A PCB surface finish is crucial for a printed circuit board as it prevents the copper (exposed traces, pads, holes and ground plains) from oxidation – a critical ingredient to high-level application performance.
A surface finish is essential for making a reliable connection between the PCB and the electronic component. A surface finish has two major functions, to provide a solderable surface for sufficient soldering components to the PCB, and to protect any exposed copper from oxidizing.
When it comes to choosing the right type of final surface finish, it is just as important as selecting the right material for your bare board. Selecting the correct surface finish for your application is critical for performance. For assistance with selecting a surface finish, send our trusted engineers an email or call.
With increasing demands for thin, high-density, fast speeds, and lightweight applications, the solder finish will make all the difference.
Modern surface finishes are lead-free, in accordance with Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) directives, and include:
Choosing the correct PCB surface finish for your application requires taking factors into account like cost, the final application environment (e.g. high-heat/thermal, vibration/stability, RF), component density/selection, lead/lead free requirements, shelf life, shock/drop resistance, production volume, and throughput.
Due to the increased demand for improved performance of electronic applications, surface finishes have also been upgrading. Because of tighter PCB topography the HASL surface finish is quickly being replaced by gold-based surface finishes like ENIG, ENEPIG, and soft/hard gold. The gold-based surface finishes have a lot to offer in terms of features and benefits when used in applications.
Diagram illustrating the difference between ENIG and other surface finishes
Example of an ENIG Surface Finish on PCB
ENIG consists of two layers of metallic coating, which are deposited on the copper surface through chemical process, 2-5μ inches of immersion Gold (Au) over 120-240μ inches of electroless Nickel (Ni).
The nickel layer protects the copper from oxidation, and the gold layer protects the nickel layer. This dramatically increases the resistance to corrosion and also maintains a flat surface which is crucial for assembly of leadless components. ENIG usage has become very common now due to the accountability for lead-free regulations.
ENEPIG is probably not new to you, but it is a surface finish that has become increasingly popular recently due to the price reductions in palladium. It also has increased in popularity and use because of its many features and benefits. It also has a distinct advantage over the ENIG surface finish, which we’ll get into below.
The ENEPIG surface finish is composed of four metal layers:
Diagram illustrating the difference between ENEPIG and other surface finishes.
The difference between ENEPIG and ENIG is the added layer of palladium. The palladium helps protect the nickel layer from corrosion, which helps prevent “black pad” from occurring. This is one of the distinct advantages of ENEPIG over the ENIG surface finish. The immersion gold layer that sits atop the palladium offers nearly-complete board protection by protecting and preserving the palladium underneath.
When compared to other surface finishes, like hard gold and soft gold plating, ENEPIG has become more affordable in recent years. The cost of the ENEPIG surface finish decreased because the added palladium layer reduces the required thickness of the more expensive gold layer.
The ENEPIG finish is capable of the majority of the ultra-demanding requirements with multiple package types like, through-holes, SMT, BGA, wire bonding, and even press fit.
The ENEPIG surface finish is capable of ultra-demanding requirements.
The PCB surface finish with ENEPIG is very thin (between 0.05μm and 0.1μm), making the assembly and soldering process very simple and definitely more reliable when compared to ENIG. ENEPIG also has a long shelf life due to its durability and resistance from tarnishing.
Depending on applications, printed circuit board technologies use gold extensively. Gold has good electrical conductivity, tarnish resistance, solderability after storage, and being excellent etch resist.
Electrolytic Ni/Au has a layer of gold plating over a base of electroplated nickel. The purity of the gold plating categorizes this finish as either hard gold (99.6% purity) or soft gold (99.9% purity). The choice of hard gold versus soft gold depends on the type of application you are looking to create
Examples of the Hard/Soft Gold Surface Finish
Hard gold is a gold alloy with complexes of cobalt, nickel or iron. A low-stress nickel is used between gold over-plating and copper. Hard gold is not suitable for wire bonding.
We recommend using the hard gold surface finish for components and applications with heavy usage and a high likelihood of wear and tear, like:
The thickness of the hard gold surface finish will vary depending on the applications. When hard gold is used for compliance in military applications, the minimum thickness shall be 50 -100 micro inch.
Nonmilitary applications require 25 to 50 micro inches. Here are the recommended minimum and maximum thickness values:
A soft gold finish, as the name suggests, contains a higher gold purity on the outer gold plating. Soft gold has a 99.9% purity.
An example of the soft gold surface finish on a PCB
Soft gold finish is used for boards designed mostly for applications that require wire bonding, high solderability and weldability. Soft gold produces a much stronger welded joint when compared to hard gold.
Electrolytic Ni/Au has a layer of gold plating of varying thickness
Generally speaking, and with all other factors being equal, Electrolytic Nickel/Gold is the most expensive PCB surface finish. However, some applications do require the Electrolytic Nickel/Gold surface finish.
Immersion Silver (also called immersion Ag) is a RoHS compliant (lead-free) surface finish that is an ideal choice for flat surface requirements and fine pitch components with reasonable costs. This surface finish can also replace Immersion Gold over Electroless Nickel (Au/Ni) for most applications. And the thickness ranges from 0.12 to 0.40 μm.
It has a decent shelf life of 6 to 12 months, but it can be sensitive to contaminants found in the air and on some surfaces. This makes it critical to have a highly qualified boardhouse to properly control and expedite the board packaging process. This surface finish is also subject to tarnishing, which is something to take into account.
As mentioned above, common applications include flat surface requirements, which may include:
Immersion Tin is a is a RoHS compliant (lead-free) surface finish that is an ideal choice for flat surface requirements and fine pitch components. White Tin (Immersion Tin) is a deposit of a thin layer of Tin on Copper layer of a PCBs. It is not used as often as other surface finishes because not all PCB manufacturers offer it. The flatness of this particular coating makes this an ideal surface finish choice for small geometries and components.
Tin also is the least expensive type of immersion coatings. Although it is an economical choice, it does come with some drawbacks.
Immersion Tin has its advantages, but there are a few disadvantages to using this surface finish. As you may know, Copper and Tin have a strong affinity for one another. This means, over time, diffusion of one metal into the other will occur. It directly impacts the shelf life of the Immersion Tin deposit and ultimately the performance of the ?nish.
Immersion Tin has a limited shelf life, 3-6 months. For the best results assembly (soldering) shall happen during 30 days.
Tin Whiskering is also another downside to using Immersion Tin as a surface finish. The negative effects of tin whiskers growth are well described in industry related literature and topics of several published papers.
The Immersion Tin finish is not very durable, therefore, PCBs with Immersion Tin finish must be handled with caution.It provides a consistently flat surface approximately 20-40 micro inches in thickness.
Immersion Tin has been primarily used as an alternative to a lead-based surface finish. It is also used for its very flat and smooth finish making it ideal for fine geometries and fine pitch surface mount components.
Another reason to use immersion tin is sustainability. Elements that may be difficult to source on a consistent basis are used in other finishes, such as ENIG or HASL. Additionally, it uses less water and chemicals in the application process.
It is also easier to re-work.
As with all of the surface finishes we’ve compared so far, there are a number of advantages and disadvantages. Here are the advantages and disadvantages for Immersion Tin.
Here is a full table with some of the pros and cons along with some of the most common uses for the Immersion Tin surface finish.
Hot Air Solder Leveling (HASL) is one of the most commonly used surface finishes in the industry. HASL is divided into two types, and there are two types because one contains tin with lead and the other contains tin without lead. HASL is also one of the least expensive types of PCB surface finishes available. This means access to this surface finish is widely available and it's also very economical.
To create a HASL surface finish, a board is submerged in molten solder (tin / lead). The solder then covers all of the exposed copper surfaces on the board. Upon leaving the molten solder, high pressure hot air is blown over the surface through air knives, this levels the solder deposit and removes the excess solder from the board surface.
HASL provides a very reliable solder joint and shelf life. HASL makes component soldering very effective, but due to the thickness of HASL coating, the planarity of the surface may be unsuitable for fine-pitch components. The HASL deposit is made from a eutectic blend of tin and lead.
If you're using through-hole or larger SMT components, HASL can work well. However, if your board will have SMT components smaller than 0805 or SOIC, it is not ideal.
One of the unintended benefits of the HASL process is that it will expose the PCB to temperatures up to 265°C which will identify any potential delamination issues well before any expensive components are attached to the board.
There are some distinct advantages and disadvantages with the HASL surface finish.
For decades HASL was the go-to choice for surface finishes. However, recently, manufacturers have realized it does have limitations. While HASL may be low-cost, the rise of improved surface mount technologies have exposed some shortcomings. One of the downsides of HASL is that it leaves uneven surfaces and is not suitable for fine pitch components. Another downside is that HASL is not lead-free unless requested and having a different process in place. There are other lead-free options that likely makes more sense for a high-reliability product. If a lead-free surface finish is required you may want to try to use Lead-Free HASL.
The HASL surface surface is also not completely level, which can cause issues when used with smaller components. Since the solder is typically Tin/Lead, that means this surface finish is not RoHS compliant. In fact, due to anti-lead laws in the EU, China, California, and many other locations, demands for other lead-free finishes like ENIG, ENEPIG, and other immersion finishes instead of ENEPIG have become better options.
Unfortunately, HASL is also not very environmentally-friendly. Compared to ENIG, HASL is not a good environmental choice. ENIG is a better choice because it reduces the use of lead and emission from flux and fusing oil in the manufacturing process. And in terms of cost, it might appear at first that HASL is a cheaper option than other options like ENIG. And in most cases HASL is cheaper when being used for a smaller number of boards. But once you start going through two or more boards the cost of HASL is comparable to others like ENIG. And with the other benefits that ENIG offers there's not much reason to choose HASL as your surface finish.
Here is a helpful table to give you a better visual representation of some of the pros and cons of HASL.
|HASL & Lead Free HASL (Hot Air Solder Leveling) / Sn/Pb|
|PROS||CONS||MOST COMMON USES|
Not good for fine pitch components
Not good for plated through-hole (PTH)
As seen above, HASL can still be a good option for a surface finish, but improvements in other surface finishes are making it obsolete. HASL does have a few benefits, but compared to others like ENIG, ENEPIG, and other immersion finishes, it is falling short. For example, ENIG is corrosion resistant, good for aluminum wire bonding, excellent for fine-pitch technology, excellent solder-ability, has a great shelf-life, and so much more. The features and benefits that made HASL a great choice in the past just cannot compare to the improvements of the other surface finishes.
OSP is a different kind of surface finish than we have seen so far. OSP is specifically designed to produce the thin, protective, and uniform layer on the copper surface of PCB’s. This coating actually protects the circuitry from oxidation during storage and assembly. It is applied over copper pads to protect the pad before soldering. This finish actually preserves the copper surface from oxidation by providing an organometallic layer protecting the copper prior to the soldering process. It is also a water-based organic surface finish, which means this surface finish is environmentally friendly. Compared to other surface finishes that have high levels of toxicity or consume large amounts of energy, this surface finish is relatively safe and “green” for the environment.
This surface finish is increasing in popularity due to an increased interest in lead-free techniques and fine pitch solutions.
It also requires low equipment maintenance.
The manufacturing process for OSP is straightforward as seen in the image above. “Clean” is the starting point with cleaning off organic contaminants such as oil or fingerprints, to not get the copper foil surface contaminated before application. For the best results and performance, the cleaning process should be checked as often as possible. If it does not maintain the standards for cleanliness the performance of the application will be negatively affected.
For Topography enhancement, micro etching is applied to eliminate the oxidation generated on top of the copper foil. This is necessary for the bonding forces to be improved between the copper foil and the OSP solution. Speed is also a very important factor. In order to obtain a smooth and even film, the stability of the micro etching speed needs to be at a level of 1.0?m to 1.5?m per minute.
In the DI rinse stage after the OSP application, preservatives need to be added in order to protect the OSP solution from being polluted by other ions. If the OSP solution is polluted, tarnishing will occur soon after the reflow soldering stage. It is best that DI rinse should be utilized with preservatives that have a PH value between 4.0 and 7.0.
This surface finish is very simple and easy to control. It also has an advantage over traditional HASL with regards to coplanarity, and its solderability. However, this finish does require significant changes with the type of flux and number of heat cycles used during assembly process.
Regarding handling of this particular surface finish, there are some definite differences. You cannot touch it with your hands unprotected. Acidic fingerprints will degrade the OSP and leave the copper susceptible to oxidation. Most assemblers prefer to work with metal finishes that are more flexible and endure more heat cycles
We compiled a list of some of the advantages and disadvantages of the OSP surface finish below:
OSP (Organic Solderability Preservatives) is the go-to surface finish for copper pad applications. It is a very specific surface finish and as you can see has several cons associated with it. But for the most part, OSP is a great choice for an organic solution for your surface finish.
If you are looking for the right surface finish for your PCB assembly, we would love to have a conversation with you. San Francisco Circuits is your partner for all your PCB needs. We are flexible and dependable for your most complicated requests and needs!
We are experts in PCB surface finishes. Call us for help in choosing the right surface finish for your PCB assembly process: 800.SFC.5143 or chat with one of our expert staff members.
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These are the lead-free PCB surface finishes we have available for your PCB order.
For more information, please contact our experts at 1.800.SFC.5143 or get a PCB production quote.
San Francisco Circuits specializes in helping you choose the right PCB surface finish. You have a lot of factors to think about like cost, application environment, lead/lead free requirements, shelf life, and so much more! Let us worry about those things and give you the right PCB surface finish.
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