We discuss the composition of a PCB, including some terminology, a brief assembly method, and an introduction to the PCB design process.
PCB (Printed circuit board) is the most common name, and it can also be called "printed wiring boards" or "printed wiring cards". Before the advent of PCBs, circuits were made up by point-to-point wiring. The reliability of this method is very low, because as the circuit ages, the rupture of the line will cause the circuit node to open or short.
Wire winding technology is a major advancement in circuit technology. This method improves the durability and replaceability of the circuit by winding small-diameter wires around the posts at the connection
point.
As the electronics industry evolved from vacuum tubes and relays to silicon semiconductors and integrated circuits, the size and price of electronic components decreased. Electronic products appear more and more frequently in the consumer field, prompting manufacturers to look for smaller and more cost-effective solutions. Thus, PCB was born.
composition (composition)
PCBs look like multi-layered cakes or lasagnas—layers of different materials are pressed together with heat and adhesives.
Let's start with the middle layer.
FR4
The base material of PCB is generally glass fiber. In most cases, the glass fiber substrate of PCB generally refers to the material of "FR4". "FR4" is a solid material that gives the PCB rigidity and thickness. In addition to the substrate of FR4, there are also flexible circuit boards produced on flexible high-temperature plastics (polyimide or similar).
You may find PCBs with different thicknesses; however SparkFun's products are mostly 1.6mm (0.063') thick. Some products also use other thicknesses, such as LilyPad and Arudino Pro Micro boards use a board thickness of 0.8mm.
Inexpensive PCBs and perfboards (pictured above) are made of materials like epoxy or phenol, which lack the durability of FR4, but are much cheaper. When soldering something on such a board, it's going to smell really bad. This type of substrate is often used in very low-end consumer products. Phenolic feces substances have a low thermal decomposition temperature, and if the welding time is too long, it will decompose and carbonize. And it smells bad.
copper
Next is a very thin layer of copper foil, which is pressed onto the substrate by heat and adhesive during production. On a double-sided board, copper foil is pressed onto both the front and back sides of the substrate. In some low-cost applications, copper foil may only be pressed on one side of the substrate. When we refer to "double-sided board" or "two-layer board", we mean that there are two layers of copper foil on our lasagna. Of course, in different PCB designs, the number of copper foil layers may be as small as 1 layer. Or more than 16 floors.
There are many types of copper layer thickness, and the unit is weight, which is generally represented by the weight of a square foot of copper evenly covered (ounces). The copper thickness of most PCBs is 1oz, but some high-power PCBs may use 2oz or 3oz copper thickness. Convert the plate (oz) per square foot, which is about 35um or 1.4mil copper thickness.
Soldermask (solder mask)
Above the copper layer is the solder mask. This layer makes the PCB hairy green (or red for SparkFun). The solder resist layer should cover the traces on the copper layer to prevent the traces on the PCB from contacting with other metals, solder or other conductive objects to cause a short circuit. The existence of the solder mask allows everyone to solder in the correct place and prevents solder bridging.
The solder mask covers most of the PCB (including the traces), but exposes the silver annular ring and SMD pads for easy soldering.
Generally speaking, solder mask is green, but almost any color can be used for solder mask. Most SparkFun boards are red, but the IOI0 board is white, and the LilyPad board is purple.
Silkscreen (silk screen)
On top of the solder mask, there is a white silkscreen layer. Letters, numbers and symbols are printed on the silk screen layer of the PCB, which can facilitate assembly and guide everyone to better understand the design of the board. We often use silk screen symbols to mark certain pins or LED functions.
The most common color for silkscreen is white, and likewise, silkscreen can be made in almost any color. Black, grey, red and even yellow silkscreen layers are not uncommon. However, it is rare to see multiple silkscreen colors on a single board.
