Meaning
Electrodeposited metal layers often contain mechanical tension that develops during the crystallization process on a polymer substrate. This internal plating stress can be tensile, which tends to contract the layer, or compressive, which tends to expand it. When thermoplastic parts receive a metallic coating for conductivity or shield properties, high levels of this tension can lead to blistering or cracking of the thin metal film.
Controlling the chemical composition of the plating bath is critical to keeping this value low. Such mechanical tension can also cause the metallised plastic component to fail prematurely during thermal cycling in the field.
Stress Generation
Plating bath chemistry and deposition rate govern the mechanical tension in the metal layer. As metal ions deposit on the polymer substrate, grain boundaries form and create lattice mismatches that generate internal plating stress. Small organic additives are added to the bath to balance these forces.
Part Deformation
Excessively high mechanical tension causes thin-walled polymer housings to warp or bend. When the polymer substrate is thin, the force exerted by the metal layer overcomes the structural rigidity of the moulded part. Moulders can increase the part thickness or use polymers with higher modulus to resist this deformation.
Adhesion Threshold
When the tension within the metal layer exceeds the bond strength between the metal and the polymer, delamination occurs. This failure often starts at sharp corners where the stress concentrates during thermal cycling. Using a chemical etch step on the polymer before plating increases the mechanical interlocking and raises the adhesion limit.
