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.
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.
At MagicPCB, 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.
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.
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:
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:
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.
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.
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.
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. MagicPCB 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.
