1, every PCB needs a good foundation: assembly instructions
　　The fundamental aspects of a PCB include dielectric materials, copper and alignment dimensions, and mechanical or dimensional layers. Materials used as dielectrics provide two basic functions for PCBs. When we build complex PCBs capable of handling high-speed signals, the dielectric material isolates signals found on adjacent layers of the PCB.The stability of the PCB depends on the consistent impedance of the dielectric across the entire plane as well as over a wide frequency range.
　　While it may seem obvious that copper acts as a conductor, there are other functions. Different weights and thicknesses of copper affect the ability of a circuit to achieve the correct amount of current flow and define the amount of loss. In the case of the ground and power layers, the quality of the copper layer affects the impedance of the ground layer and the thermal conductivity of the power layer. Matching the thickness and length of differential signal pairs solidifies the stability and integrity of the circuit, especially for high frequency signals.
　　Physical dimension lines, dimensional markings, data sheets, cutout information, through-hole information, tooling information, and assembly instructions not only describe the mechanical or dimensional layers, but also act as metrics for the PCB foundation. Assembly information controls the mounting and location of electronic components. Because the “printed circuit assembly” process connects functional components to alignments on the PCB, the assembly process requires the design team to focus on signal management, thermal management, pad placement, the relationship between electrical and mechanical assembly rules, and the physical mounting of components to meet mechanical requirements.
　　The assembly documentation in IPC-2581 is required for each PCB design. Other documents include the bill of materials, Gerber data, CAD data, schematics, fabrication drawings, notes, assembly drawings, any test specifications, any quality specifications, and all regulatory requirements. The accuracy and detail contained in these documents reduces any chance of error in the design process.
　　2. Rules that must be followed: exclusions and wiring layers
　　Electricians installing wiring in a house must follow rules to ensure that wires do not bend sharply or become vulnerable to nails or screws used to install sheetrock. Routing wires through a wall of double studs requires a consistent approach to the depth and height of the wiring path.
　　The Holding Layer and the Routing Layer establish the same constraints for the PCB design. Holding layers define physical constraints (such as component placement or mechanical clearances) or electrical constraints (such as cabling holds) for the design software. The cabling layer establishes the interconnections between components. Depending on the application and type of PCB, cabling layers can be placed in the top and bottom or internal layers of the PCB.
　　Finding space for ground planes and power planes
　　Every house has a main electrical service panel or load center that receives incoming power from the utility company and distributes it to the circuits that power lights, outlets, appliances, and devices.The grounding plane and power plane of a PCB provide the same function by grounding the circuits and distributing the different on-board voltages to the components. As with service panels, the power and ground layers can contain multiple copper segments that allow circuits and subcircuits to be connected to different potentials.
　　Protect the board, protect the alignment
　　Professional house painters will carefully document the colors and finishes of ceilings, walls and trim. On PCBs, screen-printed layers use text to designate the location of components on the top and bottom layers. Having the information available through screen printing saves the design team from having to refer to the assembly documentation.
　　Primers, paints, stains and varnishes applied by house painters can add intriguing colors and textures. In addition, these finishes protect the surface from deterioration. Similarly, a thin soldermask on the PCB helps the PCB prevent shorting of the alignment when some type of debris falls on the alignment.
　　PCB Stacking Rules
　　As PCB technology has improved and consumer demand for faster, more powerful products has increased, PCBs have gone from basic two-layer boards to boards with four, six, and as many as ten to thirty layers of dielectrics and conductors. Why add more layers? Having more layers improves the board's ability to distribute power, reduce crosstalk, eliminate electromagnetic interference and support high-speed signals. The number of layers used for a PCB depends on the application, operating frequency, pin density and signal layer requirements.
　　With two-layer stacking, the top layer (i.e., layer 1) is used as the signal layer. Four-layer stacking uses the top and bottom layers (or layers 1 and 4) as signal layers, with layers 2 and 3 used as planar surfaces in this configuration. The prepreg layer bonds two or more double-sided boards together and acts as a dielectric between the layers. The six-layer PCB adds two copper layers, with layers 2 and 5 serving as planar surfaces. Layers 1, 3, 4 and 6 carry signals.
　　Moving on to the six-layer structure, the inner layers two and three (when double-sided boards) and four and five (when double-sided boards) are core board layers, with semi-cured sheets (PP) sandwiched between the core boards. Since the semi-cured sheet material is not yet fully cured, the material is softer than the core material. the PCB manufacturing process applies heat and pressure to the entire stack and melts the semi-cured sheet and the core so that the layers can be bonded together.
　　Multilayer boards add more copper and dielectric layers to the stack. In an eight-layer PCB, seven internal rows of dielectric bond the four planar and four signal layers together. Ten- to twelve-layer boards increase the number of dielectric layers, retain the four planar layers, and increase the number of signal layers.