Computer-aided design environments
The program does not draw for you: it speeds up what you already know how to do. What CAD changes is that the drawing becomes a model with data, and from it come the bill of materials and the board.
01What changes when the drawing becomes a model
A paper drawing is a set of lines: the meaning is supplied by whoever reads it. A CAD drawing is a database: the program knows that this rectangle is a 470 Ω resistor, that it is connected to that node and that its package measures so much. All the advantages come from that difference.
| On paper | In CAD or EDA |
|---|---|
| It is redrawn to change a dimension | You change the parameter and everything updates |
| Dimensions are written by hand | They are generated from the geometry and cannot lie |
| The bill of materials is copied by hand | It comes out of the drawing itself |
| Connection errors are hunted by eye | The rule checker flags them |
| There is only one good copy | There is version control and history |
It does not decide which views are needed, nor which dimensions are functional, nor how to organize a schematic so that it is understood. All of that is supplied by whoever draws, and that is why the three previous pages of this course come before this one.
02Mechanical CAD
- Parametric drawing. Geometry is defined by constraints—parallelism, tangency, dimensions—and not by coordinates. Changing a dimension rebuilds the whole part.
- Layers. Outlines, dimensions, centerlines and annotations on different layers, each with its own color and line weight. They can be turned off while working.
- Blocks. Repeated elements—a screw, a symbol, the title block—defined once and reused; if the definition changes, all the instances change.
- 3D modeling and automatic views. The solid is modeled and the program generates views, sections and perspectives that are consistent with each other.
- Exchange. DXF and DWG for 2D drawing, STEP for solids, STL for 3D printing, PDF for reading. Choosing the wrong format means losing information.
03EDA: from schematic to board
In electronics the tool is called EDA, and the workflow is always the same:
| Step | What is done | What you get |
|---|---|---|
| 1 · Capture | Draw the schematic with library symbols | Schematic |
| 2 · Electrical check | Check for unconnected pins and conflicting outputs | Rule report |
| 3 · Netlist | Extract the list of nodes and components | Netlist and bill of materials |
| 4 · Simulation | Run the circuit in SPICE | Waveforms, frequency response |
| 5 · Assign footprints | Link each symbol to its real package | Components with dimensions |
| 6 · Routing | Place and connect on the board | PCB layout |
| 7 · Manufacturing | Generate files for the manufacturer | Gerber, drill files, BOM, pick and place |
It is a text file that says which components there are and which pins share a node. It has no geometry: it is the pure topology of the circuit. It is used to simulate, it is used to route the board and it is used at the end to verify that the board matches the schematic.
Choose a circuit and move the adjustable value: the schematic, the netlist and the calculation update together, as in an EDA program.
04Board design
With the netlist loaded, the program shows the pending connections as straight lines—the ratsnest—and the work consists of placing the components and turning those lines into real traces, respecting design rules: minimum trace width, clearance, width according to current, distance to edges, and isolation between sections at different voltages.
- Board layers: top and bottom copper—or more, on multilayer boards—, solder mask, silkscreen and drill holes.
- Output files: one Gerber per layer, Excellon for the drill holes, BOM and a placement file for automated assembly.
- Final check: the Gerber viewer shows what is actually going to be manufactured. Looking at it before sending is the last chance to find an error.
Everything done there is seen afterward in the fabrication of the printed circuit board, which is where the design becomes physical.
05Practices that do not depend on the program
- Name things well. Files and versions with a clear scheme; no “final2_really”.
- Your own libraries. A wrong symbol or footprint ruins a batch of boards: it is worth checking each new footprint against the datasheet.
- Save the complete project. Schematic, board, libraries and manufacturing files together; a loose Gerber cannot be modified.
- Document the why. A note on the schematic explaining an odd decision is worth more than ten comments in an email that nobody will find.
- Open formats when possible. A project tied to one version of a commercial program can become unreadable in a few years.
06In the lab
Capture a five-component circuit in KiCad, assign footprints, route a single-sided board and generate the manufacturing files. Review them in a Gerber viewer before calling it finished.
Simulate in SPICE the RC filter from this page’s lab, obtain its frequency response and compare the simulated cutoff frequency with the calculated one and with the one measured on the breadboard.
Model in CAD the enclosure cover with the cutouts for the connectors of the board you designed. Check that they match by exporting the board in 3D and inserting it into the model.
07Common mistakes
- Drawing without constraints in parametric CAD and losing the ability to modify.
- Using a footprint without checking it against the component’s datasheet.
- Routing without having defined the design rules and discovering later that the traces are too thin for the current.
- Sending to manufacture without looking at the Gerbers.
- Trusting the program to correct your judgment. A messy schematic is still unreadable even if it was made on a computer.
- Not saving the libraries with the project, so that symbols are missing on another machine.
08Self-assessment
What is a netlist?
The list of components and of the nodes their pins share: the topology of the circuit without geometry. It is the bridge between the schematic, the simulation and the board.
Which format is used to exchange a solid between CAD programs?
STEP. DXF and DWG are for 2D drawing and STL for 3D printing.
Which files does a board manufacturer ask for?
A Gerber for each layer, a drill file, and for assembly also the BOM and a placement file.
What are layers for in CAD?
To separate outlines, dimensions, centerlines and annotations, and to be able to show or hide them with different colors and line weights.
A divider of 10 kΩ and 4.7 kΩ with 12 V. What is the output?
12·4700/14700 ≈ 3.84 V. It is the calculation the lab on this page does.
Why is it a good idea to simulate before building?
Because a conceptual error is detected in seconds and without spending components. It does not replace measurement, but it avoids the most expensive surprises.
09Further reading
- Official KiCad documentation. KiCad Documentation. KiCad EDA, current version. The manual of the most widely used free tool: capture, footprints, routing and manufacturing.
- Chris Simpson. PCB Design Guidelines. Manufacturer application notes. Practical rules for trace width, ground planes and component placement.
- Frederick E. Giesecke et al. Technical Drawing with Engineering Graphics. 14th ed. (Spanish edition, “Dibujo técnico con gráficas de ingeniería”), Pearson, 2013. The CAD chapters, for the mechanical side of the work.
- Paul Horowitz and Winfield Hill. The Art of Electronics. 3rd ed., Cambridge University Press, 2015. The appendix on prototype construction and board design.