Choosing the optimal surface finish for your printed circuit board (PCB) is critical to its functionality, performance, and reliability. There are a number of factors to consider including:

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• Environmental exposure - Will the PCB be used primarily indoors or outdoors? Will it need to withstand harsh conditions like heat, humidity and salt air?
• End-user application - Will the PCB go in a consumer electronic or a medical device? This impacts safety regulations.
• Primary purpose - Is the priority electrical conductivity? Corrosion prevention? Solderability? Aesthetics or color matching?
• PCB design - Is it lead-free? What types of pads and traces are on the board?

This article provides a comprehensive overview of common PCB surface finish processes with a comparison of their respective pros and cons to help identify the optimal choice based on your specific project requirements.

Leaded vs Lead-Free Surface Finishes

One of the first considerations is whether your PCB design uses leaded or lead-free solder. This impacts compatibility and performance with certain finish options.

Leaded solders contain tin combined with lead. These solders melt at lower temperatures, are easier to work with, and less expensive than lead-free solders. However, due to health and environmental regulations, leaded solders usage has declined.

Lead-free solders replace lead with other metals like silver, copper and bismuth. They have higher melting points which can be more difficult to work with, but meet emerging health, safety and environmental standards.

Here is a comparison between common leaded and lead-free PCB finishes:

Leaded vs Lead-Free Surface Finishes

Primary Surface Finishes for Modern PCBs

Now that leaded PCB production has mostly been phased out, what are some of the common options for lead-free designs? Here are some of the most used today:

Organic Solderability Preservative (OSP)

OSP PCB

Overview

• Composition: Clear organic coating (azoles and imidazoles plus other synthetic resins) applied over bare copper
• Thickness: Very thin and lightweight, less than 1 micron
• Key Properties: Low-cost, allows visual inspection of traces, re-workable/coat-able, high electrical conductivity

Advantages

• Inexpensive for high volume production
• No tin whiskers risks
• Compatible with all common solders and fluxes
• Quick and simple processing after copper etching
• Minimal impact on signal integrity/data transmission

Disadvantages

• Shelf life only 3-6 months until oxidation starts
• Not resistant to abrasion/wear over time
• Not suitable for outdoor environments
• Stained easily and hard to clean after usage
• Can have inconsistent coating thickness

Applications

• Consumer electronics
• Double sided PCBs
• Budget sensitive projects

Immersion Tin

Overview

• Composition: Pure tin (Sn) plating
• Thickness: 1 to 2 microns
• Key Properties: Excellent solderability, reusable with multiple reflows

Advantages

• Longer shelf life (6-12 months)
• Little/no copper oxidation over time
• Easy visual inspection of PCB
• Good thermal cycling capabilities
• Can wipe clean after reflows to expose fresh tin finish

Disadvantages

• More prone to growth of conductive tin whiskers over time
• Higher material cost compared to OSP
• Can have dull uneven matte appearance

Applications

• Lead-free consumer electronics requiring some shelf-life
• Budget sensitive telecom infrastructure equipment
• Aerospace PCBs requiring temperature cycling

Immersion Silver

Overview

• Composition: Pure silver (Ag) plating
• Thickness: 0.1 to 0.38 microns
• Key Properties: Excellent shelf life, outstanding conductivity

Advantages

• Long shelf life 24+ months due to tarnish resistance
• Bright uniform shiny silver aesthetic
• Highest conductivity rating of PCB finishes
• Compatible with probe testing post assembly
• Lead-free compatible with excellent solderability

Disadvantages

• Higher material costs
• Can have adhesion problems during thermal shocks
• Not easy to visually inspect traces pre-assembly

Applications

• High frequency analog circuits and power electronics
• Lead-free consumer devices with long stocking periods
• Automotive electronics exposed to vibration/shock
• Mission critical electronics where conductivity is vital

Comparing PCB Surface Finishes

Here is a comparative overview of popular modern lead-free surface finishes used on printed circuit boards:

Comparing PCB Surface Finishes

To summarize:

• Immersion Silver offers the best shelf-life and conductivity but at higher cost
• Immersion Tin provides good lead-free solderability at a moderate price point
• OSP is the most inexpensive option but has shortest shelf life

Other Secondary PCB Surface Finishes

While less common today for most PCB applications, here are some additional specialty finish processes:

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Electroless Nickel Immersion Gold (ENIG) provides excellent wear resistance and bondability. Its cost is quite high so mostly used for high-reliability boards in aerospace, defense, telecom infrastructure, and medical devices.

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) is an alternative to ENIG but with lower overall thickness and material costs while still providing good connectivity and long-term stability.

Hot Air Solder Leveling (HASL) applies molten solder to provide shiny leaded finish. It has fallen out of favor due to the phase out of leaded solders but is still used where tin-lead plating provides unique soldering requirements.

Gold Plating while expensive, thick gold plating yields excellent conductivity, corrosion resistance and long-term reliability in harsh conditions like salt spray or extreme vibration. Mostly used only in specialized military-spec boards where cost sensitivity is lower.

Environmental Considerations

With growing awareness on reducing environmental impacts, certain PCB finish processes can use hazardous chemicals or produce metallic waste byproducts. When selecting a surface finish, consider options with more “green” and sustainable processing.

Immersion Tin for example does not require electrolytic plating baths while ENIG consumes a substantial amount of electricity and chemical processing agents. Properly recycling metals reclaimed after PCB production is also important for minimizing ecological footprint.

Questions and Answers

Q: What surface finish should be used for automotive engine control units?

A: In high-temperature automotive under-hood applications, ENIG would provide the best solder joint reliability with long-term stability to withstand vibration, shock and thermal cycling.

Q: Can PCB finishes be mixed and matched on the same board?

A: It's generally not recommended to combine finishes unless required for unique soldering or connectivity needs. The various platings often have different heights and textures that pose risks of bridging shorts.

Q: How long does the process take to apply different PCB surface finishes?

A: In general, plating finishes with higher thickness like ENIG take longer, while thin coatings like OSP can be completed faster. Modern equipment though now allows rapid immersion silver and immersion tin plating.

Q: What are tin whiskers and why are they a concern?

A: Tin whiskers are small conductive crystals that can grow over time on pure tin surfaces and potentially cause electrical shorts. Mitigation options like surface treatment or conformal coatings may be needed for pure tin finishes.

Q: Can PCB finishes be upgraded or changed post-production?

A: It is possible to strip old surface finishes and replace with newer ones after assembly through intensive chemical processing, but this is expensive, risky to the board's integrity, and generally not recommended.

Conclusion

Selecting the optimal PCB surface finish requires balancing factors such as cost, solderability shelf-life, conductivity, environmental resilience, reworkability and regulatory compliance. Modern lead-free finishes offer a range of advantages with tin, silver and organic coatings being most common for current electronics production. Considering the application environment and product lifecycle early when designing the PCB stackup allows matching the best surface plating process to reliability and budget needs.

 

HASL, OSP, ENIG Explained and Compared

 

Having trouble deciding which surface finish to use? You aren’t alone!

 

Surface finishes serve a two-fold purpose. One is to ensure solderability and the other to protect the conductibility of the surface against oxidation and corrosion. 

 

There is no “x is best” for PCB surface finishes. Each finish comes with its own trade-offs that you’ll have to weigh. Which surface finish you choose will ultimately come down to the design of the PCB, the assembly of its components, and it’s end application/environment. 

 

Questions to ask yourself before deciding:

How important is pad flatness for assembly?
How reliable do the solder joints need to be/solderability?
Does it need to be RoHS compliant?
Is cost important?
How long is the minimum shelf life?

 

 

HASL (Hot Air Solder Leveling) 

HASL is by far the most common surface finish used the industry. It’s an inexpensive option that is frequently used in boards that don’t need to be RoHS compliant, especially in boards meant for prototyping or personal use. As an industry default, manufacturers have a high familiarity with the application process and are less prone to error.

 

Another benefit is the excellent solderability that this finish creates for the plating and IC components. It’s applied by first getting immersed in a tin/lead alloy. Then, hot air is blown across the surface ‘cutting off’ excess solder, hence it’s unabbreviated name: Hot Air Solder Leveling. If you need a finish that has good shelf-life, this may be the surface finish for you. Its solderability can also withstand multiple manufacturing processes such as reflowing and washing.

 

However, this application process yields topographical differences between plates which can cause assembly issues and is not aesthetically pleasing. Therefore, it is not ideal for HDI (high density interconnect) PCBs or those with fine pitching, particularly <20mil between BGA and SMD.

 

HASL Lead-Free

Lead-free HASL not only keeps the benefits of leaded HASL, but also has additional benefits. Namely, that it is lead-free and therefore RoHS compliant. This is necessary if you do business in the EU and California. And by utilizing copper, nickel, tin, cobalt, or silver solder alloys, copper dissolution and heat damage issues have mostly been overcome. 

 

While slightly more expensive then leaded HASL finish, among the lead-free finishes lead-free HASL is relatively inexpensive. However, since the application process is similar to HASL, it is also not recommended for those PCBs that contain small distances between BGA and SMTs. During heating, the gap between components is frequently bridged. High temperatures are also required to melt lead-free solder, so it might be bothersome to DIYer’s who don’t have the proper equipment. 

 

Again, the main selling point of lead-free HASL is its relatively inexpensive cost and its exceptional solderability.

 

 

OSP (Organic Solderability Preservative)

 

Where Lead-Free HASL loses on PCBs with fine pitches, as lead-free and organic, OSP is an excellent choice for an inexpensive, environmentally friendly surface finish for your fine pitches. This surface finish even beats other lead-free solutions which still use toxic materials in their processes or consume more energy to apply.

 

As you might guess from P standing for preservative, OSP preserves the copper surfaces of plates from oxidation by way of a very thin protective layer of material. After a cleaning process, a water based compound selectively bonds to the copper and then protects the copper pad before soldering. However, the downside of this finish material is that the oxidation protection is not absolute. Careful handling of the boards is necessary during assembly as acid from fingerprints may cause oxidation anyway. It also does not allow wave soldering for double-sided boards because the thermal exposure might also cause oxidation of the copper. Oxidation will reduce the solderability of the copper.  

 

Although it can be difficult to test, as previously mentioned, it is very well suited for fine pitches, BGA, and small components as the topography of the coating is very flat. It also doesn’t demand high equipment maintenance and is highly repairable.

 

ENIG (Electroless Nickel Immersion Gold)

ENIG is the best lead-free solution for PCBs (it’s also an expensive one). An excellent option for fine pitches and flat surfaces; it suits the modern-day PCB design trend of HDI and tight pitches between BGAs and SMTs.

 

It’s composed of a double layer of metallic coating. Nickel acts as a barrier to copper and a surface for SMT placement and a thin layer of gold protects the nickel from oxidation. A couple downsides arise from the metallic composition however, one being undesirable magnetic properties and the other is known as “black pad” where a buildup of phosphorous between the layers causes faulty connections and fractured surfaces.

 

Although a more complex and pricey process, thanks to the rise of RoHS compliance and environmental consciousness, ENIG is perhaps the fastest growing trends in the industry. Although it cannot be easily reworked, its excellent planarity matches the needs of technological advances.

 

 HASL

HASL

Pb Free

OSPENIG

RoHS Compliant

NoYesYesYesFlatnessPoor PoorExcellentExcellentFine Pitch/SMT/BGANoNoYesYesSolderabilityBestBetterGoodGoodShelf-Life>1 year>1 year

9-12 Months

>1 yearCostLowLowLowMedium

 

 

This surface finish list is by no means exhaustive, but these are the relatively more common types used in manufacturing. Ultimately, you will need to weigh the trade-offs against the PCB’s purpose to choose a surface finish. You can see for yourself the price difference between the four surface finishes discussed above on our online quotation page. 

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