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:

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• 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.

Contents

A PCB surface finish protects the circuit from any damage due to harsh operating and storage environments. Selecting the right coating plays an important role in enhancing the board&#;s solderability, and shelf-life.

Why do you need a surface finish on a circuit board?

The surface finish is a coating between the bare copper at the solderable area and the components on the circuit board. It protects the copper surface from oxidation. During assembly, the surface coating prepares the board surface for soldering components.

Types of surface coating

Surface coating for printed circuit boards can be classified broadly into metallic and organic.

Metallic surface finishes

Metallic surface finishes use metallic layers. Some examples of these finishes are discussed below.

Hot air solder leveling (HASL)

HASL is a traditional surface finish. In this method, the board is dipped into a molten solder bath to encase all the exposed copper surfaces with solder. The extra solder is removed by passing the circuit board between hot-air knives. The solder used here is a mix of tin and lead. HASL can make a device drop-resistant.

Lead-free HASL

Lead-free HASL is identical to the HASL method. The only difference is that the solder is made up of a lead-free alloy such as Sn-Cu-Ni-Ge, Sn-Ag-Cu (SAC), and Sn-Cu-Co.

Electroless nickel immersion gold plating (ENIG)

ENIG refers to electroless nickel plating covered with a thin coating of immersion gold.

Electroless nickel deposition is an auto-catalytic process. Here nickel is deposited on the copper surface which is catalyzed using palladium. To obtain a consistent coating, the reducing agent that contains nickel ions needs to be restored. This also provides a proper concentration and temperature for plating. Subsequently, gold will adhere to the areas that have nickel plating. ENIG can be used to protect medical devices from shock and drop issues.

 

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HASL vs. ENIG: Which surface finish is better?

ParameterHASLENIG Type of metal coatingLead variant: tin-lead solder
Lead-free variant: tin with small amounts of silver and copperNickel and gold Plating thicknessA thicker solder layer is platedA thinner gold layer is deposited Adhesion to copperThe solder layer in HASL adheres well to the copper pads due to the metallurgical bond formed during the soldering processThe nickel layer serves as a barrier between the copper and the gold, ensuring good adhesion between copper-nickel and nickel-gold Thermal stressChances of damage are high since the board is exposed to a higher temperature during hot-air knives processAs the process does not require high temperature, it reduces the risk of warping Electrical performanceLower electrical performance compared to ENIGSince gold is a better electrical conductor than tin, it enhances the electrical performance FlatnessCan create an uneven surface due to the thickness of the solder layerProvides a smooth surface finish SolderabilityProvides good solderability for manual soldering and simple assembly processCompatible with all advanced and automated soldering techniques Component compatibilitySuitable for both through-hole and SMT components. However, the thickness of the solder layer can create bridges between adjacent pads and cause issues in fine-pitch component placementAllows all types of component placement. It is preferred for fine-pitch and wire-bond components, as the thin gold layer does not create any bridging issues Operational environmentNot recommended for harsh environmentsCan withstand harsh environments CostCost effective due to its simple process and lower material costsExpensive as it undergoes the gold immersion processes Shelf lifeLower shelf life as it can be oxidized and corroded over time, leading to solderability issuesLonger shelf life as the gold layer provides protection against oxidation and corrosion Eco-friendlyThe HASL lead variant is not eco-friendly. It can cause health and safety issues in the working environmentENIG is environment safe RoHS complianceThe lead variant does not comply with RoHS regulationAdheres to RoHS regulation

Electroless nickel electroless palladium immersion gold (ENEPIG)

ENEPIG is similar to ENIG. The difference is a palladium coating is implemented as the protective layer to prevent nickel oxidation and stop diffusion to the copper layer.

ENIG and ENEPIG provide great solderability at a higher cost compared to other surface finishes.

Hard gold

Hard gold includes a gold layer that is plated over a nickel coating. The hard gold finish consists of gold alloying with nickel, cobalt, or iron. It is best suited for components with a higher risk of wear and tear such as edge connectors, interconnected carrier boards, contacts, and keypads.

Immersion silver (ImAg)

Immersion silver surface finish provides a lead-free layer of silver and protects the copper traces from corrosion. An electroless immersion reaction is used to apply ImAg to the copper traces.

Immersion tin (ImSn)

Immersion tin surface finish consists of a thin layer of tin. It is RoHS-compliant and a great choice when it comes to small geometries and components. It is also well-suited for flat surface requirements and fine-pitch components.

These are all the commonly used metallic surface finishes. Now let us take a look at the organic ones.

Organic

An organic finish includes organic compounds (compounds with carbon). Some of the popular ones are listed below.

Organic solderability preservative (OSP)

OSP is a water-based organic finish used for copper pads. It protects the copper pad prior to soldering.

Carbon ink

This consists of a layer of carbon ink. This method is used in welding equipment, remote controllers, automotive, keyboards, keypads, and RF shielding. The primary factors involved in board production with the carbon ink surface finish are printing, baking, and resistance control.

Choosing the right surface finish for a board

Selecting the right surface finish is essential since it decides the reliability of the board. Some important factors to keep in mind while choosing a surface finish are as follows:

• Operating environment
• Board aesthetics
• Shock and drop issues
• Corrosion
• Presence of fine pitch components (BGA)
• Flatness of SMT pads

The types of surface finishes available at Sierra Circuits are listed below:

 

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