PCB design basics
This article discusses the major steps in the PCB design flow, from basic terminology to the primary steps required to move an example design through the schematic, layout, and manufacturing stages.
Understanding the Terminology
Schematic capture and simulation tools – A schematic capture program allows the user to draw a document representing the electrical component symbols and the interconnections between them in a graphical way. Before generating a PCB, the symbols are mapped to component footprints and the symbol interconnections are converted to a netlist that specifies the connections between the component footprints in the layout process. A schematic tool that allows the user to do interactive circuit simulation with the same schematic circuit representation used for layout is advantageous. Circuit simulation can be useful for both initial design analysis and testing the design (i.e. verification testing and troubleshooting) once complete.
PCB layout tools – A PCB layout program generates the mechanical and wiring connection structure of the PCB from the netlist. The layout program allows the wiring connection structure to be placed on multiple layers and, once complete, allows the user to generate the computer aided design (CAD) files needed to manufacture a PCB.
Gerber files – The CAD files that need to be sent to a PCB manufacturer so it can build the PCB layer structure are called Gerber files. The RS-274X is the most commonly supported Gerber file format.
NC drill files – The numerically controlled (NC) drill files indicate the size and position of holes used for unplated holes, plated through-holes, or holes for vias. Some quick-turn PCB manufacturers have only select hole sizes available.
Printed circuit board (PCB) – A wafer board defining the mechanical and copper wire structure of the circuit. (It is sometimes called a PWB for printed wiring board).
PCB structure and details
A PCB can be considered a layered structure, usually with multiple copper and insulating layers. The main portion is a non-conductive (insulative) material (substrate) usually made from fiber glass, and epoxy. The substrate material used to separate layers comes in different thicknesses, from 0.005” to 0.038”. Conducting layers consist of copper (Cu) foils that are etched away in specific areas where the user does NOT want connections to occur. A single-layer PCB has the substrate with one layer of copper foil on the top (see Figure 1).
A double layer PCB (see Figure 2) has two layers of copper foil (one on the top and one on the bottom).
If more than two layers are required due to increased complexity of the PCB, other layers of copper can be built-up or added to the ones shown above (usually in pairs). For example, a 4-layer PCB can be made up of two double-layered PCBs laminated (sandwiched) together with a core material in between. Made out of epoxy/fiber, this core layer is called a prepreg (pre-impregnated), and it insulates and supports the other layer structures. It is common for modestly complex boards to have 6, 8, or 10 layers (with increased manufacturing cost). Some highly complex PCBs have up to 32 layers or more of traces and copper planes (see Figure 3).
The height of the substrate is usually the thickness of one or multiple sheets of laminate material and is usually much smaller than the height of the core prepreg material layer.
Multilayer PCB – A PCB with more than one copper foil layer. The layers are preferably renamed in the design tool to unique names (such as power or ground) as desired by the user.
Layer Stack Up – The copper organization of multiple layer PCBs with the intent of having specific signal and ground planes on certain layers for routing convenience and electromagnetic shielding purpose. A four-layer board will typically have the following layer structure, where the top and bottom layers are reserved for signal routing and the inner layers are reserved for ground and power planes:
• Copper Top
• Inner 1
• Inner 2
• Copper Bottom
Finished PCB Height - Standard finished PCB thicknesses are commonly found as shown – this thickness includes all copper, substrate and prepeg layers:
• .031” (also .039" is common)
• .062" (most commonly used size)
Shown in Figure 4 is a more realistic layer stackup of a four-layer PCB showing the various thicknesses of the layer structures from a typical PCB manufacturer yielding the common 0.062” finished PCB height.