In two-component injection moulding (2-shot injection moulding), the circuit carrier is produced in two successive injection moulding processes. For this purpose, two different plastic components are used, one of which can be metallised and the other cannot. This allows the structures specified by the mould to be metallised in a self-defining manner.
The short process times are a particular advantage of the two-shot injection moulding process. This is why it is often the more economical alternative in projects with high quantities. In terms of 3D design, two-shot injection moulding MIDs are also more flexible. Conductor paths are also possible, for example, in cavities that a laser would not reach.
At the beginning of two-component injection moulding, a pre-moulded part is produced, which is then directly overmoulded with the second component. The mechanically more stable component is produced first. Whether the metallisable or non-metallisable component in the product is the more stable one depends on the geometry of the component. Since the circuit layout in 2-shot injection moulding technology is based on the geometry of the injection mould, there are only limited possibilities for further miniaturisation.
In this process, a wet chemical pre-treatment produces the micro-roughness of the surface and the exposure of the catalysts. In general, a basic solution – e.g. hot potassium hydroxide solution – is used. The adhesion of the metallisation is defined by the surface roughness. However, if the surface roughness is too pronounced, it is possible that this will negatively affect the bonding technique. During the pre-treatment process, only the surface is exposed. In the subsequent neutralisation step, an acidic solution removes the adhering residues of the pretreatment media and the detached plastic particles. In addition, non-core catalytic materials require germination, e.g. with Pd.
In 2-shot injection moulding technology, the metallisation is also carried out by means of chemical-reductive metallisation baths without external current. The layer structure, the layer thickness and the process control are comparable to those of the LDS technology.
After metallisation, the circuit carrier is inspected using AOI (Automatic Optical Inspection). The process is the same as for laser direct structuring.
In 2-shot injection moulding technology, special material combinations are used. A catalyst, such as Pd or Fe, is compounded into one of the two materials. To enable selective metallisation, the materials used must have different behaviour in the activation and materialisation process.
For series applications, the material LCP is used almost exclusively. Here, LCP Vectra® E820i Pd serves as the metallisable component. The non-metallisable component consists of LCP Vectra® E130i.
In principle, other material combinations are available. For example, polycarbonate/acryilnitrite-butadiene-styrene as the metallisable component and a combination of polycarbonate as the non-metallisable component. However, such combinations are only suitable for low-cost applications that are not soldered. Basically, it is important that both plastics used have almost identical coefficients of thermal expansion.
Basic rules of design
In contrast to laser direct structuring, the conductor paths in 2-shot injection moulding technology are defined by the injection moulding process. The achievable widths and distances are therefore very much dependent on the material properties and the geometry of the moulded part.
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