How to do the surface treatment of electronic product hardware stamping?
The surface treatment of metal stamping parts of electronic products should give consideration to corrosion resistance, conductivity/insulation performance, appearance accuracy and assembly compatibility, and the process should be selected according to materials (such as stainless steel, copper, aluminum and cold-rolled steel) and application scenarios (such as internal structural parts, external decorative parts and conductive contacts). The following are the main surface treatment methods, applicable scenarios and operation points:
A, basic protective treatment (corrosion, rust)
This kind of technology is mainly used to improve the environmental resistance of stamping parts and is suitable for internal structural parts.
Passivation treatment
Applicable materials: stainless steel (304/316), aluminum alloy, galvanized sheet.
Process principle: A dense oxide film (such as chromium passivation film of stainless steel and oxide film of aluminum alloy) is formed on the metal surface by chemical or electrochemical methods to isolate air and moisture.
Key points of operation
Stamping parts need to be degreased and pickled first to remove surface oil stain and scale;
Stainless steel is usually passivated by chromate or environment-friendly chromium-free passivation (in line with RoHS standards), and aluminum alloy can be passivated at room temperature for 5-10 minutes;
Rinse with deionized water after passivation, and control the drying temperature at 80-100℃ to avoid cracking of passivation film.
Advantages: low cost, no thickness increase, no influence on the dimensional accuracy of stamping parts, and the neutral salt spray resistance test can reach more than 200 hours.
Galvanizing treatment
Applicable materials: cold-rolled steel and low-carbon steel (internal structural parts)
Process type: electro-zinc plating (acidic zinc plating, alkaline zinc plating) and hot-dip zinc plating (less used in electronic products, thicker coating).
Key points of operation
The thickness of electro-galvanized coating is controlled at 5-15μm, which can be passivated with colored zinc and black zinc to improve corrosion resistance;
Key requirements: the coating is uniform and pinhole-free, and the adhesion of the coating should be ensured for the conductive structural parts (cross-check test ≥4B level) to avoid falling off during assembly.
Advantages: high cost performance, strong rust prevention ability, and certain conductivity.
Second, the conductive/shielding treatment (meet EMC)
Stamping parts of electronic products often need conductive or electromagnetic shielding function, which is suitable for shielding covers, conductive shrapnel and connector terminals.
Nickel plating
Applicable materials: copper, copper alloy, steel.
Process characteristics: It is divided into dark nickel and bright nickel, and the coating thickness is 3-8μm, which has good conductivity, wear resistance and corrosion resistance.
Key points of operation
It is necessary to activate before plating to remove the surface oxide layer;
Control the pH value (4-6) and temperature (50-60℃) of the plating solution to avoid pinhole and pitting defects;
Contact resistance (≤50mΩ) should be detected after nickel plating on the shield to ensure the electromagnetic shielding effect.
Gold plating treatment
Applicable materials: copper terminals, precision conductive contacts.
Process advantages: excellent conductivity, oxidation resistance, wear resistance and stable contact resistance, suitable for high-reliability electronic products (such as mobile phone connectors and sensor terminals).
Key points of operation
Using flash nickel primer+gold plating process, the thickness of nickel layer is 2-3μm, and the thickness of gold layer is 0.1-1μm (thick gold is used for wear-resistant scenes according to requirements);
The impurity content of gold plating solution should be strictly controlled to ensure that the coating is bright and color-free;
Comply with RoHS and REACH standards, and avoid using cyanide-containing gold plating solution.
Tin plating treatment
Applicable materials: copper bars, welding terminals.
Process type: hot dip tin, electroplating tin, coating thickness of 8-20μm, with good weldability and conductivity.
Key points of operation
Hot melt treatment (temperature 230-250℃) is needed after tin plating to eliminate internal stress of the coating and prevent tin whisker growth (to avoid short circuit risk);
Stamping parts for welding should ensure that the coating is free of oxidation and the surface tension meets the welding requirements.
Iii. Insulation/decoration treatment (external parts, insulation structural parts)
Electrophoretic coating
Applicable materials: aluminum alloy, steel (external decoration)
Process characteristics: uniform coating (thickness 10-25μm), strong adhesion, various appearances such as matte and high gloss, and excellent insulation performance (breakdown voltage ≥300V).
Key points of operation
The pretreatment should be thorough (degreasing → derusting → phosphating), and the weight of phosphating film should be controlled at 1-3g/m;
The pH value of the electrophoresis bath is maintained at 5.5-6.5, and the voltage is 60-120V V. The electrophoresis time is adjusted according to the film thickness.
powder spraying
Applicable materials: aluminum alloy, cold-rolled steel (shell)
Process advantages: no solvent pollution, coating thickness of 20-100μm, good scratch resistance and weather resistance, and various colors can be customized.
Key points of operation
Electrostatic precipitation is required before spraying to ensure the surface is clean;
The curing temperature is 180-200℃, and the temperature is kept for 15-20 minutes, so as to avoid deformation of stamping parts caused by too high temperature.
Anodic oxidation (hard oxidation)
Applicable material: aluminum alloy (such as mobile phone middle frame, heat sink)
Process characteristics: porous oxide film is formed on aluminum surface, which can be dyed (black, silver, etc.), and the hardness of hard oxide film can reach HV500 or above, which has both wear resistance and insulation performance.
Key points of operation
Pure aluminum or aluminum-magnesium alloy has the best oxidation effect, and the oxidation process needs to be adjusted for aluminum alloy with high silicon content.
The hard oxidation temperature is controlled at 5-15℃ (low temperature oxidation), and the current density is 1-3a/dm, so as to avoid the film loosening caused by high temperature.
Iv. General process requirements for surface treatment (core standard for electronic products)
Cleanliness control
Before all treatments, ultrasonic degreasing and deionized water rinsing are required, and the residual oil on the surface is ≤ 5 mg/m to avoid affecting the adhesion of the coating.
Environmental compliance
Harmful substances such as hexavalent chromium, lead and mercury are prohibited, and chromium-free passivation and cyanide-free electroplating processes are preferred, which meet the environmental protection standards of electronic products such as RoHS, REACH and WEEE.
Dimensional accuracy control
The thickness of thin coating (such as gold plating and nickel plating) should be controlled within the tolerance range to avoid affecting the assembly gap of stamping parts (such as terminal insertion and withdrawal force); For thick coatings (such as powder spraying), the size allowance should be reserved in advance.
Adhesion test
All coatings must pass the scratch test (ISO 2409), and the adhesion is ≥4B; After bending test, the coating has no cracking and falling off.
V. Examples of typical product surface treatment schemes
Recommended surface treatment technology for stamping parts types and materials using scenarios.
Electro-zinc plating and color zinc passivation for electromagnetic shielding of cold-rolled steel main board with shield cover
Flash nickel plating+thin gold plating on brass conductive contact of connector terminal
External decoration of 6061 aluminum alloy in the middle frame of mobile phone+hard anodizing of structural support+dyeing.
Anti-rust requirements of SPCC cold-rolled steel with internal fixed bracket chromium-free passivation
Soldering terminal phosphor copper PCB board welding tin plating+hot melt treatment
