Why "Gold" on PCB Boards

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I. PCB Surface Finishes

• Anti-oxidation, HASL, Lead-free HASL, ENIG, Immersion Tin, Immersion Silver, Hard Gold, Full-board Gold, Gold Finger, ENEPIG, OSP
• OSP: Low cost, good solderability, strict storage conditions, short shelf life, eco-friendly, good soldering, flat.

II. Why Gold Plating
As IC integration increases, pins become denser. HASL can't flatten fine pads, affecting SMT. Also, HASL has short shelf life. Gold plating solves this:
1. Flatness ensures solder paste printing quality for fine-pitch SMT (0603/0402).
2. Longer shelf life, good for prototyping.
3. But with finer traces (3–4 mil), gold plating causes gold whiskers and skin effect issues at high frequencies.

III. Why ENIG (Electroless Nickel Immersion Gold)
ENIG solves gold plating problems:
1. Brighter gold color.
2. Better solderability.
3. Gold only on pads → skin effect doesn't impact signal.
4. Denser crystal structure, less oxidation.
5. No gold whiskers (no shorts).
6. Better solder mask adhesion.
7. No trace spacing compensation issues.
8. Lower stress, better for wire bonding. But softer → not wear-resistant for gold fingers.
9. Same flatness and shelf life as gold plating.

IV. ENIG vs. Gold Plating
In practice, gold plating processes fall into two categories: electroplated gold and electroless nickel immersion gold (ENIG) .
Electroplated gold significantly compromises solderability, whereas ENIG offers superior solderability. Unless wire bonding is explicitly required by the customer, most manufacturers now prefer ENIG.
Common PCB surface finishes include:

• Gold-based (electroplated gold, ENIG)
• Immersion silver
• OSP
• HASL (leaded and lead-free)
• These finishes are primarily applied to FR-4 or CEM-3 substrates. For paper-based materials, flux-coated finishes are also used.

Regarding poor solderability (i.e., poor wetting), after excluding factors related to the solder paste or the SMT assembly process—such as materials or manufacturing procedures—the potential PCB-related causes are as follows:

From a PCB perspective, the main causes are:
1. Solder mask bleed onto the pad during PCB printing, which can impede solder wetting. This can be verified through a floating solder test.
2. Pad wetting area not meeting design requirements—i.e., whether the pad design adequately supports component placement.
3. Pad contamination, which can be identified through ionic contamination testing.
The above three points are the primary considerations for PCB manufacturers.

Surface Finish Comparison:

• Gold plating: longest shelf life (~1 year), stable.
• HASL: medium.
• OSP: shortest.
• Immersion silver: expensive, strict storage (sulfur-free paper), ~3 months shelf life.
• Solderability is similar across ENIG, OSP, HASL. Choice is mainly cost-performance.
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