Multilayer PCBs, with their multi-layer conductive structure and stable electrical performance, are widely used in high-end fields such as communications, servers, and industrial control. 8-layer and 12-layer PCBs fall into the category of high-multilayer circuit boards, with complex processing procedures and extremely high requirements for equipment precision, process parameters, and environmental control. While their overall process framework is similar, significant differences exist in core aspects such as stacking, lamination times, alignment accuracy, drilling, and electroplating due to the increasing number of layers. The following section, based on practical production experience, outlines the complete processing flow and key technological points for both types of multilayer PCBs.
1. Pre-processing of Materials and Stacking Planning
Material pre-processing is fundamental to ensuring board stability. Both 8-layer and 12-layer PCBs use copper-clad laminate (CCL) core boards, prepreg (PP sheets), and copper foil as core materials, differing only in the stacking method. Upon arrival of raw materials, the first step is cutting. The entire roll of CCL is cut into standard-sized core boards according to production specifications. The cut edges must be smooth and burr-free to avoid delamination issues in subsequent processes. Following this, the boards are baked to remove moisture. High-layer multilayer boards easily absorb moisture; high-temperature baking completely removes this moisture, preventing defects such as bubbles and delamination after lamination. For layer stacking, 8-layer PCBs typically use a symmetrical 2+4+2 or 4+4 stacking pattern, divided into two or three modules. Only two complete laminations are needed for overall fusion. The symmetrical structure effectively offsets thermal stress and reduces the probability of board warping. 12-layer PCBs have even more layers, doubling the number of internal conductive and insulating layers. A 4+4+4 three-segment stacking structure is commonly used, requiring three independent lamination operations. Each segment of the stack must independently complete inner layer fabrication, browning, and pre-stacking processes. The more layers, the higher the complexity of interlayer stacking and the greater the risk of cumulative deviation, making the control of the stacking sequence more stringent. Prepreg serves as interlayer insulation and bonding material. For both types of boards, appropriate specifications are selected based on insulation thickness and impedance requirements, and the stacking strictly adheres to the rule of alternating arrangement of the core board and prepreg.
2. Inner Layer Circuit Fabrication Process
The inner layers are the core carriers for signal transmission in multilayer PCBs. The manufacturing process for the inner layers of 8-layer and 12-layer PCBs is the same, including board surface treatment, pattern transfer, etching, film removal, inspection, and browning. 12-layer PCBs, due to the larger number of inner layers, have higher requirements for batch processing consistency.
Board Surface Cleaning and Roughening
After cutting, the core board first enters the cleaning production line, where chemical solutions and high-pressure water washing remove oil, dust, and oxide layers from the board surface. Subsequently, the copper surface undergoes micro-roughening treatment to improve the adhesion between the photosensitive film and the copper surface, preventing film detachment and circuit defects during pattern transfer. The entire cleaning process is completed in a cleanroom, and airborne particles are strictly controlled to prevent impurities from adhering to the board surface.
Pattern Transfer and Etching
After cleaning, the photosensitive dry film is evenly applied to the core board. Pattern replication is completed using laser direct imaging equipment. This method offers high alignment accuracy and can meet the processing requirements of fine circuits. After imaging, a developing solution is used to dissolve the dry film in the unexposed areas, exposing the copper foil areas to be etched. Next, the etching process begins, using a specialized etching solution to remove excess exposed copper foil, leaving only the complete inner layer conductive lines. During processing, the solution temperature and spray pressure are monitored in real time, and the amount of lateral etching is strictly controlled to ensure neat line edges. After etching, a stripping solution is used to remove any remaining dry film from the board surface, exposing the pure copper lines.
Inner Layer Inspection and Browning Treatment All inner layer core boards undergo automated optical inspection to quickly identify defects such as open circuits, short circuits, and gaps. Defective boards are directly sorted and reworked. Qualified core boards enter the browning process, a crucial step before multilayer board lamination. Through chemical treatment, a uniform, rough oxide layer is generated on the copper circuit surface, significantly improving the bonding strength between the copper surface and the prepreg, preventing interlayer delamination. For 12-layer PCBs with more than 8 inner layers, the browning parameters must be uniformly calibrated to ensure consistent roughening effects on each inner layer core board.
3. Core Process of Layered Lamination
Lamination is the core process in multilayer PCB manufacturing and a significant difference between 8-layer and 12-layer PCB processes. Increasing the number of layers leads to a comprehensive upgrade in the number of laminations, alignment accuracy, temperature control profiles, and pressure parameters, directly determining the overall structural strength and yield of the board.
1). Pre-stacking and Positioning According to the pre-set stack structure, the browned inner core board, prepreg, and outer copper foil are stacked sequentially. 8-layer PCBs, due to fewer laminations, mostly use mechanical pins for positioning, allowing interlayer offset to be controlled within a reasonable range. 12-layer PCBs undergo three segmented laminations, significantly increasing the risk of cumulative interlayer deviation. Mid-segment stacking and overall stacking require a laser positioning system, with positioning accuracy far exceeding traditional mechanical positioning, ensuring precise alignment of dozens of layers. Temporary riveting is performed after stacking to prevent layer misalignment during transport.
2). Vacuum Hot Press Curing The stacked boards are sent to a vacuum laminator. The vacuum environment completely removes interlayer air, preventing bubble formation. Both types of boards employ a segmented heating and gradient pressurization process. First, low-temperature pre-pressing allows the prepreg to initially melt and flow, filling tiny gaps between layers. Then, the temperature and pressure are increased for main pressurization, allowing the prepreg to fully impregnate the core board surface. Finally, curing is performed at a constant temperature, completely hardening the resin and bonding all layers into a unified structure. The temperature and pressure parameters for the two laminations of the 8-layer PCB are relatively wide, resulting in a higher tolerance for process errors. For the 12-layer PCB, each segmented lamination requires a separately set temperature control curve. Multiple hot presses continuously accumulate thermal stress, therefore, the heating and cooling rates, holding times, and pressure levels all need precise control. The holding time for each lamination is longer to ensure simultaneous curing of the multiple resin layers, preventing board deformation and uneven internal stress. After each round of lamination, the boards are allowed to cool naturally to room temperature before the next round of lamination and pressing begins, until all layers are integrated.
4. Drilling and Hole Metallization Process
After lamination, the entire board enters the drilling process. Holes are the key channels for interlayer conductivity. Differences in board thickness and number of layers result in significant differences in drilling parameters, processing difficulty, and electroplating processes between 8-layer and 12-layer PCBs.
Mechanical Drilling High-multilayer PCBs use carbide drill bits for mechanical drilling, with corresponding drill bit specifications matched to different hole diameter requirements. 8-layer PCBs have a thinner board and shallower drilling depth; drilling speed and feed rate parameters are set conventionally, and hole position accuracy is easily controlled. 12-layer PCBs have a greater overall thickness and increased drilling depth; therefore, the drill feed rate needs to be reduced, the speed optimized, and the frequency of chip removal increased to prevent high-temperature wear of the drill bit, burrs on the hole walls, and hole position misalignment. After drilling, all holes are deburred and descaling is performed to remove residual resin debris and copper shavings, ensuring smooth and clean hole walls.
Hole Metallization and Electroplating After drilling, the board material first undergoes chemical copper plating, depositing a thin layer of copper on the insulating hole walls to build a basic conductive layer and achieve initial conductivity within the hole walls. This is followed by a full-board electroplating process, where electrolysis thickens the copper layer on the hole walls and the copper layer on the board surface, ensuring the stability of interlayer conductivity. For 8-layer PCBs, conventional DC electroplating is sufficient to meet the hole copper thickness requirements. For 12-layer PCBs, with more layers and longer conductive layer connections, uneven current distribution in some densely packed hole areas necessitates segmented electroplating or pulse electroplating processes to ensure uniform copper layer thickness on the hole walls and prevent conductivity failures caused by excessively thin copper layers in certain areas. For some high-density 12-layer boards, a fill-in electroplating process may be used to accommodate complex hole structures. After electroplating, the hole copper thickness and conductivity are inspected, and boards with damaged copper layers on the hole walls are discarded.
5. Outer Layer Circuit Fabrication Process
After drilling and metallizing the entire board, the outer layer circuitry is fabricated. This process is largely the same as the inner layer circuitry, but because the board is already formed, the control standards are more stringent. First, the board surface is thoroughly cleaned and roughened to remove electroplating residues. Then, a photosensitive dry film is applied. Through a series of processes including laser imaging, development, etching, and film removal, the outer conductive circuitry is formed. Compared to the inner layer, the outer layer circuitry is directly exposed to the external environment. After etching, the edges and line width are carefully inspected to ensure that the appearance and performance meet standards. After the process is completed, optical inspection equipment is used for a full inspection to check for any appearance defects in the outer layer circuitry.
6. Solder Mask, Surface Treatment, and Shape Processing
Solder Mask Coating After the outer layer circuitry passes inspection, solder mask ink is coated onto the entire board surface. After pre-baking, exposure, development, and curing, only the conductive areas are exposed, while the rest of the board surface is covered by the solder mask layer. The solder mask layer isolates external moisture and dust, preventing circuit oxidation and short circuits, while also improving the insulation performance of the board. The process for 8-layer and 12-layer PCBs is basically the same in this step, with the only difference being the choice of solder resist ink based on the application scenario.
Surface Treatment The appropriate surface treatment method is selected according to the application scenario. Common processes include chemical nickel-gold plating, organic solder resist film, and hot air leveling. The main function of this process is to protect exposed conductive areas, improve oxidation resistance and connection performance. The two types of multilayer boards can be flexibly matched with the process according to the end-user requirements.
Shape Processing Using CNC milling machines or die punching, the board material is processed into the customer's required shape and dimensions, and excess edges are trimmed. After processing, the board edges are sanded to ensure that the edges are flat and smooth, without sharp edges, delamination, chipping, or other problems.
7. Final Inspection and Finished Product Sorting
A comprehensive inspection is carried out at the finished product stage, divided into three main sections: appearance inspection, dimensional inspection, and electrical continuity inspection. Visual inspection relies on a combination of manual labor and optical equipment to check for damage, stains, and color differences in the ink, circuitry, and holes on the board surface. Dimensional inspection uses precision instruments to verify board thickness, overall dimensions, and hole coordinates, ensuring all parameters meet standards. Electrical inspection employs specialized equipment to test interlayer continuity and insulation performance point-by-point, identifying hidden open circuits and short circuits. After passing all inspections, 8-layer and 12-layer multilayer PCBs are cleaned, moisture-proofed, and packaged before being stored. Defective products are categorized and recorded according to defect type, providing data support for subsequent process optimization.
The processing of 8-layer and 12-layer multilayer PCBs is a highly sophisticated system of interconnected processes. While their basic processing logic is similar, the core differences lie in four key areas: layer stacking, lamination count, alignment accuracy, and drilling/plating. The 12-layer PCB, with its doubled layer count, presents significantly increased complexity, processing difficulty, and precision standards, placing far higher demands on production equipment, process parameter adjustments, and workshop environmental control compared to 8-layer PCBs. In actual production, the fine-tuning of parameters and the control of details in each process directly affect the yield and stability of the finished product. Only by strictly following the multilayer board processing specifications and optimizing the corresponding process solutions for different layers of boards can we produce high-multilayer PCB products with stable performance and meeting quality standards, thus satisfying the application needs of various high-end electronic devices.