1. Root Cause of High-Current Burn
At high cathode current density, nickel ions deplete rapidly near workpiece surface. Fast metal deposition generates coarse, burnt, black brittle nickel with severe edge burning, poor coverage on recessed areas. Overdosed brighteners worsen this issue by accelerating uneven crystal growth.
2. Core Mechanism of SAS to Eliminate Burning
Uniform Cathodic Adsorption
Sodium Allyl Sulfonate (SAS) carries negatively charged sulfonate groups that evenly adsorb across all workpiece surfaces, including sharp edges with high current load. It moderately slows nickel deposition rate at high-current zones, balancing deposition speed between edges and concave low-current regions.
Grain Refining & Throwing Power Boost
SAS refines nickel crystal grains into tiny compact equiaxed crystals, replacing rough dendritic burnt deposits. It raises bath throwing power, replenishes nickel ion transport to high-current areas and prevents localized metal ion depletion.
Relieve Excessive Cathodic Polarization
Strong secondary brighteners create ultra-high polarization leading to rapid, uncontrolled metal precipitation on edges. SAS acts as a mild carrier brightener to moderate polarization, avoiding over-fast deposition and black burn marks.
3. Proper Dosage to Prevent Burn
Pure SAS target concentration: 0.8–1.6 g/L in bath
35% liquid SAS dosage: 2.3–4.6 mL/L per liter plating solution
- Under-dosing: Weak coverage, obvious edge burning
- Over-dosing: Hazy nickel coating, slightly increased internal stress
4. Auxiliary Operation Tips
- Keep bath temperature 50–60℃ and gentle air stirring to accelerate nickel ion circulation
- Control secondary brightener dosage to avoid over-polarization
- Conduct regular carbon filtration to remove decomposed organic impurities that damage bath performance
