What is PCB Surface Finish?
PCB surface finish is the intermetallic connection between the bare copper and components in the solderable area of a printed circuit board. The circuit board has a base copper surface that can easily oxidize without a protective coating, so a surface finish is required.
PCB surface finish is the most important step in the PCB manufacturing and assembly process and has two main functions, one is to protect the exposed copper circuitry, and the other is to provide a solderable surface when soldering components to the PCB. As shown in the figure below, the surface finish is located on the outermost layer of the PCB, above the copper layer, and acts as a copper "coating".
Types of PCB surface treatment
1. Hot air solder leveling (HASL)
2. Immersion tin (ImSn)
3. Electroless nickel immersion gold (ENIG)
4. Organic solderability preservative (OSP)
5. Immersion silver (ImAg)
6. Electroless nickel immersion gold (ENEPIG)
7. Hard gold (electrolytic hard gold)
3. PCB Surface Treatment Process
Hot Air Solder Leveling (HASL)
Hot Air Solder Leveling (HASL) is one of the most commonly used surface treatment methods in the industry. There are two types of HASL, one with lead tin and one without lead tin. HASL is also one of the cheapest types of PCB surface treatment available.
To form the HASL surface finish, the circuit board is dipped into molten solder (tin/lead), and the solder then covers all exposed copper surfaces on the board. After leaving the molten solder, high-pressure hot air is blown across the surface through an air knife, which flattens the solder deposit and removes excess solder from the surface of the circuit board.
In this process, the following important parameters need to be mastered: soldering temperature, air knife air temperature, air knife pressure, dip soldering time, lifting speed, etc.
Hot air solder leveling (HASL) is now widely used in SMT processes. There are 3 main points for PCB hot air leveling:
The PCB should be immersed in molten solder;
The air knife blows away the liquid solder before the solder solidifies;
The air knife can minimize the meniscus of solder on the copper surface and prevent solder bridging.
Due to surface roughness issues, SMT has limitations and cannot use touch switches.
If the circuit board is subjected to high temperature, the copper will dissolve
Especially for thick or thin boards, tin spraying is limited and production and processing are inconvenient.
Advantages and Disadvantages of Hot Air Solder Leveling (HASL)
Advantages of Hot Air Solder Leveling (HASL):
Available in stock
Reworkable
Excellent shelf life
Excellent solderability
Inexpensive/low cost
Allows a large processing window
Longer storage time
PCB is completed and pads are completely covered with tin before soldering
Suitable for lead-free soldering
Proven surface treatment options
Visual inspection and electrical measurements are possible
Disadvantages of Hot Air Solder Leveling (HASL):
Uneven surface
Not suitable for fine pitch
Contains lead (HASL)
Thermal shock
Solder bridging
Plugging or reduced PTH (plated through hole)
Thickness/topography difference between large and small pads
Not suitable for SMD and BGA with pitch less than 20 mil
Not suitable for HDI products
Not suitable for wire binding;
Immersion Tin
Immersion Tin (ImSn) is a metal finish deposited by a chemical replacement reaction, applied directly to the base metal (i.e. copper) of the circuit board.
ImSn protects the underlying copper from oxidation during its expected shelf life. Since all solders are tin-based, the tin layer can match any type of solder.
After adding organic additives to the immersion tin solution, the tin layer structure is granular, overcoming the problems caused by tin whiskers and tin migration, while also having good thermal stability and solderability.
The immersion tin process can form a flat copper-tin intermetallic compound, making immersion tin have good solderability, no flatness problems and intermetallic compound diffusion problems.
Advantages and Disadvantages of Immersion Tin
Advantages of Immersion Tin:
ImSn surface treatment can achieve excellent flatness (suitable for SMT), suitable for fine pitch/BGA/smaller components
ImSn has a medium-cost lead-free surface treatment technology
Press-fit suitable finish
Maintain good solderability after multiple thermal excursions
Suitable for horizontal production lines.
Suitable for fine geometry processing, lead-free assembly.
Disadvantages of Immersion Tin:
Sensitive to handling.
Short shelf life, tin whiskers will appear after 6 months
Aggressive to solder mask
Not recommended for use with peelable mask
Not a suitable choice for contact switches.
Special setup required for electrical testing (soft probe landing)
ENIG (Electroless Nickel Immersion Gold)
ENIG (Electroless Nickel Immersion Gold) surface finish has historically been the best fine pitch (flat) surface and lead-free option.
ENIG is a two-step process that coats a thin layer of gold over a thin layer of nickel. The nickel acts as a barrier to the copper and is the surface that the components are actually soldered to, while the gold protects the nickel during storage.
The inner layer of Ni is generally 3~6μm thick, and the outer layer of Au is generally deposited at a thickness of 0.05~0.1μm.
Ni forms a barrier between the solder and the copper.
The role of Au is to prevent Ni oxidation during storage, thereby extending shelf life, but the immersion gold process also produces excellent surface flatness.
The processing flow of ENIG is as follows: cleaning->etching->catalyst->chemical nickel plating->immersion gold->residue cleaning
While this coating process has a long shelf life and is good for plated through holes, it is a complex and expensive process that is not reworkable and has been known to cause signal losses in RF circuits.
Advantages and Disadvantages of ENIG
Advantages of ENIG:
Flat surface
Lead-free
Suitable for PTH (Plated Through Hole)
Long shelf life
Disadvantages of ENIG:
Expensive
Not reworkable
Black pad/Black nickel
Damage from aliens
Signal loss (RF)
Complex process
Organic solderability preservative (OSP)
OSP (Organic solderability preservative) or anti-rusting agent is a very thin protective layer of material applied to exposed copper, usually using a conveyor belt process, to protect the copper surface from oxidation.
This film must have anti-oxidation, thermal shock, moisture resistance, etc. to protect the copper surface from rusting (oxidation or sulfidation, etc.) under normal conditions.
However, in the subsequent high-temperature soldering, this protective film must be easily removed by the flux so that the exposed clean copper surface can immediately combine with the molten solder to form a strong solder joint in a very short time.
In other words, the role of OSP is to act as a barrier between copper and air.
The general process of OSP is: degreasing->micro-etching->pickling->pure water cleaning->organic coating->cleaning.
OSP uses a water-based organic compound that selectively bonds with copper and provides an organic metal layer that protects the copper prior to soldering. It is also very environmentally friendly compared to other common lead-free finishes that are more toxic or energy intensive.
Advantages and Disadvantages of Organic Solderability Preservatives (OSP)
Advantages of Organic Solderability Preservatives (OSP):
Flat surface
Simple process, very smooth surface, lead-free soldering and SMT
Reworkable, suitable for horizontal production lines
Cost-effective
Environmentally friendly
Disadvantages of Organic Solderability Preservatives (OSP):
Cannot measure thickness
Not suitable for PTH (Plated Through Hole)
Short shelf life
May cause ICT issues
Exposed copper at final assembly
Sensitive handling
Cannot be soldered (reworked) more than twice
Not suitable for crimping technology and wire binding
Not convenient for visual and electrical testing
Requires SMT with NOx injection
Immersion Silver
Immersion Silver is a non-electrolytic chemical surface treatment applied by immersing the copper PCB in a bath of silver ions. It is ideal for circuit boards with EMI shielding and is also used for dome contacts and wire bonding. The average surface thickness of silver is 5-18 micro inches.
Considering modern environmental issues such as RoHS and WEE, immersion silver is more environmentally friendly than HASL and ENIG. It is also popular because it costs less than ENIG.
Even if a PCB processed by immersion silver is exposed to high temperature, humidity and pollution, it can still provide good electrical properties and maintain good solderability, even if it loses its luster.
Immersion silver is a displacement reaction that directly plates copper with a layer of pure silver.
Sometimes immersion silver is combined with OSP coating to prevent the silver from reacting with sulfides in the environment.
Common applications include planar requirements, which may include:
Membrane switches
EMI shielding
Aluminum wire bonding
Very fine traces
Advantages and disadvantages of immersion silver
Advantages of immersion silver
High solderability.
Good surface flatness.
Low cost and lead-free (RoHS compliant).
Suitable for aluminum wire bonding.
Disadvantages of immersion silver
High storage requirements.
Easily contaminated.
Short assembly window after removal from packaging.
Difficult to perform electrical testing.
Electroless Nickel-Electroless Palladium Immersion Gold (ENEPIG)
The electroless immersion gold plating material has a copper-nickel-palladium-gold layer structure that allows for direct wire bonding to the plating. The final gold layer is very thin, as is the case in ENIG. The gold layer is soft, as is the case in ENIG, so excessive mechanical damage or deep scratches can expose the palladium layer.
Compared to ENIG, ENEPIG has an additional palladium layer between the nickel and gold, further protecting the nickel layer from corrosion and preventing the black pad that can occur with ENIG finishes.
The deposition thickness of Ni is approximately 3~6μm, the thickness of palladium is approximately 0.1~0.5μm, and the thickness of gold is 0.02~0.1μm.
The ENEPIG surface finish consists of four metal layers:
Gold
Palladium
Nickel
Copper
Matrix
The difference between ENEPIG and ENIG is the added palladium layer. The palladium helps protect the nickel layer from corrosion, which helps prevent "black pads". This is one of the significant advantages of ENEPIG over ENIG finishes. The immersion gold layer on top of the palladium provides nearly complete board protection by protecting and preserving the palladium underneath.
Advantages and Disadvantages of the Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) Finish
Advantages of the Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) Finish:
Extremely flat surface
Lead-free content
Multi-cycle assembly
Excellent solder joints
Wire bonding
No corrosion risk
Shelf life of 12 months or more
No risk of black pads
Disadvantages of the Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) Finish:
Still a bit expensive
Reusable but with some limitations
Handling limitations
Hard Electrolytic Gold
Hard gold, technically known as hard electrolytic gold, consists of a layer of gold plated over a nickel coating. The purity of the gold plating categorizes this finish as hard gold (99.6% pure) or soft gold (99.9% pure), which is often used in high wear areas such as edge connector fingers.
The choice of hard gold vs. soft gold depends on the type of application you are creating.
Hard gold plating, or electrolytic gold plating, uses a thin gold overlay over a nickel layer on a copper surface. The process creates a very durable gold layer, which makes hard gold plating very common in the PCB industry. Although the presence of gold makes the process expensive, it provides a perfect surface for soldering.
Hard Gold Finish
Hard gold is a gold alloy that contains a complex of cobalt, nickel, or iron. A low stress nickel is used between the gold plating and the copper. Hard gold is not suitable for wire bonding.
Hard gold finish is recommended for components and applications that experience heavy use and high potential for wear, such as:
Interconnect carriers
Edge connector fingers
Keyboards
Contacts
The thickness of the hard gold finish will vary depending on the application. When hard gold is used in military applications, the minimum thickness should be 50 -100 micro inches.
Non-military applications require 25 to 50 micro inches. The following are recommended minimum and maximum thickness values:
IPC maximum solderable thickness recommendation is 17.8 μin
25 μin gold over 100 μin nickel for IPC Class 1 and 2 applications
50 μin gold over 100 μin nickel for IPC Class 3 applications
Hard Gold Advantages and Disadvantages
Hard Gold Advantages:
Hard, durable surface
Lead-free
Long shelf life
Hard Gold Disadvantages:
Very expensive
Additional processing/labor intensive
Resist/tape use
Plating/busbaring required
Demarcation
Difficulty with other surface treatments
Etch undercuts can cause cracking/flaking
Not solderable above 17 μin
Finish does not fully encapsulate trace sidewalls except in finger areas
Soft Gold Finish
As the name implies, soft gold finishes contain a higher purity of gold over the outer gold plating. Soft gold has a purity of 99.9%.
Soft gold finishes are used for circuit boards designed primarily for applications that require wire bonding, high solderability, and solderability. Soft gold produces stronger solder joints than hard gold.
In general, electrolytic nickel/gold is the most expensive PCB finish, all other factors being equal. However, some applications do require an electrolytic nickel/gold finish.
How to Choose a PCB Surface Finish?
The choice of surface finish for a PCB is the most important step in PCB manufacturing because it directly affects process yield, number of reworks, field failure rates, test capabilities, scrap rates, and costs. To ensure high quality and performance of the final product, all important considerations regarding assembly must be considered in the surface finish selection.
1. PCB Surface Finish - Pad Flatness
As mentioned earlier, certain surface finishes can result in an uneven surface, which can affect performance, solderability, and other factors. If flatness is an important factor, consider a surface finish with a thin, uniform layer. In this case, suitable options include ENIG, ENEPIG, and OSP.
2. Solderability and Wettability
When working with PCBs, solderability is always a key factor. Certain surface finishes such as OSP and ENEPIG have been shown to hinder solderability, while other surface finishes such as HASL are well suited.
3. Gold or aluminum wire bonding
If your PCB requires gold or aluminum wire bonding, you may be limited to ENIG and ENEPIG.
4. Storage conditions
As mentioned earlier, some surface finishes, such as OSP, make the PCB fragile when handled, while other surface finishes improve durability. Consideration should be given in advance when considering storage and handling requirements, and surface finishes that make the PCB fragile should only be used when risk-free storage and handling requirements can be met.
5. Soldering cycle
How many times will the PCB be soldered and reworked? Many surface finishes are ideal for rework. However, other methods such as immersion tin are not suitable for rework.
6. PCB surface treatment - RoHS compliance
RoHS compliance is crucial when determining the surface finish to be used. Generally, all surface finishes using lead are not suitable for RoHS compliance and should be avoided.
Based on the above introduction to each surface finish, some properties are the most important elements as selection criteria. The following table shows the properties that each surface finish has and does not have.
Based on the specific requirements and characteristics of the PCB product, you can follow this table to choose the perfect surface finish option.
In summary, for the type of surface finish selection, the best type must be selected to complete numerous functions. Each type of surface treatment has its own advantages and disadvantages. There are some engineering tricks to solve the problems caused by the shortcomings of the surface finish. For example, for the disadvantage of low wetting force of OSP, there are some solutions, such as changing the board solderability plating or wave soldering alloy, increasing top surface preheating, etc. The key is that all possible factors must be considered to obtain ideal performance.