ENIG versus hard gold: start with the pad function

Choose a PCB gold finish by the job the surface must perform. A solderable component pad, a repeatedly mated contact, and a bonding surface should not be treated as interchangeable simply because each may look gold. Define the surface function first, then agree the finish system, thickness requirements, and acceptance method.

ENIG means electroless nickel/immersion gold. MacDermid Alpha lists it among its PCB surface-finish systems. A board manufacturer’s surface-finish comparison separately identifies hard gold as an electrolytic deposit. These process names describe different systems; specifying only “gold plating” leaves important requirements unresolved.

Write down the pad function

For a soldered pad, describe the assembly process and relevant solderability requirements. For a mating contact, describe the connector interface and expected use. For a bonding surface, identify the bonding process and seek compatibility evidence from the responsible process owner. A surface selection should follow these requirements rather than a blanket preference for a more expensive finish.

Make thickness units impossible to misread

One microinch equals 0.0254 micrometres. In an illustrative conversion, 10 microinches equals 0.254 micrometres, while 30 microinches equals 0.762 micrometres. These examples demonstrate unit conversion only; neither value is a recommendation for your finish.

Write both the material layer and its unit: a gold thickness is not the same requirement as a nickel thickness. Also state whether the number is a nominal target, minimum, or permitted interval. Two quotations can appear to match while one gives a nominal thickness and the other guarantees a minimum.

A mixed-function drawing example

Imagine a rigid-flex assembly with component pads on a rigid island and a separate connector-contact area. Label the surface groups S1 and C1 on a finish map. Put each finish requirement next to its group in a table, and identify any area where the boundary could be misunderstood. This is a documentation example, not an assertion that a specific selective process is available for every construction.

Ask the supplier to review feasibility before freezing the layout. Record any process-related restriction and have the design owner approve an affected geometry change. Keep the finish map with the controlled fabrication drawing so a purchasing note cannot silently override it.

Compare the finish system, not the visible color

A gold-colored surface does not identify its deposition process, underlying layers, thickness, or intended use. Put the complete finish name into the requirement and identify the surface group to which it applies. If a quotation says only gold, ask for clarification before using its price or delivery estimate in a comparison. Similar appearance is not evidence of equivalent function.

ENIG combines an electroless nickel layer with immersion gold. Hard gold is a different finish category associated with an electrolytic gold deposit. The distinction matters because the process and finished system must support the specific surface function. Do not use a plating label as a shortcut for a solderability, wear, or bonding qualification that has not actually been established.

For a soldered area, discuss the finish with the assembly process owner using the intended soldering materials and thermal process. For a contact area, use the mating-interface requirements. For a bonding area, involve the owner of the bonding process. A single board can contain more than one surface role, and a choice appropriate for one role may create additional questions for another.

Define what a mating cycle means for the contact

Start with the actual mating interface rather than selecting a thickness from an unrelated product. Identify the connector part number, contact geometry, expected insertion and removal events, and operating environment. Record whether the contact is intended for occasional service, repeated user interaction, or another defined duty. Do not turn a marketing description such as durable into a numerical endurance requirement.

Samtec’s connector plating FAQ discusses the role of plating choices in connector applications. Treat connector-specific information as evidence for the configuration it describes. It does not automatically establish the mating life of a different PCB contact, gold thickness, contact force, or environment. Ask the interface owner which qualification evidence applies to the complete mating system.

In an original usage example, assume a product is connected once during assembly and then disconnected and reconnected during four planned service visits. Record the initial connection separately from the service events, and define how the selected connector specification counts mating cycles. This prevents a maintenance worksheet and a test report from using the same word to mean different operations. The example is an accounting exercise, not a proposed endurance rating.

State thickness requirements in a comparison table

The first two illustrative statements contain numerically equivalent thickness values after unit conversion, but they impose different obligations. A nominal value does not establish the same lower bound as a minimum requirement. Neither statement identifies all properties needed to approve a finish. Use the table to compare wording before concluding that two offers describe the same deliverable.

Also keep the gold layer separate from the underlayer. If a report includes several thickness columns, label each material and measurement unit. Request a clear explanation of any combined or averaged value. An average across several locations should not be treated as proof that every location meets a minimum unless the agreed inspection and acceptance rules support that conclusion.

Make a selective-finish map readable without color

Use stable surface-group identifiers, hatch patterns, and a legend to show which finish applies to each area. Identify both sides where needed and provide sufficient dimensions to locate the boundaries. If a contact extends toward a board edge, describe the intended finished geometry and the related mechanical requirements in the controlled drawing. Do not rely on a colored screenshot whose meaning disappears when printed.

In the S1/C1 example, review the finish boundary with the pad and protection-layer geometry visible. Ask whether every exposed conductive feature has a defined treatment. Where process feasibility requires a geometry adjustment, have the design owner review the resulting electrical and mechanical consequences. Record the accepted change in the released data rather than leaving the supplier to interpret an informal sketch.

For a rigid-flex board, include the region identity in this map. A feature on a rigid island and a feature on a flexible tail may have different handling and support requirements during manufacturing and assembly. The map should help the fabricator connect the finish requirement to the actual construction. It should not assume that a process available for one region is automatically applicable everywhere.

Agree how the requirement will be verified

Specify the evidence needed to accept the finish before placing the order. This may include agreed thickness measurements, inspection records, or other project-specific documentation. Name the relevant surface groups and measurement locations so that the supplied report can be traced to the drawing. The exact verification method and sampling plan require agreement with the responsible technical parties.

Keep appearance inspection and functional evidence separate. A uniform-looking surface does not by itself demonstrate a mating-life requirement, solderability after a defined storage period, or bonding compatibility. Conversely, a visual concern should be evaluated against the applicable acceptance criteria rather than dismissed because the nominal plating thickness is correct. Define the question each inspection is intended to answer.

Record packaging, handling, and storage expectations with the assembly plan. Do not copy a general shelf-life number from a comparison chart and treat it as a guarantee for the actual packaging and environment. When parts have been stored or exposed differently from the agreed conditions, ask the responsible quality and assembly teams to determine what reassessment is needed.

Resolve finish substitutions before release

If a supplier proposes a different finish, compare the proposal by surface function. List the affected groups, the original requirement, the proposed system, and the evidence supporting compatibility. Include the assembly owner for soldered surfaces and the interface owner for mating contacts. A cost reduction alone does not establish that the substitute meets the product requirement.

Update the controlled drawing and purchasing description together after approval. Preserve the previous revision and the decision record, but make the active requirement unambiguous. This prevents a repeat order from reverting to an older finish merely because an outdated quotation or file name was reused. The result should be a precise specification that manufacturing, assembly, and inspection teams can interpret consistently.

Compare quotations on the same basis

• Do both quotations name the same finish system?
• Do they apply it to the same marked areas?
• Are underlayer and gold requirements both stated where needed?
• Are the thickness units and minimum-versus-nominal definitions consistent?
• Does the inspection record identify the relevant measurement locations?
• Have substitutions been explicitly reviewed?

Use our surface-finishes guide and engineering tools when preparing requirements. Send your finish map for review through the inquiry form and describe what each exposed surface must do.