Today's society has entered a highly information-based society. With the coming 5G era, driverless, mobile communication equipment, Internet and other extensive applications, the demand for high-frequency printed circuit boards has also increased. In the resin used for high-frequency and high-speed materials, PTFE had a market share of over 90% during its peak period and has been widely used in the high-frequency field. However, PTFE has strong inertness, low surface energy, and hydrophobicity, making it difficult for the surface to be hydrophilic. The interlayer electrical interconnection of multi-layer high-frequency printed circuit boards is achieved through metalized holes, which is particularly important for PCB to achieve special surface functions. The reliability of metalized holes directly affects the success or failure of PCB. If chemical copper plating or conventional potassium permanganate pretreatment processes are directly used for PTFE materials, it is difficult to change their surface structure, resulting in problems such as copper free pores and poor copper adhesion in pores. At the same time, due to the softness of PTFE resin, its ability to resist elastic deformation is poor, and its coefficient of thermal expansion is high. Therefore, ceramic fillers with higher hardness are often added to it to reduce Z-direction expansion and increase the reliability of PTFE sheets. This undoubtedly increases the difficulty and complexity of de gluing and hole making. At present, sodium naphthalene treatment and plasma treatment are mainly used in the industry for chemical copper deposition pretreatment of polytetrafluoroethylene circuit boards. The sodium naphthalene treatment method is achieved by etching the surface atoms of PTFE with sodium naphthalene solution to activate and wet the PTFE surface. However, this method of treating PTFE will cause its surface to turn black and dark, affecting the appearance of the material. At the same time, it poses certain risks to operators and can easily cause pollution to the environment. Compared with sodium naphthalene treatment, plasma treatment can change the surface of PTFE without affecting its physical and chemical properties. At the same time, this method is easy to operate and environmentally friendly. With the strengthening of environmental and safety awareness, plasma treatment will undoubtedly be an advanced and promising treatment method compared to sodium naphthalene treatment.

This article uses O2/CF4 combined plasma gas to treat PTFE high-frequency plates after drilling. The effect of O2/CF4 combined plasma treatment on through holes was studied through metallographic sectioning, thermal stress testing, contact angle testing, and XPS testing.

The influence of O2/CF4 combined plasma treatment on through holes

This experiment studied the effect of O2/CF4 combined plasma treatment on through holes through metallographic sectioning, thermal stress testing, contact angle testing, and XPS testing. The plasma treatment conditions were O2 flow rate of 300mL/min, CF4 flow rate of 200mL/min, treatment time of 30min, treatment power of 8000W, treatment temperature of 35 ℃, and system vacuum degree of 35Pa.

Cross section &Microscope 

Using a metallographic microscope to examine the cross-sectional morphology of copper plated holes and investigate the effect of O2/CF4 plasma on hole reliability. Figures 1-1 (a) and 1-1 (b) show metallographic microscope images of untreated and plasma treated pore samples, respectively. When there is no plasma treatment before copper plating through the through-hole, and the copper plating is completed, it can be observed that the hole wall has a rough contour, and some areas are not coated with copper. On the contrary, as shown in Figure 1-1 (b), after plasma treatment, the through-hole wall achieved relatively low hole roughness, and copper was uniformly plated on the PTFE drilled hole wall.

Figure 1-1 Cross sectional morphology of pores without plasma treatment (a) and plasma treated pores (b)

Thermal Stress

Figure 2-1 shows the thermal reliability test results of PTFE copper plated holes with and without plasma treatment. After three 288 ℃/10s thermal stress experiments (IPC-TM-650), unfilled tin and copper cracks, copper film peeling, and blasting were found in the holes in Figure 2-1 (a). On the contrary, samples with plasma treatment exhibit higher reliability, as shown in Figure 2-1 (b), where the surface morphology of the pore wall remains well preserved without any structural damage. Therefore, appropriate parameters of O2/CF4 plasma treatment are necessary steps to obtain copper plated active surfaces on PTFE boreholes.

Figure 2-1 Cross sectional view of PTFE pore reliability inspection for samples without plasma treatment (a) and plasma treatment (b)

Contact angle analysis

The contact angle test results show that the lower surface roughness of PTFE pore walls effectively improves the copper plating effect on the pore wall surface, indicating that plasma treatment increases the surface activation effect. From Figure 3-1, it can be seen that the surface contact angle of untreated PTFE decreased from 126.6 ° to 106.1 ° for PTFE treated with plasma, indicating a significant improvement in the wettability of the PTFE surface.

Figure 3-1 Water contact angle on untreated PTFE surface (a) and water contact angle on plasma treated PTFE surface (b)

XPS analysis

Figure 4-1 shows the XPS spectra of PTFE surface before and after plasma treatment to detect changes in chemical functional groups. C, F, and O elements were found in untreated and plasma treated PTFE samples, and the functional groups formed by these elements indicate activation changes on the PTFE surface.

Figure 4-1 XPS spectra of PTFE surface before and after plasma treatment

Figure 4-2 C1s spectra of untreated PTFE surface (a) and plasma treated PTFE surface (b)

Figure 4-3 O1s spectra of untreated PTFE surface (a) and plasma treated PTFE surface (b)

Figure 4-2 shows the C1s spectra of PTFE surfaces with and without plasma treatment, respectively. In Figure 4-2 (a), three peaks attributed to C-O, CF2-CF2, and C-C/C-H groups were observed at 286.0 eV, 292.2 eV, and 284.7 eV, respectively, for the untreated sample. Hydrophobic groups, including CF2-CF2 and C-C/C-H, make up the majority of PTFE surfaces, making it difficult to copper coat them. However, as shown in Figure 3-2 (b), plasma treatment of PTFE surface clearly increased a large number of hydrophilic groups, such as C=O, CF3, COO/COF, which were detected at the peaks of 287.9 eV, 293.0 eV, and 289.0 eV in XPS spectra, respectively.

In addition, Figure 4-3 shows the O1s spectra of samples with and without plasma treatment. As shown in Figure 4-3 (a), OH (532.6 eV), CO (533.2 eV), Al-O (531.3 eV), Si-O (531.9 eV), and Ca-O/Mg-O (530.6 eV) were observed in the untreated sample. It should be noted that the detected Al, Si, Ca, and Mg elements come from reinforced glass fibers and filler particles. As shown in Figure 4-3 (b), new peaks were observed in the O1s spectrum of the plasma treated sample at binding energies of 531.6 eV, 532.6 eV, and 533.5 eV, representing chemical groups belonging to C=O, COO, and OF, respectively.

From the above results, it can be seen that after O2/CF4 plasma treatment, chemical hydrophilic groups such as CO, COO, COF, and OF were generated on the PTFE surface. After plasma treatment, it significantly improved the wettability of the inactive PTFE surface.

Through a comparative study of PTFE high-frequency plate holes treated with and without O2/CF4 plasma, it was found that plasma treatment is an effective method to improve the metallization quality of PTFE high-frequency plate holes after electroplating and observation of through-hole slices and thermal stress experiments. After contact angle testing, the surface contact angle of PTFE high-frequency plate was effectively reduced from 126.6 ° to 106.1 °, indicating that the wettability of PTFE surface was enhanced after plasma modification. XPS measurements confirmed the formation of new polar functional groups such as CO, COO, COF, and OF on the pore wall surface, enhancing the wettability of PTFE surface and improving copper adhesion. Plasma treatment is an effective method to improve the quality of PTFE high-frequency plate pore metallization.