The PCB substrate is composed of layers of Copper Clad Laminate (CCL) and Prepreg (PP). CCL consists of three main components: Copper Foil, Reinforcement, and Resin Matrix. Since the start of lead-free manufacturing processes, the fourth item of Fillers has been added to the PCB material in large quantities to improve its heat resistance.
We can imagine copper foil as the blood vessels in the human body, used to transport blood, while in PCBs, they are used to transmit electronic signals. Reinforcement materials can be imagined as the bones in the human body, used to support and strengthen PCBs so that they don’t collapse. And the resin can be thought of as the muscles in the human body, which are the main components of PCBs.
(It is important to note that this content is derived from classroom lectures, and Workingbear has not gained actual work experience in a PCB substrate or PCB manufacturing plant. Therefore, the content may not be entirely accurate. If you have any questions or concerns about the content, please feel free to let me know.)
1. Copper Foil
Electric Circuit: Conductive wire.
Signal line: Conductive wire used to transmit electronic signals.
Vcc: Power layer, working voltage. The working voltage of the earliest electronic products was mostly set at 12V. However, with the evolution of technology and the need for power-saving, the working voltage has gradually decreased to 5V, 3V, and even as low as 1V. As a result, the demand for high-quality copper foil has also increased.
GND (Grounding): Ground layer. Vcc can be thought of as a water tower in the house. When we turn on the tap, water (electrons) flows out due to the higher water pressure (working voltage). Similarly, the functioning of electronic components depends on the flow of electrons. On the other hand, GND can be imagined as a sewer, where all used or unused water flows to here. Otherwise, if the tap continues to drain water, the house will flood.
Heat Dissipation (due to high thermal conductivity): For heat dissipation. Most electronic components consume energy and generate heat. At this time, a large area of copper foil needs to be designed to release the heat into the air as soon as possible.
When selecting PCB reinforcement materials, they must possess the following excellent characteristics. The PCB reinforcement materials that we mostly see are made of glass fiber (GF). If you look closely, the material of the glass fiber is similar to very thin fishing line. It is often selected as the basic material for PCB due to the following excellent characteristics:
- High Stiffness: Possesses high “rigidity,” which prevents PCB from deforming.
- Dimension Stability: Possesses good dimensional stability.
- Low CTE: Possesses low “coefficient of thermal expansion” (CTE), preventing the internal circuit connection of PCB from disconnecting and causing failure.
- Low Warpage: Possesses low deformation, which means low board bending or warping.
- High Modules: High “Young’s modulus”.
3. Resin Matrix
Traditionally, FR4 boards are mainly made of epoxy resin. LF (Lead-Free)/HF (Halogen-Free) boards use a combination of multiple resins and different curing agents, which increases the cost by around 20% for LF and 45% for HF.
Unfortunately, HF also means that the board is more prone to being brittle, cracking, and having a higher water absorption rate. High-speed and high-frequency boards are more like to experience CAF (Conductive Anodic Filament) defects, so tighter and flatter weave fabrics must be applied, and the impregnation must be strengthened too.
Good resin must meet the following conditions:
Heat resistance: able to withstand high temperatures without delamination after two to three rounds of heating and welding.
Low water absorption: low water absorption, as it is the main cause of PCB delamination.
Flame retardance: must be flame retardant.
Peel strength: high tear resistance.
High Tg: high glass transition temperature. Materials with high Tg are generally less prone to water absorption, and it is the lack of water absorption that is the fundamental reason for preventing PCB delamination, not just the high Tg value.
Related reading: What is the Glass Transition Temperature (Tg)?
Toughness: good toughness. The greater the toughness, the less likely the board is to delaminate. Toughness is also known as “destruction energy” and the better the toughness of the material, the stronger its ability to withstand impact and resist destruction.
Dielectric properties: high dielectric properties, meaning good insulation.
4. Fillers System
In the early days of lead soldering, the reflow temperature was not very high, and the PCB substrate could still withstand it. However, since the lead-free soldering be introduced to electronic products, the melting point of solder paste has increased from 183°C (Sn63/Pb37) to 217°C (SAC305). Therefore, fillers are added to the PCB substrate to enhance its resistance to temperature.
Fillers should be coupled first to improve dispersion and adhesion in polymer-based compositions. This can enhance their thermal conductivity and dielectric properties. Fillers can also affect the drill processibility and heat dissipation of PCB substrates.
The role of filler system in PCB structure:
- Heat Resistance
- Low Water Absorption
- Flame Retardance
- High Stiffness
- Low CTE
- Dimension Stability
- Low Warpage
- Drill processibility: Because of the high rigidity and toughness of the powder, it makes PCB drilling difficult.
- High Modulus: Young’s Modulus
- Heat Dissipation (due to high thermal conductivity): For heat dissipation.