Printed circuit boards (PCBs) form the backbone of all electronic devices. They provide the mechanical support structure to mount various electronic components and interconnect them through conductive copper tracks to form functioning circuits. The copper tracks and pads on the PCB that connect the various components need to be protected from oxidation and corrosion to maintain the electrical conductivity and solderability. This is achieved by applying different types of surface finishes over the exposed copper on the PCB.
There are various types of surface finishes available for PCBs, each with their own sets of pros and cons. Selecting the right surface finish for a PCB design requires carefully weighing factors like cost, solderability, oxidation resistance, contact resistance, assembly process compatibility, reworkability and more. The most common finishes used in PCB fabrication today include:
- Hot Air Solder Leveling (HASL)
- Immersion Silver (IAg)
- Immersion Tin (ISn)
- Electroless Nickel Immersion Gold (ENIG)
- Organic Solderability Preservatives (OSP)
- Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
This article provides a detailed overview of these popular PCB surface finishes, their characteristics, typical applications and process methods used to apply them.
PCB Surface Finishing Processes
Surface finishing of PCBs is typically done after the boards have gone through the complete fabrication process of lamination, drilling, metallization and imaging. The exposed copper tracks are coated with the selected surface finish to protect the copper from oxidation and maintain solderability. There are various techniques used for depositing the different finish layers:
This involves submerging the PCB in a electrolytic bath containing the coating metal ions (tin, silver, gold etc.) and applying electric current to facilitate deposition of the metal layer onto the copper through reduction reactions. Electroplating allows deposition of uniform, smooth and shiny metallic coatings.
In this process, the PCB is dipped in a aqueous solution containing the plating metal (nickel, gold, palladium etc.) and a reducing agent. The reducing agent reacts with the metal ions leading to autocatalytic deposition of the metal layer onto the copper surface without external current.
Hot Air Solder Leveling
In this technique, the PCB is passed over a molten solder wave. The solder adheres to the copper tracks forming a coating. Hot air knives immediately blow hot air to flatten out the solder layer and give a smooth finish.
Organic Surface Preservatives
The PCB is dipped in an organic solution containing additives like imidazole, benzimidazole etc. which form an invisible thin coating and prevent copper oxidation.
With this background on the commonly used PCB surface finishing processes, let us now look at the popular finishes in more detail.
Hot Air Solder Leveling (HASL)
Hot air solder leveling using tin-lead alloy was the most prevalent PCB finish for many decades due to its low cost, ease of application, repairability and acceptable performance. However, due to the toxicity of lead and subsequent ban on lead usage in electronic assembly, lead-free solders have become standard for HASL finish.
The HASL finish consists of a thin layer of solder alloy coated over the copper conductors on the PCB. Common solder alloys used are:
- Sn96.5/Ag3/Cu0.5 (SAC305) – Melting point 217??C
- Sn95.5/Ag3.8/Cu0.7 (SAC387) – Melting point 219??C
- Sn99.3/Cu0.7 (SAC0307) – Melting point 227??C
These lead-free solders provide comparable performance to tin-lead solders. The thickness of the finish typically ranges from 1 to 3 mils.
The steps in the HASL process are:
- Flux application – The PCB goes through a foam fluxer unit containing flux solution to clean the copper and prepare it for soldering.
- Preheating – The board is preheated to about 150??C to evaporate solvents in the flux and activate it.
- Solder coating – The hot PCB (typical solder bath temperature 240-260??C) passes over a pumped solder wave which deposits and evenly coats the molten solder over the entire board surface.
- Hot air leveling – High pressure hot air knives (245-255??C) immediately blow down on the solder coated board. This flattens out the solder, removing any peaks and valleys, giving an even, smooth finish.
- Cooling and cleaning – The board cools down as it exits the HASL system. Residual flux is washed off in a cleaning unit.
- Low cost process due to high throughput.
- No special PCB fabrication processes required.
- Provides good solderability. The solder finish itself acts as a permanent coating.
- Easy rework of components by locally applying solder.
- Repairable finish unlike ENIG, ISn etc. Damaged boards can be re-finished.
- Can cause unevenness on small component leads and test pins.
- Flux residue entrapment can lead to corrosion and dendrite growth.
- Thickness control harder versus plating processes.
- Exposed finish prone to oxidation requiring nitrogen storage.
- Thermal fatigue issues due to CTE mismatch with copper.
- Unsuitable for fine pitch components due to bridging risk.
- Lead contamination risk in solder bath affects long term reliability.
HASL continues to be used for cost-sensitive applications like:
It is often avoided for high reliability or fine pitch boards. The most common lead-free solder used is SAC305 which provides good solderability.
Immersion Silver (IAg)
Immersion silver finish has emerged as a popular and cost-effective replacement to HASL for many applications. It avoids the unevenness and bridging issues faced with HASL for fine pitch ICs and offers better coplanarity. The immersion silver plating process is simple and does not need any special PCB fabrication adjustments.
The immersion silver deposit consists of pure silver. The plating thickness is typically 2 to 5 micro-inches (0.05 to 0.125 microns).
Immersion silver plating involves the following steps:
- Surface preparation – The copper pads and traces are microetched to remove oxides and activate the surface.
- Silver deposition – The PCB is immersed in a silver plating solution containing a silver salt (typically silver nitrate) and a reducing agent. The reducing agent facilitates deposition of silver ions onto the copper surfaces.
- Rinsing and drying – The plated board is rinsed thoroughly in deionized water to remove any chemicals. It is then hot air dried.
The immersion silver process typically takes just 2-3 minutes and can easily be integrated into standard PCB fabrication lines.
- Provides excellent solderability comparable to HASL.
- Uniform thickness deposition across pads, leads and test pins.
- Solves issues of HASL like icicling, flagging and bridging.
- Lower cost than other common finishes – ENIG, ENEPIG etc.
- Suitable for fine pitch components.
- RoHS compliant and halogen free process.
- Silver tarnishes and oxidizes over time which can degrade solderability. Shelf life is limited.
- Not easily repairable unlike HASL. Damaged boards need to be re-plated.
- Slightly higher cost compared to HASL process.
- Does not prevent whisker growth like immersion tin.
The low cost, excellent solderability and fine pitch compatibility of immersion silver has made it the most widely used surface finish today for a range of applications:
- Cellphones, tablets and portable electronics
- Computer motherboards and graphic cards
- Telecommunication hardware
- Automotive electronics
It provides drop-in replacement capability for HASL. The limited shelf life means Ag plated boards are best assembled within 6-12 months.
Immersion Tin (ISn)
Immersion tin finish provides an economical way to protect copper traces from oxidation while retaining excellent solderability. The matte tin coating prevents whisker growth which has made it popular for high reliability applications.
The immersion tin deposit on PCBs consists of pure tin. The thickness typically ranges from 1 to 5 micro-inches (0.025 – 0.125 microns).
The immersion tin plating process involves the following steps:
- Surface preparation – Microetching cleans the copper surfaces and activates them for plating.
- Tin deposition – The PCB is immersed in a heated tin salt solution containing a reducing agent. This causes deposition of Sn ions as a thin uniform metallic tin layer.
- Rinsing and drying – the plated board is thoroughly rinsed and then dried using hot air.
The immersion tin plating process adds minimal cost to PCB fabrication.
- Provides excellent solderability. Comparable to HASL for leaded and lead-free solders.
- Lower cost compared to other platings – ENIG, ENEPIG.
- Uniform thickness across component leads, test pads etc.
- Matte grey finish prevents tin whisker growth risks.
- Suitable even for ultra-fine pitch components.
- Long shelf life with good oxidation resistance.
- Can have adhesion issues during thermal cycling leading to cracking.
- Not easily repairable compared to HASL process.
- Higher cost versus HASL process.
- Dull cosmetic finish.
The whisker resistance, stable solderability and fine pitch compatibility of immersion tin make it well suited for high reliability applications:
- Aerospace and defense electronics
- High-end telecom, 5G and networking gear
- Automotive electronics
- Medical equipment
- Industrial electronics
Its ability to prevent tin whiskering has led to the wide adoption of this finish in these segments.
Electroless Nickel Immersion Gold (ENIG)
The electroless nickel immersion gold (ENIG) finish provides an excellent combination of durability, solderability, bondability, conductivity and corrosion resistance. This had made it the finish of choice for high-reliability and high-performance electronics over the past several decades, despite its higher cost.
ENIG finish consists of a bottom layer of nickel-phosphorus alloy topped by a thin layer of immersion gold. Typical thickness is:
- Electroless nickel: 80 to 250 micro-inches (2-6 microns)
- Immersion gold: 3 to 15 micro-inches (0.08 – 0.4 microns)
The ENIG process involves sequential plating of nickel and gold layers:
- Surface preparation – A microetch step removes oxides and activates the copper surface.
- Nickel plating – The PCB is immersed in an autocatalytic nickel bath where the exposed copper catalyzes deposition of nickel-phosphorus without any electrical current.
- Gold plating – The nickel layer is coated with a thin top layer of immersion gold by dipping in a gold plating bath containing a reducing agent.
- Rinsing and drying – Finally, the board is thoroughly rinsed in DI water and hot air dried.
- Provides excellent solderability combined with long shelf life.
- Gold layer prevents copper/nickel oxidation.
- Wear and abrasion resistant finish.
- Uniform thickness deposition across component leads, test pads etc.
- Good for both leaded & lead-free soldering.
- Strong wire bondability.
- Excellent corrosion and tarnish resistance.
- Good electrical conductivity compared to other finishes.
- Relatively higher cost finish.
- Not easily repairable or reworkable finish.
- Nickel barrier prone to cracking under thermal fatigue.
- Black pad risk due to thick nickel layer.
The superior reliability and performance characteristics have established ENIG as the finish of choice for critical applications:
- High speed digital electronics – CPUs, GPUs, chipsets, memory
- Aerospace and defense electronics
- Telecommunication infrastructure
- Automotive electronics
- High-end consumer electronics
ENIG is also used where component wire bonding is needed. The thickness of the nickel layer is adjusted based on required corrosion resistance and current density.
Organic Solderability Preservatives (OSP)
Organic solderability preservatives provide an inexpensive way to protect copper traces from oxidation without the cost of metal plating. The organic coating prevents tarnishing while allowing soldering when needed.
OSP deposits consist of organic inhibitors like imidazoles, benzimidazoles, benzotriazoles etc. They form monolayers with a thickness of only a few molecules.
The OSP application process is quick and involves:
- Surface preparation – The copper is microetched and activated prior to OSP deposition.
- OSP treatment – The PCB is immersed in a room temperature OSP solution containing organic inhibitors. These coat the copper surfaces.
- Rinsing and drying – Residual treatment solution is rinsed off, followed by drying.
The OSP process adds minimal fabrication cost and can easily be integrated.
- Extremely low cost compared to metal finishing.
- Simple room temperature process.
- Minimal effect on production process flow.
- Provides temporary protection against oxidation.
- Minimal risk of insertion issues versus thick metal layers.
- Shelf life limited to about 6 months.
- Multiple heat cycles can degrade solderability.
- Not suitable for high temperature or high reliability applications.
- Repairing and reworking is difficult.
- No wire bonding possible. Only for SMT parts.
The ultra-low cost of OSP makes it attractive for cost sensitive consumer electronic products with limited shelf lives:
- Mobile phones
- Tablets and laptops
- IoT devices
- Portable electronics
- LED lighting
For long-life or high temperature products, other finishes like ENIG or Immersion Silver are used.
Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
ENEPIG finish provides a cost-effective alternative to ENIG for many applications by replacing the immersion gold layer with electroless palladium + immersion gold. This reduces the overall gold consumption while providing excellent shelf life.
ENEPIG deposits consist of:
- Electroless nickel: 2 to 5 microns
- Electroless palladium: 0.05 – 0.3 microns
- Immersion gold: 0.025 – 0.05 microns
ENEPIG plating involves the following steps:
- Surface preparation using microetching
- Electroless nickel plating
- Electroless palladium plating
- Immersion gold – Double dip process
- Rinsing and drying
- Low gold consumption provides cost benefit over ENIG.
- Shelf life, solderability and conductivity close to that of ENIG finish.
- Good corrosion and oxidation resistance.
- Palladium/Gold resists black pad risk.
- Uniform deposition across component leads and pads.
- More expensive than tin, silver or OSP finishes.
- Slightly lower bondability than with thick immersion gold.
- Repair and rework is difficult.
The ENEPIG process allows reducing thickness (and cost) of gold layer without compromising shelf life or solderability. It is commonly used for:
- High reliability telecommunications hardware
- Aerospace and defense electronics
- Automotive electronics
- Medical electronics equipment
- High-end industrial electronics
Faster depletion of thinner gold layer limits reworkability.
Summary of Common PCB Finishes
|Low cost, repairable, good solderability
|Thermal fatigue issues, uneven deposits, bridging risks
|Low cost, excellent solderability, suitable for fine pitch
|Oxidation and tarnishing issues, short shelf life
|Cellphones, computers, telecom hardware
|Whisker-free, excellent shelf life and solderability
How to know the surface finish from PCB color?
From the PCB surface, there are three main colors: gold, silver, and light red color. The PCB with gold color is the most expensive, the silver color is cheap, and the light red color is the cheapest.
You could know if the manufacturer have cut corners or not, from the surface color.
Besides, the circuit inside the board is mainly pure copper. The Copper is easily oxidized when exposed to air, so the outer layer must have the above protective cover.
- Gold color
Some people say that gold color is copper, that is not right.
Please refer to the picture with gold plating on the board as below:
The golden circuit board is the most expensive and is the real gold. Although it is thin, it also accounts for nearly 10% of the cost of the board.
There are two advantages for using gold, One is for welding conveniently and the other is for anti-corrosion.
As shown below, this is the golden finger of the memory stick 8 years ago. It is still golden and shining.
If you find that some boards are all silver, it must be cut corners. We call it “costdown.”
Generally speaking, the motherboards of mobile phone are gold-plated boards, but computer motherboards and small digital boards are not gold-plated.
Please refer to the board of iPhone X as below, The exposed parts are all gold plated.
- Silver color
Gold color is gold, silver color is silver? Of course not, it is tin.
The silver board is called a HASL board. Spraying tin on the outer layer of the copper, it also could help to welding, but it is not as stable as gold.
There is no effect on already welded components for HASL board. However, if pads are exposed to the air for a long time, such as ground pads and pin sockets etc, It is easy to oxidize and rust, resulting in poor contact.
All the small digital products are HASL boards. There is only one reason: cheap.
- Light red color
This membrane evaporates as soon as it is heated. Then you could solder the copper wire and components together.
But it is easy to be corroded. An OSP board can’t be soldered if it is exposed to the air for more then 10 days.
There are many OSP processes in computer motherboards. Because the board size is too large.