Rigid PCB – All About Rigid Copper Clad FR-4 Laminates
A rigid PCB is commonly made up of copper clad FR4 laminates. The copper cladding is flame resistant and it is coated over a reinforced fiberglass substrate. The FR in ‘FR4’ stands for flame retardant. Brominated epoxy resin is used as an adhesive in the laminate because of the thermal resistance, high strength and exceptional dielectric properties of the reinforced fiberglass rigid PCB which allows the thru-holes to be plated with conductive metals which are favorable for manufacturing 2 layer PCBs and multilayer PCBs. Consequently, rigid fiberglass has transcended to the level of most popular copper PCBs.
Manufacturing Process of Rigid Copper Clad Copper Clad FR-4
The Manufacturing process of rigid PCB-like copper clad FR-4 can be broadly classified into 2 categories as explained below.
- Discontinuous manufacturing
- Continuous manufacturing
While a discontinuous manufacturing process is popularly used in this day and age, it has lower production efficiency and consumes more materials as compared to the continuous manufacturing process.
Copper Thickness
Whenever you are ordering copper clad rigid PCBs, copper thickness has to be specified. Generally, the thickness of copper is specified in ounce per square foot. For example, if you are ordering a rigid PCB of 1/0, it means that it will have 1 ounce of copper per square foot on just one side.
FR-4 Products
There have been diverse products manufactured that are based upon copper clad FR-4 laminates. Thus, the levels of performance vary accordingly.
Mid Tg FR-4
It is one of the most commonly used laminate in the rigid PCB domain. This laminate class is used in many industries such as the telecom, medical and industrial electronics industry. These laminates also have good performance in lead-free soldering. The glass transition temperature is 1500C.
High Tg FR-4
These laminates are used where the operating temperatures are really high which is why these are ideal for PCBs used in the automobile industry. This laminate class yields even better performance in lead-free soldering. The glass transition temperature is 1800C.
Halogen-free
These laminates are used in situations where halogen may result in fire. Even these laminates have good lead-free soldering characteristics. In these designs, phosphating nitriding epoxy resin is used as a flame retardant adhesive. The properties of halogen-free laminate vary from one type to another.
High Frequency FR-4 Laminates
These laminates are applied in places where there is a requirement of a stable Dk (dielectric) factor and a low Df (dissipation) factor. Therefore, these laminates are ideally used in amplifiers, high frequency digital circuits and radio PCBs.
Prepreg/HDI Dielectric Laminate
The most common type of HDI substrate is the one which has a thin layer of prepreg. It is not necessary that the prepreg is used as copper clad rigid PCBs. However, it can be used to manufacture these as well. Prepreg is used as an adhesive in the inner layers of multilayer PCBs.