ENIG and hard gold both put gold over nickel on a printed circuit board, but they solve different problems. ENIG is primarily a solderable surface finish applied across exposed copper. Hard gold is an electroplated, wear-resistant contact finish used selectively where a connector, switch or test point will experience repeated mechanical contact.
Choosing by color alone is a costly mistake. The deposition method, gold composition and thickness determine whether a surface is optimized for soldering or abrasion. This guide explains the differences and shows what to put in the fabrication data.
ENIG vs. Hard Gold at a Glance#
| Decision factor | ENIG | Hard gold |
|---|---|---|
| Full name | Electroless nickel immersion gold | Electroplated hard gold over nickel |
| Main purpose | Solderable, planar surface finish | Wear-resistant contact surface |
| Deposition | Chemical, no plating bus required | Electrolytic; features need electrical connection during plating |
| Gold layer | Very thin immersion layer | Thicker alloyed deposit, often cobalt-hardened |
| Repeated insertion | Not preferred for high-wear contacts | Designed for repeated mating cycles |
| Soldering | Commonly used on SMT and PTH pads | Not a default solderable finish; composition and thickness matter |
| Typical location | Exposed pads over most of the PCB | Edge fingers or selected contact areas |
| Relative cost | Moderate | Higher and more process-dependent |
What Is ENIG?#
ENIG deposits electroless nickel onto copper and then replaces a very small amount of nickel with immersion gold. Nickel is the main barrier and solderable layer; the gold protects the nickel from oxidation during storage and assembly. Because the process is chemical, it can coat isolated pads without adding a temporary electrical plating connection.
The result is flat compared with hot-air solder leveling, so ENIG works well for fine-pitch components, BGAs and other pads where coplanarity matters. It also has a useful shelf life when fabrication, packaging and storage are controlled. IPC identifies IPC-4552B as the performance specification for ENIG; procurement documents should name the applicable revision and product requirements rather than relying on an informal “gold finish” note.
ENIG is not thick gold plating. Its gold is intentionally thin. It protects the nickel before soldering and dissolves into the solder joint during reflow. It therefore should not be selected as a wear coating for a connector that will be inserted hundreds or thousands of times.
What Is Hard Gold?#
Hard gold is an electrolytic nickel/gold system. A small amount of an alloying element, commonly cobalt, increases deposit hardness and wear resistance. The gold can be substantially thicker than immersion gold, but the exact value is an engineering and supplier decision based on mating cycles, contact force, environment and connector requirements.
Electroplating needs current to reach the plated features. Edge fingers are convenient because they can be connected to a plating bar and separated when the panel is routed or beveled. Selective hard gold elsewhere on a board may require special bussing, masking and panel design. Ask the fabricator to approve the geometry before layout release.
Eurocircuits' official edge-connector process guide makes the functional distinction clear: it offers ENIG as a whole-board finish and hard plated gold over nickel for edge fingers because the thicker, harder deposit withstands repeated abrasion.
Why They Are Not Interchangeable#
Wear resistance
A sliding contact cuts through a thin soft coating. ENIG may be acceptable for a contact with very few mating cycles when the connector owner approves it, but hard gold is the conventional choice for repeated insertion. Connector life is a system property: gold thickness, nickel underplate, contact material, normal force, wipe length, contamination and environment all matter.
Solderability
ENIG is designed and controlled as a solderable finish. Hard gold contains hardening additives and can introduce excessive gold into a solder joint if a thick deposit is soldered. Do not place the same hard-gold callout on edge contacts and solder pads unless the assembly process and reliability owner have qualified it.
Planarity and coverage
ENIG coats exposed copper across the board and provides a planar surface suitable for dense SMT. Hard gold is usually selective. A design can use ENIG on solder pads and hard gold on edge fingers, but that is a dual-finish process that needs explicit drawing notes and supplier confirmation.
Cost and panel constraints
Hard gold uses more gold and adds masking, electroplating and inspection. The layout may need fingers at a panel edge, a removable plating rail, thieving or other process features. ENIG avoids an electrical plating bus, although it still requires disciplined bath control and thickness inspection.
Gold Fingers and Edge Connectors#
For a card-edge connector, define the contact geometry and finish as a complete feature. Include finished gold thickness, nickel underplate, bevel angle and depth, finger length, chamfer limits, solder-mask setback, copper-to-edge relationship and any unequal-length sequencing contacts. Refer to the mating connector specification for the permitted finish and contact dimensions.
Keep holes, vias and components away from the bevel and routing zone. If fingers are joined to a plating bar, the supplier needs a path that can be removed without damaging the finished contact. Our PCB gold fingers guide covers bevels, layout and connector-specific checks in more detail.
Surface-Finish Selection by Use Case#
| Use case | Usually preferred | Reason |
|---|---|---|
| BGA and fine-pitch SMT pads | ENIG | Planar, solderable surface |
| General mixed SMT/PTH board | ENIG or another qualified solderable finish | Broad pad coverage and assembly compatibility |
| Repeatedly mated card edge | Hard gold | Abrasion resistance |
| Membrane or dome contact | Application-specific | Contact force, wipe and cycle life require qualification |
| Temporary production test pad | Application-specific | Probe type and expected hits determine wear need |
| Solder pads plus card edge | ENIG plus selective hard gold | Each area receives a finish suited to its function |
How to Specify ENIG Correctly#
- Name ENIG and the applicable IPC-4552 revision or agreed supplier specification.
- State any product class, thickness, solderability, corrosion and inspection requirements that apply.
- Identify exposed copper that must or must not receive the finish.
- Coordinate solder-mask-defined and copper-defined pads with the assembly land pattern.
- Define packaging, shelf-life or cleanliness requirements when the product needs them.
“Gold plating” is not enough. It does not tell the fabricator whether you want immersion or electrolytic gold, what the underplate is, or what performance the finish must provide.
How to Specify Hard Gold Correctly#
- Mark every plated contact area on a controlled drawing.
- Specify finished nickel and hard-gold thicknesses with units and tolerances.
- Reference the connector manufacturer's finish and mating-cycle requirements.
- Define bevel, edge, mask and copper geometry.
- Confirm that panelization gives the supplier plating access.
- State inspection and thickness-measurement locations.
- Keep thick hard gold off solder joints unless it has been explicitly qualified.
Common Selection Mistakes#
Calling ENIG “24-karat hard gold”
Purity and hardness are different concepts. ENIG's immersion layer and alloyed hard electroplate have different structures and functions.
Using ENIG for unlimited mating cycles
The attractive gold appearance does not provide the thickness or hardness needed for repeated abrasion.
Specifying hard gold over the entire PCB
This can add unnecessary cost and create solderability concerns. Use selective hard gold only where wear demands it.
Leaving thickness units ambiguous
Micrometres and microinches differ by a factor of 25.4. Put both the unit and measurement method in the purchase documentation.
Ignoring the plating bus
A hard-gold feature that cannot be electrically connected during panel plating may be impractical or require a different process.
ENIG vs. Hard Gold Decision Checklist#
Start with function. If the surface will be soldered and needs a flat whole-board finish, ENIG is usually the relevant option. If it is a repeatedly wiped contact, investigate hard gold. If the board has both, use a controlled dual-finish drawing and confirm panel feasibility early.
Then quantify rather than guess: mating cycles, contact force, environment, solder process, minimum shelf life, geometry, finish thickness and inspection. Review the broader PCB plating guide and the PCB manufacturing process. For a stack-up and finish review, send PCBPit your fabrication drawing and connector requirements.
Frequently Asked Questions#
Is ENIG the same as gold plating?
ENIG is one nickel/gold surface-finish process. “Gold plating” can also mean electrolytic soft or hard gold, so the generic phrase is not a complete manufacturing specification.
Can ENIG be used on PCB edge fingers?
It may work for low-cycle applications if the connector owner approves it, but its thin, soft immersion gold is not intended for repeated abrasion. Hard gold is normally selected for frequently mated card edges.
Can hard gold be soldered?
Some deposits can be soldered under controlled conditions, but thick or alloyed gold can affect solder-joint reliability. Use a qualified solderable finish on assembly pads unless the product process specifically approves hard gold.
Can a PCB use ENIG and hard gold together?
Yes. ENIG can cover solderable pads while selective hard gold covers edge contacts. The fabricator must approve the extra process, plating access and drawing callouts.
Which finish costs more?
Selective hard gold is generally more expensive because it uses more gold and adds electroplating, masking and inspection. Actual cost depends on area, thickness, panel arrangement and supplier capability.