The surface finish of PCB board may be an organic substance or a metallic substance. Comparing the two types and all the existing options, you can quickly see the corresponding advantages and disadvantages. In general, when determining the most suitable surface treatment, the decisive factors are the end use of the product, the assembly process and the design of the PCB itself. The following will briefly introduce some of the most commonly used surface finish type.
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Hot Air Solder Leveling (HASL)
Hot air leveling, also known as hot air solder leveling, is a process of coating molten tin-lead solder on the PCB surface and leveling (blow-out) with heated compressed air to form a layer that resists copper oxidation and provides good solderability. When the hot air is leveled, the solder and copper form a copper-tin metal compound at the joint, and its thickness is about 1 to 2 mils.
The PCB is immersed in molten solder when hot-air leveling. The air knife blows the liquid solder flat before the solder solidifies, and can minimize the meniscus shape of the solder on the copper surface and prevent solder bridging.
Hot air leveling is divided into two types: vertical and horizontal. Generally, the horizontal type is considered to be better. The horizontal hot air leveling coating is relatively uniform, which can realize automatic production.
The general process is: micro-etching-> preheating-> coating flux-> spraying tin-> cleaning.
1. Excellent solderability
2. Cheap / low cost
3. Allow long processing period
4.The industry has a long history / universal surface treatment
5. Shelf life is at least 12 months
6. Multiple thermal deviations
1. There is a difference in thickness / surface shape between the large and small solder pads
2.Not applicable to SMD and BGA with pin pitch <20 mil
3. Micro-pitch formation of bridges
4. Not ideal for HDI products
1. Excellent solderability
2. Relatively cheap
3. Allow long processing period
4.The industry has a long history / universal surface treatment
5. Shelf life is at least 12 months
6. Multiple thermal deviations
1. There is a difference in thickness / surface shape between small and large solder pads, but the difference is lower than SnPb
2. High processing temperature, 260-270 degrees Celsius
3. Not suitable for SMD and BGA with pin pitch <20 mil
4. Micro-pitch formation of bridges
5. Not ideal for HDI products
In fact, HASL and LF HASL are roughly the same. The surface color and flatness of the pads presented have the same effect on the soldering quality.
OSP (Organic Solderability Preservative)
OSP differs from other surface treatment processes in that it functions as a barrier layer between copper and air; in short, OSP is a chemically grown organic film on a clean bare copper surface. This film has anti-oxidation, thermal shock resistance, and humidity resistance to protect the copper surface from rusting (oxidation or sulfurization) in the normal environment. At the same time, it must be easily assisted at the subsequent welding temperature. The flux is quickly removed for soldering.
The organic coating process is simple and low cost, which makes it widely used in the industry. The early organic coating molecules were imidazole and benzotriazole, which played a role of rust prevention, and the latest molecules were mainly benzimidazole. In order to ensure that multiple reflows can be performed, it is not possible to have only one organic coating on the copper surface. There must be many layers. This is why copper baths are usually added in chemical tanks. After coating the first layer, the coating layer adsorbs copper; then, the organic coating molecules of the second layer are combined with copper until twenty or even hundreds of organic coating molecules are aggregated on the copper surface.
The general process is: degreasing-> micro-etching-> pickling-> pure water cleaning-> organic coating-> cleaning. The process control is relatively easy compared to other methods.
OSP (Organic Solderable Protective Layer) (usual thickness 0.15 &#; 0.65 μm)
1. Excellent flatness
2.Suitable for fine pitch / BGA / smaller components
3. Cheap / low cost
4. Can be reworked
5.Clean and environmentally friendly technology
1. Sensitive to operation &#; gloves must be used and avoid scratching
2. Short processing period during assembly phase
3. Limited thermal cycling, not the first choice for multiple welding processes (> 2/3)
4. Limited shelf life &#; not suitable for certain modes of transport and long-term storage
5. Difficult to test
6. Misprinted solder paste may have an adverse effect on the OSP coating
7. Baking may be adversely affected before use
Chemical Gold
Electroless nickel plating / gold immersion is a thick layer of nickel-gold alloy with good electrical properties and can protect the PCB for a long time. Unlike OSP, which is only used as a rust-proof barrier layer, it can be useful and achieve good electrical properties during long-term use of the PCB. In addition, it also has environmental tolerance not available in other surface treatment processes.
The reason for nickel plating is that gold and copper will diffuse to each other, and the nickel layer can prevent the diffusion between them. Without the barrier of the nickel layer, gold will diffuse into the copper in a few hours. Another advantage of electroless nickel plating / immersion gold is the strength of nickel. Only 5um thickness of nickel can control the Z-direction expansion at high temperature. In addition, electroless nickel / immersion gold can prevent the dissolution of copper, which will be beneficial for lead-free soldering.
The general process is: de-acid cleaning-> micro-etching-> pre-immersion-> activation-> chemical nickel plating-> chemical immersion gold; there are 6 chemical tanks in the process, involving nearly a hundred species Chemicals, the process is more complicated.
ENIG-Immersion Gold / Electroless Nickel Plating (usually 3-6 μm (nickel) / 0.05 &#; 0.125 μm (gold)
1. Chemical immersion, excellent flatness
2.Suitable for fine pitch / BGA / smaller components
3. Process tested and tested
4. Long shelf life
1. Costly surface treatment
2.BGA has black pad problem
3. Aggressive to solder masks-especially larger solder masks
4. Avoid BGA defined by solder mask
5. Should not be plugged on one side
Immersion Silver
The silver immersion process is between OSP and electroless nickel / gold plating, and the process is simple and fast. Immersion silver is not a thick armor for PCB. Even if exposed to heat, humidity and pollution, it can still provide good electrical properties and maintain good solderability, but it will lose its luster. Because there is no nickel under the silver layer, immersion silver does not have all the good physical strength of electroless nickel / immersion gold.
Immersion silver is a displacement reaction, it is almost submicron pure silver coating. Sometimes the silver immersion process also contains some organics, mainly to prevent silver corrosion and eliminate silver migration problems. It is generally difficult to measure this thin layer of organics. Analysis shows that the weight of the organism is less than 1%.
Since all solders are based on tin, any type of solder can match the tin layer. From this point of view, the immersion tin process has great development prospects. However, tin whiskers tended to occur after the previous PCBs were immersed in tin. The solder whisker and tin migration during the soldering process will bring reliability problems, so the use of tin immersion is limited. Organic additives were added to the tin immersion solution to make the tin layer structure a granular structure, which overcomes the previous problems, and also has good thermal stability and solderability.
The immersion tin process can form a flat copper-tin intermetallic compound. This feature makes the immersion tin have the same good solderability as hot air leveling without the troublesome flatness problems of hot air leveling; nor does it have electroless nickel plating / immersion Diffusion issues between gold and metal; just the tin immersion board cannot be stored for too long.
Three. The thickness of immersion silver (0.12 &#; 0.40 μm)
1. Chemical immersion, excellent flatness
2.Suitable for fine pitch / BGA / smaller components
3.Medium cost range for lead-free surface treatment
4. Can be reworked
5. Medium shelf life with proper packaging
1. Must worn very sensitive to handling / discoloration of silver surface / appearance problems-gloves.
2. Special packaging required-If the panel is not used up after the package is opened, it must be resealed quickly.
3. Short processing period during assembly phase
4. It is not recommended to use peelable glue
5. Should not be plugged on one side
6. Fewer suppliers provide this surface treatment process
There are also gold plating and flash gold processes. Used very little, customers will require different surface processes based on the difficulty of their substrates. The most widespread is still HASL. If the pads are required to be very flat and cost savings are required, then OSP is used. High-precision products, substrates with BGA and other chips are generally ENIG.
This guide covers all the critical aspects of PCB surface finishes.
So, before you start fabricating PCBs, read this guide.
A PCB surface finish is a coating applied over the conductive circuitry on an unpopulated printed circuit board surface.
You find the PCB surface finish can consist of several elements applied through different processes.
The PCB surface finishing procedure is particularly useful for PCB&#;s with copper traces and typically precedes the soldering process.
PCB surface finish
You find the application of a PCB surface finish important for two major reasons.
They include;
There are several material options for use when applying the PCB surface finish.
The materials used have different properties and capabilities.
Some of the material elements used in PCB surface finish include gold, nickel, tin, silver, and lead.
These materials also have different application procedures while they provide different solderability levels.
The choice of surface finish to use on a printed circuit board depends on several factors.
As such, you can base your choice of surface finish by considering each of the factors below.
There are many options to select from when picking a surface finish for your printed circuit board.
Some of the common options include:
HASL is a surface finish composed of solder material comprising of lead and tin elements.
You find this surface finish as one with the least cost requirements.
Furthermore, the availability of HASL makes its use popular.
HASL Surface finish
In hot air solder leveling, the printed circuit board is bathed in liquefied solder such that the conductive traces are submerged.
A stream of hot air in a line of action perpendicular to the board is used then to level the solder.
The aim is to evenly spread the solder across the board surface in equal thickness.
There are several benefits to be accrued form employing hit air solder leveling as a finish on your circuit board surface. These benefits include:
While hot air solder leveling is appreciated in many ways, its use is limited in the following instances.
Consequently, the HASL surface finish type is considered not to conform to RoHS recommendations.
Therefore, you find a thickness disparity on the board especially relating to the solder pad sizes.
Accordingly, attaching components of small sizes, with fine pitch such as the small outline chips, is a problem.
You find their high connectivity demands and the less impressive finish by HASL incompatible.
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WHILE you find the HASL composition to include tin and lead elements, sometimes lead is replaced with other environmentally friendly elements.
Replacing lead is due to the hazardous nature of the element to the environment and as a stipulation of the RoHS initiative.
It follows that lead can be replaced by elements such as copper, nickel, and germanium.
Tin is retained in this composition.
Additionally, a mixture of more than three elements can be employed in this solder type.
The HASL without lead conforms to the ideals of the RoHS initiative.
You find using lead-free solder in hot air leveling achieves the following:
Yes, there are.
You find lead-free HASL is limited as a PCB surface finish in the following ways:
Immersion coatings are surface finishes employed on printed circuit boards by submerging them in molten states of particular elements.
The formation of the coating is dependent on a chemical process that occurs during submersion.
In this process, the metal ions of the molten element are attached to the board surface.
Common elements used in the immersion process include silver, tin, and gold.
You find immersion coatings to cost more than ordinary finishing techniques such as HASL.
While immersion surface finishes are pricier than other PCB surface finishes, they find popular use for a variety of reasons.
Use of immersion coatings is influenced by the following reasons:
There are two common immersion coatings used for printed circuit boards.
You find immersion tin and immersion silver applied as popular circuit board surface finishes.
Immersion tin entails the chemical layering of the copper trace on a PCB surface with tin ions.
The result is the formation of a single coat of tin over the conductive pattern.
Also, the surface is even allowing for the use of this surface finish for small board attachments.
You find that tin provides the least cost requirement when implemented for the immersion surface finish.
Additionally, while it can support small board attachments, it responds remarkably well to the soldering process.
The surface finish achieved through immersion tin is unaffected by temperature fluctuations.
However, you find immersion tin coating delicate to touch and consequently requiring the use of protective gloves.
Also, the formation of whiskers is a concern when employing immersion tin surface finish.
Moreover, tins react harshly to solder mask applications requiring a wide dam.
It follows that its use with removable masks is highly ineffective.
In immersion silver, a coating of silver is chemically deposited over the copper pattern to form a uniform layer.
While the copper-silver compatibility is better than copper-tin compatibility, silver is tarnished on exposure to air.
Packaging of PCBs with immersion silver coating, therefore, has to be air-tight.
When securely packaged, you can take up to a year before carrying out the soldering process on the PCB.
Otherwise, the soldering process has to be carried out within twenty-four hours.
Immersion silver can be plated with a gold layer to protect from tarnishing.
Accordingly, the coating can last longer than when unplated.
While immersion tin is commonly engaged when working with large PCB volumes, its use is limited in small board numbers.
A contributing factor to this limitation is the tarnishing of the tin coating over the conductive copper trace.
Consequently, this affects solderability over time.
Green surface finish on PCB
Therefore, to assuage the impact of tarnishing on the circuit board surface, the soldering process has to quickly follow.
When soldering is done without delay, you can achieve better solder connections.
Since large volumes of PCBs are carried out on successive step processes, the time taken between processes is reduced.
As a result, when the coating process is finalized, the soldering process will follow.
Small productions are approached differently, as they may be stalled to allow for volume achievement to mitigate costs.
Immersion silver has several features that work in its favor and disfavor.
Some of the pros attributed to immersion silver include:
On the other hand, the misgivings of immersion silver as a PCB surface finish include:
ENIG is an acronym for Electroless Nickel Immersion Gold.
ENIG is a surface finish that is composed of a nickel-gold combination achieved through electroless plating and immersion respectively.
You find an excellent surface outcome with this surface finish attributing to the high cost incurred in its achievement.
The ENIG process consists of two steps, the nickel application, and the gold immersion.
Gold is applied over the nickel to prevent it from succumbing to a reaction with atmospheric oxygen.
Before that, the copper surface is catalyzed with palladium before the application of nickel.
A suitably concentrated solution composed of nickel ions is used to electrolytically apply a layer of nickel over the conductive pattern.
To ensure the formation of a uniform coat, the solution temperature and nickel concentration are controlled.
The laying of a gold coat involves the use of an immersion technique.
In this case, the nickel-coated board is submerged in a solution heavily concentrated with gold ions.
A reduction process occurs where the gold ions in the solution are reduced forming a metal deposit on the nickel surface.
The gold coating deposited over the nickel layer is made to a thickness that allows nickel to retain its solderability.
However, it also needs to prevent interaction with oxygen molecules to prevent oxidation.
Consequently, you find the need to keep close tolerance.
While executing an ENIG PCB surface finish is a costly process, the benefits derived from the finish justify the cost.
ENIG surface finish
You find the preferment of ENIG to other surface finishes for PCBs to be based on the following grounds:
A black pad is an unwanted formation that sometimes manifests when ENIG is used as a surface finish.
A black pad results from leakage of sulfur-based substance contained in the solder mask through the gold coating.
It follows that from this seepage stems the formation of a layer of phosphorus sandwiched by the nickel and gold coats.
You find this formation cannot transfer electrical charge and can cause electrical isolation.
Additionally, it can occasion cracks on the surface disrupting the evenness and subsequently the solderability.
OSP refers to organic solderability preservative.
As the name suggests, this finish type is organic and water-based.
You find the OSP finish adheres well with the copper surface offering effective cover from corrosion.
Furthermore, it offers a good soldering surface.
To kick start the application of OSP, the copper surface is cleaned to ensure it is free of contaminants.
Chemical agents are used in the cleaning process before being rinsed off.
You can also improve the board&#;s surface appearance by removing bumps and nodules.
Another rinsing process follows the surface enhancement step only this time an acidic solution is employed.
Thereafter, the organic solder preservative is applied by a conveyor mechanism ensuring a thin layer is applied.
You find the resulting coating has a thickness in the nanometer range.
Upon layering, a specialized rinsing procedure that deionizes the coat is undertaken.
Finally, the surface is dried ready for storage.
Storing a circuit board with OSP finish requires ideal temperature and humidity conditions.
Extreme temperature and humidity values will affect the board surface by causing breakage and soaking.
Also, boards with this finish type need to be kept away from direct sunlight.
Yes, it does.
You can use OSP finish when looking to attach small profile PCB components such as ball grids.
Additionally, the planarity achieved by the OSP finish is remarkable.
Also, its wettability impresses on the ability to carry out the soldering procedure on the coating.
PCB components
Other notable benefits derived from using OSP include the low cost incurred in implementing this finish and its simplicity.
Besides, it does not pose any environmental threat due to its zero lead content.
You also find this finish tolerant to thermal cycles within certain ranges and can be applied several times over.
The use of the OSP finish has been on the rise due to its low cost and environmentally friendly features.
However, this PCB surface finish has a few drawbacks related to its use.
While many surface finishes containing a single or dual coating, the ENEPIG finish implements three layers of material.
ENEPIG is an acronym for Electroless Nickel Electroless Palladium Immersion Gold which is an ENIG finish with an extra palladium layer.
The PCB finish poses no imminent harm to the environment adhering to RoHS standards.
In an ENEPIG finish, the palladium layer is furnished over the nickel coat.
You find this prevents the corrosion of the copper surface due to leakage that causes black pads formation.
A gold coat is then applied over the palladium layer.
Gold offers resistance to oxidation preserving the chemical state of the elements underneath.
Using palladium in ENEPIG finish allows for the use of a thinner gold coating saving on cost.
Additionally, you find this finish to be highly resilient with an extended life span.
You can also use ENEPIG in HDI PCBs with large speed requirements.
Equally splendid about the ENEPIG finish is its planarity and solderability.
You can contact Venture Electronics today for the best PCB surface finishes.
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