In mechatronics technology, plastic housings are used to protect components from harmful environmental influences, such as moisture, media, or reactive substances. Connections, preferably in the form of metallic lead-frame structures, through the housing wall, are required for their electrical supply or control. In this system, an insufficient connection between the plastic component and the metal surface due to the incompatibility between the two materials is often observed. In industrial applications and due to the thermal-induced stress, cracks in the polymer can be formed leading to leakage. Therefore, an adhesion promoter is implemented in the metal-polymer interface. This layer should flow into micro-damage such as interface, cracks, inhibit them from growing and close them. Based on that self-healing mechanisms can be created. The selected primer is an epoxy form DELO (DUALBOND LT2266). It is an adhesion promoter with two reaction stages. In these, the first stage provides fixation to the lead frame directly after the coating step. This can be done within seconds using UV light (UV light with 365 nm). In the second stage, the material will be thermally hardened during the injection moulding process.

In this study, the primer fluidity and ability to fill micro-cracks are investigated. For these characterizations, a pressing test was carried out using a cylindrical piston with a pressing ram. Several pistons were produced with different die diameters. The pistons and the ram were installed in a universal testing machine. The tests were done on completely cured primer at different temperatures and with different pressing speeds (100, 200, 300, 400 μm/min). Fig. 1 shows the resulting stress-strain curves from the pressing test at 150 °C and with a ram speed of 100 μm/min.

Furthermore, the primer capability to fill micro-cracks has been characterized using negative polydimethylsiloxane (PDMS) templates with micro-meter holes. The PDMS stamps could be moulded negatively using a micro-structured silicon wafer (see Fig. 2 left). Using the PDMS templates and based on the stamping process, the ability of the primer (partially cured strips used as specimens) to fill the template holes was characterized. The characterizations were done on specimens with different degrees of curing. The filling ability of the primer is then estimated by the filling depth which is here equal to the height of micro-structured of the primer surface. Using white light interferometry, a 3D-micro-profile of the primer micro-structured surface could be generated and the heights were measured (Fig. 2 right).

Finally, the penetration behavior of the selected primer in defined capillaries made in Ultramid was investigated. Holes with a diameter of 0.3 mm were produced in injection-moulded A3EG7 plates with a thickness of 4 mm. A substrate coated with an adhesion promoter was placed on the test specimen with pressing forces determined by the piston pressing test and at different temperatures. Metallographic analyses were carried out to verify the filling grade (Fig. 3).

Due to the mismatch in the coefficient of the thermal expansion (CTE) in a hybrid system among the copper and the polymer, the stress is introduced in the system upon heating and cooling down, which should be the driving forces for the self-healing mechanisms. In this study, the ability of the selected primer to flow and fill micro-cracks as long as it exposed to external impulse is proven meaning the primer can flow and fill the formed cracks in the metal-plastic system under the induced thermal stress.