PCB fabrication
Printed circuit boards (PCBs) are built using layers of dielectric and conductive materials. The layers are bonded together under viable pressure to form a stable substrate, called the core. Different types of cores are used depending on the application. The substrate is usually made of a fiberglass-reinforced epoxy, or some other laminate material. Copper foil is then deposited on the surface of the board. The thickness of the copper layer is determined by the application. Various factors, such as cost and performance priority, determine the material of choice.
The pcb core is a laminate material composed of a matrix or filler with reinforcement. The most common choice is FR-4, a glass-reinforced epoxy composite. This material has good electrical and mechanical properties, including shear and tensile strength. Other common choices are FR-2, FR-3, and FR-5.
Conductors on PCBs are made of various metals, such as gold, nickel, and copper. The choice of metal depends on the type of circuits required and the physical characteristics needed for the board. Metallurgical differences in metals affect their electrical, thermal, and mechanical properties. For example, aluminum and copper offer good thermal conductivity. The insulating property of copper is better than that of nickel, and it has higher corrosion resistance.
What materials are used in PCB fabrication?
Another factor in the selection of a material for a PCB is its dielectric constant, which affects signal transmission speed. A low dielectric constant allows fast edges of signals to reflect more than slower ones. However, a low dielectric constant also increases the loss of electromagnetic energy in the signal. Choosing unnecessarily low-loss materials can increase the cost of the board without an associated benefit.
Other important properties of PCB materials are their thermal expansion coefficient and Tg (glass transition temperature). Choosing a material with a compatible thermal expansion coefficient with the components reduces stress during temperature fluctuations. A high Tg helps prevent damage from exposure to soldering heat.
The insulating properties of a material are also determined by its thickness. Thicker materials have a lower dielectric constant, while thinner materials have a higher one. The selection of a material for a specific application requires careful consideration of its performance, cost, and manufacturability. The versatility of PCBs is one of their greatest strengths. They can be single-sided, with components mounted on one side of the board, or double-sided, with components mounted on both sides. Multilayer PCBs take this a step further, incorporating multiple layers of conductive pathways separated by insulating layers, allowing for complex circuits in compact designs.
Some PCBs use other materials, such as phenolics and epoxies, for the substrate layer. These are less expensive but have inferior mechanical and electrical properties to FR-4. These are more common in inexpensive consumer electronics. These cheaper substrates delaminate and smoke when exposed to hot temperatures. Moreover, they can be identified by their characteristic bad smell when being soldered. A more durable alternative to these is polyimide, which can withstand temperatures up to 260 degrees Celsius. It is also a good choice for flexible PCBs.