A PCB surface finish covers between components and a bare printed circuit board. It is used for two main reasons: solderability and protection against exposing the copper circuitry. Because there are so many different types of surface finishes, choosing the proper one can be difficult. In this post, we are going to list the pros and cons of the 7 important types of PCB surface finishes, and how to choose the proper surface finish type.
#1. Hot Air Solder Leveling (HASL)
HASL is a common surface finish for business usage. Circuit boards are immersed in a molten pot containing tin or lead alloy, then the excess solder is removed with "air knives", which allow hot air across the surface of the board to flatten it. This process generates a coating layer that resists copper oxidation and improves solderability. You have to consider the following important points during this process.
- The PCB must be absorbed in melted solder.
- The wind knife is used to blow the melted solder earlier than the solder solidifies.
- The wind knife should be able to lessen the meniscus of the solder on the surface of the copper and stop the solder from bridging.
Pros of HASL
- A relatively long storage time.
- Good pad wetness and copper coverage.
- A lead-free (RoHS-compliant) variant is widely available.
- HASL is a mature technology.
- The surface finish has a low price.
- It is suitable for optical examination and electrical testing.
Cons of HASL
- It is not suitable for wire bonding.
- It has a low quality of exactness because of the natural meniscus of the molten solder.
- It is not suitable for capacitive touch switches.
- For particularly thin panels, HASL may not be suitable. The high heat of the bath may cause board warping.
#2. Organic Solderability Preservative (OSP)
Organic Solderability Preservatives, often known as Preflux, are abbreviated as OSP. It is sprayed onto the copper pads' surface to give an organic chemical-based protective coating to prevent the copper surface from rusting in a normal environment. Moreover, this layer of the film must have oxidation resistance, heat shock resistance, moisture resistance, and other properties.
Pros of OSP
- It is simple and cheap; the surface finish is just sprayed on.
- Its pad surface is extremely smooth, with a pattern similar to the ENIG.
- It is lead-free and environmentally friendly.
- It is reworkable
Cons of OSP
- There is bad weather and direct extreme weather can affect its performance.
- The clear and thin nature of the film means it is difficult to gauge quality with visual inspection and perform in-circuit testing.
- There's a short life, as well as high storage and handling requirements.
- It has poor protection of plates from holes.
#3. Immersion Silver
Silver possesses stable chemical characteristics. Even though the immersion silver-processed PCB is subjected to heat, humidity, and pollution, in any case, it can still have good electrical properties and solderability. Moreover, immersion of silver is a displacement reaction that coats the copper with a pure silver film immediately.
Pros of Immersion Silver
- It has high solderability.
- It has good surface quality.
- It is also low-cost and lead-free (RoHS compliant).
- It is suitable for aluminum wire bonding.
Cons of Immersion Silver
- It needs high storage requirements and is easily contaminated.
- The next one is its short assembly window once removed from the packaging.
- It is hard to electrically test.
#4. Immersion Tin
The tin layer can match any type of solder because all solders have a tin base. The tin layer structure takes on a granular structure after adding organic additives to the tin immersion solution, which overcomes the issues caused by tin whiskers and tin migration, as well as having good thermal stability and solderability.
Pros of Immersion Tin
- It is suitable for horizontal production lines.
- It is suitable for fine-trace processing
- It is suitable for lead-free soldering, especially for crimping technology.
- It also has very good flatness, suitable for SMT.
Cons of Immersion Tin
- Tarnishing of fingerprints can occur in high storage conditions.
- Tin whiskers can cause short and soldered joint issues, reducing shelf life.
- It also has the drawback that it is hard to electrically test.
- The process involves a carcinogen.
#5. Immersion Gold (ENIG)
Electroless Nickel Immersion Gold, or ENIG, is a popular surface treatment coating having two metallic layers. A displacement reaction coats the copper with gold atoms after nickel is put directly on it. The immersion gold method also results in outstanding surface planarity.
Pros of Immersion Gold (ENIG)
- It is suitable for lead-free (RoHS-compliant) soldering.
- It has an excellent surface.
- The surface has a long shelf life and is very durable.
- It is suitable for aluminum wire bonding.
Cons of Immersion Gold (ENIG)
- It is very expensive because it uses gold.
- This process is complex and difficult to control.
- Another drawback is that it is prone to the black pad effect.
#6. Electrolytic Nickel/Gold
"Hard gold" and "soft gold" are two types of electrolytic nickel gold. Hard gold is utilized for gold fingers, PCB contacts, and other hard-wearing locations because of its lower purity. The thickness of gold may vary depending on the requirements. Soft gold is purer and has frequent usage in wire bonding.
Pros of Electrolytic Nickel/Gold
- It has a longer shelf life.
- It can be used for contact switches and wire bonding.
- Hard gold is suitable for electric tests.
- This process is lead-free (RoHS compliant).
Cons of Electrolytic Nickel/Gold
- This is a more expensive surface finish.
- Electroplating the gold fingers requires an additional conductive trace.
- Hard gold has poor solderability. Due to the thickness of the gold, thicker layers are more difficult to solder to.
#7. Nickel Palladium (ENEPIG)
Nickel with no electrolyte ENEPIG is becoming more popular as a PCB surface treatment. ENEPIG features an additional layer of palladium between nickel and gold. It protects the nickel layer from corrosion. Moreover, it prevents the black pad that can appear with ENIG. In addition, recent palladium price drops have made ENEPIG more accessible.
Pros of Nickel Palladium (ENEPIG)
- It has all the benefits of ENIG without the black pad problems.
- This process is more suitable for wire bonding than ENIG.
- Its other advantage is that it has no corrosion risk.
- It also has a very long, very long, very long storage time.
Cons of Nickel Palladium (ENEPIG)
- It is a very complicated process.
- It is very hard to control.
- It has a very high cost.
- ENEPIG is a relatively new method and is not mature for many PCB manufacturers.
However, PCBONLINE, a leading high-tech PCB manufacturer for medical, aerospace, defense, automotive, industrial, and consumer electronics, has mature technology for ENEPIG. All the above PCB surface finishes can be done for PCB manufacturing at PCBONLINE. Besides the above-mentioned PCB surface finish types, PCBONLINE can also do the newest immersion silver/immersion gold (ISIG). The maximum hard gold plating thickness for PCBONLINE is 100u.
Choose PCBONLINE as your advanced PCB manufacturer and assembler, you will enjoy free one-on-one engineering support (DFX) and customer services. Online register and get $100 coupons for online purchases.
Which Type of PCB Finish Should I Use?
The easiest method to answer this question is to weigh the various PCB surface finishes against a set of well-defined characteristics that will almost certainly influence your decision. Here is a list of these characteristics.
- Lead-free solder - adheres to restrictions on hazardous substances (ROHS).
- Handling sensitivity - Susceptible to contamination or breakage from handling.
- Wire bondable - Capable of forming good wire-bound connections.
- Tight pitch - a good choice for tight pitch components, such as ball grid arrays (BGAs).
- Contact usage - Is it possible to use the contact for contact?
- It has a long shelf life and can be stored for six months or more.
- Additional costs are typically added to your PCB manufacturing costs.
How to Choose a PCB Surface Finish?
Choosing a surface finish for your printed circuit boards is an important step in the design process. There are other things to think about, including:
- The components you use,
- Your expected production volume,
- Your requirements for durability,
- Environmental impact,
- And cost.
Conclusion
During PCB design, the PCB surface finish is sometimes ignored. However, surface finish is a critical factor that influences PCB assembly and board reliability by shielding copper traces and strengthening solder connections. I hope the above pros and cons of the 7 PCB surface finish types may help you choose the proper surface finish for your design. If you still have concerns, welcome to contact PCBONLINE online or send an email to info@pcbonline.com for free advice.
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