The Core Links from Base Materials to Finished Products

I. Four Commonly Used Types of PCB Boards8 layer pcb: Characteristics and Application Scenarios

1. Epoxy – resin – based Boards: FR – 4 and High – TG Boards (Accounting for over 70%)

   As the most basic rigid boards, FR – 4 is made by impregnating brominated epoxy resin and pressing with fiberglass cloth as the reinforcing material. Its dielectric constant (Dk) ranges from 4.0 – 4.7, and the flame – retardant grade is UL 94 V – 0. The cost is only 1/5 – 1/10 of that of special boards. The glass – transition temperature of high – TG FR – 4 is increased to over 170℃, replacing ordinary FR – 4 in high – temperature scenarios such as automotive electronics.

   Typical applications: Xiaomi mobile phone motherboards, Haier refrigerator control boards, automotive central control systems.

2. Metal Substrates: Aluminum – based and Copper – based (Special for Heat Dissipation)

   With an aluminum alloy or pure copper as the base, a surface – composite insulating dielectric layer and copper foil are added. The thermal conductivity of an aluminum substrate is 20 – 30W/(m・K), and that of a copper substrate can reach over 200W/(m・K), which can quickly conduct away the heat from power devices. However, metal substrates are relatively brittle, and mechanical stress needs to be controlled during processing.

   Typical applications: 100W LED street lights, new – energy vehicle OBC modules, industrial power supplies.

3. High – frequency Boards: PTFE and BT Resin Boards (Low – signal Loss)

   Polytetrafluoroethylene (PTFE) boards have a low dielectric constant of 2.0 – 2.2 and a dielectric loss (Df) < 0.001, making them the core base material for 5G millimeter – wave radars. BT resin boards have a dielectric constant of 3.0 – 3.5, heat – resistance above 260℃, and the cost is only 60% of that of PTFE, suitable for mobile phone RF modules.

   Typical applications: Tesla autopilot radars, Huawei 5G base – station power amplifiers.

4. Flexible Boards: PI Polyimide Boards (Bendable)

   The thickness can be as thin as 12μm, and it can withstand 100,000 times of 180° bending. Its performance is stable in the environment from – 20℃ to 150℃. PI substrates account for 45% of the material cost in flexible PCBs. However, PI materials have weak chemical corrosion resistance, and the concentration of the etchant needs to be strictly controlled in the process.

   Typical applications: The hinge of Huawei Mate X5 folding screen, smart bracelet flex cables.

II. General Manufacturing Process: Taking an FR – 4 Four – layer Board as an Example (Standardized Process)

1. Base – material Pretreatment and Layout Transfer
   • Board Cutting and Cleaning: Cut the FR – 4 copper – clad laminate into the designed size and remove oil stains through ultrasonic cleaning to avoid subsequent circuit short – circuits.
   • Photosensitive Imaging: Coat a photosensitive film on the copper – foil surface, irradiate it with a UV lamp to cure the light – transmitting areas, and wash the uncured parts with an alkaline solution to expose the copper foil to be etched.
   • Etching into Circuits: Etch the exposed copper foil with a NaOH solution. After removing the film, inner – layer circuits are formed with an accuracy of up to ±3μm.
2. Lamination and Drilling
   • Core – board Inspection: Use an AOI device to automatically compare the circuits with the design drawing, and the defect rate needs to be controlled below 0.1%.
   • Lamination Assembly: Stack in the order of “copper foil – prepreg – inner – layer core board – prepreg – copper foil”. The prepreg melts at high temperatures to form an insulating adhesive layer.
   • Positioning Drilling: Use an X – ray drilling machine for positioning and drill inter – layer connection holes with an aperture tolerance of ±5μm.
3. Via Metallization and Outer – layer Processing
   • Electroless Copper Plating: Deposit a 1μm conductive layer on the hole wall to ensure the insulating hole wall is conductive, and then electro – plate to thicken it to 25μm.
   • Outer – layer Imaging: Transfer the outer – layer circuits using the positive – film process. Electro – plate a tin layer to protect the circuits, and after etching, remove the tin to expose the copper foil.
   • Surface Treatment and Shaping: Spray tin or plate gold to prevent oxidation. Cut the shape with a CNC milling machine and finally test the conductivity.

III. Process Breakthroughs for Special Boards: Customized Solutions

1. Metal Substrates: Temperature Control and Anti – oxidation Processes
   • Lamination Pressure Control: The lamination pressure of aluminum substrates needs to be 30% higher than that of FR – 4 to avoid bubbles in the dielectric layer. Shenlian Circuit has increased the lamination yield to 99.2% through a vacuum hot – press.
   • Reflow – soldering Control: Use nitrogen reflow – soldering with an oxygen content < 1000ppm and a peak temperature controlled at 235 – 245℃ to prevent aluminum oxidation.
   • Cutting Protection: The V – cut depth should not exceed 1/3 of the metal layer. Add a 3mm process edge to the panel to enhance strength.
2. Ceramic Substrates: Laser Processing and Metallization Technologies
   • Laser Drilling: Aluminum nitride ceramics have high hardness and need to be drilled step – by – step with an ultraviolet laser. The minimum aperture can reach 50μm. The efficiency is 40% lower than mechanical drilling but with higher precision.
   • Thick – film Metallization: Screen – print silver paste and sinter it at 850℃ to form a conductive layer, replacing traditional electro – plating to adapt to the non – conductive characteristics of ceramics.
3. Flexible PI Substrates: Low – stress Process Design
   • Etching and Thinning: Reduce the copper – foil thickness from 18μm to 9μm through multiple shallow – etching processes to reduce stress concentration during bending.
   • Cover – film Lamination: Use a PI cover – film to protect the circuits. Control the lamination temperature at 180℃ and the pressure at 0.3MPa to avoid substrate deformation.
   • Bending Test: The finished product needs to pass 100,000 times of 180° bending tests with a circuit resistance change rate < 10%.

IV. Core Principles of Process Selection: The Triangle Balance of Board – Performance – Cost