JLCPCB can provide three surface finish options: HASL, Lead-free HASL, ENIG. PCB surface finish is applied to the exposed copper pads to protect from oxidation, which would strongly inhibit the contact pad's ability to bond with molten solder. 

HASL - Hot Air Solder Leveling

HASL is a type of finish used on printed circuit boards (PCBs). For hot air solder leveling finish, the board will be dipped in a bath of molten solder and then passed through a conveyor of hot air knives that brush the excess solder off. Basically, the copper pads are pre-tinned with the thinnest layer of solder, and the tinning protects the bare copper underneath. which can provide the board with a good shelf-life, and the existing layer of solder makes the molten joints much easier to join. It is a commonly used and cheap finish for most DIY projects. However, it is not suitable for fine pitch components since HASL leaves uneven surfaces.

Pros:

• Cheap
• Avoids copper corrosion

Cons:

• Uneven Surfaces
• Not good for fine pitch components
• Thermal Shock
• Poor wetting
• High thermal stress during process may introduce defects into PCB

Lead-free HASL

Lead-free HASL is variant from HASL, which is lead-free and qualified for ROHS compliance products. Please note that higher soldering temperatures are required for lead-free solder. 

ENIG _ Electro-less Nickel / Immersion Gold

ENIG is quickly becoming the most popular surface finish for industry use. In the ENIG plating process, the copper pads are protected by a coating of nickel, and a layer of gold protects the nickel during storage. 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 an ideal answer to major industry trends such as lead-free requirements and rise of complex surface components (especially BGAs and flip chips), which require flat surfaces.

Pros:

• Flat surfaces
• Strong Lead-free
• Good for PTH

Cons:

• Black pad syndrome
• Expensive
• Not good for rework