which surface finish for PCB

Please find here under a list of PCB surface finish.

OSP (Organic Solderability Preservative) or Entek or passivation is an organic finish that bond with copper. It is probably the most eco-friendly surface finish, it requires little energy and no toxic materials are used. Pros: the simplicity of the process, a flat surface and a low cost. Cons: sensitive to storage and heat cycles, also not ideal for plated vias.

HASL or HASL LF (lead free), Hot Air Levelling is still one of the most popular surface finish choices. But, in recent years, it has lost some of its shine, as manufacturers have realized its limitations. More complex products, higher level of integration, triggered the development of smaller components and the need for smaller traces, smaller pads and higher density PCBs. HASL leaves uneven surfaces that are not suitable for fine pitch components. Pros: good solderability and low cost Cons: uneven surfaces, which might make it difficult to assemble fine pitch components.

ENIG (Electroless Nickel Immersion Gold) or Immersion Gold or Cheminical Gold has widely replaced ‘Flash Gold’ (electro), which was so widely used in the 1990s in Asia. ENIG has become ‘the’ surface finish of reference for multi-layers PCB and fine pitch boards. Note: the components are soldered on the nickel; gold is only present to protect the former one. 
Pros: smooth & flat surface, good wetting, multiple heat shock, storage time.
Cons: black pads and cost

ENEPIG (Electroless Nickel / Electroless Palladium / Immersion Gold) is a similar process to ENIG, though we add a layer of palladium to prevent the nickel to oxidise.
Pros: perfect for Al or Au wire bonding
Con: high cost

Immersion Tin (Chemical Tin) is reviving. A few years ago, its process has been improved and with the lead free directive, Chemical Sn is yet again getting more popular.
Pros: flat surface, good wetting, and multiple heat shock, ideal for pressfit and backplane applications.
Cons: must be handled carefully and whiskers.

Chemical Silver (Immersion Silver) is achieved during an operation where we deep the working panels into an acid solution with silver salts. Immersion silver is a good alternative to ENIG.
Pros: uniform layer, good wetting, ideal for silver wire bonding.
Cons: oxidation by sulphur presence, silver whiskering

Hard Gold is an electro process, where adjusting the duration of the plating cycle can control gold thickness. This type of surface finish is normally used for areas, which are exposed to repetitive contact, friction, rubbing parts or plug in and out.
Pros: very hard and durable surface, very long shelf life
Cons: high cost related to the material thickness, but also the process.
 

Surface finish proposed by www.PCBprototype.com

Our company is in a position to offer all surface finish mentioned here above. Surface finish is a critical aspect of your PCB, therefore you must make the right choice. The repeatability of the surface finish process is equally important in order to obtain the same result for every single production batch. Our team is well trained and experienced in this field so that your finished goods will meet your highest expectation. Depending on the nature of your application, we would be happy to advise on the most appropriate surface finish.

Do not hesitate to contact us for more information.