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ENIG (Electroless Nickel Immersion Gold) and Hard Gold are two common PCB surface treatment processes, which have significant differences in principles, characteristics and application.
ENIG(Immersion gold):
ENIG is an electroless plating process that deposits a layer of nickel and then a layer of gold on the copper surface through a chemical reaction.
The nickel layer acts as a barrier layer to prevent copper from diffusing into the gold layer; the gold layer protects the nickel layer from pcb oxidation and provides good soldering properties.
The gold layer is thin (usually gold thickness:0.025-0.127 microns Or 1u”-5u”) and is an "immersion gold" process.
Extension: According to IPC II Standard(IPC-4552 ),The minimum immersion gold deposit thickness shall be 0.05 µm [1.97 µin] at -4 sigma from the mean as measured on a pad size of 1.5 mm x 1.5 mm [0.060 in x 0.060 in] or equivalent area.The minimum Electroless Nickel shall be 3 µm [118 µin]
Hard Gold:
Hard Gold is an electroplating process that electrolytically deposits a thick layer of gold on a copper or nickel substrate.
The thickness of the gold layer is usually between 0.5-3 microns(20u”-120u”), with higher hardness and greater wear resistance.
ENIG:
Advantages:
The surface is very flat, suitable for welding fine-pitch components (such as BGA).
It has strong corrosion resistance, and the gold layer protects the nickel layer from oxidation.
It is suitable for most welding situation both for SMT and THT.
Disadvantages:
The ENIG normally use in the whole board, the price is expensive while the gold dimension is large and the gold thickness thick.
There is a "black disk" problem (oxidation of the nickel layer leads to reduced reliability of the solder joint).
Not suitable for high wear situation.
Hard Gold:
Advantages:
High hardness, good wear resistance, suitable for contact points that are frequently plugged in and out.
The thickness of the gold layer can be customized to meet different needs with huge thick gold requirement.
It is not easy to oxidize and has a long service life.
Disadvantages:
The surface is not flat enough, not suitable for welding fine-pitch components.
The process is complicated and the cost is much higher than ENIG.
It is usually only applied to specific areas (such as gold fingers).
ENIG:
Widely used in high-density PCBs that need a flat surface finish treatment , especially those that require soldering of fine-pitch components (such as BGA, QFP).
Applicable to consumer electronics, communication equipment, computer motherboards, etc.
Hard Gold:
Mainly used in areas where mechanical contact is required, such as:
Gold finger: the connection between PCB plug and socket.
Key contact point: such as mobile phone keys, remote control keys.
High-frequency connector: an electrical connection that requires long-term stable contact.
Both have their own advantages and disadvantages, and the specific choice depends on the design requirements and application scenarios of the PCB:
If your PCB needs to solder fine-pitch components and requires high flatness surface, it is more appropriate to choose ENIG.
If your PCB requires wear-resistant and plug-resistant contact points (such as gold fingers pcb), it is better to choose Hard Gold.
