PCB surface finish is the coating between the component and the bare PCB. There are two basic reasons for its application: ensuring solderability and protecting bare copper circuits. Since there are many types of surface treatments, choosing the right surface treatment is not easy, especially as surface mounts become more complex, and regulations such as RoHS and WEEE have changed industry standards.

HASL (hot air solder leveling) / lead-free HASL
HASL is the primary surface treatment used in the industry. The process involves immersing the board in a tin/lead alloy melting tank and then using an "air knife" to remove excess solder that blows hot air across the surface of the board.
Advantages: low cost, available, repairable.
Disadvantages: uneven surface, not suitable for fine pitch components, thermal shock, not suitable for plated through holes (PTH), poor wettability.

OSP (Organic Solderability Preservative)
OSP is a water-based organic surface treatment agent commonly used in copper pads. It selectively bonds to the copper and protects the copper pad prior to soldering. OSP is environmentally friendly, provides a coplanar surface, is lead free, and requires low equipment maintenance. However, it is not as powerful as HASL and can be sensitive to processing.
Advantages: lead-free, flat surface, simple process, repairable.
Disadvantages: not suitable for PTH, sensitive, short shelf life.

ENIG (electroless nickel plating)
ENIG is quickly becoming the most popular surface treatment in the industry. It is a two-layer metal coating that is both a barrier to copper and the surface of the welded component. A layer of gold protects nickel during storage. ENIG is the answer to key industry trends, such as lead-free requirements and the addition of complex surface components (especially BGA and flip chip) that require a flat surface. However, ENIG can be expensive and sometimes leads to what is commonly referred to as "black mat syndrome", the accumulation of phosphorus between the gold and nickel layers, which can lead to surface cracking and connection errors.
Advantages: flat surface, sturdy, lead-free, suitable for PTH.
Disadvantages: black pad syndrome, expensive, not suitable for rework.