There are several different surfaces to choose from when making your PCB, including two types of gold, tin, and silver. The choice of surface material is just as important as other parts of the process. If you choose the wrong material, you may have problems in the long run, both when the product needs to be assembled and in terms of reliability.
It is therefore essential to spend some time evaluating the advantages and disadvantages of the different surfaces, while at the same time considering your product, storage, functionality, etc. It is also crucial to keep in mind whether the factory mounting the components will be able to easily do so. The choice of surface is, therefore, an element you must get right from the very beginning.
There are pros and cons to all surfaces.
ENIG gold is one of the types of gold surfaces you can use. Here, nickel and gold are placed on top of copper. A disadvantage of ENIG gold is that, in some cases, the material may crack when you solder. The advantage, on the other hand, is the very smooth surface it gives, making it very solderable. It is the main reason why it is the most frequently used surface for PCB design.
In the gold category, there is also ENEPIG gold, where the P stands for palladium. Palladium is added in the material, to avoid any potential cracks after soldering.
HASL (Hot Air Solder Level)
When creating a HASL surface, the PCB is immersed in liquid tin, after which high-pressure hot air is blown over the surface. The HASL surface is not as smooth as the gold surface. HASL surfaces are generally great to solder on, however, be aware that you may not be able to solder the smallest components. It can also be a disadvantage, because flecks of tin residue may remain on the PCB.
Keep in mind that tin is both the cheapest but also the worst surface for fine electronics.
You will not see chemical silver in use very often. The reason for this, is that the surface can oxidize, making it difficult to solder on.
OSP (Organic Solderability Preservative)
OSP provides a smooth surface that is suitable for small components. However, this surface is not recommended if the PCB is to be stored for a long time before being fitted with components.