Hard chrome plating is a surface treatment process applied to rigid boards (often metal substrates or metal-clad PCBs) to enhance hardness, wear resistance, corrosion resistance, and surface smoothness. This electrolytic process deposits a layer of chromium (Cr) onto the substrate, creating a durable surface that extends the board’s lifespan in harsh operating environments.

The process begins with thorough surface preparation, which is critical for adhesion and coating quality. The substrate (typically steel, aluminum, or copper alloys) is cleaned to remove oils, oxides, and contaminants. This involves degreasing with solvents or alkaline cleaners, followed by acid etching (e.g., with sulfuric or hydrochloric acid) to create a micro-rough surface that improves chromium adhesion. For aluminum substrates, a zincate treatment is often used to promote bonding between the aluminum and chromium layers.

The plating bath is a key component, consisting of chromic acid (CrO3) and sulfuric acid (H2SO4) in a specific ratio (usually 100:1 by weight). The substrate is immersed in the bath as the cathode, while lead or lead-alloy plates act as anodes. When an electric current is applied (typically 20–60 A/dm²), chromium ions are reduced at the cathode surface, forming a hard, crystalline layer. The thickness of the chrome layer varies based on application—wear-resistant applications may require 5–50 μm, while decorative or corrosion-resistant coatings can be thinner (1–5 μm).

The properties of the hard chrome layer are highly desirable for rigid boards. It has a hardness of 800–1000 HV (Vickers), significantly higher than the substrate material, making it resistant to abrasion and indentation. This is beneficial for boards used in industrial machinery or automotive applications, where they may come into contact with moving parts or debris. The chromium layer also provides excellent corrosion resistance, as it forms a passive oxide layer (Cr2O3) that prevents further oxidation of the underlying metal. Additionally, the smooth, low-friction surface reduces the buildup of dirt and contaminants, making the boards easier to clean and maintain.

Post-treatment steps ensure optimal performance. After plating, the board is rinsed to remove residual chemicals, then dried. Some applications require grinding or polishing to achieve a specific surface finish (e.g., mirror-like for decorative purposes). Quality control involves measuring thickness with X-ray fluorescence, testing adhesion with bend or scratch tests, and inspecting for defects like cracks, pits, or peeling, ensuring the coating meets industry standards such as ASTM B117 for corrosion resistance.