Structure and ApplICation Value of Metal Substrates

2. Special Processing Requirements for Metal Substrates (Aluminum Substrates)

2.1 Special Requirements for Material Pretreatment

• Metal Base Surface Treatment: The surface of the aluminum base material (commonly used 1060 and 5052 aluminum alloys) of aluminum substrates is prone to form a dense oxide film (Al₂O₃), which reduces the bonding force between the insulating layer and the metal base. A two-step "alkaline cleaning-acid cleaning" process is required: first, soak in 5%-8% sodium hydroxide solution (temperature 50-60℃) for 30-60 seconds to remove the oxide film and oil; then neutralize with 10%-15% nitric acid solution (room temperature) for 10-20 seconds to form a uniform micro-rough surface (Ra=0.3-0.5μm), and finally rinse with deionized water and dry (100-120℃, 10-15 minutes) to ensure the insulating layer adhesion ≥1.5kN/m (in line with IPC-TM-650 2.4.9 standard).
• Insulating Layer Pretreatment Control: The insulating and thermally conductive layer (usually 50-200μm thick) of aluminum substrates is mostly epoxy resin composite material. Before processing, it is necessary to check the flatness (deviation ≤5μm/mm) and bubble rate (＜0.1%) of the insulating layer. If there are scratches or impurities on the surface of the insulating layer, it should be slightly polished with 1000-mesh sandpaper, then subjected to plasma cleaning (power 400-600W, time 60-90 seconds) to remove surface organic contamination and avoid circuit delamination during subsequent etching.
• Copper Foil Pretreatment: The copper foil (18-70μm thick) on the surface of aluminum substrates is mostly rolled copper, which needs to be micro-etched (the micro-etching solution is ammonium persulfate solution with concentration 80-100g/L, temperature 30-40℃), and the micro-etching amount is controlled at 1-2μm to enhance the bonding force between the copper foil and photoresist and prevent photoresist warping during pattern transfer.

2.2 Special Control of Drilling Process

• Tool Selection and Parameter Optimization: Special cemented carbide drills should be used, with a cutting edge angle of 135°-140°, a helix angle of 35°-40°, and the drill tip should be specially polished to reduce aluminum sticking. Drilling parameters need to be accurately matched: the speed is controlled at 20000-30000rpm, and the feed rate is 5-10mm/min (adjusted according to the total thickness of the aluminum substrate, 5mm/min for thickness below 1mm, 10mm/min for thickness above 2mm). During drilling, compressed air (pressure 0.5-0.6MPa) should be used for continuous cooling and chip removal to avoid aluminum chip accumulation leading to rough hole walls or drill breakage.
• Hole Wall Quality Requirements: After drilling, the hole wall should be free of burrs and aluminum chip residues, the hole wall roughness Ra≤1.0μm, and the hole diameter tolerance is controlled at ±0.05mm. For via holes, it is necessary to check whether the hole wall penetrates the insulating layer and is in reliable contact with the aluminum base material (for insulating holes, it is necessary to ensure that the insulating layer is not damaged). The insulation performance can be verified by a high-voltage test (500V DC, 1 minute), and the leakage current ≤10μA.
• Stacking Method Adjustment: The number of stacked layers should not be too large (usually 1-2 layers) when drilling aluminum substrates, and a polyethylene film (50-100μm thick) should be placed between each layer to prevent aluminum chips from transferring between layers and scratching the substrate surface. A hardwood backing plate (10-15mm thick) should be placed at the bottom to avoid damage to the workbench surface after the drill penetrates the substrate and enhance drilling stability.

2.3 Special Requirements for Etching Process

• Etchant Selection and Parameter Control: Acidic copper chloride etchant (Cu²+ concentration 120-150g/L, HCl concentration 150-200mL/L) is used, the temperature is controlled at 45-50℃, and the etching rate is 1.5-2μm/min. This etchant has a fast etching rate for copper foil and a low corrosion rate for aluminum base material (＜0.1μm/min), which can reduce metal base material loss. Continuous stirring (stirring speed 30-50rpm) is required during the etching process to ensure uniform etching and avoid "side etching" (side etching amount ≤3μm).
• Edge Protection and Mask Design: The edge of the aluminum substrate should be masked with acid-resistant tape or ink to prevent the etchant from eroding the edge of the metal base material and causing substrate warping. During circuit pattern design, an "insulation isolation zone" (width ≥0.2mm) should be reserved in the exposed area of the copper foil and aluminum base material to avoid short circuits caused by direct contact between the copper circuit and aluminum base material after etching.
• Post-etching Treatment: After etching, it is necessary to immediately rinse with deionized water (pressure 0.3-0.4MPa) to remove residual etchant, then neutralize with 5%-8% sodium carbonate solution (temperature 40-50℃) for 20-30 seconds, and finally dry (110-120℃, 15-20 minutes). After drying, check the circuit pattern accuracy (line width tolerance ±0.03mm) and the integrity of the insulating layer. If pinholes are found in the insulating layer, repair with insulating ink and cure.

2.4 Special Requirements for Insulating Layer and Surface Treatment

• Insulating Layer Voltage Resistance and Thermal Conductivity Test: After processing, the insulating layer should be subjected to a voltage resistance test (AC 1000V, 1 minute) to ensure no breakdown; the thermal conductivity test uses the laser flash method, and the thermal conductivity should meet the design requirements (usually ≥1.5W/(m·K)). For aluminum substrates used in power modules, a thermal cycle test (-40℃~125℃, 500 cycles) should also be conducted. After the test, the insulating layer should have no cracking or delamination, and the voltage resistance performance attenuation ≤10%.
• Solder Mask Selection and Coating: A high-temperature resistant and high-insulation solder mask (Tg≥150℃, dielectric strength ≥20kV/mm) should be selected, and the coating thickness is controlled at 20-30μm. The curing temperature of the solder mask should be lower than the heat resistance limit of the aluminum substrate (usually below 200℃), and the curing time is 60-90 minutes to avoid delamination between the insulating layer and the metal base material due to high temperature.
• Metal Base Surface Treatment: The metal back surface (non-circuit surface) of the aluminum substrate usually needs to be anodized to form an oxide film (Al₂O₃) with a thickness of 5-10μm, which enhances corrosion resistance and heat dissipation performance. After anodization, thermal conductive silicone grease can be further sprayed or thermal conductive gaskets can be pasted to improve the contact thermal resistance with heat dissipation devices (≤0.1℃·in²/W).

2.5 Special Requirements for Shape Processing and Machining

• Selection of Shape Cutting Method: For simple shapes (such as rectangles), CNC milling can be used (milling cutter diameter 1-2mm, speed 15000-20000rpm, feed rate 8-12mm/min); for complex shapes or small components, laser cutting (CO₂ laser, wavelength 10.6μm, cutting speed 50-100mm/s) is required, with cutting accuracy ±0.02mm and edge roughness Ra≤0.3μm. After cutting, the edge should be processed with a deburring machine (speed 3000-4000rpm) to remove aluminum chips and burrs.
• Bending and Stamping Process Control: Some aluminum substrates need to be bent (such as LED light strip substrates), the bending radius should be ≥3 times the substrate thickness, the bending angle tolerance is ±1°, and the insulating layer at the bending place has no cracking. For stamping processing, a high-precision stamping die (accuracy ±0.01mm) should be selected, and the stamping pressure is adjusted according to the substrate thickness (5-8 tons for 1mm thick substrates) to avoid damage to the insulating layer during stamping.
• Mechanical Strength Test: After processing, the bending strength (≥150MPa) and impact strength (≥5kJ/m²) of the aluminum substrate should be tested to ensure that the substrate will not break or the circuit will not be damaged due to mechanical stress during assembly and use.

3. Common Problems and Solutions in Processing