Hard Gold versus Soft Gold PCB Plating: Which to Use

Gold finishes on a board are not interchangeable. A gold finish chosen for a soldering surface behaves differently from one chosen for a connector that will be inserted a thousand times, and using the wrong one produces either a finish that wears through or a joint that will not form. The distinction starts with what is underneath the gold.

The Two Families

Hard gold is an electroplated layer of gold with a small amount of a hardening element, deposited over a nickel barrier. The alloying element raises the hardness substantially, which is what makes it suitable for surfaces that slide against another contact.

Soft gold is pure gold, usually applied by immersion over a thin nickel layer. It is soft and has excellent solderability, but it wears quickly under repeated mechanical contact. The name immersion gold is often used for this finish when it is applied selectively over copper.

Hardness and Wear

Wear resistance is the property that separates them, and it is a function of the deposit rather than of the appearance, which is why two visually identical gold surfaces can behave completely differently in a socket. Hard gold typically reaches several times the hardness of pure gold, and it is specified with a minimum thickness, commonly around zero point eight to one point five micrometres, on contact surfaces that see repeated insertion.

Soft gold is measured in tenths of a micrometre and wears through within a small number of mating cycles. Where a connector is mated once during assembly and never again, that difference is irrelevant, and paying for hard gold on such a surface adds cost without adding life. Where it is a card edge inserted into a socket repeatedly, it is the difference between a product that lasts and one that develops intermittent contacts.

Contact Resistance and Oxide Formation

Gold does not form a significant oxide, which is why it is used on low level signal contacts where the voltage is too small to break down a film. Both finishes share that advantage over tin, which forms an oxide that raises contact resistance unless the contact pressure is high enough to break it.

The nickel barrier under both finishes matters as much as the gold. Nickel diffuses into gold over time, and if the gold is too thin the diffusion reaches the surface and raises the contact resistance. Hard gold over a controlled nickel layer therefore has a defined minimum thickness for both metals rather than for the gold alone.

Solderability

Soft gold is easy to solder because it dissolves readily into the molten alloy, exposing a clean nickel surface that wets well. Hard gold is much less cooperative, and a joint made to a thick hard gold surface can be brittle because the hardening element dissolves into the alloy.

The practical consequence is that hard gold is used only where a connection must be made by contact rather than by solder, and the transition to a solderable area is arranged before the plating begins. A board that requires both usually has the gold applied selectively, with the contact fingers plated hard gold and the rest of the board finished with something else. The coating was covered in the material on lead-free versus leaded solder and the finish selection in general follows from the same reasoning.

Selective Plating and Its Cost

Selective plating applies the gold only where it is required, which saves material and avoids the solderability penalty elsewhere. The process involves masking the rest of the board, plating the exposed areas and removing the mask, and it adds steps that must be sequenced correctly with the solder mask.

Because the gold is expensive and the masking is labour intensive, the cost rises steeply with the plated area. Designs should therefore keep the gold confined to the contact area and avoid decorative plating, which is a cost with no electrical function.

Where Each Finish Is Used

Hard gold appears on edge connectors, on this type of card interface, on membrane keypad contacts and on any surface that slides or is mated repeatedly. Its thickness and the underlying nickel are specified by the number of mating cycles the product must survive.

Soft gold and the immersion gold finishes appear on solder pads where a flat surface and good wetting are needed, including fine pitch pads and areas that must survive a long storage period before assembly. Where a board carries both functions, the two finishes are applied in different areas.

Electrolytic Nickel Gold Compared With Immersion Gold

The two gold finishes are often confused. Electrolytic plating requires a connection to the pattern and deposits a thicker layer, which is why it is used for hard gold contacts. Immersion gold deposits by a chemical displacement reaction, without an electrical connection, and it produces a very thin layer that stops growing once the surface is covered.

The immersion process is simpler because it does not require the board to be wired up, but it cannot build the thickness a contact surface needs. That is the real distinction: not the cost, but whether the layer is thick enough to survive mechanical wear.

Design and Manufacturing Considerations

The gold area should be defined on a dedicated layer of the design data, with the outline of the plated region shown clearly, and the drawing should state the thickness of both the gold and the nickel underneath.

The transition between the plated edge and the solder mask is where defects appear, and the mask should be pulled back from the plated area to avoid plating onto a resist edge. Where the contact fingers are at the board edge, the outline and the chamfer also matter, because a rough edge will damage the socket. The pad layout that the fingers follow is governed by the same rules as any other pad, described in PCB pad design standards.

Verification

Plating thickness is verified by X-ray fluorescence measurement or by microsection, and both the gold and the nickel are measured. Hardness is specified from the bath chemistry rather than measured on the finished part, which is why the process control around the plating bath is the real guarantee, and the controls that matter are the ones described in PCB design guidelines for manufacturability.

For a contact surface, wear testing on a sample is the definitive check, and it should be repeated whenever the plating bath is changed, because a small change in chemistry shows up first as a reduction in wear life rather than as a visible defect. Cycling a sample connector to the specified number of insertions and then measuring the contact resistance shows whether the thickness and the hardness are adequate, and it is far more informative than a thickness measurement alone.

FAQ

Can hard gold be soldered? It can be soldered with difficulty, and the joint can be brittle if the layer is thick. Solderable areas should be finished differently.

How thick should hard gold be? Around zero point eight to one point five micrometres over a nickel barrier for typical connectors, with more where many mating cycles are expected.

Is gold plating always the best contact finish? For low level signals and for humid or polluted environments, gold is the standard choice. Where contact pressure is high and the environment is benign, a tin finish can be adequate at much lower cost.