How to Choose the Right PCB Surface Finish for Your Project
Selecting the right surface finish for your PCB project is important in order to ensure optimal electrical and mechanical performance. There are many different types of surface finishes available, each with their own benefits and drawbacks.
In this blog post, we will explore the different types of surface finishes and how to choose the right one for your next project.
Electroless Nickel Immersion Gold (ENIG)
The working principle of most finish processes — covering the solderable surfaces of a PCB with some sort of protective material. In this case, ENIG comprises two-layer metallic coatings deposited onto the flat surface of PCB previously covered with copper — electroless nickel plating with immersion cold.
The deposition is done through the chemical process — 2-5μ inches of Gold plating is applied over 120-240μ inches of Nickel applied over copper pads. It is known as electroless Nickel and gold deposition. The application process is done under 80°C in the bath with the solution of a proper concentration of gold and nickel ions respectively.
Pros:
- Ensures a perfectly flat surface that is suitable for micro-electronics and is good for assembly.
- Ensures a great shelf life and prolonged serviceable life overall.
- Belongs to one of the few lead-free finishes and is RoHS compliant.
- Perfect for plated through-hole (PTH) the mounting scheme.
Cons:
- Hardly can be reworked or repaired.
- It is one of the most expensive methods.
- The coating may interfere with electrical signal leading to signal losses.
- Sometimes lead to the “black pad syndrome” phenomenon which is the buildup of phosphorus in between gold and nickel layers. It may result in a faulty board connection.
- A complex process to execute.
Application:
- High-density PCBs.
- Rigid PCBs.
- High-performance PCBs.
- High-frequency PCBs.
- Flex PCBs.
- Aerospace, Broadcast, Medical, Chemical, and Automotive industries.
Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
ENEPIG is a modified, from a certain standpoint, version of ENIG plating. It has grown in its popularity over the past years. This coating comprises three metallic layers — Gold, Palladium, and Nickel applied over flat or uneven surfaces of copper.
The deposition is done through the chemical process — 0.03μm inches of Gold plating is applied over 0.05-0.3μm of Palladium applied over 3-6 μm of Nickel over copper pads. The added layer of Nickel additionally helps to prevent both the corrosion and the black pad concerns from occurring.
See what ENEPIG stackup looks like in the image below.
Pros:
- Nearly no risk of black pad occurrence due to the layer of Palladium.
- Wire bondable method.
- Protects surface from oxidation, making a PCB almost invulnerable to corrosion.
- Ensures excellent solderability.
- Belongs to Lead-Free HASL surface finishes.
- Ensures nearly the year of shelf life.
- Ensures a flat, smooth surface.
Cons:
- It’s even more expensive than ENIG
- Not that reliable in the matter of gold wire bonding.
- Can be somewhat reworked but with limitations.
Application:
- Multi-layer PCBs.
- High-density PCBs.
- High-performance PCBs.
- Mixed Package technologies.
- Military, Aerospace, Medical, Chemical, Consumer Electronics, and Automotive industry
Hot Air Solder Leveling (HASL)
HASL is probably, the predominant surface finish method. From the technical perspective, it’s immersion tin. But, the excess of tin/lead ally further is removed with hot-air knives. You may visualize it as blowing the hot air across a board’s surfaces.
The interesting thing about the process is that exposure to the air heated to 265°C instantly identifies weaknesses of the lamination. Besides, it can spot poor solderability manifested in solder bridging and other issues.
Pros:
- It’s a cost-effective type of surface finish.
- It is widely available.
- It can be fairly easily reworked and repaired.
- Ensures good shelf life.
Cons:
- It’s not an organic surface finish since it contains lead.
- Not the best choice for fine pitch components.
- Leaves uneven surfaces normally.
- Does not provide excellent protection of copper from oxidation.
- Reduced possibility of PTH surface mounts.
- Lead is toxic to the human body so it conceives some cosmetic concerns.
Application:
- Multi-layer PCBs.
- Rigid PCBs.
- Consumer Electronics and Automotive industries.
Lead-free HASL
This process is much similar to HASL, but it does not involve using tin-lead molten solder. Instead, it features Tin-Copper, Tin-Nickel, or Tin-Copper-Nickel Germanium liquid solder mask. The remaining process is exactly the same and requires blowing excess solder with hot air knives.
Pros:
- It is safe for the human body.
- Has a higher melting point than lead HASL, ensuring the high strength of solder joints.
- It’s a cost-effective type of surface finish.
- It is widely used.
- Can still be easily reworked and repaired.
- Ensures long shelf life.
Cons:
- The activity of tin wire is reduced, so it’s harder to handle than lead wire.
- May lead to solder bridging that is unacceptable as per industry standards.
- Does not provide excellent protection of copper from oxidation.
- Reduced possibility of PTH surface mounts.
- Poor wetting.
- May be prone to thermal shock.
Application:
- Multi-layer PCBs.
- Rigid PCBs.
- Mil-spec.
- Chemical and Consumer Electronics industries.
Immersion Tin
Technically, it’s simply the application of a thin layer of tin over the copper layer surfaces. It’s another lead-free finish that constantly leads to sufficient results. The obtained thin layer is mostly white in color. Like other types of coating, immersion tin is applied through the electroless chemical bath.
The notable limitation of the process is the immersion of copper into tin and vice versa, which somewhat reduces the quality of soldering.
Pros:
- It’s a lead-free process.
- It’s a cost-effective type of surface finish.
- It a substitute for reflowed solder
- Can still be reworked and repaired.
- Ensures long shelf life.
- Produces flat surface.
Cons:
- Leads to tin whiskers.
- Reduced possibility of PTH surface mounts.
- Can damage solder mask.
- Causes difficulty handling.
- Interfere with some assembly processes.
Application:
- Multi-layer PCBs.
- Rigid PCBs.
- Copper Pad Applications.
- Chemical, Automotive, and Consumer Electronics industries.
Organic Solderability Preservative (OSP)
It’s a unique type of PCB surface finish method intended to produce thin, uniform, protective layers on copper circuitry. OSP surface finishes extremely well protects the copper from oxidation during storage and assembly due to the organometallic layer. Yet, the window between assembly stages and fabrication should be short because of difficulty handling.
Its working principle is cleaning the PCB surface, typography magnification, acid rinse, rinsing with OSP solution, and rinsing with DI solution.
Pros:
- Is a water-based organic surface finish
- Mid-range cost finish method.
- Can be reworked multiple times.
- Flat surface.
- Environment-friendly method.
Cons:
- Does not withstand many thermal cycles.
- Requires nitrogen oxide infused SMT.
- Require special packaging because of the difficulty handling.
- Shows a significant difference in thickness from board to board.
- Can Cause ICT Issues.
- Not perfect for PTH.
- Industry experience shows that the method requires changing the assembly process.
Application:
- High-density PCBs.
- Rigid PCBs.
- Multi-layer PCBs.
- Copper Pad Applications.
- Chemical, Aerospace, Automotive, Medical, Chemical, and Consumer Electronics industries.
Hard Gold
Gold or hard gold coating is a plating of gold over a barrier coat of Nickel. Such coating is an expensive type of surface finish yet durable. Generally, it’s applied in a way immersion silver or immersion gold does.
The gold thickness may be controlled during the processing window. The minimal values are 30 μ inches gold over 100 μ inches nickel for Classes 1 and 2. 50 μ inches gold over 100 μ inches nickel for Class 3.
See what a hard gold stackup looks like in the image below.
Pros:
- Durable surface.
- High-reliability products.
- The electrical properties of PCBs are not altered.
- Long shelf life.
- It’s a thin board finish.
Cons:
- Extremely expensive process.
- Labor-intensive process.
- Demarcation.
- Not Solderable Above 17 μ inches of thickness.
- It cannot intersect with other surface finishes.
Application:
- Rigid PCBs.
- Multi-layer PCBs.
- High-performance PCBs.
- Chemical, Aerospace, Automotive, Medical, and Chemical industries.
Soft Bondable Gold
You may visualize this immersion finish as simply the hard gold coating, but with the usage of higher gold purity (99.9%). Soft gold finish serves for applications that require ire bonding, excellent solderability, and weldability.
Pros:
- Withstand multiple thermal excursions.
- Long shelf life.
- Durable surface.
Cons:
- The most expensive of all the types of finish.
- Does not fully cover the sidewalls of the trace.
- Involves more processing.
- Plating required.
Application:
- Rigid PCBs.
- Multi-layer PCBs.
- High-performance PCBs.
- Premium Consumer Electronics, Military, Aerospace, and Automotive industries.
Immersion Silver
Immersion Ag is one of the most frequently used immersion finishes. It ensures RoHS compliant process that complies with lead-free requirements. Silver surface finish is intended to replace Immersion Gold over Electroless Nickel with a consistent coating suitable for many applications.
The average surface thickness of the silver coating is about 0.12 to 0.40 μm. This non-electric board surface coating is way safer and environment-friendly than its alternatives.
Pros:
- Cost-effective surface finish.
- Highly stable.
- Lead-based surface finish.
- Can be reworked multiple times.
- Corresponds to flat surface requirements.
- High operating window.
- Excellent electrical properties.
- Simple processing.
Cons:
- Extremely sensitive to handling and contaminants.
- Stick storage requirements.
- Difficulty electric testing of such PCBs.
- Subject to tarnishing.
- Prone to electromigration.
Application:
- Rigid PCBs.
- Multi-layer PCBs.
- High-performance PCBs.
- High-frequency PCBs.
- Flex PCBs.
- Premium and Regular Consumer Electronics, Military, Aerospace, Chemical, Medical, and Automotive industries.
How to Choose Surface Finish for Your PCB Design?
You should get the idea of how to choose a suitable PCB surface finish for your purposes. The most obvious difference between the two methods, like, for example, between immersion silver and immersion tin, is the scope of application and your design limitations.
The other important factors to consider are the following:
- Lead-free solder production. If you require RoHS compliant (lead-free) surface finish, then your selection is already limited.
- Sensitivity of handling. Sometimes it’s just impossible to approach boards in the right way.
- Wire bondable property. Probably, you need PCBs to form a good wire-bound connection.
- Pitch tightness. It should be considered when there is a need for right pitch components such as BGAs.
- Shelf life duration. Should you need 6-month storage of as long as 12 months.
- Cost. The cheapest option is, normally, HASL, with the bondable gold being the most expensive one.
Keep in mind these attributes while selecting the perfect-fit surface treatment processes.
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