HASL - Hot Air Solder Levelling
For HASL finishes, the boards are dipped in a bath of molten solder and then passed through a conveyor of hot air knives that brush the excess solder off. Essentially, the copper pads are pre-tinned with the thinnest layer of solder possible, and the tinning protects the bare copper underneath. This gives the boards good shelf-life and the existing layer of solder makes the molten joints much easier to join. The HASL surface finish is the simplest and most common surface finish and sufficient for most projects. However, it is not ideal for fine pitch components since, the thin layer of solder still leaves a rounded surface and the components will not lie completely flat against the surface of the pad.
HASL Lead Free - Lead Free Hot Air Soldering Levelling
In order to be RoHs compliant, regular lead solder cannot be used and this includes as a surface finish. Therefore, Seeed Fusion offers a Lead Free option as well for those intending on commercial distribution. However, lead free solder has a higher melting point, so it may not be ideal for the average hobbyist with regular soldering equipment.
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 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 reserved for gold fingers which require durability and excellent resistance to corrosion. It is not suitable for soldering due its poor solderability and the high cost of gold. If you select the Hard Gold option, we will plate the entire board with Hard Gold. If your boards have gold fingers and you would like the rest of the pads to have an ENIG finish, please use the Advanced PCB service.
The following options are only available in the Advanced PCB service.
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.
Hard Gold + ENIG
Select this option if your board has gold fingers and you would like the rest of the pads to have ENIG finish.