PCB surface finish refers to the coating applied between a component and a bare PCB. Its primary purposes are to ensure solderability and to protect exposed copper circuitry. With a variety of surface finishes available, selecting the appropriate one can be challenging, especially as surface-mount technology becomes more complex and regulations such as RoHS and WEEE evolve.

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

HASL is the most commonly used surface finish in the industry. This process involves immersing the circuit board in a molten tin/lead alloy and then using ‘air knives’ to blow hot air across the board to remove excess solder.

• Pros: Cost-effective, Widely available, Repairable
• Cons: Uneven surface, Not ideal for fine-pitch components, Prone to thermal shock, Not suitable for plated through-hole (PTH), Poor wetting

OSP (Organic Solderability Preservative)

OSP is a water-based, organic surface finish mainly used for copper pads. It selectively bonds with copper, offering protection before soldering. This finish is environmentally friendly, provides a flat surface, and is lead-free, requiring minimal equipment maintenance. However, it is less durable than HASL and can be sensitive to handling.

• Pros: Lead-free, Flat surface, Simple process, Repairable
• Cons: Not suitable for PTH, Sensitive, Short shelf life

ENIG (Electroless Nickel Immersion Gold)

ENIG has rapidly become one of the most popular surface finishes. This process creates a two-layer metallic coating, with nickel serving as a barrier to the copper and a surface for component soldering, while a gold layer protects the nickel during storage. ENIG addresses key industry trends, such as the need for lead-free solutions and the demand for flat surfaces, especially for complex components like BGAs and flip chips. However, ENIG can be expensive and may lead to “black pad syndrome,” where phosphorus buildup between the gold and nickel layers can result in weak solder joints and potential failures.

• Pros: Flat surface, Strong, Lead-free, Ideal for PTH
• Cons: Black pad syndrome, Expensive, Not suitable for rework

Other surface finishes commonly used in the industry include Immersion Silver, Immersion Tin, Hard Gold, and Nickel Palladium (ENEPIG). Based on your specific requirements, you can select the most suitable surface finish.

If you have any questions about PCBs or PCBA, please feel free to contact me at info@wellcircuits.com.