An Expert's Guide to PCB Surface Finishes

An Expert's Guide to PCB Surface Finishes

Applying a surface finish is one of the final steps in the manufacturing process of printed circuit boards (PCBs). Typically, this coating is applied after the solder mask process. The main objective of a PCB surface finish is to protect the exposed copper from oxidation, as oxidized copper cannot be soldered effectively. The solder mask generally covers most of the circuitry, thus preserving functionality.

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In cases where a PCB necessitates a body gold finish, this surface treatment is typically applied before the solder mask. Gold finishes tend to incur higher costs, yet they ensure a uniform coating that can be particularly beneficial for designs requiring precise surface mounts, which may justify the investment.

Below is an overview of some of the widely used PCB finishes, along with their applications and critical characteristics.

Leaded Solder (HASL)

What is it?

Leaded solder is one of the most common PCB finishes, consisting of a mixture of tin and lead, often in a 63/37 ratio. It's also known as SN67 or SN66, with a melting point of approximately 485 degrees Fahrenheit, resulting in a shiny silver appearance.

Why is it used?

This finish serves as a default process, particularly suitable for boards utilizing thru-hole technology, although it may not yield a flat surface adequate for Surface Mount Devices (SMDs) or Ball Grid Arrays (BGAs).

Application Process:

The standard application method is Hot Air Leveling (HAL), where the boards are submerged in molten solder and air knives remove any excess.

Critical Measurements:

The thickness of leaded solder is typically not measured, with the IPC standard focusing on visual coverage of copper. Also, this option remains inexpensive compared to alternatives.

Lead Free Solder (HASL Lead-Free)

What is it?

This finish closely resembles leaded solder but is composed of about 99.3% tin and 0.6% copper, known as SN100CL with a flow temperature around 515 degrees Fahrenheit.

Why is it used?

Lead-free solder is necessary in applications that prohibit lead usage. However, a higher temperature laminate like 370HR may be required to mitigate the higher melting point during assembly.

Electroless Nickel Immersion Gold (ENIG)

What is it?

ENIG involves plating a thin layer of gold over nickel, which is applied first to act as a barrier to prevent gold and copper migration. This finish provides a flat surface ideal for SMDs and BGAs.

Why is it used?

Due to its exceptional conductivity, ENIG is preferred by many contract assemblers.

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Conclusion:

Understanding the various PCB surface finishes is crucial for ensuring reliability in electronic components. Each type of finish has its distinct advantages and applicable scenarios for use. Selecting the right finish significantly impacts performance, durability, and assembly quality.