Crystal Oscillator Pad Surface Finish: The Necessity of Gold Plating, Process Methods, and Comparative Analysis

1.Excellent Solderability and Solder Joint Reliability (Core Reason)

Prevents Oxidation: Gold (Au) is a very stable metal and does not easily oxidize in air. Other common pad surface finishes, such as tin plating (HASL), are prone to oxidation during storage, forming a tin oxide film that leads to decreased solderability.

Ensures Solder Joint Strength: A clean gold surface provides excellent wettability, allowing the solder to spread easily and uniformly, forming strong and reliable solder joints. This is crucial for automated SMT production, significantly reducing defect rates such as cold solder joints and false soldering.

2.Ensures High-Frequency Electrical Performance

Excellent Conductivity: Gold is an excellent electrical conductor with very low surface resistance. For high-frequency components like crystal oscillators (especially those at tens of MHz or even hundreds of MHz), tiny differences in pad surface resistance can introduce unnecessary losses or signal integrity issues.

Stable Contact: The gold-plated layer is flat and smooth (referred to as "planarization treatment"), enabling uniform and stable electrical contact with the crystal oscillator's gold-plated electrodes or solder balls. This reduces loss and reflection during signal transmission, which is very important for ensuring the purity and stability of the clock signal.

3.Suitable for Gold Wire Bonding (Wire Bonding)

The internal chips of some high-end or specially packaged crystal oscillators (such as some OCXOs) need to be connected to external pins using very fine gold wires. This process needs to be performed on the oscillator's package pads.

Only gold-to-gold bonding can achieve the most reliable and stable connections. Gold-plated pads provide the necessary condition for this process.

4.Extended Shelf Life

Because gold does not easily oxidize, the solderability of gold-plated PCBs can be maintained for a long time (typically over 1 year), facilitating material management and inventory turnover. Tin-plated boards, however, may develop solderability issues after just a few months in humid environments.

5.Suitable for Multiple Reflow Soldering (Reflow)

During complex PCBA assembly, the board may need to undergo multiple high-temperature reflow soldering cycles. The gold plating remains stable at high temperatures and will not easily melt or develop "tin whiskers" like tin plating, ensuring the pads maintain good solderability after each reflow pass.

Choice of Gold Plating Process: ENIG
The most commonly used gold plating process for crystal oscillator pads is ENIG (Electroless Nickel Immersion Gold).

Base Nickel (Ni) Layer: This is key. The nickel layer acts as a barrier layer, preventing the upper gold layer and the underlying copper (Cu) from inter-diffusing at high temperatures to form brittle intermetallic compounds (IMCs), which severely affect the mechanical strength of the solder joint.

Surface Gold (Au) Layer: The gold layer is very thin (typically 0.05-0.1μm), used only to protect the nickel layer from oxidation and to provide an excellent solderable surface. At the moment of soldering, the gold quickly dissolves into the solder, and the actual solder joint is formed by the combination of the underlying nickel layer and the solder (Sn) into a Ni-Sn alloy.

Comparison with Other Surface Finish Methods
Surface Finish Method -- Advantages, Disadvantages: (For chip crystal oscillators)
ENIG (Electroless Ni/Immersion Au) -- Advantages: High flatness, good solderability, oxidation resistance, suitable for wire bonding. Disadvantages: Higher cost.
HASL (Hot Air Solder Leveling) -- Advantages: Low cost. Disadvantages: Uneven surface, can lead to poor soldering of small components; prone to oxidation.
OSP (Organic Solderability Preservative) -- Advantages: Low cost, very flat. Disadvantages: Protective film is fragile, not resistant to multiple reflow cycles; short shelf life.
Immersion Silver -- Advantages: Flat, good solderability, moderate cost. Disadvantages: Prone to oxidation and sulfidation (tarnishing); long-term reliability not as good as ENIG.
ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) -- Advantages: Optimal performance, very suitable for gold wire bonding. Disadvantages: Highest cost.

Summary
Gold plating (ENIG) the pads for chip crystal oscillators is primarily intended to ensure this critical component, which provides the "heartbeat" for the system, is successfully soldered on the first attempt during high-speed, automated SMT production, and to form long-term stable and reliable electrical and mechanical connections.
Although gold plating has a higher cost, this investment is entirely worthwhile in the vast majority of electronic products that require high reliability (such as communication equipment, industrial controls, automotive electronics, and medical devices) to avoid system clock failures, batch rework, or even product recalls caused by soldering defects.