Printed Circuit Boards (PCBs) are associated with innumerable applications across industries. Predominantly, PCBs are designed for applications in environments containing dust, harsh chemicals/solvents, and elevated/fluctuating temperatures and pressures, among others.
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Since PCBs are manufactured with copper traces, they must be coated comprehensively with suitable surface finish materials. This is done to prevent deterioration/oxidation of copper, along with augmenting the service life of the assembly as a whole.
What are the Most Commonly used PCB Finish Materials?
PCB finishing materials must be chosen after considering several factors. These include materials used in the assembly, manufacturing methods, application areas and intended service period.
Some of the generally used surface finishes are listed below with their pros and cons.
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Leaded Solder
Leaded solder, which is the most common in the list, is produced by mixing tin and lead. The finish is easy to apply and provides a comprehensive coating that lasts for years. It is applied by immersing the board completely into a mixture of molten lead and tin. Following the dipping stage, an air knife is used to blow off the excess mixture from the board’s surface. This technique is the oldest, but still popular, due to low cost and wide availability. However, the lead content is high, posing a threat to environmental sustainability.
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Lead-Free Solder
Whenever the requirement calls for low cost and lead-free surface finish, this is the best solution. Here, the PCB is dipped in the molten mixture of tin and copper and then the excess solder is removed using an air knife. It offers a shiny silver colored finish that lasts for years. The type is known for its easy applications and wide availability.
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Electroless Nickel Immersion Gold
Abbreviated as ENIG, this method is the best lead-free finish used in the PCB manufacturing. At first, nickel, about 150 to 200 micro inches, is coated onto the copper surface. This is followed by the application of a thin layer of gold (approximately 3 to 10 micro inches) over nickel. The coating provides comprehensive protection to the PCBs from hostile environmental conditions. However, the coating process is laborious and expensive.
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Immersion Silver
There are several benefits associated with the immersion silver finishing technique. These include the ability to create a flat surface, outstanding solderability, and environmentally suitable lead free coating, among others. In this finishing process, one-micron thick layer of silver is comprehensively coated over the copper surface through the electroless plating process. The method is cost advantageous and compatible with most assembly processes. However, the coating is susceptible to damage with the exposure to high temperature, humidity, and improper handling.
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Immersion Tin
It is a lead-free finish ideal for SMT assembly. Here, a thin layer of tin is applied over copper, resulting in a whitish finish. Immersion tin finish is cost-effective when compared to silver and gold immersion. However, it has limited shelf life and is liable to damage faster than gold and silver finishes. Due to this reason, PCBs coated with tin must be used within 6 to 12 months.
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Organic Surface Protectant
It is a ‘green’ finish compared to others. Here, a water based organic compound is applied over copper using a conveyorized process. The coating substantially protects the copper from oxidation when used in aggressive industrial environments. It is a cost-effective finish that features simple application.
Along with protecting the copper circuitry, these surface finishes provide a solderable surface to assemble PCB components with ease. Selecting the right type of the several forms is critical, as it determines the efficiency and durability of the PCB.
A printed circuit board surface finish can be either organic or metallic in nature. Comparing both types and all available options can quickly demonstrate the relative benefits or drawbacks. Typically, the decisive factors when it comes to selecting the most suitable finish is the end application, the assembly process and the design of the circuit board itself. Below you can find a brief summary of the most common finishes, however for further or more detailed information about any specific PCB surface finish, please contact NCAB Group and we will be more than happy to answer any of your questions.
- Hard Gold
- Enig
- Hasl
- Lead Free Hasl
- Immersion Silver
- Immersion Tin
- OSP
- Edge Contacts
- ENEPIG
Hard Gold / Edge Contacts
Minimum Gold thickness for Class 1 & 2 boards is 0.8 µm, and for Class 3 boards 1.25 µm. Minimum Nickel thickness for Class 1 boards is 2 µm, and for Class 2 & 3 boards 2.5 µm. Shelf life: 12 months
- Durable surface resistant to mechanical friction. This is where it performs best.
- Long service life
- RoHS compliant
- Expensive
- Poor solderability, therefore selective plating only is recommended.
HASL – Tin/Lead hot air solder level
Typical thickness 1 – 40um. Shelf life:12 months
- Excellent solderability
- Inexpensive / Low cost
- Allows large processing window
- Long industry experience / well known finish
- Multiple thermal excursions
- Difference in thickness / topography between large and small pads
- Not suited for < 0,5µm pitch SMD & BGA
- Bridging on fine pitch
- Not ideal for HDI products
LF HASL – Lead Free hot air solder level
Typical thickness 1 – 40um. Shelf life: 12 months
- Excellent solderability
- Relatively inexpensive
- Allows large processing window
- Multiple thermal excursions
- Difference in thickness / topography between large and small pads – but to a lesser degree than SnPb
- High processing temperature – 260-270 degrees C
- Not suited for < 0,5µm pitch SMD & BGA
- Bridging on fine pitch
- Not ideal for HDI products
ENIG – Immersion gold / Electroless Nickel Immersion Gold
Typical thickness 3 – 6um Nickel / 0.05 – 0.125um Gold. Shelf life: 12 months
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- Immersion finish = excellent flatness
- Good for fine pitch / BGA / smaller components
- Tried and tested process
- Wire bondable
- Expensive finish
- Black pad concerns on BGA
- Can be aggressive to soldermask – larger soldermask dam preferred
- Avoid soldermask defined BGA’s
- Should not plug holes on one side only
ENEPIG – Electroless Nickel Electroless Palladium Immersion Gold
Typical thickness = Nickel 3 – 6 µm / Palladium is 0.05 – 0.3 µm / Gold 0.05 – 0.125 µm Shelf life = 12 months
- Excellent for wire bonding
- Free from black pad concerns
- Immersion finish = excellent flatness
- Palladium reduces impact of nickel on high speed designs
- Expensive finish
- Not widely available
- Solderability influenced by palladium deposit
Immersion Sn – Immersion Tin
Typical thickness ≥ 1.0µm. Shelf life: 6 months
- Immersion finish = excellent flatness
- Good for fine pitch / BGA / smaller components
- Mid range cost for lead free finish
- Press fit suitable finish
- Good solderability
- Very sensitive to handling – gloves must be used
- Tin whisker concerns
- Aggressive to soldermask – soldermask dam shall be ≥ 127µm
- Baking prior to use can have a negative effect
- Not recommended to use peelable masks
- Should not plug holes on one side only
Immersion Ag – Immersion Silver
Typical thickness 0.12 – 0.40um. Shelf life: 6 months
- Immersion finish = excellent flatness
- Good for fine pitch / BGA / smaller components
- Mid range cost for lead free finish
- Very sensitive to handling / tarnishing / cosmetic concerns – gloves must be used
- Special packaging required – if packaged opened and not all boards used, it must be resealed quickly.
- Short operating window between assembly stages
- Not recommended to use peelable masks
- Should not plug holes from one side only
- Reduced supply chain options to support this finish
OSP (Organic Solderability Preservative)
Typical thickness 0.20-0.65µm. Shelf life: 6 months
- Excellent flatness
- Good for fine pitch / BGA / smaller components
- Inexpensive / Low cost
- Can be reworked
- Clean, environmentally friendly process
- Very sensitive to handling – gloves must be used and scratches avoided
- Short operating window between assembly stages
- Limited thermal cycles so not preferred for multiple soldering processes (>2/3)
- Limited shelf life – not ideal for specific freight modes and long stock holding
- Very difficult to inspect
- Cleaning misprinted solderpaste can have a negative effect on the OSP coating
- Baking prior to use can have a negative effect
ENEPIG
Electroless Nickel Electroless Palladium Immersion Gold. Typical thickness = Nickel 3 – 6um / Palladium is 0.05 – 0.3um / Gold 0.05 – 0.125um. Shelf life = 12 months
- Excellent for wire bonding
- Free from black pad concerns
- Immersion finish = excellent flatness
- Palladium reduces impact of nickel on high speed designs
- Expensive finish
- Not widely available
- Solderability influenced by palladium deposit
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