Introduction to the structure and function of PCB boardPosted on 2022-04-27
Today’s PCB substrate outsole is composed of three main components: copper foil layer (Copper Foil), reinforcement (Reinforcement), and resin (Epoxy). Powder (Fillers) was added to the PCB in large quantities to improve the heat resistance of the PCB.
We can think of copper foil as the blood vessels of the human body, which are used to transport important blood and allow the PCB to function; the reinforcing material can be imagined as the bones of the human body, which is used to support and strengthen the PCB so as not to slack off; and Resin can be imagined as the muscle of the human body and is the main component of PCB.
The uses, characteristics and precautions of these four PCB materials are explained below:
1. Copper Foil (copper foil layer)
Electric Circuit: Conductive circuit.
Signal line: The signal (signal) line that transmits the message.
Vcc: power layer, working voltage. The working voltage of the earliest electronic products was mostly set to 12V. With the evolution of technology and the requirement of power saving, the working voltage has gradually become 5V and 3V, and now it is gradually moving to 1V, and the requirements for copper foil are also increasing. Come higher.
GND (Grounding): ground plane. Think of Vcc as a water tower in the home. When we turn on the faucet, water (electrons) will flow out through the pressure of the water (working voltage), because the actions of electronic parts are determined by the flow of electrons. GND can be imagined as a sewer, all used or unused water will flow away through the sewer, otherwise the faucet will keep draining and the house will be flooded.
Heat Dissipation (due to high thermal conductivity): for heat dissipation. Have you ever 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. At this time, a large area of copper foil needs to be designed to release the heat to the air as soon as possible. Among them, otherwise not only human beings can’t stand it, but even electronic parts will also crash.
When choosing a PCB reinforcement material, it must have the following excellent characteristics. Most of the PCB reinforcing materials we see are made of glass fiber (GF, Glass Fiber). If you look closely, the material of glass fiber is a bit like a very thin fishing line. Because of the following individual advantages, it is often selected. When the basic material of PCB.
High Stiffness: With high “rigidity”, the PCB is not easily deformed.
Dimension Stability: Has good dimensional stability.
Low CTE: It has a low “thermal expansion rate” to prevent the circuit contacts inside the PCB from detaching and causing failure.
Low Warpage: It has low deformation, that is, low plate bending and plate warpage.
High Modules: High “Young’s modulus”
3. Resin Matrix (resin mixture)
The traditional FR4 sheet is mainly based on Epoxy (epoxy resin), while the LF (Lead Free)/HF (Halogen Free) sheet uses a combination of various resins and different curing agents, which increases the cost. LF is about 20%, and HF is about 45%. .
HF sheet is brittle and easy to crack, and the water absorption rate increases. Thick and large sheets are prone to CAF. It is necessary to use open fiber cloth and flat fiber cloth, and strengthen the impregnated material.
A good resin must have the following conditions:
Heat Resistance: Good heat resistance. The heat resistance is good if the board will not explode after two to three times of heating and welding.
Low Water Absorption: Low water absorption. Water absorption is the main cause of PCB explosion.
Flame Retardance: Must be flame retardant.
Peel Strength: Has high “tear strength”.
High Tg: High glass transition point. Most of the materials with high Tg are not easy to absorb water, and the lack of water absorption is the fundamental reason for not breaking the board, not because of high Tg PCB.
Related reading: What is glass transition temperature (Tg, Glass Transition Temperature)
Toughness: Good “toughness”. The greater the toughness, the less likely it is to burst. Toughness is also known as “destruction energy”, and the tougher the material, the stronger the ability to withstand impact and damage.
Dielectric properties: High dielectric properties, that is, insulating materials.
4. Fillers System (powder, filler)
In the early days of lead soldering, the temperature was not very high, and the original board of the PCB was still tolerable. Since the lead-free soldering was used, the temperature increased, so the powder was added to the PCB board to make the PCB strong against temperature.
Fillers should be coupled first to improve dispersion and adhesion.
Low Water Absorption
Drill processability: Due to the high rigidity and high toughness of the powder, it is difficult to drill a PCB.
High Modulus: Young’s modulus
Heat Dissipation(due to high thermal conductivity): for heat dissipation.