5 Types of PCB Surface Finishes: Is One of Them Right For ...

Choosing the right PCB surface finish involves evaluating numerous factors, such as the components used, expected production volume, durability requirements, environmental impact, and cost. According to a snippet from Google, when selecting a PCB surface finish, "each type of finish has its advantages and disadvantages, and the suitability depends on the specific needs and constraints of the PCB design." In this article, we delve into the five main types of PCB surface finishes to help you determine which one might be suitable for your needs.

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The PCB surface finish helps protect copper circuitry from corrosion. It also provides a solderable surface for your components. There are a number of factors to consider, including:

1. The components you use
2. Your expected production volume
3. Your requirements for durability
4. Environmental impact, and
5. Cost

Below are 5 types of PCB surface finishes, along with their benefits and drawbacks.

Hot Air Solder Leveling (HASL)

Hot Air Solder Leveling is one of the least expensive PCB surface finishes available.

It is widely used due to its economic efficiency. The board is dipped in molten solder and then leveled off with a hot air knife. HASL is suitable for through-hole or larger SMT components, but it’s not ideal for smaller SMT components like 0805 or SOIC. This is mainly because the surface is not completely level, which can cause issues with smaller components. HASL typically uses Tin-Lead solder, which is not RoHS compliant, so if reducing lead is important, consider Lead-Free HASL.

Advantages:

• Excellent solderability
• Inexpensive / Low cost
• Allows a large processing window
• Long-standing industry experience

Disadvantages:

• Inconsistent thickness/topography between pads of different sizes
• Not suitable for < 20mil pitch SMD & BGA
• Risk of bridging on fine pitch
• Less ideal for HDI products

Lead-Free HASL

Lead-Free HASL is similar to standard HASL but does not use Tin-Lead solder. Instead, it uses Tin-Copper, Tin-Nickel, or Tin-Copper-Nickel Germanium, making it RoHS compliant while remaining economical. However, like standard HASL, it isn't ideal for smaller components. Here, immersion coatings might be a better option despite being slightly more expensive.

Advantages:

• Excellent solderability
• Relatively inexpensive
• Allows large processing window
• Handles multiple thermal excursions well

Disadvantages:

• Inconsistent thickness/topography between different pad sizes
• High processing temperature
• Not suitable for < 20mil pitch SMD & BGA
• Risk of bridging on fine pitch

Immersion Tin (ISn)

All immersion coatings are applied through a chemical process. This yields a flat metal layer on copper traces, making it ideal for small components. Tin is the most economical immersion coating but can tarnish, reducing solder joint quality if not soldered within 30 days.

High production volumes can mitigate tarnishing. However, for lower production volumes, alternative coatings like immersion silver might be preferable.

Advantages:

• Excellent flatness
• Good for fine pitch/BGA/smaller components
• Mid-range lead-free finish cost
• Press-fit suitable
• Good solderability after multiple thermal excursions

Disadvantages:

• Highly sensitive to handling; gloves needed
• Tin whisker formation concerns
• Aggressive to solder mask
• Negative effect of baking before use
• Not recommended with peelable masks

Immersion Silver (IAg)

Unlike tin, immersion silver doesn’t react with copper, but it tarnishes when exposed to air. Proper anti-tarnish packaging is crucial. Stored correctly, the finish is solderable for 6-12 months, but once opened, soldering should occur within a day. Gold plating can offer a higher shelf life.

Advantages:

• Excellent flatness
• Good for fine pitch/BGA/smaller components
• Mid-range cost for a lead-free finish
• Reworkable

Disadvantages:

• Highly sensitive to handling/tarnishing
• Requires special packaging
• Short operating window between assembly stages
• Not recommended with peelable masks

Electroless Nickel Immersion Gold (ENIG)

Electroless Nickel Immersion Gold (ENIG) comprises a thin gold layer over nickel. Despite its durability and long shelf life, these qualities make it more expensive than other finishes. Additional details can be found on our page about the Best Formats for 4-Layer PCB Stackups.

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Advantages:

• Excellent flatness
• Good for fine pitch/BGA/smaller components
• Proven process
• Wire bondable

Disadvantages:

• Expensive
• Black pad concerns for BGA
• Aggressive to solder mask
• Not ideal for solder mask-defined BGAs

Choose the Right Finish for Your PCBs

When selecting a PCB finish, consider your component types, production volume, durability needs, environmental impact, and cost. Taking these factors into account will help you make the right choice for your project.