The plating processes for PCB circuit boards are diverse, with each process serving specific purposes and applications. Below is a detailed description of the main and common plating processes in PCB manufacturing:
1. Classification by Purpose and Location (Core Processes)
1. Full Board Copper Plating
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Purpose: Mainly used to increase the copper thickness of the hole walls, ensuring that the hole walls are not etched away in subsequent processes, thus achieving electrical connections between layers. This is one of the most critical plating steps in PCB manufacturing.
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Process: After drilling, the board undergoes a copper deposition (chemical copper deposition, which generates a thin layer of chemical copper on the insulating walls of the holes) process, and then the entire circuit board is immersed in a copper plating tank, where copper layers are deposited simultaneously on the board surface and inside the holes through an electrolytic reaction.
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Characteristics: Provides conductivity and current-carrying capacity for the circuit.
2. Pattern Plating
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Purpose: On boards where the circuit pattern has been completed, selectively thickens the copper layer on exposed copper traces (including pads and hole walls) and plates a layer of etch-resistant metal (usually tin).
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Process: After applying and developing the dry film, the circuit portions are exposed. The board is then plated, first thickening the copper, followed by plating a layer of tin as a protective layer on top of the copper.
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Characteristics: This “tin protective layer” protects the underlying copper traces from being etched away during subsequent alkaline etching, thus forming the final circuit pattern.
3. Chemical Copper / Hole Metallization
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Strictly speaking, this is not electroplating (electrolytic plating), but chemical plating.
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Purpose: To deposit a thin layer of chemical copper (usually 0.3-0.5μm) on the non-conductive hole walls, making the hole walls conductive and laying the foundation for subsequent full board copper plating. This is the cornerstone for achieving electrical interconnection through vias.
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Process: After a series of chemical treatments (such as cleaning, micro-etching, activation, and acceleration), under the catalysis of palladium, copper ions in the solution are reduced to metallic copper on the insulating substrate.
4. Tin Plating
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Purpose: Mainly serves as an etch-resistant protective layer (in pattern plating), and is also used as one of the final surface treatments (as a solderable layer).
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Pattern Plating Tin: Acts as an etching barrier.
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Final Surface Treatment Tin: Provides good solderability and a flat surface.
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Types: Includes matte tin, bright tin, etc.
5. Nickel/Gold Plating
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Purpose: Mainly used in areas requiring high wear resistance, high oxidation resistance, or good contact performance, such as keys, gold fingers, bonding, etc.
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Process: Typically, a layer of nickel is plated first, followed by a layer of gold on top of the nickel.
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Hard Gold Plating: Contains elements like cobalt or nickel, has high hardness, excellent wear resistance, and is mainly used for gold fingers.
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Soft Gold Plating: Very high purity (99.99%), soft texture, mainly used for chip bonding.
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Nickel Layer: Acts as a barrier layer to prevent mutual diffusion between copper and gold, while providing hardness and wear resistance.
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Gold Layer: Provides excellent conductivity, oxidation resistance, and corrosion resistance. Gold is further divided into:
6. Copper Pillar Plating
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Purpose: Mainly used for advanced packaging with high-density interconnections, such as forming tiny copper bumps in Flip-Chip technology for connecting chips to substrates.
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Characteristics: This is a high-precision, micron-level plating technology.
2. Final Surface Treatment Processes
These processes are applied to the final pads of the PCB, primarily aimed at protecting the pads from oxidation and providing good solderability. Some of these are chemical plating/immersion plating rather than electroplating.
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HASL – Hot Air Solder Leveling
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Process: The PCB is immersed in molten tin-lead or lead-free tin, and then excess tin is blown off with a hot air knife, forming a flat, uniform tin layer.
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Advantages: Low cost, good soldering performance.
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Disadvantages: Uneven surface, not suitable for fine-pitch components.
ENIG – Electroless Nickel Immersion Gold
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Process: A layer of nickel is first deposited on the copper pads, followed by a thin layer of gold deposited through a chemical displacement reaction on the nickel.
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Advantages: Flat surface, high hardness, good oxidation resistance, long lifespan.
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Disadvantages: Higher cost, potential “black pad” issue.
Immersion Tin
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Process: A layer of tin is deposited on the copper surface through a chemical displacement reaction.
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Advantages: Flat surface, good compatibility with tin solder.
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Disadvantages: Sensitive to storage conditions, prone to tin whiskers.
Immersion Silver
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Process: A layer of silver is deposited on the copper surface through a chemical displacement reaction.
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Advantages: Flat surface, good electrical performance, excellent solderability.
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Disadvantages: Prone to oxidation and sulfide, leading to yellowing.
ENEPIG – Electroless Nickel Palladium Immersion Gold
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Process: A thin layer of palladium is added between the nickel and gold.
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Advantages: Solves the “black pad” issue of ENIG, suitable for wire bonding and soldering, with excellent performance.
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Disadvantages: Highest cost.
OSP – Organic Solderability Preservative
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Process: Forms an organic thin film on the copper surface to prevent oxidation.
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Advantages: Low cost, very flat surface, simple process.
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Disadvantages: Very thin protective layer, not durable for multiple soldering and short storage time.
3. Special Plating Technologies
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Pulsed Plating
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Principle: Uses pulsed current instead of direct current for plating.
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Advantages: Can achieve denser, more uniform, and lower porosity plating, especially suitable for uniformity of plating in deep holes and small holes.
Horizontal Plating
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Principle: The PCB is placed horizontally and passed through the plating tank, where a special spraying device flushes the electrolyte from below to the top of the board.
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Advantages: Very suitable for the manufacturing of thin boards, small holes, and high-end HDI boards, significantly improving the uniformity of copper thickness in holes at different positions.
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When selecting specific processes, decisions should be based on the product’s application scenarios, cost budget, and performance requirements.
