7 Common Processes for PCB Surface Treatment

With the continuous advancement of science and technology, the quality control of the internal PCB circuit board has become more and more stringent, and the development and upgrading of PCB surface treatment technology has become more and more urgent. The editor will introduce to you the common PCB surface treatment technology on the market.

Why is surface treatment needed?

Since the copper layer on the PCB is easily oxidized, the resulting copper oxide layer will seriously reduce the welding quality, thereby reducing the reliability and effectiveness of the final product. In order to avoid this situation, the PCB needs to be surface treated. Some materials will talk about surface forming, and the board factory will make a distinction in the process. It is believed that the outermost layer of the PCB and on the copper plays the role of copper "coating".

What is surface treatment technology?

PCB surface treatment technology refers to the process method of artificially forming a surface layer with different mechanical, physical and chemical properties from the substrate on the PCB components and electrical connection points.

Historical evolution of common surface treatment methods

The surface treatment process will use flux to solder components on the copper surface from the beginning. When the product needs to be in large quantities, electroplating tin-lead will be used. When the PCB is coated with solder mask, hot air solder leveling (HASL) begins to appear. As product wiring density increases, organic solderability protective film (OSP) appears. Due to the need for clean assembly operations, ENIG process is widely used. Later, environmentally friendly lead-free

requirements appeared, and chemical immersion silver, chemical immersion tin, chemical nickel palladium gold, chemical nickel silver (ENIAg), etc. began to appear.

There are two main types of surface treatment processes: metallic and organic.

1. HASL, ENIG / ENEPIG, immersion gold and immersion tin are all metal surface molding,

2. OSP belongs to organic surface molding.

When choosing a PCB surface treatment, several key factors must be considered to ensure the final product meets performance, reliability, and cost requirements. Here’s a concise overview of common surface treatments and how to select the right one for your needs.

Common PCB Surface Treatments

1. Organic Solderability Preservative (OSP)
   OSP forms a thin organic film on copper to prevent oxidation. It’s RoHS-compliant and cost-effective, but the film is fragile. It works well for single reflow processes but struggles with multiple soldering steps. The main drawback is that thick films can hinder soldering, and there’s no easy way to check if the copper is fully protected. OSP is not suitable for wave soldering.

2. Hot Air Solder Levelling (HASL)
   HASL coats the PCB with a layer of solder (either leaded or lead-free). Leaded HASL is cheaper and easier to work with but contains toxic lead. Lead-free HASL is环保 (environmentally friendly) but has a higher melting point and may create a less smooth surface. HASL is durable and suitable for most soldering methods, but its uneven surface can be an issue for fine-pitch components.

3. Electroless Nickel Immersion Gold (ENIG)
   ENIG deposits a nickel layer followed by a thin gold layer. The nickel provides a stable base for soldering, while the gold prevents oxidation. It offers excellent flatness and is ideal for fine-pitch components and wire bonding. However, it’s more expensive and can face “black pad” issues if the nickel layer oxidizes over time.

4. Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
   Similar to ENIG but with a palladium layer between nickel and gold. The palladium reduces the risk of nickel corrosion during the gold immersion process, improving reliability for multiple reflows. ENEPIG is often used in high-reliability applications like semiconductor packaging but comes at a higher cost.

5. Immersion Silver (IMS)
   Immersion silver deposits a thin silver layer on copper via a chemical reaction. It offers good solderability and flatness at a lower cost than ENIG. However, silver can tarnish over time, and the lack of a nickel barrier means it’s less durable in harsh environments. It’s suitable for short-term storage and moderate-reliability applications.

6. Immersion Tin (IMT)
   Immersion tin provides a smooth, solderable surface and is lead-free. It’s a good alternative to HASL for simple boards but is sensitive to strong acids and alkalis. Tin can also form whiskers over time, which may cause electrical shorts in long-term use.

7. Mixed Surface Treatments
   Combining treatments (e.g., ENIG + OSP or HASL + gold fingers) allows designers to balance cost and performance. For example, using HASL for most of the board and gold plating for connectors or contact points ensures durability where needed.

How to Choose PCB Surface Finish?

1. Pad Flatness
   Flatness is critical for fine-pitch components or high-density boards. ENIG, ENEPIG, and OSP offer the most uniform and smooth surfaces. HASL, with its uneven solder layer, is less suitable for precise applications.

2. Solderability and Wetting
   HASL provides excellent wetting for soldering, making it ideal for simple, high-volume boards. OSP requires careful control of film thickness to ensure solderability, while ENIG/ENEPIG offer consistent results but may require more precise soldering temperatures.

3. Wire Bonding Needs
   If your PCB requires金线 (gold wire) or铝线 (aluminum wire) bonding (e.g., for semiconductor packages), ENIG and ENEPIG are the primary choices. Their nickel-gold layers provide a stable surface for bonding.

4. Storage Conditions
   OSP and immersion silver are sensitive to moisture and oxidation, so they require dry, controlled storage and should be used quickly after production. ENIG, ENEPIG, and HASL are more durable for long-term storage or harsh environments.

5. Number of Reflows
   For boards that need multiple reflow cycles (e.g., during prototyping or complex assembly), ENIG and ENEPIG are preferred because their layers withstand heat better. OSP and immersion tin may degrade after repeated heating.

6. RoHS Compliance
   Avoid leaded HASL for RoHS-compliant projects. Opt for lead-free HASL, OSP, ENIG, ENEPIG, immersion silver, or immersion tin instead.

My Experience and Recommendations

In my work, I’ve found that OSP is a go-to for low-cost, single-reflow projects, like simple consumer electronics. For example, I once designed a basic IoT sensor board where cost was a priority, and OSP worked perfectly for the single assembly step. However, for a medical device that needed to withstand frequent sterilization and had fine-pitch components, ENIG was worth the extra cost for its durability and flatness.

For prototyping, I often use lead-free HASL because it’s reliable and widely available, even if the surface isn’t perfectly smooth. When working on high-reliability projects, like automotive electronics, ENEPIG’s resistance to corrosion and multiple reflows gives me confidence in the long-term performance.

Conclusion

Choosing the right PCB surface treatment boils down to balancing your project’s specific needs: cost, reliability, soldering requirements, and environmental factors. Start by listing your priorities (e.g., “must be RoHS-compliant” or “needs to support fine-pitch components”) and then match them to the strengths of each treatment. Don’t hesitate to consult with manufacturers, as they can offer insights into process compatibility and potential pitfalls. With careful consideration, you can ensure your PCB performs well now and into the future.