I. Materials: Ensuring Thick - copper Support and Temperature Resistance

• Thermal Stability: High - Tg base materials are less likely to soften at high temperatures (such as a soldering temperature of 260℃), avoiding the risk of thick - copper layer detachment due to base - material deformation. Since the thickness of an 8OZ copper foil is 8 times that of a regular 1OZ copper foil, the base material needs to provide stronger support. Low - Tg base materials may cause the copper foil to warp at high temperatures.
• Mechanical Strength: The base material has a flexural strength of ≥150MPa and a tensile strength of ≥200MPa, which can withstand the stress after thick - copper etching and prevent the substrate from warping (the warpage of the finished board should be ≤0.75%).
• Dielectric Properties: The dielectric constant (at 1MHz) is ≤4.5, and the dielectric loss is ≤0.02, ensuring low signal interference during large - current transmission and adapting to the high - frequency requirements of industrial power supplies (such as a switching frequency of ≥100kHz).
• Common Specifications: The thickness of the base material is mostly 1.6mm (suitable for regular high - power modules), and for some large - current scenarios, a 2.0mm - thick base material is used to enhance the heat - dissipation path (increasing the thickness of the base material can improve the heat - conduction efficiency and reduce the local temperature rise of the substrate).

II. Copper Foil: Ensuring Large - current Carrying Capacity and Processability

• Mechanical Properties: The tensile strength is ≥300MPa, and the elongation is ≥8% (at room temperature) to avoid the breakage of thick copper during etching and bending. Due to the large thickness of the 8OZ copper foil, stress concentration occurs at the circuit edges after etching, and copper foil with a low elongation is prone to cracking.
• Surface Roughness: The roughness Ra of the copper - foil surface in contact with the base material is ≤3μm, and that of the non - contact surface is ≤1.5μm. Excessive roughness on the contact surface will increase the difficulty of etching (copper slag is likely to remain), while too low roughness will reduce the bonding force with the base material (which needs to be compensated by subsequent blackening treatment).
• Impurity Content: The content of impurities such as iron, zinc, and nickel is ≤50ppm to avoid the influence of impurities on the conductivity of the copper foil. In large - current scenarios, impurities can cause local resistance to increase, resulting in additional temperature rise.
• Selection Differences: Standard electrolytic copper foil is used in regular scenarios, and for high - frequency and large - current scenarios (such as new - energy inverters), high - ductility electrolytic copper foil (elongation ≥ 10%) can be selected to improve the thermal - cycling resistance (-40℃~125℃ for 100 cycles without cracks).

III. Adhesive: Ensuring the Bond between Thick Copper and the Base Material

• Resin Content: The resin content is ≥50%, and the fluidity (at 170℃/10min) is ≥20%, ensuring that the resin can fully fill the gaps between thick - copper circuits during lamination (the line spacing of 8OZ copper foil is usually ≥0.3mm, and sufficient resin is required to avoid inter - layer bubbles).
• Temperature Resistance: After curing, Tg≥160℃, and the coefficient of thermal expansion (CTE) at the glass - transition temperature is ≤25ppm/℃, matching the CTE of the base material and copper foil to reduce inter - layer stress during thermal cycling.
• Common Models: 7628 fabric - based PP (thickness of 0.18mm) or 2116 fabric - based PP (thickness of 0.1mm) is selected, adjusted according to the substrate thickness and thick - copper gaps to ensure the uniform thickness of the laminated substrate.

IV. Other Auxiliary Materials

• Solder - mask Ink: High - temperature - resistant epoxy - resin solder - mask ink (such as the Taiyo Ink PSR - 4000 series) is selected. After curing, Tg≥150℃, and the temperature resistance meets the soldering requirement of 260℃/10s. The insulation resistance is ≥10¹²Ω (500V DC) to prevent the risk of electric leakage in large - current scenarios. The color is mostly green (conventional) or black (for high - heat - dissipation requirements, as black ink can assist in heat dissipation after absorbing heat).
• Surface - treatment Materials:
  • Soldering Area: Electro - plated nickel - gold (nickel thickness of 8 - 10μm, gold thickness of 1 - 3μm) is used. The nickel layer can enhance the bonding force with the copper foil, and the gold layer improves soldering reliability and anti - oxidation properties (large - current solder joints are prone to oxidation, and the gold layer can extend the service life).
  • Non - soldering Area: Tin - plating (tin thickness of 5 - 8μm) is used, which is lower in cost than nickel - gold, and the tin layer can be directly soldered, suitable for mass - production scenarios.
• Drilling Consumables: Tungsten - steel drill bits (diameter of 0.3 - 1.0mm) with a hardness of ≥HRC65 are used to ensure that they can penetrate the thick - copper layer (275μm copper foil + 1.6mm base material) during drilling. The hole - wall roughness Ra≤2μm to avoid excessive via impedance.

II. Complete Manufacturing Process: Optimizing the Process around the Characteristics of Thick Copper

(1) Base - material Pre - treatment: Laying the Foundation for Thick - copper Processing

• Base - material Cutting: Cut the high - Tg FR - 4 base material to the designed size (with an error of ±0.1mm) using a CNC cutting machine. After cutting, chamfer the board edges at 45° (radius of 0.5mm) to prevent the board edges from scratching the copper foil during subsequent processes.
• Degreasing and Cleaning: Put the base material into a 5% sodium hydroxide solution (temperature of 50℃, time of 20min) for ultrasonic cleaning to remove surface oil stains and dust. After cleaning, rinse it 3 times with deionized water and then dry it with hot air (80℃, 30min) to ensure that the surface water content of the base material is ≤0.1%. Residual oil stains will reduce the bonding force between the copper foil and the base material, and the thick - copper layer is likely to fall off.
• Copper - foil Lamination: Use the hot - press lamination process to laminate the 8OZ electrolytic copper foil on both sides of the base material (temperature of 120℃, pressure of 20kg/cm², time of 30min). During the lamination process, monitor the pressure uniformity in real - time (pressure deviation ≤ 5%) to avoid bubbles or wrinkles in the copper foil. Due to the large area of the thick - copper foil, bubbles will cause incomplete circuits during subsequent etching.

(2) Circuit Formation: Ensuring Uniform Thick - copper Etching

• Dry - film Lamination: Select a 50μm - thick high - temperature - resistant dry - film (the conventional dry - film thickness is 35μm, and the thick dry - film can cover the unevenness of the thick - copper surface). Laminate it onto the copper - foil surface using a hot - press roller (temperature of 90℃, pressure of 0.8kg/cm², speed of 1m/min). After lamination, use a vacuum laminator to remove the bubbles under the dry - film (vacuum degree ≤ 10Pa).
• Exposure: Use an ultraviolet exposure machine (exposure energy of 150 - 200mJ/cm², while it is 80 - 120mJ/cm² for conventional double - sided PCBs) and extend the exposure time to ensure the full curing of the thick dry - film. If the thick dry - film is not fully cured, it is likely to fall off during development, resulting in etching errors.
• Development: Develop with a 1.5% sodium carbonate solution (temperature of 32℃, spray pressure of 2.0kg/cm²) for 90s (the conventional time is 60s) to ensure the complete removal of the unexposed dry - film. After development, rinse it with deionized water to avoid the influence of residual liquid on etching.
• Etching: Adopt the "segmented etching" process. First, use a 15% ferric chloride solution (temperature of 55℃, spray pressure of 2.5kg/cm²) to etch 60% of the copper thickness, and then switch to a 10% ferric chloride solution (temperature of 50℃, spray pressure of 2.0kg/cm²) to etch the remaining 40%. The high - concentration etchant quickly removes most of the copper thickness, and the low - concentration etchant finely trims the circuit edges to reduce under - etching (under - etching amount ≤ 10μm). After etching, use a 5% hydrochloric acid solution to remove the remaining dry - film, and detect the circuit width through AOI (error ≤ ±15%, while it is ±10% for conventional double - sided PCBs. A slightly larger error is allowed for thick - copper circuits).

(3) Via Metallization: Ensuring Inter - layer Interconnection of Thick Copper

• Drilling: Drill vias (hole diameter of 0.3 - 1.0mm) with a CNC drilling machine (rotation speed of 25000rpm, feed rate of 40mm/min). After drilling, blow out the copper slag in the holes with compressed air (pressure of 0.5MPa). Copper slag is likely to be generated and block the holes during the drilling of the thick - copper layer, affecting the subsequent electroless copper plating.
• Hole - wall Treatment:
  • Alkaline Degreasing: Use a 5% sodium hydroxide solution (50℃, 15min) to remove the oil stains and resin powder on the hole wall.
  • Plasma Cleaning: Activate the hole wall with oxygen plasma (power of 600W, time of 8min) to improve the adhesion of electroless copper plating. The large surface area of the thick - copper hole wall requires sufficient activation to prevent the electroless copper - plating layer from falling off.
  • Micro - etching: Slightly etch the copper layer on the hole wall with a 10% ammonium persulfate solution (45℃, 8min) to remove the oxide layer and expose a fresh copper surface.
• Electroless Copper Plating and Electro - plating Copper:
  • Electroless Copper Plating: Use a copper sulfate solution (temperature of 48℃, pH value of 12.5, time of 30min) to form a 1.0 - 1.5μm - thick copper layer on the hole wall (0.5μm for conventional double - sided PCBs), ensuring no copper leakage on the hole wall.
  • Electro - plating Copper: Use an acidic copper sulfate plating solution (temperature of 28℃, current density of 2.0A/dm², time of 120min) to increase the copper thickness on the hole wall to over 30μm. The thick - copper vias need a sufficient copper thickness to carry large currents and avoid via heating. During the electro - plating process, stir the plating solution regularly to prevent uneven plating in the holes.

(4) Surface Treatment and Shape Processing

• Solder - mask Ink Printing: Use screen - printing to print the high - temperature - resistant solder - mask ink on the substrate surface (thickness of 25 - 30μm) to cover the non - soldering areas. After printing, pre - bake it (80℃, 30min), then expose it to ultraviolet light (energy of 180mJ/cm²), develop it (with a 1.2% sodium carbonate solution), and finally bake it at 150℃ for 60min for curing. Since thick - copper circuits dissipate heat quickly, the solder - mask ink needs to have good high - temperature resistance to avoid ink falling off at high temperatures.
• Surface Treatment: Electro - plate nickel - gold (nickel layer of 8 - 10μm, gold layer of 1 - 3μm) or tin - plate (tin layer of 5 - 8μm) in the soldering area, and coat OSP (Organic Solderability Preservative) in the non - soldering area. After surface treatment, test the coating adhesion (cross - cut test ≥ 4B level) to ensure that the coating does not fall off in large - current scenarios.
• Shape Processing: Process according to the design size using a CNC punching machine (for simple shapes) or laser cutting (for complex shapes), with a cutting accuracy of ±0.05mm and edge burrs ≤ 0.02mm. After processing, chamfer the fixing holes (such as screw holes) (45°, depth of 0.1mm) to avoid scratching the bolts during installation.

(5) Finished - product Inspection: Ensuring Large - current Reliability

• Electrical Performance Testing: Use ICT in - circuit testing to check for open - circuits and short - circuits. Use a flying - probe tester to test via impedance (≤5mΩ) and line resistance (according to the line length, for example, the resistance of a 100mm - long and 2mm - wide line is ≤50mΩ). Conduct a withstand - voltage test (1000V AC, 1min without breakdown) to prevent electric leakage in large - current scenarios.
• Large - current Testing: Pass the designed current (such as 50A) for 1h and monitor the line temperature rise (≤30℃, with an ambient temperature of 25℃) to avoid the overheating and burning of the circuit. If there are etching defects in the thick - copper circuit (such as the thinning of the circuit), it will cause the local resistance to increase and the temperature rise to exceed the standard.
• Environmental Reliability Testing: Conduct a thermal - shock test (-40℃/30min→125℃/30min, 100 cycles) to test the inter - layer bonding force (peel strength ≥ 1.2N/mm), and a damp - heat test (85℃/85% RH, 500h) to test the insulation resistance (≥10¹⁰Ω), ensuring stable operation in harsh environments.

III. Key Technical Difficulties and Solutions

(1) Uneven Thick - copper Etching: Segmented Etching + Real - time Monitoring

• Difficulty: Due to the large thickness of the 8OZ copper foil, it is difficult for the etchant to penetrate uniformly during etching, and the circuit edges are prone to serration (under - etching amount exceeds 15μm), affecting the current - carrying capacity.
• Solution: Adopt the "segmented etching" process (high - concentration fast etching + low - concentration fine etching), and install an on - line monitoring camera in the etching machine to observe the state of the circuit edges in real - time. Dynamically adjust the concentration of the etchant and the spray pressure (for example, when the under - etching amount is too large, reduce the etchant concentration to 8% and increase the spray pressure to 2.8kg/cm²) to ensure that the under - etching amount is ≤10μm.

(2) Low Via - metallization Reliability: Strengthening Hole - wall Treatment + Thick Electroless Copper - plating Layer

• Difficulty: The large hole - wall area of thick - copper vias and the possible gaps at the junction of the copper foil and the base material make it easy to have "broken copper layer on the hole wall" during electroless copper plating, resulting in via open - circuits.