HASL - Hot Air Solder Levelling
ENIG - Electroless Nickel / Immersion Gold
In the ENIG plating process, the copper pads are protected by a coating of nickel, and then the nickel is protected with a fine layer of gold particles. This offers superior corrosion resistance compared to HASL due to the high corrosion resistance of gold. In addition, the excellent surface planarity of the nickel/gold finish allows components to be soldered flat onto the pad, making it ideal for BGA pads and other fine-pitch components. The disadvantages include the higher cost of the gold solution and soldering is not as easy compared to HASL since there is no pre-tinned surface. Solderability is still good, however.
Similar to ENIG, the pads are plated with nickel but then with a much thicker layer of gold. Hard gold is common for gold fingers or other hard wear areas which require durability and excellent resistance to corrosion. Like ENIG, solderability is poor and the cost is even higher since a thicker layer of gold is applied. If you select the Hard Gold option, we will plate the entire board with Hard Gold. For small batches and prototyping orders, a different plating process is used compared to large batch processing lines. With this process, it is much more difficult and labor intensive to apply more than one surface finish on the boards, therefore, dual surface finish boards are currently not available at Seeed Fusion.
OSP - Organic Solderability Preservative
A thin layer of a water-soluble organic compound is applied to the surface of the boards, which then selectively adheres to the copper surfaces, forming an almost undetectable protective layer. This makes it ideal for fine-pitched devices but the shelf life is relatively short - up to one year, so it is more suited to batch assembly rather than prototyping. It is also clear in color, making it difficult to inspect the coverage of the compound. It is the most environmentally friendly of all the available finishes.
Tin is deposited onto the surface of the copper via a displacement reaction which removes some of the surface copper in the process. The resulting finish has excellent planarity and the reliability is comparable to ENIG. The thickness of the tin layer is also much easier to control, but the process involves the use of a carcinogen and the boards must be handled carefully to avoid contamination.
Similar to immersion tin, immersion silver produces exceptionally flat surfaces, has great solderability and can serve as an ENIG alternative but it also has a poor shelf life and does not respond well to handling. The immersion silver process however, does not use highly toxic chemicals and does not affect the final diameter of plated holes. Due to a high friction coefficient, immersion silver surface finishes are not suited for pin insertion or press-fit machines.