Materials need to achieve a match in the coefficient of thermal expansion (CTE) among "copper foil - base material - adhesive", and at the same time meet the requirements of thick - copper carrying and insulation. The core material selections are as follows:
- Base Material: Select high - Tg FR - 4 copper - clad laminates (Tg≥170℃) with a CTE≤18ppm/℃, which meet the IPC - 4101 standard. The board thickness is designed to be 2.0 - 3.0mm according to current requirements. For a 3.0mm - thick board, pre - bake it at 120℃ for 2h to reduce the moisture absorption rate and avoid lamination bubbles.
- Copper Foil: Use 5OZ high - purity electrolytic copper foil (purity≥99.9%) for all layers, with a tensile strength of ≥300MPa and an elongation of ≥8%. Control the surface roughness of the copper foil at Ra 0.8 - 1.2μm to ensure both the bonding force with the resin and reduce the difficulty of etching.
- Prepreg (PP): Select 2116 fabric - based PP with a high resin content (55% - 65%) and a fluidity controlled at 25 - 35%. For areas where the inner - layer circuit gap is ≥0.3mm, switch to non - flow PP (fluidity < 10%) to prevent excessive resin loss.
- Etch - resistant and Solder - mask Materials: Use a 50μm - thick high - temperature - resistant dry - film (conventional is 35μm) with an adhesion of 5B level to avoid pattern collapse during etching. Select epoxy - resin - type solder - mask ink with a Tg≥150℃ and an insulation resistance of ≥10¹²Ω to meet the insulation requirements for large currents.
The process focuses on "inner - layer formation → lamination integration → outer - layer processing → finished - product inspection", and optimizes key parameters according to the characteristics of thick copper and multi - layers:
- Dry - film Lamination: In a Class 10000 clean room, laminate the dry - film by hot - pressing at 90℃ and a pressure of 0.8kg/cm², and use a vacuum laminator to remove bubbles (vacuum degree ≤ 10Pa).
- Exposure and Etching: Use LDI laser exposure (accuracy of ±10μm) and increase the exposure energy to 180 - 200mJ/cm². For etching, adopt a "segmented process". First, use a 15% ferric chloride solution to quickly etch 60% of the copper thickness, and then use a 10% solution for fine trimming, controlling the under - etching amount to ≤10μm.
- Inspection and Blackening: After etching, conduct a full inspection of circuit defects by AOI, with a qualification rate of ≥99.8%. 随后进行黑化处理，增强铜箔与 PP 的结合力。
- Stacking and Positioning: Stack alternately according to the "signal layer - ground layer - power layer" (e.g., Top: signal → 2: ground → 3: power → 4: power → 5: ground → Bottom: signal). Use "copper - foil positioning holes + laser alignment" to keep the inter - layer misalignment ≤ ±0.03mm.
- Step - by - step Pressing:
- Low - pressure Penetration (10kg/cm², 120℃): Exhaust air and allow the resin to flow initially.
- Medium - pressure Diffusion (25kg/cm², 160℃): Fill the circuit gaps and continue for 40 minutes.
- High - pressure Curing (35kg/cm², 180℃): Ensure the inter - layer bonding force is ≥1.2N/mm.
- Cooling Control: Cool from 180℃ to 120℃ at a rate of 5℃/min, and then cool naturally to reduce warping caused by thermal stress.
- Drilling and Hole Treatment: Drill 0.3 - 1.0mm vias with a tungsten - steel drill bit (rotation speed of 25000rpm). After drilling, activate the hole wall through plasma cleaning (600W, 8min).
- Via Metallization: Use a vertical pulse electro - plating system with a plating solution containing polyether - type additives to achieve a copper thickness of over 30μm on the hole wall and improve uniformity by 20%.
- Surface Treatment: Plate nickel - gold (8μm nickel, 2μm gold) in the soldering area and tin - plate in the non - soldering area. Verify the adhesion by a cross - cut test with an adhesion of ≥4B level.
- Electrical Performance Testing: Use ICT to check for open - circuits and short - circuits. The via impedance tested by a flying - probe tester should be ≤5mΩ, and withstand a voltage test of 3000V AC/1min without breakdown.
- Environmental Reliability: No delamination after 3 times of 288℃ tin - immersion, pass 100 cycles of - 40℃ ~ 125℃ thermal shock, and the temperature rise should be < 30K when 100A current is applied for 1 hour.
For the core pain points in production, improve the yield through process innovation:
- Difficulty: During the etching of 175μm thick copper, the etchant penetrates unevenly, and serrated edges are likely to occur.
- Solution: Use 355nm UV laser etching, and with an AI - based concentration monitoring system, adjust the etchant parameters in real - time to control the line - width accuracy within ±10μm.
- Difficulty: The large gaps between thick - copper circuits may cause insufficient resin flow and form voids.
- Solution: Switch to 1080 high - resin - content PP, apply an impact pressure of 200PSI at 140℃, and use - 80kPa vacuum - assisted lamination. This reduces the bubble rate from 8% to 1%.
- Difficulty: The full - layer thick copper leads to stress imbalance, and the warpage is likely to exceed 0.8mm/m.
- Solution: Adopt a symmetrical lamination design, add a 2mm silica - gel buffer layer during lamination, and conduct stress - relief at 120℃ for 2h after pressing. This keeps the warpage ≤ 0.5mm/m.
