A 4 - layer 4OZ power thick - copper board has a core structure of "2 signal layers + 2 4OZ thick - copper power/ground layers". The single - side copper thickness reaches 140μm (far exceeding the conventional 1OZ/35μm), and its current - carrying capacity can reach over 30A. It is the core carrier for new - energy vehicle OBCs and industrial power modules. However, the differences in physical properties between the thick - copper layer and the base material pose multiple challenges during the manufacturing process, such as insufficient lamination resin filling and out - of - control etching precision. This article, combined with industry practical experience, disassembles the complete manufacturing process and analyzes four core difficulties and their solutions.
The 4 - layer 4OZ power thick - copper board requires special steps like thick - copper pretreatment and step - by - step electro - plating based on the conventional PCB process. The full - process cycle is about 12 - 15 days, and leading enterprises can stabilize the yield above 85%.
- Base - material Selection and Pretreatment
Select high - Tg (≥170℃), low - CTE (Z - direction ≤ 35ppm/℃) FR - 4 base material, and match it with 4OZ high - purity electrolytic copper foil (copper content ≥ 99.98%) to ensure temperature resistance and current - carrying capacity. After cutting, first conduct ultrasonic cleaning (40℃ neutral cleaning agent), and then pre - bake at 150℃ for 2 hours to reduce the water absorption rate of the base material to ≤0.04%, avoiding lamination bubbles. Especially, the surface of the copper foil needs to be micro - etched to form a rough surface with Ra1.2μm, enhancing the bonding force with the resin. - Inner - layer Thick - copper Circuit Manufacturing
The inner layer (power/ground layer) adopts the process of "pattern transfer - etching compensation - secondary inspection": Coat a high - temperature - resistant photosensitive film (thickness of 25μm), and form patterns through LDI laser exposure (accuracy of ±1μm). During etching, use the "multiple rapid - etching method", conduct acidic etching three times (copper chloride concentration of 200g/L), and rinse with clear water after each etching to control the under - etching amount within 12μm, which is usually 25μm. After etching, double - check with AOI and a metallographic microscope to ensure the line - width deviation is ≤±5μm. - Lamination Assembly and Precise Pressing
The stacking order is "outer 1OZ copper foil → 1080 prepreg (2 pieces) → inner 4OZ thick - copper layer → 106 prepreg (1 piece) → inner 4OZ thick - copper layer → 1080 prepreg (2 pieces) → outer 1OZ copper foil". To solve the problem of resin filling in thick - copper gaps, select high - fluidity prepreg with a resin content of 65%, and add copper - laying blocks in copper - free areas to increase the residual copper rate and reduce the filling pressure. The lamination uses a step - by - step temperature - control program: 80℃/10kg/cm² (30min) for exhaust → 150℃/25kg/cm² (40min) for resin flow → 180℃/30kg/cm² (90min) for curing, extending the high - temperature stage by 30 minutes compared to the conventional process to ensure complete resin cross - linking. - High - precision Drilling and Desmear
Use a CNC drilling machine (rotation speed of 80000r/min) to process through - holes and blind holes, controlling the aperture deviation within ±0.01mm. After drilling, use the plasma desmear process (O₂/CF₄ mixed gas) to remove the resin residue and carbonized layer on the hole wall, avoiding poor bonding of the hole wall in thick - copper areas. For small holes with a diameter ≤ 0.3mm, adopt the "laser pre - drilling + mechanical fine - drilling" composite process to prevent drill breakage and hole - position deviation. - Step - by - step Electro - plating for Thick - hole Copper
Via metallization is the core of the power - board reliability. Adopt the "copper - deposition - three - time electro - plating" process: First, electroless copper - plating to form a 1μm thin copper layer. The first electro - plating (current density of 1A/dm²) increases the copper thickness on the hole wall to 20μm. The second time, open windows with a dry - film to electro - plate the areas that need to be plugged separately, making the copper thickness on the hole wall reach over 35μm. The third time, electro - plate the whole board for thickening. Finally, the surface copper thickness reaches 140μm, and the copper thickness on the hole wall is ≥ 38μm. During electro - plating, monitor the solution temperature (25±2℃) and pH value (10±0.5) in real - time to avoid uneven thick - copper deposition. - Outer - layer Circuit and Impedance Control
The outer - layer signal layer adopts the process of "pattern transfer - etching - impedance calibration". The line - width offsets the under - etching effect through etching compensation (increasing the design value by 10μm). Use a TDR time - domain reflectometer to detect the impedance online, controlling the 50Ω single - ended impedance tolerance within ±5%. For the pad areas connected to the thick - copper layer, reserve a 0.2mm transition area to reduce the heating problem caused by current mutation. - Step - by - step Solder - mask Printing
Due to the 50 - 80μm height difference between the thick - copper circuits and the base material, two - time solder - mask printing is required: The first time, fill and level the gaps in the thick - copper area (film thickness of 40μm), and cure at 150℃ for 30 minutes. The second time, print the solder - mask ink on the whole board (thickness of 20μm), ensuring the surface flatness is ≤ 15μm. The solder - mask curing needs to be heated in stages to avoid ink cracking caused by thermal - stress concentration in the thick - copper area. - Surface Treatment and Finished - product Inspection
The pads are treated with gold - plating (2μm gold layer, 5μm nickel layer), and the non - pad areas are coated with OSP for anti - oxidation. The finished product needs to pass three core tests: Electrical performance (flying - probe test for open - circuits and short - circuits, insulation resistance ≥ 10¹⁰Ω), thermal reliability (-40℃~125℃ cycle for 1000 times, no delamination), and current - carrying test (applying 30A current for 1 hour, temperature rise ≤ 50K). Randomly select 5% of the samples from each batch for metallographic sectioning to check the inter - layer bonding and the copper thickness on the hole wall.
- Insufficient Lamination Resin Filling and Layer Misalignment
- Difficulty: The gaps between 4OZ thick - copper circuits reach 80 - 100μm. The resin flow of conventional prepreg is insufficient, easily forming voids. The CTE difference between copper and the base material is large (copper 17ppm/℃, resin 35ppm/℃), and layer misalignment of ≥ 20μm is likely to occur during lamination.
- Solution: Stack 3 pieces of 1080 prepreg for resin filling, increasing the resin content to 65%. Before lamination, add rivets to the edge of the core board for fixation, and achieve an alignment accuracy of ±2μm through the CCD optical positioning system. Set misalignment targets at the board edge, and after lamination, detect the layer misalignment with X - ray. Rework the boards that exceed the tolerance.
- Out - of - control Thick - copper Etching Precision
- Difficulty: During the etching of 140μm thick copper, it is difficult to exchange the etching solution. The under - etching amount can reach 30μm, resulting in a line - width deviation exceeding ±15μm, and even "incomplete etching" residues may occur.
- Solution: Use a "high - pressure spray + segmented etching" device, increase the spray pressure to 3kg/cm², and control the etchant temperature at 45±1℃. Set dynamic etching compensation through CAM software, adjust the compensation coefficient (0.1 - 0.15mm) according to the line width. Check the line width every 50 boards and adjust the etching rate in real - time.
- Electro - plating Concavity of Thick - hole Copper and Exposed Base Material at the Hole Mouth
- Difficulty: During deep - hole electro - plating, the current distribution is uneven. The copper thickness at the hole center is only 60% of that at the hole mouth. After plugging and grinding, the copper layer at the hole mouth is easily worn through, resulting in exposed base material.
- Solution: Use a pulse electro - plating power supply (frequency of 20kHz) to improve the current distribution, reducing the copper - thickness difference between the hole center and the hole mouth to within 20%. Pre - plate and thicken the hole mouth of the holes that need to be plugged by 5μm. When grinding, use a fine - grit grinding wheel (800# mesh) to control the grinding amount within ≤ 5μm. Select high - temperature - resistant plugging resin with a hardness of ≥ 80D after curing.
- Reliability Failure Caused by Thick - copper Thermal Stress
- Difficulty: When the power board is working, the temperature of the thick - copper layer can reach over 100℃. The stress generated by thermal expansion is likely to cause inter - layer delamination and hole - copper fracture.
- Solution: Select a base material with a matched CTE (filler content ≥ 60%), controlling the CTE difference between the copper layer and the base material within 10ppm/℃. After lamination, conduct an annealing treatment at 180℃ for 4 hours to release internal stress. Design a heat - dissipation hole array in the thick - copper area to reduce the working temperature rise and extend the service life.
