Multilayer PCBs - Points to be considered while Manufacturing

Multilayer PCBs are considered the epitome of Design for Manufacturability (DFM). They comprise complex designs, and are produced using high grade materials. They are designed to support intricate applications such as industrial controls, CAT scanning equipment, weather analysis machines, and satellite systems.

One of the key aspects to the successful manufacturing and use of a multilayer PCB is its circuit board stackup. This configuration improves the PCB’s performance in an application. Let’s take a look at the important points that need to be considered when designing or selecting a multilayer PCB.

Planning Multilayer PCB Stackups

Before initiating any work on the PCB, the manufacturer must first consider the following points:

• Signal Layers: As the name suggests, these layers are designed to maintain proper integrity and transmission of signals.
  • Ground Planes: These are conductive layers used for shielding, circuit returns, and heat sinking purposes.
  • Signal Path: The path that the signal will have to travel from the PCB to the equipment.
  • Cost: This is dependent on the PCB design. More investment will allow the manufacturer to design a more complex PCB.

Discussing these points further, the manufacturer must also keep in mind:

• Signal layers should always be next to adjacent planes, and should be tightly coupled with the planes.
• The return path for a signal should be determined in the layout diagram. Power and ground planes can be used for the signal path.

Factors Affecting PCB Stackup Design

When manufacturing multilayer PCBs, the goal of a manufacturer is to improve signal integrity, reduce crosstalk and electromagnetic emissions, and have a low power inductance network. The following factors are extremely important when creating a multilayer PCB that achieves these goals:

• Soldermask: This needs to be taken into consideration when calculating impedance levels. A soldermask coating can reduce microstrip impedance, especially with thin traces. Generally, soldermask can reduce impedance by 2 or 3.5 ohms.

• Dielectric Materials: These materials are used to provide insulation between single and multiple connectors. Generally, FR4 is the material of choice, and can be either core or impregnated (prepreg). Core materials are fiberglass epoxy resins with copper foils. Prepreg materials are fiberglass sheets, which are impregnated with epoxy resin.

The easiest method to support multilayer PCBs with dielectric materials is to incorporate the Foil Method. This method consists of two points. First, prepreg materials should be bonded on the top and bottom layers. The second point is to alternate between core and prepreg materials through the layers.

• Ball Grid Arrays (BGA): This is a chip package provided in a grid array style. Solder balls are used by external terminals to carry electrical connections from the PCB to the device. The number of layers in the PCB are decided after considering the number of power supplies the BGA requires, and signal nets extruding from the BGA.

These are the factors that need to be considered when designing multilayer PCBs. A proper design can lead to the manufacture of a printed circuit boards on time, and within budget.