Today's PCB substrate substrate is composed of three main components: copper foil, reinforcement, and epoxy. However, since the lead free process began, the fourth category of fillers has been added to PCB boards in large quantities to improve the heat resistance of PCB.
We can think of copper foil as a blood vessel of the human body, whICh is used to transport important blood and enable PCB to function; The reinforcing material can be imagined as the skeleton of the human body, which is used to support and strengthen the PCB without being soft; Resin can be considered as the muscle of human body and the main component of PCB.
The following describes the use, characteristics and precautions of the four PCB materials:
1. Copper Foil
Electric Circuit: conductive circuit.
Signal line: the signal line for sending messages.
Vcc: power supply layer, working voltage. The working voltage of the earliest electronic products was mostly set at 12V. With the development of technology and the requirement of power saving, the working voltage gradually changed to 5V and 3V, and now it is gradually moving to 1V, and the requirement for copper foil is also getting higher and higher.
GND (Grounding): ground plane. We can think of Vcc as a water tower at home. When we turn on the tap, water (electrons) will flow out through the pressure of water (working voltage), because the action of electronIC parts is determined by the flow of electrons; GND can be imagined as a sewer. All used or unused water flows through the sewer. Otherwise, the faucet will drain all the time and the home will be flooded.
Heat Dissipation (due to high thermal conductivity): used for heat dissipation. Have you heard that some CPUs are so hot that they can cook eggs? This is not an exaggeration. Most electronIC components consume energy to generate heat energy. At this time, a large area of copper foil needs to be designed to release heat energy into the air as soon as possible, otherwise not only humans can't stand it, but also electronic parts will crash.
When selecting PCB reinforcement materials, they must have the following excellent characteristics. Most of the PCB reinforcement materials we see are made of glass fiber (GF). If you look carefully, the material of glass fiber is a little like a thin fishing line. Because it has the following personality advantages, it is often used as the basic material of PCB.
High Stiffness: It has high "rigidity", so that PCB is not easy to deform.
Dimension Stability: Good dimensional stability.
Low CTE: It has a low "thermal expansion rate" to prevent the circuit contacts inside the PCB from separating and causing failure.
Low Warpage: low deformation, that is, low bending and warping.
3. Resin Matrix
The traditional FR4 plate is mainly made of epoxy, while LF (Lead Free)/HF (Halogen Free) plate uses a variety of resins and different curing agents, which increases the cost. LF is about 20%, HF is about 45%.
HF board is easy to be brittle and cracked, and the water absorption rate becomes larger. Thick and large boards are prone to CAF. It is necessary to use fiber cloth and flat fiber cloth, and strengthen the materials with uniform immersion.
Good resin must meet the following conditions:
Heat Resistance: good heat resistance. The heat resistance is good only if the plate will not burst after heating and welding for two to three times.
Low Water Absorption: Low Water Absorption. Water absorption is the main cause of PCB bursting.
Flame Retardance: Must be Flame retardant.
Peel Strength: It has high "tear strength".
High Tg: high glass transition point. Most materials with high Tg are not easy to absorb water, and non absorption is the root cause of non explosion, not because of high Tg.
Toughness: Good "Toughness". The greater the toughness, the less likely it is to burst. Toughness is also called "damage energy". The better the toughness is, the stronger the ability of materials to withstand impact and damage.
Dielectric properties: High dielectric properties, that is, insulating materials.
4. Fillers System
In the early days when there was lead welding, the temperature was not very high, and the original PCB board was tolerable. Since the lead-free welding, the temperature has increased, so the powder was added to the PCB board to make the PCB strong temperature resistant materials.
Fillers should be coupled first to improve dispersion and cohesion.
Drill processibility: Because of the high rigidity and toughness of the powder, it is difficult to Drill PCB holes.
High Modulus: Young's modulus
Heat Dissipation (due to high thermal conductivity): used for heat dissipation.