What's a PCB?

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What's a PCB?

Printed circuit board is the most common name but may also be called “printed wiring boards” or “printed wiring cards”. Before the advent of the PCB circuits were constructed through a laborious process of point-to-point wiring. This led to frequent failures at wire junctions and short circuits when wire insulation began to age and crack.
A significant advance was the development ofwire wrapping, where a small gauge wire is literally
wrapped around a post at each connection point, creating a gas-tight connection which is highly
durable and easily changeable.

As electronics moved from vacuum tubes and relays to silicon and integrated circuits, the size and
cost of electronic components began to decrease. Electronics became more prevalent in consumer
goods, and the pressure to reduce the size and manufacturing costs of electronic products drove
manufacturers to look for better solutions. Thus was born the PCB.

PCB is an acronym for printed circuit board. It is a board that has lines and pads that connect various points together. In the picture above, there are traces that electrically connect the various connectors and components to each other. A PCB allows signals and power to be routed between physical devices. Solder is the metal that makes the electrical connections between the surface of the PCB and the electronic components. Being metal, solder also serves as a strong mechanical adhesive.

Composition

A PCB is sort of like a layer cake or lasagna- there are alternating layers of different materials which

are laminated together with heat and adhesive such that the result is a single object.

FR4

The base material, or substrate, is usually fiberglass. Historically, the most common designator for
this fiberglass is “FR4”. This solid core gives the PCB its rigidity and thickness. There are also
flexible PCBs built on flexible high-temperature plastic (Kapton or the equivalent).

You will find many different thickness PCBs; the most common thickness for SparkFun products is
1.6mm (0.063"). Some of our products- LilyPad boards and Arudino Pro Micro boards- use a 0.8mm
thick board.

Cheaper PCBs and perf boards (shown above) will be made with other materials such as epoxies or phenolics which lack the durability of FR4 but are much less expensive. You will know you are working with this type of PCB when you solder to it - they have a very distictive bad smell. These types of substrates are also typically found in low-end consumer electronics. Phenolics have a low thermal decomposition temperature which causes them to delaminate, smoke and char when the soldering iron is held too long on the board.

Copper

The next layer is a thin copper foil, which is laminated to the board with heat and adhesive. On
common, double sided PCBs, copper is applied to both sides of the substrate. In lower cost
electronic gadgets the PCB may have copper on only one side. When we refer to a double sided or
2-layer board we are referring to the number of copper layers (2) in our lasagna. This can be as
few as 1 layer or as many as 16 layers or more.

The copper thickness can vary and is specified by weight, in ounces per square foot. The vast majority of PCBs have 1 ounce of copper per square foot but some PCBs that handle very high power may use 2 or 3 ounce copper. Each ounce per square translates to about 35 micrometers or 1.4 thousandths of an inch of thickness of copper.

Soldermask 

The layer on top of the copper foil is called the soldermask layer. This layer gives the PCB its green (or, at SparkFun, red) color. It is overlaid onto the copper layer to insulate the copper traces from accidental contact with other metal, solder, or conductive bits. This layer helps the user to solder to the correct places and prevent solder jumpers. In the example below, the green solder mask is applied to the majority of the PCB, covering up the small traces but leaving the silver rings and SMD pads exposed so they can be soldered to.