Basics Pro Reviews System

From the schematic to the finished PCB layout – generally explained

I have already tried to explain the rough course of a development with an article. I also touched the circuit diagram and the PCB layout very superficially. Afterwards, Igor had addressed several details in the article "Why the performance increase of Nvidia's Ampere could be quite enormous – the board partners already have to go into the practice phase" several details, which in retrospect had led to a longer discussion in the forum.

Since there has always been a great deal of interest in the subject and it has been clearly noted that there are still many ambiguities about it, I would like to explain the whole thing a little more closely (and above all enough in general terms) by means of a fine example. The article may still be a little more difficult to understand than the usual text cost, even if I have reused a few pictures and texts, but I trust in the expertise of the readers of the diieser side. Insights and practice are always an interesting pairing.

Since I can of course not publish data from my employer here, I have been able to use the iMX6 Rex including baseboard ( https://www.imx6rex.com/ ) by Robert Feranec (FEDEVEL Academy) for the example. The project was kindly published as open source including the data for the program Circuitmaker© and Altium Designer© and I was thankfully given permission to use the data for articles. This data is currently also used for rates at The Fedevel Academy. In addition, I use an example that I started privately in the program Circuitmaker© with components from the Octopart Library. Let's go!

What is another schematic please?

So let's start with a repetition of part of the development article. A schematic contains a schematic that normally represents all electrical connections. This usually requires a few pages depending on the project size. And now symbols and the component library come into play. The individual components in the schematics are represented with simplified symbols. Of course, there are also different modes of presentation here: the IEC 60617, which is commonly used in the English-speaking world, and the EN-60617, which is common in Germany. Here is a small selection (above the usual symbol in Germany, below the usual in the English-speaking area; the colors are freely chosen):

Figure 1: Symbol examples of a schematic

Components with many connectors are often displayed only as a rectangle and then the connectors are named accordingly.

Figure 2: Symbol example of an IC in a schematic

Of course, these symbols must first be created. In fact, this is largely manual work. It is really the case that each port should be named and the corresponding properties (is it an input or output, etc.) should be assigned. However, the libraries contain not only the symbols but also the part footprints. A footprint here is nothing more than an exact drawing of what the component will look like later on the printed circuit board. Depending on the EDA program, a footprint can also contain the 3D data of a component. Here's a rough look at what you mean by it:

Figure 3: Building a Footprint

 

What is here what?

  • The solder stop coating release marks the area that must be exempted from the solder ingess coating. Otherwise, nothing could be soldered there, as the soldering stop varnish, as the name suggests, effectively prevents soldering.
  • The solder paste seam construction saving is really just that. The soldering paste is applied over a sieve. And logically, only solder paste can be found where the sieve was omitted
  • Copper is ultimately the actual soldering pad. Of course, this must also be numbered or otherwise labelled.
  • You don't really have to say much about the contour or the model. You just have to know how big the component is.
  • Screen printing is not found on every printed circuit board, but can still be practical
  • The center point is usually also the point used for SMD placement machines. The free area is a help for the PCB layout

Some EDA programs support footprint creation with footprint generators.  With the help of these generators, many common footprints can be created with relatively little effort. You virtually type the dimensions out of the data sheet and the generator does the rest. It goes so far that e.g. in the Altium Designer© even the 3D model can be created in step format.

Figure 4: Circuitmaker© IPC® Compilant Footprint Wizard

 However, it is important to keep in mind that not every footprint can be created with it. Exotic footprints are therefore mostly still handmade. Of course, the solder pads of the footprint still have to be linked to the connectors of the symbol. Most of the EDA programs I know are still done in the library.

Figure 5: Linking Symbol and Footprint

Additional information that is usually included in the component library is:

  • For passive components, the respective values (e.g. resistance value) including tolerances
  • Manufacturer
  • Certifications of the component e.g. AEC-Q
  • Supplier and item number at the supplier. If possible, several are packed here
  • Availability
  • Price
  • Operating voltage range
  • Temperature range
  • Interface types (e.g. SPI, I2C, LVDS, etc.)
  • Replacement types
  • design (e.g. as in this example SOT-23)

1.1.2.   Connections

The connections of the components are connected to lines representing the electrical connections. These are usually referred to as networks. For example, this looks like this:

Figure 6: Example of electrical connections in a schematic

Connections between 2 lines, so-called nodes, are usually represented with a thick dot. That is the first time we are through here.