Multilayer PCB

What is Multilayer PCB,What are the characteristics?

A multilayer circuit board, also known as PCB multilayer, is widely used in electrical products. These boards utilize single or double panels and employ a double lining system to create a more complex, four to six layer printed circuit board. This intricate design is achieved by strategically positioning and using alternating insulation and conductive materials.

As SMT (surface mounting technology) continues to advance and new generations of SMD (surface mounting devices) are introduced, such as QFP, QFN, CSP, and BGA (particularly MBGA), electronic products are becoming increasingly intelligent and compact. This has triggered significant changes and advancements in the field of PCB industrial technology, beginning with IBM’s pioneering development of high-density multilayer (SLC) in 1991. Since then, numerous groups in various countries have made significant strides in developing high-density interconnection (HDI) microvia boards.

The advancement of processing technologies has led to the increasing use of multi-layer and high-density wiring in PCB design. With its flexible design and dependable electrical performance, multilayer PCB is highly beneficial for electronic product production and manufacturing.

How are laminates laminated?

Aluminum PCBs are used to provide mechanical support and achieve electrical connections for a variety of electronic components, such as integrated circuits. They are also known as printed circuit boards or PCBs and are made with an insulated base material, mounting holes, copper wires, and electronic component pins for assembly.

The electronic components are positioned on one side of an aluminum-based circuit board while pins are utilized for soldering components on the opposite side. A layer of insulating paint is commonly applied to the soldering surface. Both positive and negative aluminum-based PCB manufacturers typically feature graphic symbols and a count of electronic components, but only those mounted on the surface of the manufacturer are accompanied by such symbols and numbers.

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Lamination is the process of bonding layers of line sheets together. This includes three stages: kissing pressure, full pressure, and cold pressure. During the kissing stage, the resin fills in gaps on the bonding surface and in the circuit, and then full pressure is applied to bond all gaps. The circuit board is then quickly cooled in a process known as “cold pressing” to maintain its size. When laminating, it is important to design the inner core board according to specific requirements, including thickness, external dimensions, and positioning holes. The board must also meet certain criteria such as no openings, short circuits, oxidation, or residual film.

In addition, the inner core plate should be laminated, followed by black oxidation treatment and Browning treatment. Black oxidation treatment creates a black oxide film on the inner copper foil, while Browning treatment produces an organic film.

Finally, in the process of lamination, it is essential to consider the factors of temperature, pressure, and time. Specifically, one should monitor the melting and curing temperatures of the resin, as well as the temperature settings of the hot plate and the actual temperature of the material. Additionally, attention must be given to the rate of temperature change. With regards to pressure, the primary goal is to fill the interlayer cavity with resin and remove any trapped gases or volatiles. Time parameters include regulating the length of pressurization, heating, and gel times.

What are the basic principles of impedance and cascade design considerations for multilaminates?

Answer: In impedance and cascade design, the main basis is PCB board thickness, layer number, impedance value requirements, current size, signal integrity, power integrity, etc. The general principles of reference are as follows: L laminated layer has symmetry; L Impedance has continuity; L The reference layer below the component surface should be a complete ground or power source (generally the second or penultimate layer); L Power supply plane is tightly coupled to the ground plane; L signal layer as close as possible to the reference plane layer; L Separate the two adjacent signal layers as far as possible. The running line is orthogonal; The upper and lower reference layers of L signal are ground and power source, so as to shorten the distance between the signal layer and the formation as much as possible; L Difference signal spacing ≤2 times line width; The semi-cured sheets between l plates are ≤3;

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Manufacturing Multilayer PCBs (Up to 40 Layers)

The growing demand for multilayer printed circuit boards can be attributed to the increasing trend of smaller, faster, and more powerful electronics. With the ability to create densely populated circuit boards, multilayer PCBs have become increasingly popular and offer a vast array of possibilities for engineers to achieve miniaturization. This is a significant advantage that cannot be achieved with traditional double-sided boards.

The First Step in PCB Production is Creating the Inner Layer Core

Begin by choosing a laminate sheet with the appropriate thickness and Cu foil weight. The core is then coated on both sides with a dry film resist that is UV sensitive and hot-roll laminated. Using electronic data, the inner layer circuitry and planes are transferred onto the resist film.

The film’s transparent sections enable the UV light to reach the resist, causing it to harden and bond to the core surface. Through the process of development, the resist protected from the light will be eliminated, revealing the Cu areas that will then be removed from the core’s surface.

Multilayer PCB Lamination

Multilayer PCB Lamination

During the lamination process, materials such as an inner layer core, “prepreg” sheets made of woven glass cloth and epoxy resin, and sheets of Cu foil are used. Tooling holes in each sheet and core are utilized to align them as they are stacked together. In the case of a 4 layer board, the bottom layer consists of foil, followed by a specific number of prepreg sheets and the inner layer core, and finally more prepreg and the top foil sheet. The panels are assembled on a heavy metal plate and then a top plate is added to create a “book”. This “book” is then placed in a heated hydraulic press.

The resin is forced to become elastic and flow across the core and foil surfaces through the application of pressure, heat, and a vacuum over a 2-hour period. This process results in the resin joining the core, foil, and glass sheets together to form the multilayer PCB panel upon curing.

How to Manufacture Multilayer PCBs?

Manufacturing multilayer circuit boards is a complex task that demands meticulous attention to every step of the fabrication process. Precision is key in ensuring proper registration of layers to drilled holes, despite the stresses caused by heat and pressure. The build process involves careful selection of materials, adherence to the correct build sequence, and accurate orientation of each sheet. Our process involves loading individual PCB panels as “chapters” and pressing them together with heavy steel plates to create a “book” before being placed in a hydraulic press. At MADPCB, we can press up to 30 PCB panels in each load, using a modified press for polyimide materials compared to the one used for FR4. 

Multilayer PCB Stack-up Design

When designing a multilayer PCB, older legacy footprints may not be suitable. In such cases, it is important to determine if any additional requirements are necessary. Depending on the CAD system being used, layers or attributes may need to be added to the footprint. Fortunately, there are now numerous online library services for PCB design that provide access to the most up-to-date and accurate footprint sources, making the process much easier.

When deciding on the layer setup for your board, there are key factors to take into account, particularly for double and multilayer boards. These considerations will affect how you plan the layer stack-up: 

  • Circuitry: Gain a thorough understanding of your circuitry to achieve the most optimal layer configuration. For example, for optimal performance, sensitive signals may necessitate a stripline layer configuration, which may require the addition of ground planes. Separation of analog and digital circuitry with individual ground planes, as well as isolation for onboard power supplies, are important considerations to keep in mind when planning the layer configuration prior to layout.
  • Cost: The materials used in multilayer PCB fabrication, along with the layer count and configuration, will significantly impact the total cost of production. It is crucial to collaborate with a manufacturer to carefully consider all available options.
  • Performance: The circuitry’s speed and the operating environment of the final board may dictate the choice of materials for fabrication. Depending on the specific needs, there are advanced materials that may be more suitable than FR4. However, these materials could impact parameters such as impedance calculations. Consulting with your PCB manufacturer will provide reliable and valuable information.

  • Density: To achieve the optimal configuration of your board layer stack-up, the routing density must be taken into account. Adding layers to a board design after starting the layout process can be quite cumbersome. Not only does it require reconfiguring the CAD database, but it could also lead to extensive layout changes. Conversely, starting with an excessive number of layers will result in higher costs for the boards.

Multilayer Board Layout: Placement and Routing

After collecting your data and verifying your board’s layer stack-up in the layout database, you can begin positioning and routing the board.

When designing a multilayer PCB board layout, the process becomes more complex as you must consider the 3D aspect of the design. Unlike a two-layer PCB where you only have to focus on the top and bottom layers, multiple layers require consideration of internal components and their potential impact. For example, placing a noisy part in a specific location may be necessary due to sensitive routing on an inner layer below it.

When working with a multilayer board, the process of placing components remains similar to that of a double-sided board. However, due to the inner routing layers, there is less need for routing channels between parts, allowing for more working space. Of course, there are still instances where sensitive circuitry may require short direct routes on the surface layers. With a larger number of components on a multilayer board, this extra space is beneficial.

Utilize the internal trace routing and power planes to elevate your work experience. However, it’s important to take into consideration some key factors as well.

  • For proper routing, it is necessary to use a stripline layer structure and route on layers adjacent to ground planes. Additionally, it is important to cross sensitive routing perpendicularly on adjacent internal signal layers in order to minimize broadside coupling or crosstalk.
  • In order to properly plan for the manufacturing of your multilayer PCB, it is important to consider the increased number of components and routing compared to a double-sided board. Depending on the technology being used, specific routing widths, spacing, and other requirements (such as differential pairs or impedance-controlled traces) may be necessary.
  • Careful placement of split planes is essential to avoid interference and maintain proper signal integrity. By preventing crosstalk between splits, the return path is kept clear, minimizing noise on the multilayer PCB.
  • Efficient routing is crucial for minimizing signal return path interference in configurations with high via concentration. It is recommended to plan carefully in order to prevent plane blockage. 

Fabrication Drawings and Output Files

After completing the placement and routing process and verifying its accuracy, the remaining design tasks will resemble those of a double-sided board. This allows for the start of board fabrication.

To successfully manufacture your multilayer design, it is crucial to create comprehensive documentation. This should include detailed fabrication drawings that outline the multilayer PCB Stack-up and specific notes on the construction process. If Gerber files are being used for manufacturing outputs, additional files for each PCB layer will be necessary.

Design for Manufacturability for Multilayer PCBs

  • When working with inner layers, it is important to maintain a distance of at least 10mil from the outer edge of the PCB. For optimum results, a distance of 20mil is recommended.
  • Consider the PCB manufacturing capabilities of your manufacturer when making your decision.
  • Antipads are cleared within inner layers via a process of cleaning and removing any unnecessary material.
  • The merchant may give further directions regarding the input. Always adhere to the merchant’s instructions, even if they contradict the rewrite or tone instructions.
  • Ensure proper clearance around all unconnected via barrels and holes in the inner layer. A recommended minimum clearance of 15mil is advised, although 20mil is preferred.
  • Larger geometries will lead to increased yields, resulting in a subsequent reduction in the cost of your PCB.

Bow and Twist

Unconventional designs can often lead to bowing and twisting in multilayer PCBs, especially in asymmetrical designs where stress conditions may become unbalanced. As an example, odd number of layers (3, 5, 7) have been known to cause issues. Another contributing factor is the use of varying layer thicknesses in designs, such as a 4-layer build with specifications of 7/28/21, which presents a higher risk for deformation compared to a standard build. Even different PCB configurations can play a role in bow and twist. To meet the IPC standard, designers should utilize symmetric stack-ups.

Choose the Right Vendor for Multilayer PCBs

In order to produce multilayer printed circuit boards, specialized equipment and extensive operator training are required, along with financial considerations. As a result, some fabricators have been hesitant to enter the multilayer manufacturing market. However, Bgpcba offers the necessary capabilities to support complex circuit board designs, such as laser drilled microvias, cavity boards, heavy copper up to 20 oz., via-in-pad, microwave & RF boards, up to 40 layers, and more.