PCB Surface Finish
PCB surface finish is necessary in PCB manufacturing, which acts as a protective covering on the exposed copper on a PCB and a soldering surface to which components are soldered. It directly affects the solderability, reliability and shelf life of your boards. At MOKOPCB, we provide different RoHS compliant surface finish options like HASL, ENIG, OSP, immersion silver, etc. to cater to various applications.
PCB Surface Finish: What It Is and Why It Matters
PCB surface finish is a thin coating applied on the exposed copper areas of printed circuit board. Bare copper is extremely prone to oxidation, and the surface finish protects the copper against environmental damage and also enables components to be soldered reliably to the board in the assembly process.
It forms the critical interface between the PCB and mounted components, and it influences the performance and longevity of the final product. A good surface finish provides high solderability, strengthens intermetallic bonding, and increases the shelf life of the PCBs.
7 Popular PCB Surface Finishes Types
1. HASL/Lead-free HASL
In PCB manufacturing, Hot Air Solder Leveling (HASL) is a widely used surface finish. Throughout this procedure, the PCB is covered with molten solder, and the excess solder is removed by using selective hot air knives, and hence a layer of solder coating is formed to protect the exposed copper. There are two types of HASL finishes namely, traditional HASL and lead-free HASL finish. The former is made of tin-lead alloy and the latter one is made of RoHS compliant solder alloys.
Pros:
- Cost-effective
- Suitable for most components
- Good protection against copper oxidation
- Compatible with standard assembly processes
Cons:
- Uneven surface
- Traditional HASL finish is not RoHS-compliant
- Risk of solder bridging
- Not ideal for high-density PCBs
2.Immersion Silver
Immersion silver is a non-lead surface finish whereby a thin film of silver is deposited on the copper traces of a PCB by an electroless chemical reaction. This helps keep the copper free of oxidation as well as providing good solderability.
Pros:
- Excellent flatness, ideal for SMT and BGA components
- Superior electrical performance
- RoHS compliant
Cons:
- Prone to tarnishing and creep corrosion
- Shorter shelf life compared to ENIG PCB finish
- Requires careful storage and environmental control
3. Immersion Tin
Immersion Tin (ISn) is a metallic coating applied by means of an electroless displacement process directly onto the copper substrate of printed circuit boards. This coating layer works as a protective layer to prevent oxidation of copper during storage periods.
Pros:
- Smooth & flat finish
- Works well with press-fit
- More affordable than ENIG or ENEPIG
Cons:
- Susceptible to tin whisker formation
- Limited shelf life
- Difficult to measure the coating thickness
4. Electroless Nickel Immersion Gold (ENIG)
The ENIG PCB finish is a commonly used surface treatment in which a nickel plating is deposited over the copper pads by an electroless chemical deposition and then a thin layer of immersion gold is coated upon it.
Pros:
- Ideal for fine-pitch and BGA components
- Excellent corrosion resistance and durability
- Come with long shelf-life
Cons:
- Not re-workable
- Higher cost than basic finishes like HASL or OSP
- Risk of black pad if the process is not well-controlled
5. ENEPIG
ENEPIG, which is also known as electroless nickel electroless palladium immersion gold, is a variation of ENIG. In this case, palladium is applied as a protective coating to prevent nickel oxidation and its diffusion to the copper surface. Although ENIG and ENEPIG are more expensive compared to other finishes, they provide excellent solderability.
Pros:
- Suitable for wire bonding &soldering
- Strong resistance to corrosion and oxidation
- Long shelf life and high reliability
Cons:
- More expensive than ENIG and other finishes
- Requires strict process control
- Mostly applied in high-end or mission-critical PCBs
6. Hard Gold
Hard gold, also known as Electrolytic Gold, is a layer of gold that is applied over a nickel coating. Unlike pure gold, hard gold is a gold alloy that contains nickel, cobalt, or iron, and is most suitable for parts where wear and tear is more likely to occur. The required thickness of this finish varies based on its application area.
Pros:
- Extremely durable and wear-resistant
- Lead-free, RoHS-compliant
- Long shelf life
Cons:
- Costly as a result of higher thickness of gold use
- It involves more processing
- Only used to a few selected areas
7. OSP
OSP (Organic Solderability Preservative) is a thin layer of film that is normally done on a conveyorized process. It applies water based organic compound, which is selectively bonds to copper and gives an organometallic coating that protects the copper before soldering.
Pros:
- Low-cost and environmentally friendly
- Flat surface for fine-pitch SMT
- Easy to rework
Cons:
- Short shelf life and poor durability in harsh environments
- Not suitable for PTH
- Limited Corrosion Protection
PCB Surface Finish Comparison
| Surface Finish | Solderability | Cost | Shelf Life | Planarity | RoHS Compliant | Durability | Corrosion Resistance |
|---|---|---|---|---|---|---|---|
| HASL (Traditional) | Good | Low | Long | Poor (uneven) | No | Good | Good |
| Lead-free HASL | Good | Low | Long | Poor (uneven) | Yes | Good | Good |
| Immersion Silver | Excellent | Medium | Short | Excellent (flat) | Yes | Poor | Poor |
| Immersion Tin | Good | Medium | Short | Excellent (flat) | Yes | Fair | Fair |
| ENIG | Excellent | High | Long | Excellent (flat) | Yes | Excellent | Excellent |
| ENEPIG | Excellent | Very High | Long | Excellent (flat) | Yes | Excellent | Excellent |
| Hard Gold | Good | Very High | Long | Good | Yes | Excellent | Excellent |
| OSP | Good | Very Low | Very Short | Excellent (flat) | Yes | Poor | Poor |
How to Choose the Right Surface Finish for Your PCB Project?
PCB surface finish selection has increasingly become an important issue due to the growing complexity of assembly processes and the need to meet regulatory requirements such as RoHS and WEEE. Selecting a PCB surface finish requires attention to these main factors:
- Operating Environment
External factors such as moisture, high temperatures or corrosive materials affect your decision. As an example, silver finishes may suffer from corrosion in highly humid conditions.
- Mechanical Stress and Shock
For applications that might be subjected to frequent drops, such as smartphones, surface finishes with a tin-copper bond would be preferred over a tin-nickel bond as the former can minimize the risk of component fracture. However, ENIG remains one of the most popular finishes as long as they are in a stable environment such as medical devices.
- Reliability Requirements
In cases where the consequences of product failure are significant (e.g., aerospace, medical), more durable finishes such as ENIG or ENEPIG are typically used, although they come at a higher cost.
- Corrosion Resistance
For harsh environments, use corrosion-resistant finishes such as ENIG and ENEPIG that offer superior protection, while OSP is not a good choice in this case.
- Component Type and Pitch
Fine-pitch or BGA components usually require very flat surfaces, and ENIG or immersion silver are ideal for these applications.
- Pad Flatness for SMT/BGA
Flatness guarantees uniform solder joints, especially in BGAs. ENIG and ENEPIG provide great coplanarity that reduces solder defects.
- Cost Considerations
OSP and HASL are cost-effective on mass-usage or consumer products whereas ENIG PCB finish is more applicable on high-reliability devices.
Contact MOKOPCB for Expert Surface Finish Solutions
Selecting the correct surface finish has a critical impact on PCB performance, reliability, and cost. MOKOPCB provides multiple surface finish solutions tailored to your project’s unique needs. Having accumulated rich experience in PCB manufacturing and assembly, we can guide you through the entire process—from surface finish selection to final production.
Have questions or not sure which finish is best for your application? Contact our team to receive a free expert consultation on your PCB needs.
Contact Us
Got any questions or inquiries? Fill out the form and we will get back to you soon
