Schematic to PCB Development

A simple example for a very small project circa 1994

Here is an example of a rapid printed circuit design. Someone sent me this schematic image, and the next day I sent the PCB (printed circuit board) artwork images back. Larger schematics, more specific or more vague component definitions, or tight space requirements will substantially lengthen this process. A professional result requires expensive equipment, yet a few PCBs of this size can be shipped within less than a week for about $100, once I had completed the PCB files described below.

However, this client elected to pursue the experience of etching their own copper-clad material and drilling their own holes.

First I create (or obtain / adopt / revise) a Schematic which represents the electrical signal paths and electronic components desired in the system. In this example the client provided a photocopy or scanned image.  I used a schematic editor to "capture" the schematic. I used OrCAD for this one using a manual interactive process of placing electrical symbols using its GUI graphical working environment. (Update: For more recent work I used Eagle and KiCad. Search "Schematic Editor" or "EDA tools". Most recent workplaces also used Altium, Cadence Allegro, and Synopsis). 

There are eight transistors and a voltage regulator in this schematic. This circuit could be built with a variety of transistor shapes and at least three voltage regulator shapes. The schematic editor need not require this information, so the designer may pursue an accurate representation of the electrical interconnections without worrying about the actual shapes and sizes of the actual components. In this case the circuit has repeating portions. The schematic editor program can capture portions of the circuit, much as a "cut/paste" feature in a text or graphic processor program. Here is my capture of this schematic:

The schematic editor, however, make associations between the endpoints of lines, junctions, and component leads, so that "netlist" information may be generated. The netlist may be printed, for one-of-a-kind and wire-wrap construction, or passed to the Printed Circuit Board (PCB) editor to facilitate the placement of printed or wired circuit paths. Here is an excerpt from the netlist (this is only one, primitive, netlist format):

    ( N00001  U1,3   J3,1 )
    ( N00002  U1,2   R18,2   SW1,1 )
    ( N00003  SW1,2  LED9,2  J2,4  J2,5 )
    ( N00004  U1,1   VR1,3   R18,1 )

Single and two-layer PCBs (printed circuit boards) are quite common; midplanes are frequently added for power and ground which greatly improve electromagnetic emissions characteristics. Additional signal planes are present in more advanced designs and may substantially extend the lead time to a successful prototype. This image is a composite of several "layers" - a printed circuit board house will require several CAM (Computer Aided Manufacturing) files separately in order to represent the copper layer(s), solder-mask material (that covers the copper on a finished board), 'silkscreen' layer to designate component shapes and reference to the assembly instructions, holes and sizes, and routing (outline cutting and mounting hole) instructions. The CAM files are simple text files containing feature and coordinate information; they cannot be viewed with conventional GIF / JPG image viewers - instead a Gerber format viewer is utilized. This format associates a set of apertures with X,Y positions, or Lines or Tracks drawn between X,Y positions. Typically letters and numbers and literally any shape that is not round, square, or a line, is made up of draw commands using the simpler shapes. A Design-Rule Check (DRC) is performed to insure that lines and shapes do not touch each other, and maintain proper minimum distances so that the finished PCB will not have shorts, solder will not bridge the gaps during assembly, and voltages will not jump across during operation.

Here is an excerpt from the Aperture file:

                                Aperture and Tool Descriptions
  ========================================================================

    Code   Shape    X    Y    Hole  Type   Comment
    ------------------------------------------------------------------------
    D10    Ellipse  010  010   000   Draw    10 Mil Round  Draw
    D11    Ellipse  012  012   000   Draw    12 Mil Round  Draw
    D12    Ellipse  020  020   000   Draw    20 Mil Round  Draw
    D13    Ellipse  030  030   000   Draw    30 Mil Round  Draw
    D14    Ellipse  050  050   000   Draw    50 Mil Round  Draw

    G54D13*
    X3425Y7225D02*
    X3550Y7225D01*
    X2575Y7225D02*
    X2675Y7225D01*
    D02*
    G54D10*
    X2030Y7495D02*
    X2205Y7495D01*
    ...
    D02M02*