Copper & Finish

The copper layer of a Printed Circuit Board (PCB) refers to the thin copper foils laminated onto the core layers (substrate) of the PCB. These layers serve to conduct electrical current through the board, connecting various components together.

Number of Layers

The number of layers depends on the design requirements, with a standard PCB having either one (single-sided) or two layers (double-sided), while multi-layer PCBs have additional inner layers in addition to the two outer layers. The various layers are generally insulated by a dielectric, either a core layer (substrate) or a pre-impregnated fiber layer ("pre-preg").

Copper Thickness

The thickness of the copper layer on a PCB is specified in micrometers (µm) but can also be measured as weight in ounces per square foot (oz/ft²). A typical copper layer often has a thickness of 35µm (1 oz/ft²), but depending on the application requirements, the thickness can vary. A higher copper thickness can carry more current but is also more expensive and can complicate the manufacturing process.

Manufacturing

To create the desired traces and structures on a PCB, an etching process is applied to the flush copper. A protective material is applied to the areas of the copper layer that are to be retained. The unprotected areas are then chemically removed or etched, leaving only the desired copper structures.

Surface Treatment (Finish)

The finish of a circuit board refers to the surface treatment of the copper surface after the circuit board manufacturing process. The finish has several important functions and impacts on the performance and reliability of the circuit board. Here are some common types of circuit board finishes:

HSL SnPb (Hot Air Solder Leveling Tin-Lead)

HSL SnPb is a traditional finishing process where the PCB is dipped into molten tin-lead. Hot air is used to remove excess material, creating a smooth, solderable surface.

Advantages/Disadvantages

Proven method with known reliability.

Good for Plated Through Hole (PTH) applications.

Cost-effective.

Contains environmentally and health-hazardous lead.

Possible uneven surfaces.

Limited storage (tin pest).

HASL Lead-Free (Hot Air Solder Leveling Lead-Free)

HASL lead-free is similar to HSL but uses lead-free alloys. It provides an environmentally friendly alternative while ensuring a solderable surface.

Advantages/Disadvantages

Environmentally friendly.

Uniform surface.

Relatively cost-effective.

Higher processing temperatures.

Possible uneven surfaces.

Storage limitations.

Chemical Tin

In the chemical tin process, the PCB is dipped into a special solution that creates a pure tin layer on the surface. It protects the copper from oxidation and prepares it for soldering.

Advantages/Disadvantages

Uniform surface.

Lead-free and environmentally friendly.

Good for fine-pitch components.

Whisker formation.

Limited shelf life.

Higher costs than other methods.

Chemical Nickel

The chemical nickel process involves dipping the PCB into a nickel solution, creating a thin nickel coating. This serves as a barrier layer and promotes solderability.

Advantages/Disadvantages

Corrosion resistant.

Good barrier layer.

Improved solderability.

Often not used as a sole surface.

Higher costs.

Compatibility issues with some soldering processes.

Chemical Silver (Immersion Silver)

In the chemical silver process, the PCB is dipped into a silver solution, leaving a thin silver layer. The silver protects the copper from oxidation and improves solderability.

Advantages/Disadvantages

Good solderability.

Uniform surface.

Longer storage life.

Susceptible to sulfur.

Higher costs than other methods.

Limited reworkability.

ENIG (Electroless Nickel Immersion Gold)

ENIG first applies a nickel barrier followed by a thin gold layer. This provides protection, excellent solderability, and an aesthetically pleasing surface.

Advantages/Disadvantages

Long-lasting storage life.

Flat surface.

Suitable for fine-pitch components.

Higher costs.

Risk of "Black Pad" phenomenon.

Not ideal for thicker components.

ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)

ENEPIG forms a triple layer of nickel, palladium, and gold. This combination offers a robust surface, providing versatile soldering and wire bonding capabilities.

Advantages/Disadvantages

No "Black Pad" risk.

Excellent wire bonding capability.

Versatile solderability.

Higher costs than ENIG.

Complex process.

Thickening control can be challenging.

Hard Gold

Hard gold is applied through electrolysis, creating a thick gold layer on the PCB. This makes it particularly wear-resistant and suitable for connectors or buttons.

Advantages/Disadvantages

Wear-resistant.

Very good conductivity.

Aesthetically pleasing.

Very high costs.

Not suitable for solder areas.

Limited applications.

OSP (Organic Solderability Preservative)

OSP is an organic compound applied directly to the copper. It protects the copper from oxidation and prepares it for soldering without using metals.

Advantages/Disadvantages

Environmentally friendly.

Cost-effective.

Flat surface.

Limited reworkability.

Susceptible to handling.

Limited storage life.

HT_OSP (High-Temperature Organic Solderability Preservative)

HT_OSP is an enhanced version of OSP, optimized for higher temperatures. It provides improved performance in high-temperature soldering processes.

Advantages/Disadvantages

Resistant to high temperatures.

Cost-effective.

Flat surface.

Limited reworkability.

Remains sensitive to handling.

Limited storage life.

No Finish

A PCB without finish means that the copper of the traces remains unprotected and can easily oxidize. This condition can affect the solderability and performance of the PCB.

Advantages/Disadvantages

No additional costs for finish.

Direct contact with copper.

Simple process.

Rapid copper oxidation.

Limited solderability after prolonged storage.

Not recommended for many applications.