Surface finishing in printed circuit boards is part of the manufacturing process, which refers to the process of treating the surface of the PCB to improve its functionality, durability, and also appearance. The processes included are electroplating, coating, and polishing. Some of the commonly used surface finishes in printed circuit board manufacturing are as follows:
HASL (Hot Air Solder Leveling): This process involves immersing the printed circuit board in a bath of molten solder, then using hot air to level and smooth the surface.
ENIG (Electroless Nickel Immersion Gold): This process involves applying a thin layer of electroless nickel to the printed circuit board surface, followed by a thin layer of immersion gold. This finish is known for its durability and resistance to oxidation.
OSP (Organic Solderability Preservatives): This process involves applying a thin layer of an organic compound to the printed circuit board surface to protect it from oxidation and improve its solderability.
Immersion Tin: In this, the surface finish involves coating the printed circuit board with a layer of tin, which provides good corrosion resistance and low contact resistance.
ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold): In this, the surface finish is an advanced version of ENIG with improved corrosion resistance and excellent shelf life.
Immersion Silver: The surface finish involves the coating of the printed circuit board with a layer of silver, similar to Immersion Tin, and provides good corrosion resistance.
Gold Plating: As the name suggests, the surface finish involves the coating of the printed circuit board with a layer of gold.
The selection of the surface finish depends on the requirements of the specific application, such as the type of components to be used, the environment in which the printed circuit board will be used, and the cost analysis. The factors to be considered as solderability, cost, reliability, processability, lead-free requirements, etc. HASL and ENIG are common surface finishes that have good solderability.
Sometimes, if the printed circuit board is used in a lead-free application, then the surface finish should comply with RoHS (Restriction of Hazardous Substances) and WEEE (Waste Electrical and Electronic Equipment) regulations.
Hot Air Solder Leveling is considered to be one of the most popular surface finishes used on printed circuit boards. It involves immersing the printed circuit board in a bath of molten solder, which is then leveled off using hot air. The process is used to develop a uniform surface finish on the printed circuit board, which is important for proper soldering and to ensure that the components are stable. The molten solder Tin/Lead combination with an approximate proportion of 63 percent Tin and 37 percent Lead, but HASL can also be used for a lead-free printed circuit board with a minor change in the process.
The lead-free Hot Air Solder Leveling has a proportion of 99.3 percent Tin and 0.6 percent Copper but is similar to HASL in terms of appearance and usage. But there is also a change in the manufacturing process, as the lead-free HASL alloy has a higher melting point for lead-free solder when compared with the lead solder; this requires a modification for the Reflow Soldering process.
Even though Hot Air Solder Leveling is widely used, it has some disadvantages, such as high lead content, poor reliability in high temperature and humidity conditions, and the potential for poor soldering performance. There are chances of uneven surfaces, which are not ideal for fine-pitch components, and have poor thermal shock resistance. This type of surface finish is also not ideal for plated through-hole components.
As briefly explained in the earlier section, there are two types of Hot Air Solder Leveling surface finishes used on printed circuit boards– Lead-based HASL and Lead-free HASL.
Lead-based HASL process: This is the conventional way of surface finishing with molten solder that contains lead. A lead-based HASL has a brighter appearance of surface finish on the printed circuit board.
Lead-free HASL process: In this new type of HASL finish that does not contain lead, the alloy has a tin-silver or tin-copper combination. It has high reliability in high temperature and humidity conditions.
Still, HASL is currently being replaced by other lead-free surface finishes, such as Electroless Nickel Immersion Gold (ENIG) and Immersion Tin, which are highly reliable and environmentally friendly.
Electroless Nickel Immersion Gold is a type of surface finish used in printed circuit board manufacturing as it involves applying a thin layer of nickel using an electroless plating process. The chemical process does not require any external electrical source. The coating provides a smooth surface for the gold plating to adhere to and has excellent corrosion resistance. Then it is immersed in a bath of gold, which is deposited onto the Nickel surface used in a process called immersion plating. This gives the board enhanced electrical conductivity and low contact resistance.
Electroless Nickel Immersion Gold has a two–layer metallic surface– one of Nickel and a thin layer of Gold onto it. This surface finish process is suitable for mounting parts like SMDs and ball grid arrays. The Nickel thickness ranges from 4 to 7 um, while the gold thickness ranges from 0.05 to 0.23 um. This method is RoHS compliant with environmentally friendly usage.
Electroless Nickel Immersion Gold provides excellent corrosion resistance, low contact resistance, and good adhesion; suitable for SMTs, lead-free, low cost, consistency, and has a long shelf life.
The combination of Nickel and Gold provides good corrosion resistance which makes it suitable for electronic devices to withstand harsh environmental conditions.
The Gold layer on top of the Nickel provides good electrical conductivity and low contact resistance, which is important for proper soldering and ensures that the components are stable.
Electroless Nickel Immersion Gold is suitable for surface mount devices and components as it does not require a separate solder mask layer and provides a flat surface for SMT components.
Electroless Nickel Immersion Gold provides a uniform and smooth surface finish to the printed circuit board.
In Electroless Nickel Immersion Gold plating, the reliability is not known that this type of leaf had dependability issues. The plating does not uniformly wet, resulting in the disuniting of the copper pads from the finish. To ensure that the coating provides a proper temperature and concentration, the reducing agent consisting of Nickel ions must be changed regularly.
There are several applications of surface finishes in printed circuit board manufacturing. The following are a few of them:
Copper circuitry: The surface finishing process in printed circuit board manufacturing helps protect the exposed copper circuitry on the board. It prevents oxidation of the copper as the coating is done on the base copper surface of the outer layer of a printed circuit board.
PCB assembly: The process also provides a solderable surface when assembling electrical components to the printed circuit board.
There are many advantages of using surface finishes in printed circuit board manufacturing. Following is a list of advantages:
Ease of soldering: After surface finishing using HASL or ENIG, the coating provides a smooth surface on the printed circuit board that allows for good soldering of electrical components.
Corrosion resistance: When surface finishing is done using ENIG or immersion tin, it provides a corrosion-resistant layer that protects the printed circuit board from damage, even when deployed in harsh environments.
Improved electrical performance: When immersion silver or OSP is used as a surface finish, they improve the printed circuit board's electrical performance by reducing the surface's impedance.
Enhanced appearance: Surface finishing that includes gold and specific plating can create an aesthetic appearance on the printed circuit board.
Low cost: Out of many, HASL and OSP are comparatively inexpensive surface finishing and are widely used in the PCB manufacturing process.
Similarly, there are several downsides to the use of each surface finish; however, below are general disadvantages that are common to all of them:
Complicated system: The surface finishing of the printed circuit boards can be a complex process that demands specialized equipment and skills. This can increase the production cost and time required to manufacture the PCB.
Environmental problems: Several surface finishes need to be more environmentally friendly. HASL, for example, can release harmful fumes into the environment during the coating process, which can become a concern for workers and the surrounding community.
Product shelf life: If the printed circuit board manufacturer decides to use OSP, the printed circuit board has a limited shelf life and can degrade over time.
Compatibility issues: Not all surface finishes are compatible with all printed circuit board materials and components. The manufacturer needs to take note of compatibility during the design and manufacturing process.
Application-specific downsides: Certain surface finishes are not ideal for high-frequency applications due to their electrical properties, such as impedance and loss tangent. Similarly, specific surface finishes may not be suitable for high-temperature applications due to the thermal properties of the surface finish.
Other than the two commonly used surface finishes– HASL and ENIG- many more are widely used depending on the application requirements. Some of these are Organic Solderability Preservative, Immersion Tin, Immersion Silver, Electroless Nickel Electroless Palladium Immersion Gold, Electrolytic Nickel/Gold– hard gold surface finish, and soft gold surface finish.
Since Gold has excellent electrical conductivity and corrosion resistance, it is extensively used in printed circuit board manufacturing. The electrolytic Nickel and Gold have a layer of gold plating over the base layer of electroplated Nickel. The purity of gold depends on the type of application. When the purity of gold is 99.6 percent, it is categorized as hard gold, while when the purity of gold is 99.9 percent, it is considered to be soft gold.
Hard gold is a gold alloy with some other metal which can be Iron in common use cases. This type of gold plating is not ideal for wire bonding, and they are usually considered for heavy usage applications, such as interconnected carrier boards, edge connector fingers, keypads, etc. On the other hand, a soft gold finish is used for boards that require wire bonding, high solderability, and weldability.
Organic Solderability Preservative is a type of surface finish used in PCB manufacturing that has a thin organic coating that is applied to the copper traces of the printed circuit board to protect it from oxidation and corrosion. This surface finish uses a water-based organic compound that bonds to the copper and provides an organometallic layer. It is an environmentally friendly surface finish in comparison with the other common lead-free finishes.
There are three basic steps for this type of surface finish:
PCB cleaning: The printed circuit board is first cleaned to remove any contaminants, such as oil, dust, and fingerprints. If the contaminants are not removed properly, it will result in uneven film thickness. This is done using a solvent cleaner followed by a rinse with water.
PCB coating: The printed circuit board is first dipped into an acidic solution which prepares the surface of the copper traces for the OSP coating. The OSP coating is applied using a spray or immersion processor with a typical thickness between 2 and 4 microns.
Film formation: The applied OSP coating is cured at high temperatures between 120 and 150 degrees Celsius. This causes the coating to harden and adhere to the copper traces.
However, Organic Solderability Preservative has some disadvantages– it is not suitable for plated through holes and generally has a short shelf life. The process requires expertise and is sensitive to handling.
Immersion Tin is also a type of surface finish used in printed circuit board manufacturing which involves the coating of a thin layer of tin over the copper traces. It is applied by immersing the printed circuit board in a tin bath, which causes the tin to bond to the copper surface. According to IPC, Immersion Tin is a metallic finish deposited by a chemical displacement reaction that is directly over the basic metal of the circuit board, which is copper. This is RoHS compliant, which means a lead-free surface finish becomes an ideal choice for flat surface requirements and fine-pitch components. Immersion Tin is also commonly referred to as White Tin.
Compared to other surface finishes of PCB manufacturing, Immersion Tin is one of the least expensive types of immersion coating. Irrespective of the advantages, there are a few disadvantages to using this surface finish. This immersion coating impacts the shelf life of PCB products and ultimately affects the overall performance. Immersion Tin has a shelf life of three to six months, but to improve the PCB assembly and soldering process, it has to be done during 30 days of the surface coating.
The immersion silver surface finish is similar to Immersion Tin, but in this process, silver is electroplated onto the surface of the printed circuit board. This creates a thin layer of silver on the surface of the board to improve the electrical conductivity and provide durability along with corrosion resistance properties. Manufacturers that deal with aluminum wire bonding, EMI shielding, and metallic dome contacts use Immersion Silver with a typical thickness of 5 to 18 micro inches. It is a non-electrolytic chemical finish done by immersing the copper-printed circuit board in a tank of silver ions. Even this surface finish is RoHS compliant, which means it is a lead-free surface finish becoming an ideal choice for flat surface requirements. It is suitable for specific applications that want to avoid any magnetic materials in the printed circuit board.
The process of Immersion Silver surface finish is slow, which helps in building a dense crystalline structure forming a high-density layer of silver on the copper surface of the printed circuit board. This slow process also helps in avoiding particle growth from agglomeration and precipitation. It is possible to store Immersion silver PCB for a minimum of 12 months; however, once the user removes the board from storage conditions, it must be soldered within 24 hours.
Electroless Nickel Electroless Palladium Immersion Gold is another type of surface finish that has a multi-layered process. In this, a thin layer of nickel is electrolessly deposited onto the surface of the printed circuit board, followed by a thin layer of Palladium. Once these two layers are coated on the circuit board, a layer of gold is immersion plated on top of the Palladium. The overall process delivers high reliability and performance of the circuit board for aerospace applications and also medical devices.
Electroless Nickel Electroless Palladium Immersion Gold surface finish delivers excellent ball shear solderability performance while adhering to IPC4556 thickness specification for the coating. SEM studies and elemental analysis have proved that the presence of Palladium at the solder joints interfaces reduces intermetallic propagation. The surface finish has no requirements for bussing lines, a parameter that translates to additional flexibility in the design of the circuits. The Electroless Nickel Electroless Palladium Immersion Gold surface finish has immunity against black pad because of the Palladium plating.
Surface finishing in printed circuit boards is part of the manufacturing process, which refers to the process of treating the surface of the PCB to improve its functionality, durability, and also appearance.
HASL involves immersing the printed circuit board in a bath of molten solder, then using hot air to level and smooth the surface.
ENIG involves applying a thin layer of electroless nickel to the printed circuit board surface, followed by a thin layer of immersion gold.
OSP involves applying a thin layer of an organic compound to the printed circuit board surface to protect it from oxidation and improve its solderability.
ENEPIG is an advanced version of ENIG with improved corrosion resistance and an excellent shelf life