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:

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• 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.

Contents

There is a huge demand for the replacement of tin-lead (Sn/Pb) solders with lead-free solders in the electronics industry because of health and environmental concerns. Lead-free HASL is a popular finish that provides the most durable finish for PCBs.

What is lead-free HASL PCB surface finish?

Lead-free HASL is a type of surface finish that uses lead-free alloys instead of tin-lead (Sn-37Pb) eutectic alloys. Initially, molten solder is applied on a board (after cleaning and fluxing) to achieve wetting and excess solder is scraped with the help of air knives (a tool that is set at a temperature above the melting point of the solder). After the solder solidifies, the circuit board is passed through the washer where the solder flux residues are removed.

There are two methods of applying lead-free HASL:

• Vertical process: Vertical process involves a panel of boards mounted on a frame that moves vertically downward into the solder bath, holds for a certain time (usually 2-3 seconds), and then lifts the panel through the operating air knives. These knives are adjusted to clear through-holes and vias while leaving a sufficient thickness of solder on all areas where a solderable finish is essential. After the solder cools and solidifies, the panel passes through the washer/dryer to remove flux residues. The operator then inspects the quality of the finish.

• Horizontal process: In the horizontal process, the board is flooded with solder applied through nozzles or between rollers on the top and bottom of the panel before being moved between hot air knives placed above and below the panel.

Usually, HASL lines are of vertical design and deliver a high-quality finish. Fully automated vertical lines can deliver rack-to-rack performance including fluxing, preheating, washing, and drying. The finish in the horizontal process is more uniform in thickness compared to the vertical process.

The lead (Pb) level in lead-free solder is less than 0.1%. Castin (a composition of Sn 93­ 98%, Ag 1.5-3.5%, Cu 0.2-2%, and Sb 0.2-2%) alloy and SnCu 0.7 are widely used in this process.

 

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Why is lead-free HASL used?

There are several problems associated with the HASL process as given below:

• Damage due to additional thermal deviations
• Coplanarity issues because of  pad to pad variation in coating thickness

• Stencil gasketing issue related to the difference in coating thickness and mushroom cap profile (of the coating) found on small pads.

• Heat and fumes associated with the process were considered out of place in modern board shops.

The difference in the solder coating thickness of a lead-free HASL finish is usually about half that of the tin-lead HASL finish. The coplanarity is also good in this finish. The modern HASL line is designed to provide the operating conditions in line with other equipment in modern board shops to avoid issues with heat and fumes.

Coating thickness

The solder coating profile is defined by the volume of solder left on the pad after passing the board through the hot air knife and surface tension forces. Due to surface tension, the coating has a tendency to be thicker on smaller pads.

Because of the higher surface tension of lead-free solder, the coating formed in the HASL process is both thinner and uniform compared to tin-lead solder formed in similar conditions.

The coating thickness can be checked by XRF (X-ray fluorescence spectroscopy) technique.

Whisker formation

Compressive stress is the driving force for whiskers (a thin filament protruding from a board surface having tin as a final finish). According to the JESD22A121 specification, whiskers appear on the hot-dipped lead-free finish on copper only in areas where compressive stress is intentionally introduced and under the most severe conditions such as the combination of elevated temperature (60°C) and high humidity (87%RH).

Lead-free HASL provides a corrosion-resistant finish that can assure solderability over long periods.

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