Surface Finishes Used on Board PCB Assemblys
The surface finish on a board is a crucial aspect of the circuit board that protects the copper layer from oxidation and enhances its durability and reliability. It also prepares the copper for soldering components onto it during assembly and ensures that the connections can be maintained over a long period of time. Choosing the wrong surface finish can lead to poor performance and failure of the circuit board. Hence, it is essential to understand the different types of surface finishes that are available so that you can choose one that meets the specifications of your board pcb assemblys.
The main types of PCB surface finishes are metallic and organic. Metallic surface finishes use a metal plating to protect the copper from oxidation and improve conductivity. Organic surface finishes are mainly made of organic compounds with carbon. Some popular examples include OSP (Organic Solderability Preservative), OHT, and ENIG. Each type of surface finish has its own advantages and disadvantages. Here is a brief overview of the most common ones:
HASL has been a standard for PCBs for decades, but as component density has increased so too has the need for better surface finishes. HASL is easy to apply, manufacturers simply dip the PCB in a mixture of tin and lead, which then encases all the copper areas on the board pcb assembly. The excess is blown off with an air knife, leaving the board with a flat surface that’s suitable for surface mount devices (SMD).
Types of Surface Finishes Used on Board PCB Assemblys
This is the oldest of the metallic surface finishes. Traditionally, it was used on boards with through-hole technology, however it is now becoming increasingly obsolete and being replaced by more advanced surfaces such as ENEPIG and soft/hard gold. This is because HASL has a high copper content, which can cause problems with soldering and heat transfer.
Another classic metallic finish is immersion tin, which is RoHS compliant and offers a good solution for flat surface requirements. This is achieved by dipping the copper layer of the PCB in a molten mixture of tin and lead, which leaves a coating on the copper that’s ideal for fine pitch components.
ENIG is a more expensive metallic surface finish, but it overcomes the shortcomings of other surface finishes by applying a layer of nickel over the copper. This creates a tin-nickel bond, which prevents copper from oxidising and offers excellent conductivity. It is a good choice for use in applications that are susceptible to shock and vibration, such as mobile phones and medical equipment. It is also useful for applications that require a durable finish, such as RF shielding, remote controllers, and automotive vehicles. This finish requires special care when handling the PCB as it can be damaged by static charge. For this reason, it is important to place the boards in antistatic bags before assembling them.
PCBs are complex assemblies composed of various materials, including fiberglass, epoxy resin, and multiple metals like copper, gold, silver, and palladium. They also contain hazardous substances such as lead, mercury, and brominated flame retardants. These materials make recycling both a challenging and necessary process. The valuable metals within PCBs provide a strong incentive for recycling, as they can be recovered and reused, reducing the demand for new mining operations.
