Anatomy of a PCB24 Jul 2013
First we should understand what materials go into a PCB. At the most basic level, the base of the PCB is formed out of some sort of solid, non-conductive, material. This material is then laminated with a copper (or other metal) sheet, this creates the conductive surface.
The base material is usually a type of glass-reinforced epoxy known as FR-4. This is the most common material because it is flame resistant, cheap, and of course has a low conductivity.
For higher performance circuits (RF), there are other types of materials to consider such as ceramic or PTFE bases with various fillers. Since this article is more focused on general PCB design I will not go into details about designing for RF. Fortunately the EDN Network has posted a useful article on choosing PCB materials for high-frequency circuits
Really these two materials are about all that goes into a bare PCB. When you send your design for manufacture (or do it yourself) the electrical connections are usually created by removing select copper portions of the bare PCB.
The cheapest PCBs are single sided boards. This means that they are just made of the base material, with a single sheet of metal over the top.
Single sided boards are incredibly easy to work with, if you are making your own PCB at home you will most likely be designing for a single sided board.
While the single sided boards are simple to manufacture and understand, they can also be a pain when laying out your PCB. Since you only have one layer of metal to work with, you cannot cross electrical connections without the help of some external jumper.
As a result of this complication the majority of simple commercial and hobbyist boards are created on double sided PCBs. On a double sided PCB it is a simple matter to cross electrical connections and this fact allows for more complex yet elegant designs.
For all but the simplest of designs I recommend designing for a double sided PCB. This is generally the most cost effective method that will leave you with the least headache possible.
As designs become even more complex, it may even become necessary to add additional layers to your design. This can be useful if your board has incredibly complex signal paths or if you are aiming for a compact design.
For the majority of users I do not recommend using more than two layers, if you do this and do not need to you will end up with an unnecessarily complex design that will cost more to produce.
The copper traces on your PCB are easily the most important part of the design so it is important to understand what they’re doing and what limitations you should consider.
As I mentioned earlier, copper traces are created by removing copper from the solid sheet that sits on top of the base material.
This means that the traces on your board are in fact just thin layers of copper. I don’t know about you, but when I discovered that it came as a bit of a surprise (relax I didn’t just find this out). For the longest time I assumed that PCBs were manufactured by pouring copper into a mold, letting it cool, and then somehow melting an insulator around the copper.
The thin sheet nature of these traces mean that there are some constraints to consider when routing your traces, most importantly, size considerations.
The final “main component” of a PCB would be the ever useful via. Vias are used in multi-layer boards to electrically connect one layer to another.
There are essentially three types of vias, only one of which is common in the hobbyist world. These via types are:
Through hole - Most common type of via, a hole is drilled through the whole board and then electroplated so that it is conductive.
Blind - A blind via is used in designs with more than two layers to connect a surface layer to an internal layer without going all the way through.
Buried - Buried vias are similar to blind vias but are only used to connect internal layers.
After discussing PCB materials, possible layering options, copper traces, and vias we have pretty much covered the basics for what makes a PCB what it is. Understanding these things certainly gives you enough information to make your own working design, but there are still some other concepts to consider.
Some other PCB concepts to explore:
Soldermask - If I had to guess what you think of when I say “PCB” I would bet that it is probably something green. Did you know that PCB’s are not this color as a result of what material is used? This is in fact another layer that is applied to the board after manufacturing. The purpose is to keep solder paste from spreading where it shouldn’t be, but it also has the effect of giving the board a definite style.
Fiducials - These are special markings on your board that allow a pick-and-place automated assembly machine to calibrate itself. Fiducials are usually just a circle where the soldermask has not been applied with copper circle in the middle. This makes the point appear fairly reflective.
Silkscreen - This is also another layer of the PCB added after fabrication. Silkscreen is used to provide visual cues to the user, document board information, identify proper component placement, or for branding. There are conflicting suggestions on proper usage of silkscreen which I address in Design the Perfect PCB.
Copper fill - Deciding whether to use a ground/power plane in your design will be an important decision to make during the design stage. The most common reasons for using a copper fill are to suppress noise on the ground circuitry, dissipate heat from a particularly active device, or because someone told you that was the way to do it.