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Understanding the Various Categorizations of Rigid PCBs

Rigid Printed Circuit Boards (PCBs) can be designed and manufactured in a variety of ways. There are three basic types of rigid PCBs. These include:

  • Single Layer: This is basic type offered to customers. This rigid PCB consists of a conductive layer of copper on one side of the circuit board.
  • Double Layer: As the name suggests this PCB is designed with conductive copper layers on both sides of the board.
  • Multilayer: These rigid PCBs consist of multiple conductive copper layers. Such PCBs are designed for high density applications. Multilayer PCBs can be designed with up to 14 or 16 copper layers.

Categorization via Surface Finishes

Besides the number of conductive layers, rigid PCBs can also be categorized by the type of surface finish that has been used in the manufacturing process. There are a variety of materials that can be used to create the surface finishes. Some of the most popular materials used include:

  • Electrolytic Gold: Also known as hard gold, this surface finish consists of applying a layer of gold over a coat of nickel. The advantage of this finish is that it is extremely durable, and is used for areas that are prone to a lot of wear. A good example of the application area would be edge connector fingers.
  • Immersion Tin: This is a metallic finish that is applied directly onto the copper of the circuit board. The primary purpose of immersion tin is to protect the copper from oxidation, thus extending its service life. The advantages of this finish is that it creates a flat surface, and can be reworked if there are any discrepancies.
  • Electroless Nickel Immersion Gold: One of the most popular surface finishes, Electroless Nickel Immersion Gold (ENIG) is a metallic coating comprising two layers. The nickel layer is about 120 to 250 micro inches, while the gold layer is about 2 to 8 micro inches. The nickel layer is designed to act as a barrier to the copper, while the gold layer protects nickel. Components that need to be soldered are installed onto the nickel layer. The reason for ENIG’s popularity is mainly due to the fact that it adheres to nearly all RoHS regulations.
  • Hot Air Solder Levelling: Also known as HASL, this surface finish is first choice for most PCB manufacturers. The process consists of immersing circuit boards in a molten pot of a tin/lead alloy and then removing the excess solder by using ‘air knives’, which blow hot air across the surface of the board. One of the unintended benefits of the HASL process is that it will expose the PCB to temperatures up to 265°C which will identify any potential delamination issues well before any expensive components are attached to the board. The advantages of this surface finish is its low cost and its long service life. It is easily available, and can be reworked if any errors take place during the application process.
  • Wire Bondable Gold: This surface finish consists of a combination of gold and electrolytic nickel. The nickel is designed with a high level of hardness to ensure proper protection. As the gold is bonded to electrical wires, the gold does not consist of grain stiffeners. This surface finish can be bonded to wires via thermal or aluminum wedge welding.

Understanding the different types of rigid PCBs can help you make a better decision for your application. Knowledge of the surface finishes will allow you to understand the PCB’s service life and how it will benefit your product.

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