Improvement of copper adhesion on polyimide films

The initial situation

Polyimide / PI (e. g. Kapton ®) is an advantageous substrate material for flexible printed boards, ribbon cables and multilayer PCBs because it combines mechanical strength, temperature and chemical stability with good dielectric properties. For these reasons, PI has become increasingly widely used in recent years, especially in miniature devices such as mobile phones, in automobile electronics, aerospace electronics etc..

High demands are placed on the reliability of PCBs used in these applications, many of them being safety related. In particular, the copper conductors must adhere firmly to the polyimide to withstand the thermoshock from the soldering during the PCB assembly as well as the stresses from the vibrations, shocks, temperature and humidity fluctuations during the device exploitation.

Currently, copper-polyimide laminates are made almost exclusively by adhesive bonding. The tendency towards using ever thinner PCBs means however that the adhesive makes up an ever higher proportion of the overall mass of the PCBs, which results in worsening of the electrical and mechanical properties of the system. Other methods of manufacturing copper-polyimide laminates are therefore becoming increasingly interesting.

One technology that is particularly suitable for making thin laminates is the vacuum deposition (PVD) of a thin (a few hundred nm) copper layer, followed by galvanic reinforcement. Using a semi-additive method, only the conductors are galvanically reinforced, leading to both economic and technical advantages (improved resolution). Previously, sufficient adhesion could only be achieved by using a chromium PVD intermediate layer. This technology was not widely accepted by the industry, as the chromium layer requires an additional etching process during the PCB production.

Development goals

The goal is to chemically modify the surface of the polyimide in such a way that it forms a chemical compound with metallic copper, thereby resulting in a dramatic adhesion improvement. Compared with wet chemistry, the plasma-chemical modification of the surface has the advantage , that this process can be combined with PVD copper deposition in a single unit and from roll to roll. In addition, the chemicals consumption is low and the waste disposal unproblematic.

The procedure

In order to realize this vision, three Fraunhofer Institutes have pooled their competences. These are IGB (plasma-chemical processes, analytics), IPA (vacuum deposition of copper) and IZM (galvanic reinforcement, adhesion tests, analytics).

To achieve an adhesion improvement, a very thin layer of a nitrogen-containing plasma polymer is deposited onto the surface of polyimide films, which binds copper chemically via complex building. After this a thin copper layer is deposited through PVD, and then copper galvanically reinforced. Finally, the layers are characterized and the copper adhesion is measured.

Results

Very good adhesion values have been achieved (10 to 14 N/cm in the peel test). The figure on the left shows a SEM photo of the copper side of the peeled interface. The surface has a homogeneous appearance. According to the EDX analysis, the ratio C/Cu is 7.9. After the copper deposit has been peeled off, a thin polymer layer remained on the copper surface, prooving that the break did not take place between the metal and polymer (which is the case with untreated Kapton), but in the polymer itself (cohesive failure). A strong adhesion was also retained following the climate test: 24 hours at 85°C and 85% relative humidity did not cause any significant reduction in adhesion. After one week under the same conditions, the adhesion value was still over 6 N/cm, while the copper deposited onto untreated polymer could be easily removed. In the latter case, the copper was oxidized on its interface with the film (it became dark). In contrast, in the case of the plasma-treated film there was no oxidation apparent, indicating a strong chemical bond between the metal and the plasma polymer.

The treatment time was reduced to a few seconds by optimizing the process control. This makes plasma treatment compatible with PVD copper coating. In addition to polyimide as a film material, other synthetic materials of interest to copper plating are also being investigated. The adhesion of copper to Hyflon, a fluoro-polymer, could be increased to over 4 N/cm.