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What Are PCB Surface Finishes?

PCB surface finishes, also known as PCB surface treatments, are processes used to coat the copper pads of a printed circuit board with a protective layer. This layer can be applied through chemical or physical methods. The primary purposes of PCB surface finishes are twofold: first, to prevent oxidation of the copper pads, and second, to ensure that the PCB is easy to solder when mounting electronic components.

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Comprehensive Guide to Various PCB Surface Finishes

Immersion Gold (ENIG)
- Overview: ENIG stands for Electroless Nickel Immersion Gold. This process involves applying a thick layer of nickel followed by a thin layer of gold onto the copper surface of a PCB. The gold layer protects the PCB from oxidation, ensuring a long shelf life (up to 12 months) and excellent electrical conductivity, making it ideal for high-density PCBs.
Immersion Silver
- Overview: In this process, a layer of silver is chemically deposited onto the copper pads of the PCB. Silver provides outstanding electrical performance due to its excellent conductivity and ease of soldering. However, it is more prone to tarnishing, which can cause a yellowish discoloration over time.
Immersion Tin
- Overview: Immersion tin involves depositing a layer of tin onto the copper pads through a chemical displacement reaction. Tin protects the copper from oxidation and offers excellent solderability since both the surface finish and solder are tin-based. Compared to HAL, immersion tin provides a smoother surface, though at a higher cost.
ENEPIG
- Overview: ENEPIG, or Electroless Nickel Electroless Palladium Immersion Gold, is an advanced version of ENIG. A palladium layer is added between the nickel and gold layers to prevent nickel migration, which can cause defects during PCB assembly. ENEPIG offers superior wire bonding, aging resistance, and eliminates the risk of black pad defects common in ENIG.
Hot Air Leveling (HAL)
- Overview: HAL, or Hot Air Leveling, involves melting tin and then leveling it across the PCB surface with hot air. The result is a solder coating on the copper pads that is typically 5-40 µm thick. HAL is the most widely used surface finish due to its cost-effectiveness and excellent solderability.
Organic Solderability Preservative (OSP)
- Overview: OSP is a chemical process that applies a thin organic layer onto the copper traces, protecting them from oxidation. This film is highly effective for soldering as it evaporates during the soldering process, leaving a clean copper surface. OSP is cost-effective and environmentally friendly.
Hard Gold Plating
- Overview: Hard gold plating involves first applying a nickel layer to the exposed copper, followed by a gold layer of 2-5 µm. The nickel acts as a barrier to prevent gold and copper diffusion, ensuring reliability during assembly. Hard gold contains additional elements like cobalt, making the surface harder and more durable, typically used for edge connectors.
Soft Gold Plating
- Overview: Similar to hard gold plating, soft gold plating involves nickel followed by a gold layer, usually 5-30 µm thick. The key difference is that soft gold is pure and lacks additional hardening elements, making it ideal for soldering electronic components. The surface appears less shiny compared to hard gold.
Selective Finishes
- Overview: To harness the advantages of multiple surface finishes, selective finishing combines two or more methods on a single PCB. Common combinations include ENIG with OSP, ENIG with hard gold plating on edge connectors, and HAL with hard gold plating. This approach optimizes performance across different parts of the board.

This guide should give you a clearer understanding of the various PCB surface finishes and their applications, helping you choose the best option for your specific needs.

Advantages and Disadvantages of Different PCB Surface Finishes

Surface Finish	Cost	Shelf Life	Solderability	Advantages	Disadvantages
HAL with Lead	Normal	12 months	High	– Cost-effective and efficient in production – Excellent solderability	– Not RoHS/REACH compliant – Unsuitable for solder pin distances < 0.5mm – Not ideal for BGA due to uneven surface
Pb-Free HAL	Normal	12 months	High	– Cost-effective and efficient in production – Excellent solderability	– Unsuitable for solder pin distances < 0.5mm – Not ideal for BGA due to uneven surface
OSP	Low	3 months	Normal	– Cost-effective and efficient in production	– Not suitable for PTH & SMT mixed assembly – Poor thermal stability – Must solder within 14 hours after first reflow – Unsuitable for PCBs with EMI grounding, mounting holes, and test pads – Not suitable for press-fit holes
Plating Gold	High	12 months	High	– Excellent solderability and conductivity – Long shelf life, very durable and stable – Smooth surface, ideal for fine-pitch – Best for edge connectors requiring multiple plugs	– High cost – Risk of embrittlement in solder joints
Immersion Gold	High	12 months	High	– Excellent solderability and conductivity – Long shelf life, very durable and stable – Smooth surface, ideal for fine-pitch	– Risk of “black pad” issue
Immersion Silver	Normal	12 months	High	– Excellent solderability – Cost-effective – Smooth surface, ideal for small ICs	– Prone to vulcanization (sensitive to sulfur) – Non-welding areas may discolor at high temperatures
Immersion Tin	Normal	12 months	High	– Excellent solderability – Cost-effective – Smooth surface, ideal for small ICs	– High chemical requirements for immersion tin
ENEPIG	High	12 months	High	– No risk of “black pad” – Extremely durable and stable – Ideal alternative for OSP or ENIG in certain applications	– Higher cost

This table provides a clear overview of the pros and cons of various PCB surface finishes, helping you to choose the most suitable one for your specific application.

Shelf Life and Workshop Life for PCBs with Different Surface Finishes

Surface Finish	Storage Condition & Shelf Life (from Production Date)	Storage Condition & Workshop Life After Unpacking
Immersion Silver	Temperature ≤ 27℃, humidity ≤ 60%, no acid, alkali, or organic solution	Temperature ≤ 27℃, humidity ≤ 60%, no acid, alkali, or organic solution ≤ 48 hours
(OSP) Organic Solderability Preservative	Temperature ≤ 27℃, humidity ≤ 60%, no acid, alkali, or organic solution	Temperature ≤ 27℃, humidity ≤ 60%, no acid, alkali, or organic solution ≤ 24 hours
ENIG, ENEPIG, Plating Gold	Temperature ≤ 27℃, humidity ≤ 60%, no acid, alkali, or organic solution	Temperature ≤ 27℃, humidity ≤ 60%, no acid, alkali, or organic solution ≤ 24 hours
Immersion Tin	Temperature ≤ 27℃, humidity ≤ 60%, no acid, alkali, or organic solution	Temperature ≤ 27℃, humidity ≤ 60%, no acid, alkali, or organic solution ≤ 24 hours
(HASL) Hot Air Solder Level	Temperature ≤ 27℃, humidity ≤ 60%, no acid, alkali, or organic solution	Temperature ≤ 27℃, humidity ≤ 60%, no acid, alkali, or organic solution ≤ 24 hours

Notes:

For HDI boards stored for more than three months but less than six months, PCBs need to be baked before production. After baking, the PCBs should be assembled within 8 hours.

Baking Guidelines:

Silver-Immersed PCBs: Baking temperature 120 ± 5℃, duration 2 hours, stacking height ≤ 2.5 cm.
Whole Nickel Gold PCBs: Baking temperature 120 ± 5℃, duration 2-4 hours, stacking height ≤ 2.5 cm.
OSP and Selective OSP Finish PCBs: Baking temperature 120 ± 5℃, duration 2 hours, stacking height ≤ 2.5 cm.

Ensure that the oven used for baking is clean and free from volatile gases and corrosive chemicals like acids, alkalis, and organic solvents.

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Custom PCB Assembly

What Are PCB Surface Finishes?

Request Your Free Quote Today!

Comprehensive Guide to Various PCB Surface Finishes

Advantages and Disadvantages of Different PCB Surface Finishes

Shelf Life and Workshop Life for PCBs with Different Surface Finishes

Get Your Free Instant Quote Today!

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