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

Choosing the surface finish is an essential step in the design of your printed circuit boards.

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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.

Finish Type #1 &#; Hot Air Solder Leveling (HASL)

&#;Hot Air Solder Leveling&#; is the least expensive type of PCB surface finish.

It is widely available and very economical. The board is dipped in molten solder and then leveled off with a hot air knife. If you&#;re using through-hole or larger SMT components, HASL can work well. However, if your board will have SMT components smaller than or SOIC, it is not ideal.

The surface is not completely level, so this can cause issues with small components. The solder used is typically Tin-Lead. That means that it isn&#;t RoHS compliant. And if reducing the amount of lead you use is important, you may want to choose Lead-Free HASL instead.

Advantages:

• Excellent solderability
• Inexpensive / Low cost
• Allows large processing window
• Long industry experience / well-known finish

Disadvantages:

• Difference in thickness/topography between large and small pads
• Not suited for < 20mil pitch SMD & BGA
• Bridging on fine pitch
• Not ideal for HDI products

Finish Type #2 &#; Lead-Free HASL

Lead-Free HASL is similar to standard HASL, but with an obvious difference&#; It doesn&#;t use Tin-Lead solder.

Instead, Tin-Copper, Tin-Nickel or Tin-Copper-Nickel Germanium may be used. This makes Lead-Free HASL an economical and RoHS compliant choice. But like standard HASL it is not ideal for smaller components.

For boards with smaller components, immersion coatings can be a better choice. They are slightly more expensive but more suitable for this purpose.

Advantages:

• Excellent solderability
• Relatively inexpensive
• Allows large processing window
• Multiple thermal excursions

Disadvantages:

• Difference in thickness/topography between large and small pads
• High processing temperature &#; 260-270 degrees C
• Not suited for < 20mil pitch SMD & BGA
• Bridging on fine pitch

Finish Type #3 &#; Immersion Tin (ISn)

With all immersion coatings, a chemical process is used.

A flat layer of metal is deposited on the copper traces. The flatness of the coating makes it ideal for small components. Tin is the least expensive type of immersion coating. Although it is an economical choice, it comes with some drawbacks.

The main drawback is that after the tin is deposited onto the copper it begins to tarnish. That means that if you want to avoid lower quality solder joints, you need to do your soldering within 30 days.

If you are expecting a high volume of production this may not be an issue. And if you are using large batches of boards quickly, you can avoid tarnishing. However, if your production volume isn&#;t high, it may be better to choose a coating like immersion silver.

Advantages:

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

Disadvantages:

• Very sensitive to handling &#; gloves must be used
• Tin whisker concerns
• Aggressive to solder mask &#; solder mask dam shall be &#; 5 mil
• Baking prior to use can have a negative effect
• Not recommended to use peelable masks

Finish Type #4 &#; Immersion Silver (IAg)

Immersion silver does not react with copper the way that tin does. However, it does tarnish when exposed to air.

That means it needs to be stored in anti-tarnish packaging.

When stored in proper packaging it will still be solderable for 6-12 months. But once the PCB is removed from its packaging, it will need to go through solder reflow within a day. A higher shelf life can be achieved with gold plating.

Advantages:

• Immersion finish = excellent flatness
• Good for fine pitch / BGA / smaller components
• Mid-range cost for lead-free finish
• Can be reworked

Disadvantages:

• Very sensitive to handling/tarnishing/cosmetic concerns
• Special packaging required
• Short operating window between assembly stages
• Not recommended to use peelable masks

Finish Type #5 &#; Electroless Nickel Immersion Gold (ENIG)

Electro gold flash plating consists of a thin layer of gold over electroless or electrolytic nickel.

This type of plating is hard and durable. It also has a long shelf life, lasting for years. However, its durability and shelf life make it more expensive than any of the finishes mentioned above.

Advantages:

• Immersion finish = excellent flatness
• Good for fine pitch / BGA / smaller components
• Tried and tested process
• Wire bondable

Disadvantages:

• Expensive finish
• Black pad concerns on BGA
• Can be aggressive to solder mask &#; larger solder mask dam preferred
• Avoid solder mask defined BGA&#;s

Choose the Right Finish for Your PCBs

Remember that when choosing the finish for your PCBs, you need to consider component types and production volume. You&#;ll also need to consider requirements for durability, environmental impact, and cost. By taking all these factors into consideration, you can be sure to make the right choice.

Introduction

The surface finish of a printed circuit board (PCB) refers to the final coating applied to the copper traces on the board. This coating protects the copper from corrosion, enhances solderability, and improves aesthetics. There are several types of PCB surface finishes, each with their own advantages and disadvantages. The most common finishes used today are:

• Hot Air Solder Leveling (HASL)
• Immersion Silver (IAg)
• Immersion Tin (ISn)
• Organic Solderability Preservative (OSP)
• Electroless Nickel Immersion Gold (ENIG)
• Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

Selecting the right surface finish is an important decision that depends on the application, assembly process, reliability requirements and cost. This article provides a detailed comparison of the most popular PCB surface finish options to help you determine the best choice.

PCB Surface Finish Types

Hot Air Solder Leveling (HASL)

HASL is the traditional and most widely used PCB surface finish. It involves applying molten solder onto the PCB, where it adheres to the copper traces and then levels out as it cools. The result is a uniform coating of solder over the entire board.

Advantages:

• Low cost
• Easy to apply
• Good solderability and compatibility with standard assembly processes
• Can withstand multiple reflows
• Repairable and reworkable

Disadvantages:

• Prone to oxidation which reduces solderability over time
• Can cause uneven board thickness due to solder buildup
• Excessive thermal stress during soldering
• Not suitable for fine pitch components
• Contains lead (unless lead-free solder is used)

Immersion Silver (IAg)

Immersion silver involves depositing a thin layer of silver onto the copper traces through chemical displacement. It provides excellent solderability while being lead-free.

Advantages:

• Lead-free and RoHS compliant
• Low contact resistance
• Excellent solderability that is maintained over time
• Compatible with a variety of assembly processes
• Withstands multiple reflows
• Thin coating does not impact trace spacing

Disadvantages:

• More expensive than HASL
• Silver oxidizes and migrates which can cause issues over time
• Not repairable or reworkable

Immersion Tin (ISn)

Immersion tin plates the copper with a thin layer of tin by placing the PCB in a heated tin-based solution. The tin protects the copper from oxidation.

Advantages:

• Lead-free and RoHS compliant
• Very flat surface finish
• Resists oxidation better than HASL
• Suitable for fine pitch components
• Lower whisker growth risk than pure tin
• Withstands repeated thermal cycling

Disadvantages:

• More expensive than HASL
• Not easily repairable or reworkable
• Can have adhesion issues with older fluxes

Organic Solderability Preservative (OSP)

OSP coatings are organic compounds applied to the copper to protect against oxidation and promote solderability. They are removed during the soldering process, exposing the fresh copper underneath.

Advantages:

• Lead-free and RoHS compliant
• Low cost
• Allows visual inspection of joints
• Suitable for fine pitch components
• Flat surface for improved lead coplanarity

Disadvantages:

• Limited shelf life as copper oxidizes over time
• Multiple reflows can deplete the coating
• Difficult repair and rework
• Sensitive to handling and humidity

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Electroless Nickel Immersion Gold (ENIG)

ENIG applies an electroless nickel plating to the copper, followed by a thin layer of immersion gold. This provides a gold surface finish over the nickel barrier layer.

Advantages:

• Excellent surface planarity
• High corrosion resistance
• Good solderability that is maintained over time
• Withstands repeated thermal cycling
• Suitable for fine pitch components
• Lead-free and RoHS compliant

Disadvantages:

• High cost compared to other finishes
• Gold embrittlement can occur
• Not easily repaired or reworked

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG is a variation of ENIG using palladium instead of nickel under the gold layer. It provides similar benefits as ENIG.

Advantages:

• Excellent solderability maintained over time
• Resists corrosion and oxidation
• Compatible with lead-free solders
• Withstands repeated thermal cycling
• Suitable for fine pitch components
• Lead-free and RoHS compliant

Disadvantages:

• Very expensive finish
• Not easily repaired or reworked
• Harder to find industry support

PCB Surface Finish Comparison

Here is a comparison of the key characteristics for the most common PCB surface finish options:

Surface FinishSolderabilityShelf LifeDurabilityRepairabilityCostHASLGoodMediumMediumHighLowIAgExcellentMediumMediumLowMediumISnVery GoodMediumHighLowMediumOSPExcellentLowLowLowLowENIGExcellentHighHighLowHighENEPIGExcellentHighVery HighLowVery High

Solderability

• HASL, IAg, ISn, OSP, ENIG, and ENEPIG all provide good to excellent solderability out of the box.
• IAg, ENIG, and ENEPIG maintain the highest solderability over time.
• OSP has short shelf life as solderability degrades quickly.

Shelf Life

• OSP has the shortest shelf life of less than 6 months.
• HASL, IAg, and ISn offer medium shelf life of about 1-2 years.
• ENIG and ENEPIG provide the longest shelf life of 3-5 years or longer.

Durability

• HASL and IAg offer medium durability with higher oxidation rates.
• ISn, ENIG, and ENEPIG provide the best durability against oxidation and corrosion.
• OSP has low durability as coating depletes after repeated reflows.

Repairability

• HASL is the most repairable finish.
• IAg, ISn, OSP, ENIG, and ENEPIG cannot be easily repaired.

Cost

• HASL and OSP are the lowest cost options.
• IAg and ISn offer medium cost.
• ENIG and ENEPIG are the most expensive surface finishes.

Selecting the Right Surface Finish

With all the options available, here are some guidelines on selecting the best surface finish for your application:

• HASL is ideal for cost-sensitive, high volume, and low mix assemblies with less demanding requirements.
• IAg offers the best solderability and shelf life balance when lead-free is required. It works for a wide range of product types.
• ISn is suitable for more demanding applications requiring lead-free compliance, fine pitch components, thermal cycling, and flexibility.
• OSP works well for quick-turn prototypes at low cost but has limited shelf life.
• ENIG provides maximum durability and corrosion resistance when budget allows. It satisfies the highest quality demands.
• ENEPIG is overkill for most applications given the very high cost. Consider only for extremely harsh environments.

Also assess your specific needs in terms of assembly process compatibility, reworkability, shelf life requirements, and reliability testing results. Partner with your PCB manufacturer early in the design process to select the optimal surface finish.

Frequently Asked Questions

Q: What is the most cost-effective PCB surface finish?

HASL offers the lowest cost PCB surface finish and provides decent solderability and shelf life for many standard applications. It allows easy rework and repair which also reduces costs.

Q: Which surface finish has the best solderability?

IAg, ENIG, and ENEPIG provide the highest and most consistent solderability over the lifetime of a PCB. IAg is the most cost-effective option that maintains excellent solderability.

Q: Is ENIG better than HASL?

ENIG is better than HASL in terms of corrosion resistance, shelf life, thermal cycling durability, and compatibility with fine pitch components. However, ENIG has significantly higher cost and is not easily repairable like HASL.

Q: What is the difference between IAg and ISn?

The main differences are cost, durability, and whisker resistance. IAg costs slightly more while ISn provides better resistance to oxidation and tin whiskers. ISn is generally the preferred finish for lead-free boards.

Q: How long does OSP last?

The shelf life of OSP is less than 6 months. OSP provides excellent initial solderability but degrades quickly compared to other finishes. It is mainly used for short-term prototyping applications.

Conclusion

Selecting the right PCB surface finish requires balancing factors like cost, solderability, shelf life, durability, assembly process compatibility and reparability. Leaded HASL is the inexpensive workhorse finish for high volume products. For lead-free boards, IAg and ISn offer the best value and performance. ENIG or ENEPIG are chosen when maximum reliability is needed despite higher costs. Collaborate with your PCB manufacturer to determine the optimal finish for your specific design needs.

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