Unpacking is one of the most important phases in the design of a printed circuit. Typically, it follows the layout of the electrical diagram and precedes the physical realization of the PCB (Printed Circuit Board). There is need to analyze the design environment in which the guidelines for setting it up, developing it and completing it successfully are inserted and defined in an organic way.

Unpacking a printed circuit mainly means defining the routing of the tracks that connect the pins of the devices that compose it. This operation is preceded by the analysis and definition of the circuit to be built, the drafting of the wiring diagram and the identification or selection of the components.

Having said that, it is vital to look at the design context of the PCB design grid system in which the unpacking of a circuit is inserted and the guidelines that allow it to be set up, developed and successfully completed in a professional manner.

Since the design of a PCB is computer-assisted, during the exposure we will refer, only in a general way, to the CAD of routing. The purpose of the discussion is, in fact, to give indications for the unpacking of a PCB that are worth as a baggage of a good designer regardless of the specific CAD employed.

A PCB-Printed Circuit Board can be single-sided, double-sided or multilayer. A single-sided printed circuit (Figure 1) is typically made up of an insulating support, a copper sheet on which to form (by subtractive techniques) the tracks and on which to weld and a component side.

In order to reproduce the design of the PCB design grid system on the unit, it is necessary to define the design of the master (on a 1: 1 scale) by performing the so-called unraveling. In the case of double-sided PCBs, the master has two layers and the unpacking both.

The connections between the two layers are made through through holes (or sometimes blind) metallized. For particularly complex circuits we resort to the design of multi-layer boards: a track can develop partially on a layer, then pass onto other layers on which it continues its path.

The problem of routing remains, in general terms, almost unchanged in the design of SMT circuits (Surface Mount Technology or SMD where D stands for Device) except for some details (pads for surface welding and smaller component dimensions, hence the greater level of integration).

Designers for PCB design grid system use CAD with the unit of measurement set in mils: this comes from operational considerations. The distance between the pads of an integrated, for example, is in many cases 100 mils and in general the distances between the pins of a component are equal to an integer number of mils.

Generally, the work grid is set to integer sub-multiple values of the typical distances between the component pins (eg 50 or 25 mils). The approach to the design of a printed circuit is of a top-down nature and the design of the PCB is to be framed in a broad and articulated context.

Design flow

In general, a phase of electrical design is distinguished (functional analysis / analysis, identification of circuit blocks, choice of components, schematic, electrical rule check) and a design of the PCB itself (definition of the board area, arrangement of components, unraveling, design rule check).

The two phases are linked by the creation of a netlist that is generated at the end of the first phase and used in the second. In industrial environments, before proceeding to production, electrical simulations are performed based on pest extraction.

These allow you to validate the work done up to that point before making the form (correcting an error has different weight and cost depending on the moment in which it is discovered).