Comparing OSP Plating with Other PCB Surface Finishes

Once a PCB passes through the fabrication process, one of the final steps is to apply a surface plating to the exposed copper on the surface layers. The exposed copper will oxidize over time if it is not treated with a surface finish, and the various surface finishes used in PCBs are used to both prevent copper degradation and provide a solderable surface.

One of these more specialized options is organic solderability preservative (OSP), which is the only organic compound-based copper surface treatment. Leveraging this surface treatment for PCBs requires correct storage and handling practices that are not implemented for metallic surface treatments. In addition to storage and handling, rework of OSP treated PCBs can be difficult without compromising the reliability of treated conductors.

Organic Solderability Preservative (OSP) plating has become a popular surface finish in PCB manufacturing due to its cost-efficiency and excellent solderability. However, despite its advantages, OSP plating presents several challenges that must be addressed to maintain PCB reliability and performance. This article explores the most common challenges associated with OSP plating and provides strategies to overcome them.

What is OSP Plating?

OSP plating involves applying a thin organic layer over the copper surface of a PCB to protect it from oxidation. OSP is a water-based organic coating applied to copper pads to protect them from oxidation before soldering. This coating is temporary and designed to preserve the solderability of copper during assembly. The coating is removed during the soldering process, leaving a clean copper surface for strong electrical connections. It is environmentally friendly due to its lead-free composition and minimal chemical impact, making it a preferred choice for RoHS-compliant manufacturing.

OSP vs Other Surface Finishes:

1. HASL:

Hot Air Solder Leveling or HASL is an affordable finishing option that utilizes tin/lead to creating a thin protective covering on a PCB. Hot air bursts are used to clear excess lead or tin from the board’s surface. Formerly the industry standard, HASL popularity has faltered due to potential RoHS compliance issues.

Some advantages include low cost, long shelf life, and HASL is reworkable. But it has an uneven surface for soldering, contains lead (Not RoHS Compliant), and cannot hold tight tolerances on plated holes.

2. Lead-Free HASL:

Non-toxic and more environmentally-friendly PCB finish types are gaining popularity due to concerns behind the use of lead in manufacturing. Pb-Free HAL finishes use tin or copper paired with nickel to create a protective coating. Pb-Free HASL has the same advantages and disadvantages as HASL except Pb-FREE HASL is RoHS compliant.

3. ENIG (Electroless Nickel Immersion Gold):

Electroless Nickel Immersion Gold is one of the most popular and widely-used circuit board finishes available today.  Constructed with two layers of coating, ENIG places 2-4 μ” Au over 120-200 μ” Ni. The gold protects the nickel from corrosion and the nickel protects the base metal board and allows for circuits to be securely soldered to its surface.

Some advantages are that it provides a flat surface to solder to, lead free and RoHS Compliant, longer shelf life, tighter tolerances can be held for plated holes. But on the other hand it is expensive, signal loss for signal integrity applications, black pad.

4. Immersion Silver(ImAg):

Immersion Silver (IAg) is applied directly to the base metal of a PCB via chemical displacement. It’s a more affordable option than ENIG, and it is also RoHS-compliant. A typical thickness for Immersion Silver is 4-12u”.  Due to the way copper and silver interact, they eventually diffuse into one another.

Some advantages are that it provides a flat surface to solder to, lead-free and RoHS compliant, tighter tolerances can be held for plated holes, low loss for signal integrity applications. but handling the PCB can cause soldering issues, it is more cost effective than ENIG but less cost effective than Immersion Tin, and the finish can tarnish and oxidize.

5. Immersion Tin:

Immersion Tin (ISn) is applied directly to the base metal of a PCB via chemical displacement. It’s a more affordable option than ENIG and Immersion Silver, and it is also RoHS-compliant. A typical thickness for Immersion Tin is 20-50u”.  Due to the way tin and copper interact, they eventually diffuse into one another. But due to soldering issues, tin whiskers, shorter shelf life than ENIG.

6. OSP:

A PCB surface finish comparison based on green appeal leaves no questions.  The Organic Solderability Preservative (OSP) doesn’t introduce any toxins into the process. Instead, an organic compound is used that bonds naturally with copper, creating an organometallic layer that protects against corrosion.

It has a flat surface for soldering, RoHS Compliant and lead-free, cost effective. But on the other hand the shortcomings are related to its short shelf life, handling the PCB can cause soldering issues, and thickness isn't measurable.

When to Use OSP Plating:

OSP is not a new coating technology, but it is less-often used than metallic finishes due to the special storage and handling procedures required before assembly. Like immersion silver coatings, which have low losses comparable to solder mask, OSP coatings offer low loss and have a shelf life that must be considered.

OSP is ideal for single- and double-sided PCBs used in consumer electronics, where cost-effectiveness and high solderability are priorities. It is less suitable for complex or multilayer PCBs requiring extended shelf life or resistance to harsh environmental conditions.

Advantages of OSP Plating:

Despite its challenges, OSP plating offers several advantages that make it a preferred choice for many PCB applications. Its environmentally friendly composition eliminates the use of harmful chemicals like lead or cyanide. Additionally, it is cost-effective, making it ideal for high-volume production. OSP also provides excellent solderability and is suitable for fine-pitch components, making it a versatile choice for modern PCB designs. Here are few points in conclusion:

• Cost-Effective: One of the most affordable surface finishes.
• Environmentally Friendly: Lead-free and non-toxic, aligning with RoHS standards.
• Excellent Solderability: Provides clean copper pads for strong, reliable solder joints.
• Planarity: Ideal for high-density PCBs and fine-pitch components.

Common Challenges in OSP Plating and How to Overcome Them:

Organic Solderability Preservative (OSP) plating is widely used in PCB manufacturing due to its cost-effectiveness and excellent solderability. However, it poses several challenges that can affect production quality if not managed carefully.

1. Sensitivity to Oxidation

OSP coatings are thin and vulnerable to oxidation when exposed to air, moisture, or contaminants. If the OSP layer is compromised, the underlying copper oxidizes, negatively affecting solderability. This challenge becomes more pronounced in high-humidity environments or during extended storage periods.

2. Limited Durability in Multi-Reflow Processes

While OSP coatings are ideal for single reflow cycles, they can degrade after multiple thermal cycles. This degradation reduces the solderability of the PCB and increases the risk of defects such as poor joint formation or voids.

3. Handling and Contamination Issues

The thin OSP coating is highly sensitive to physical handling. Fingerprints, dust, and oils can compromise the coating, leading to uneven soldering. Unlike more robust finishes like ENIG, OSP requires careful handling to prevent contamination.

4. Compatibility with Certain Fluxes

Not all flux materials work well with OSP finishes. Some fluxes fail to activate the surface properly, leading to inadequate soldering and increased defects. This makes flux selection a critical consideration for manufacturers using OSP.

Conclusion:

OSP plating is a reliable and cost-effective choice for simple PCB designs, especially in consumer electronics. While it may not offer the durability or extended shelf life of finishes like ENIG or immersion silver, its affordability and environmental benefits make it a valuable option. While it may not suit every application, OSP remains a reliable and cost-effective choice for many PCB designs, provided its limitations are well understood and addressed.

Comparing these types of surface finishes, generally speaking, in terms of cost, ImAg and OSP are the most inexpensive while ENIG is the most costly. In terms of corrosion resistance, HASL and ImSn have the best corrosion resistance capability while ImAg has the worst. In terms of ICT, only OSP is the worst while others are just similarly good. In terms of hole fill, HASL and ENIG are better than the other types.