Engscope

Before we can make a PCB, let’s take a look at its composition. If you deal with PCBs all the time (as in, you do PCB layout all day long), you can probably skip this section. However, if you’re a bit shaky on PCB concepts, it’s best to at least skim through this section.

PCBs stands for printed circuit boards. They are called “printed” because you print your circuits out onto the copper. With the design printed, you then either mill or etch your prints into the copper. The general process is actually quite complex, especially with quality control considerations and efficiency measures implemented in large fabrication houses. However, the process can be simplified into manageable steps such that home fabrication is possible. We will also be skipping a ton of steps, just because some of the more complicated features such as silkscreen and multilayer (more than two layers) is impossible at home. But I digress, here’s what you need to know about the PCB itself.

The surface of the PCB has several features. You will probably notice right away, when picking up any typical PCB, that the majority of the surface is covered with green stuff. This is called the soldermask, and it is a dielectric (insulator). It actually has several specific tasks. First, it is there to prevent corrosion, as the oxygen in our atmosphere is quite toxic to the copper on the top and bottom layers. Next, it has the job of preventing accidental shorts from occurring. The exposed copper is very vulnerable to paper clip drops and loose screws. Best to cover it up with green stuff that won’t conduct.

The next thing you will notice are the tiny lines that run across the surface of the board (albeit covered in green). They are the copper that reside on top of the PCB. This is how electrical connections are created from one electronic element to another. The term used to describe these lines is “signal trace” or just “trace”; they describe the trace that a copper takes from one point to another. Next, there are the pads. These are exposed bits of metal covered in tin (through electroplating). They are exposed so that the pins on your ICs and your resistors can be soldered onto the board. The tin does not oxidize, but is still conductive. This property protects the underlying copper, while still allowing an electrical connection to occur to the component being soldered. A plus side is that the metal tin is a major component in modern solder, such that the flow of the solder is facilitated by the tinned pads. Lastly, the colored letters and markings seen on top of the soldermask are called the silkscreens. They are aptly named since the markings are applied to the soldermask through a silkscreen process. It is essentially a stencil made with a thin membrane, onto which colored ink is applied. This layer allows the PCB designer to label the components, and indicate switches and functionality.

However, there are things underneath the surface that cannot be seen with the naked eye, but play a key role in the functionality of the board. Below is an example of a 4 layer PCB, typically very cheap to manufacture.

On any given PCB, you can only see the top and bottom copper traces. Underneath however, there may be many layers of copper creating connections between components. The cost of a PCB is generally dictated by the number of layer. These layers increase the number of possible connection options between components by allowing traces to intersect one another without shorting out. For very dense circuits such as mobile devices, more layers are need since the number of connection per area is high. On circuits with lower densities, a lower number of layers is preferred since it reduces manufacturing costs.

The round circle like things that can be seen on the surface of PCBs are called vias.

These are drilled holes that create the connections between the layers. The holes are actually drilled after the copper traces are created, and synthesized through copper electroplating. With a combination of traces and vias, the PCB designer is allowed to create circuits in three dimensions.

Finally, the layers between the copper (labeled “core” and “prepreg” in the above picture) are FR4 (most of the time). The abbreviation stands for Flame Retardant 4, created out of fiberglass and resin. These insulators create the structure of the board, and gives it rigidity. The copper on each of the conductive layers are grown onto the FR4, then etched off in acid to create the traces. Each stack, consisting of one layer of copper and one layer of FR4, are then put together on a heated vacuum press, and allowed to meld together into a single board structure.

The goal of our manufacturing process is much less ambitious. We will be constructing a two layer board, with no soldermask, no pad tinning and no silkscreens. The process is usually called “barebone” since it only contains the bare essentials of a functioning PCB. As long as the signals pass through, it can be technically called a circuit board.

I am always amazed at these marvels of manufacturing technology. I think a lot of people take for granted the plethora of electronic devices that are used in our daily lives, simply because there are so many of them. However, the production of a PCB is by no means an easy task. Next time you type on your keyboard or power up your favorite MP3 player, please refrain from being impressed by the functionality of the gear. Take a moment and marvel at the humble PCB that is surely lodged inside their plastic shells.

In the next section we will make preparations for the manufacturing process.

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