Meaning
Sputtered metal coatings provide an adhesive base for electroplated metals on low-energy polymer surfaces. When plastic parts like polyetherimide or liquid crystal polymers are metallised, a titanium seed layer is deposited first because of its high chemical affinity for both polymers and copper. This extremely thin layer prevents the subsequently plated metal from peeling or blistering under mechanical load.
It is usually deposited via physical vapour deposition before thicker conductive coatings are applied. In microwave waveguides, this adhesive layer must be kept as thin as possible to avoid increasing the RF attenuation of the signal.
Interface Adhesion
Titanium bonds chemically with the carbon and oxygen atoms in the polymer chains. When a titanium seed layer is formed, it creates a stable carbide and oxide transition region at the interface. This chemical bond provides much higher peel strength than the weak mechanical interlocking achieved on untreated plastic surfaces.
Deposition Control
The thickness of this initial metal layer must be monitored to avoid high internal stress. If the titanium seed layer is too thin, it may not cover the surface completely, which leads to plating gaps and poor conductivity. Conversely, an overly thick layer can crack under thermal cycling due to mismatched thermal expansion coefficients.
Thermal Stability
Acting as a diffusion barrier, the titanium prevents copper from migrating into the polymer substrate. During thermal stress or high-power RF transmission, this barrier ensures that the metal-polymer bond remains intact and stable. This prevents long-term degradation of the plated component in harsh operating environments.
