Do you know how many types of surface treatment processes for PCBs? Let's uncover it together!

Due to copper's susceptibility to oxidation in the air, the presence of copper oxide significantly affects soldering, leading to potential issues like false soldering and inadequate bonding, which may ultimately prevent proper soldering between the PCB pads and components. To mitigate these concerns, PCB manufacturing includes a crucial step of applying a protective coating or plating on the pad surface to prevent oxidation.

Currently, PCB manufacturers use various surface treatment processes, including Hot Air Solder Leveling (HASL), Immersion Tin, Immersion Silver, Organic Soldering Preservative (OSP), Electroless Nickel Immersion Gold (ENIG), and Electroplated Gold. Additionally, specific applications may require specialized PCB surface treatments.

Each of these surface treatment processes possesses distinct advantages and disadvantages, as well as varying costs and applicability for different scenarios. Selecting the most suitable treatment involves a balance between effectiveness and cost-efficiency. It is essential to understand and leverage the strengths of each technique to optimize PCB performance.

Let's now delve into a comparative analysis of these PCB surface treatment processes, examining their pros, cons, and suitable application scenarios.

1. Bare Copper:

Pros: Low cost, smooth surface, excellent soldering capability (under non-oxidized conditions).

Cons: Vulnerable to acid and humidity, cannot be stored for extended periods after unsealing (within 2 hours) due to copper oxidation. Not suitable for double-sided PCBs, as the second side oxidizes after the first reflow soldering. Testing points require added solder paste to prevent oxidation and ensure good probe contact.

2. Hot Air Solder Leveling (HASL):

Pros: Relatively low cost, excellent soldering performance.

Cons: Unsuitable for soldering fine-pitched pins and small components due to poor surface flatness. Prone to producing solder beads during PCB processing, which may cause short circuits in fine-pitched component pins. In double-sided SMT processes, reflowing the second side can cause remelting of the HASL coating, resulting in uneven surfaces affecting soldering.

3. Organic Soldering Preservative (OSP):

Pros: Retains all advantages of bare copper soldering, and boards expired within three months can be reprocessed once.

Cons: Susceptible to acid and humidity. Secondary reflow soldering requires a specific time frame, with the second reflow usually exhibiting inferior results. If stored for more than three months, the surface treatment needs to be reapplied. After unsealing, OSP needs to be used within 24 hours. OSP acts as an insulating layer, necessitating added solder paste to remove the OSP layer for electrical testing.

4. Electroless Nickel Immersion Gold (ENIG):

Pros: Resistant to oxidation, allowing for extended storage. Provides a smooth surface, suitable for soldering fine-pitched pins and small components. Preferred for PCBs with keypads. Can undergo multiple reflow soldering processes with minimal impact on solderability. Also used as a base material for Chip On Board (COB) wire bonding.

Cons: Higher cost, lower soldering strength due to using electroless nickel, leading to potential black pad issues. Nickel layer may oxidize over time, affecting long-term reliability.

5. Immersion Silver:

Pros: Immersion silver is more cost-effective than immersion gold. It is an excellent choice when the PCB requires connection functionality and cost reduction. With good surface flatness and contact properties, immersion silver is preferred in communication products, automotive applications, and computer peripherals. It finds application in high-speed signal design and high-frequency signal applications. EMS (Electronics Manufacturing Services) recommends immersion silver for its ease of assembly and superior inspectability.

Cons: However, immersion silver may suffer from issues such as loss of luster and solder voids, resulting in slower growth (but not decline) compared to other surface treatments.

6. Electroplated Tin:

Pros: Electroplated tin emerged in surface treatment processes around a decade ago, driven by the requirements of automated production. It does not introduce any new elements during soldering and is particularly suitable for communication backplanes.

Cons: Outside the storage period of the PCB, the tin loses its solderability, necessitating proper storage conditions. Furthermore, the presence of carcinogens in the tin plating process has led to restrictions on its use.