metal / resin adhesion include chemical etching, laser treatment, ultraviolet irradiation, plasma treatment and so on. Among them, plasma surface treatment has the advantages of less pollution, convenient operation, and little influence on the overall performance and strength of the matrix. It is a promising surface treatment method. In recent years, plasma treatment has also been gradually used to improve the interfacial bonding properties of copper foil / resin.

Surface morphology and chemical state of plasma treated resin

The bonding force of copper foil / resin interface mainly comes from two parts, mechanical anchor cooperation and chemical bond cooperation provided by roughness. In order to further improve the bonding force between low roughness copper foil and resin and make it meet the requirements of industrial application, the plasma treatment of resin semi-cured sheet is added to the conventional process route. In this paper, the surface morphology and surface chemical state of epoxy semi-cured sheet and polyphenylene ether semi-cured sheet after plasma treatment are studied.

Figure 1 is the SEM image of the surface morphology of epoxy resin semi-cured sheet and polyphenylene ether resin semi-cured sheet before and after plasma treatment. It can be seen from Fig.1 that the surface morphology of the two resin semi-cured sheets did not change significantly after plasma treatment. It is speculated that the surface changes may occur more in the surface chemical state.

Fig.1 SEM images of resin semi-cured sheet surface before and after plasma treatment

In order to further analyze the change of surface chemical state, the epoxy resin and polyphenylene ether resin semi-cured sheets before and after plasma treatment were characterized by XPS. Fig.2 shows the XPS spectra of the surface of the epoxy resin semi-cured sheet before and after plasma treatment. Among them, Fig.2a and Fig.2d are the XPS full spectrum, and the corresponding element content is shown in Table 2. It can be seen from Fig.2 that the surface of the untreated epoxy resin shows strong C1s and O1s peaks but no N1s peak. After plasma treatment, the 01s peak is further enhanced, and the N1s peak is obvious. From Table 2, it can be seen that the content of element 0 on the surface of epoxy resin treated by plasma increases significantly, and the O / C value increases from 0.140 to 0.254. The above analysis shows that more O and N groups are generated on the surface of epoxy resin after plasma treatment, and the surface polarity is enhanced, which is beneficial to the improvement of bonding strength. Fig.2b and Fig.2e are the fine spectra of C1s after peak fitting. It can be seen that the untreated epoxy resin surface is mainly composed of two main peaks : C-C / C-H and C-O. After plasma treatment, the intensity of C-O peak decreased, and a new O-C = O peak appeared, that is, a large number of strong polar groups COOH were generated. Correspondingly, the O1s fine spectra in Figure 2c and Figure 2f show that the content of O-C bonds on the epoxy surface after plasma treatment is reduced, and O = C ( corresponding to COOH groups ) is generated in large quantities. The above analysis shows that after plasma treatment, a large number of active groups such as COOH increase, and the chemical activity of the epoxy resin surface is enhanced, which will be beneficial to the improvement of chemical bonding between epoxy resin and copper foil.

Fig.2 XPS spectra of epoxy resin surface

Compared with epoxy resin, polyphenylene ether is a kind of surface ' chemical inert ' resin, which means that it is more difficult to produce good chemical bonding with the surface of copper foil, and the adhesion is poor. Therefore, it is of great significance to study the effect of plasma treatment on the surface chemical state of polyphenylene ether resin. Fig.3 shows the XPS spectra of the surface of the polyphenylene ether resin semi-cured sheet before and after plasma treatment. From the XPS full spectrum 3a, 3d and table 2, it can be seen that the O / C value of the surface of the polyphenylene ether after plasma treatment has increased, and the N1s signal has been significantly enhanced, that is, the surface has generated more O and N groups, and the chemical reactivity has been enhanced, which will be beneficial to the improvement of the bonding strength. Fig.3b and Fig.3e are the fine spectra of C1s after peak fitting. It can be seen that the surface of untreated polyphenylene ether resin mainly shows the main peaks of C-C / C-H, C-O, C = O and O-C = O ; after plasma treatment, the C = O peak disappeared and the C-O peak intensity was enhanced. Fig.3c and Fig.3f are the fine spectra of O1s.After plasma treatment, the content of O-C and O = C bonds on the surface of polyphenylene ether increased. The above analysis shows that the content of active groups COOH and C-OH on the surface of polyphenylene ether resin after plasma treatment increases, the chemical activity of the surface increases, and the chemical reactivity between the surface and copper foil also increases.

Fig. 3 XPS spectra of the surface of polyphenylene ether resin

Plasma treatment can effectively improve the bonding properties between low roughness copper foil and epoxy resin and polyphenylene ether resin. After plasma treatment, the surface morphology of epoxy / polyphenylene ether resin has no obvious change, and the surface chemical state has changed significantly, which is mainly reflected in the increase of O / C content ratio, the emergence of N element and the increase of surface active groups such as COOH and COH. The improvement of bonding strength comes from the enhancement of surface chemical activity of epoxy resin after plasma treatment, which provides a stronger chemical bond for copper foil / resin interface.