Complete Guide on Multilayer PCB

Multi-Layer or Multilayer Printed Circuit Boards (PCBs), or Multilayer Boards (MLBs), which have more than 2 copper layers. A multilayer PCB must have a minimum of 3 conductive layers of conductive material or copper layer. All the layers are interconnected with copper plated holes, including NC vias and laser microvias either in plated through, or in buried and blind.

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Layers of copper foil, prepreg (PP) or adhesive, and core materials are sandwiched together under high temperature and pressure to produce multilayer PCB boards. Pressure is needed to squeeze out air while heat is required to melt and cure the thermosetting PP or adhesive which holds the multilayer PCB together.

Layer Counts of Multilayer PCB

At MADPCB, we can fabricate, assemble and design multilayer PCBs in different types, like rigid, flex, rigid-flex and metal core PCBs, and each type has different layer counts.

• Rigid Multilayer PCBs: 4, 6, 8, &#; up to 30 layers
• Flex Multilayer Circuits: 3, 4, 5, &#; up to 8 layers
• Rigid-Flex Multilayer PCB: 2, 3, 4, &#; up to 20 layers
• Metal Core Multilayer Boards: 4 layers at most

How to Manufacture Multilayer PCBs?

Manufacturing multilayer circuit boards is a straightforward process but does require a high level of attention to detail in all fabrication processes. Care must be taken to ensure all layers are correctly registered to the required drilled holes despite the deformation stresses produced by the heat and pressure. The buildup requires the operator ensures correct materials are chosen, the build sequence is followed and the orientation of each sheet is correct. Each unpressed PCB panel is loaded as a &#;chapter&#; with up to 10 chapters presses together to form a &#;book&#; separately by heavy steel plates. These books are then loaded into each chamber of a hydraulic press. At MADPCB, we have the capacity to press up to 30 PCB panels into each load. The press for pressing polyimide material is modified slightly from the process required for FR4.

Multilayer PCB Stack-up Design

In some cases, older legacy PCB footprints may not be adequate for a multilayer design, and you need to find out if there are any additional requirements necessary. Depending on the CAD system being used, you may have to add layers or attributes to a footprint for a multilayer use. But nowadays you have access to a many PCB design system to online library services can be real benefit. It makes it much easier to have the latest and most accurate PCB footprint sources to work with.

The main difference between a double layer and multilayer board setup, will be in planning your layer setup. The following are some of the points that you will need to consider while planning your board layer stack-up:

• Performance: How fast the circuitry will operate at, and the operating environment of the final board, may make a difference in the materials that the board will be fabricated with. There are more advanced materials than FR4 that may be suited for your application depending on what the needs are, but those materials may affect parameters such as our impedance calculations. Here is where the help of your PCB manufacturer will be an invaluable source of information.

• Cost: The multilayer PCB fabrication materials as well as the layer count and configuration, will have a direct bearing on the overall cost of building the board. Here, again, you need to work with your manufacturer to consider all of the options.

• Density: The routing density of your board is another factor when determining the configuration of your board layer stack-up. It is very painful when you have to go back and add layers to a board design after you&#;ve already started your layout. Not only do you have to reconfigure your CAD database, but you may have to make a lot of changes to your layout. On the other hand, if you start with too many layers you will be paying more for the boards then you should.

• Circuitry: You also need to understand the needs of your circuitry in order to create the most optimized layer configuration. For instance, sensitive signals may require a stripline layer configuration for their best performance, which will mean adding additional ground planes. Areas of analog and digital circuitry will need to be separated with their own ground planes, and onboard power supplies will need isolation. All of this could have an effect on the layer configuration, and this should be planned for before layout starts.

Multilayer Board Layout: Placement and Routing

Once you&#;ve gathered your data and confirmed your board layer stack-up in the layout database, it will be time to start placing and routing the board.

When working on a multilayer PCB board layout, one of the first things that will be different is how much you need to start thinking in terms of &#;3D&#; design. A two-layer PCB only requires you to consider it in terms of top layer and bottom layer. Now you are in a world of multiple layers, and there are different things happening internally that could affect the top and the bottom. For instance, you may now want to place a noisy part in a certain location, because of the sensitive routing on an inner layer underneath it.

As far as the tools go, placing components will be the same as with a double-sided board, but the landscape that you are working with will be different. For instance, you don&#;t have to worry about leaving a s much space for routing channels between your parts since they will be mostly routed on the inner layers. There will still be a need for short direct routes on the surface layers for sensitive circuitry, but for the most part, you now have more room to work with. This is good thing too because with a multilayer board, there are probably a lot more components that need to be placed.

Internal trace routing and power planes will be a joy to work with, but at the same time there are some important considerations here as well:

• Multilayer PCBs will typically have more components and therefore more routing than a double-sided board, so plan ahead for it. Depending on the technology of the board, some of this routing may have required specific routing widths and spaces or other requirements, such as differential pairs or impedance-controlled traces.
• Some routing will require a stripline layer structure and must be routed on layers adjacent to ground planes. Additionally, sensitive routing must be crossed perpendicularly on adjacent internal signal layers to help reduce any possible broadside coupling or crosstalk.
• Ground planes will have a lot of vias in them for connectivity, but those vias could affect signal return paths. This requires carefully planning your routing to avoid blocking up the planes.
• Split planes need to be laid out so that sensitive signals don&#;t cross the splits and thereby ruin their return path. A situation like this can create a lot of noise on the multilayer PCB.

Fabrication Drawings and Output Files

Once the placement and routing are done and checked, the rest of the design work will be the similar to a double-sided board. Now you are ready to have the boards fabricated.

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To get your multilayer design out for manufacturing, you will need to create the documentation. In your fabrication drawings, you need a multilayer PCB Stack-up detail, and notes detailing the specifics of how the PCB will be built. If you are using Gerber files for your manufacturing outputs, you will obviously need to generate additional files for the multiple PCB layers.

Design for Manufacturability for Multilayer PCBs

• Taking PCB manufacturing capabilities of your manufacturer into considerations
• Copper areas -on inner layers, all copper should be kept at least 10mil from the outer edge of the PCB, 20mil is preferred.
• Via clearance through inner layers (antipads)
• Allow for clearance around any holes or via barrels not connected to an inner layer. This clearance should be at least 15mil although 20mil clearance is preferred.
• Thermal Relief Pads -the tie should be a minimum of 8mil while more is preferable.
• Larger geometries -will result in higher yields which will be reflected in your PCB cost.

Bow and Twist

Bow and Twist in multilayer PCBs is typically the result of unconventional designs, which is more likely occur in asymmetric designs which can result in unbalanced stress conditions. For example, odd layer counts (3, 5, 7 layers) are known to cause issues. Another source of multilayer PCB bow and twist comes from designs which specify variable layer thicknesses. For example, a 4 layers buildup specification of 7/28/21 creates more risk of deformation than a standard build. Even different PCB configurations can be influencing factors. For reaching the IPC standard bow and twist, the designers should use symmetric stack-ups.

Choosing the Right Manufacturer for Multilayer PCBs

Manufacturing multilayer printed circuit board requires specialized equipment and a significant commitment to operator training, not to mention the financial consideration when the board with complex design. This explains why some PCB fabricators have been slower to step in multilayer boards production market than us. MADPCB can provide the advanced capabilities to support advanced PCB designs with demanding requirements including laser ablated microvia, embedded passive boards, heavy copper PCB, via-in-pad, high frequency boards and others.

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Single-layer PCBs and double-layer PCBs are two basic types of boards that have only one or two layers of conductive materials over the substrate. When more routing space or signal integrity is required, multi-layer PCBs are suited to meet these demands. Unlike single-layer and double-layer PCBs, multilayer PCBs stack multiple layers of conductive and insulating materials to create a complex network of connections. The stack-up typically includes copper layers, cores, substrates, and prepreg. Prepreg and core are two types of insulating materials or dielectric materials in PCBs. 

Figure1 : Multilayer PCB stack-up

What is prepreg?

Prepreg, short for pre-impregnated, is a layer of fiberglass cloth impregnated with uncured epoxy resin but without any attached copper foil. In a PCB stack-up, prepreg serves as both an insulating layer and an adhesive layer. When constructing a PCB stack-up, the prepreg is placed between adjacent copper layers or between a copper layer and a core material. During the manufacturing process, high temperature and pressure cause the epoxy resin in the prepreg to flow and bond the layers together, resulting in a solid, rigid board. 

Figure2 : Prepreg

What is a core of a PCB?

The core of a PCB is the central layer of the board that provides mechanical support and rigidity. It typically consists of a laminate material made of thin layers of copper foil bonded to a rigid substrate material. The copper foil on the core can serve multiple purposes, such as providing a ground plane or a signal plane. In multilayer PCBs, there can be multiple core layers, with prepreg layers between each core layer and the outer copper layers.

Figure3 : PCB core

HDI Stack-up

Compared to traditional PCB stack-ups that rely on through-hole vias to interconnect layers, HDI stack-ups utilize micro-vias, blind vias, and buried vias to achieve higher routing density and better electrical performance. The X-N-X structure is a common way to describe the HDI stack-up configuration. Here, X represents the number of HDI layers with micro-vias, while N represents the number of core layers sandwiched between these HDI layers. Some common stack-up configurations include 1-N-1, 2-N-2, and 3-N-3.

To further illustrate how the X-N-X structure is applied in practice, let's consider the example of a 1-4-1 stack-up configuration. In a 1-4-1 structure, there is one HDI layer with micro-vias at the top of the PCB, followed by four core layers with no micro-vias in between, and finally another HDI layer with micro-vias at the bottom of the PCB.

Figure4&#;HDI Stack-up example

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