A printed circuit board is a complex assembly of various components, working harmoniously to create a functional unit. Among these, the solder mask serves as a protective shield, while the copper layer acts as the conductor – both pivotal elements in constructing a PCB. However, a facet often overlooked aspect – Surface Finish. Whether to employ a finish in the manufacturing process rests on the engineer’s discernment, hinging on factors such as enhancing solderability, safeguarding against oxidation, and more. In this TechSparks article, we unravel the enigma surrounding PCB surface finishes, delving into the decision-making process and exploring the range of options available.
What is Surface finish in PCB and What is its Purpose
Surface finish or surface treatment pertains to the application of a coating, overlay, or intermetallic compound layer onto a circuit board’s surface. This results in a reflective portion under sunlight. However, while aesthetics play a role, they are among the least significant aspects of its multifaceted functions. Instead, it confers substantial practical benefits. Firstly, it serves as a protective shield against the onset of oxidation defects due to copper layer exposure, thus elongating the board’s lifespan. Secondly, it furnishes a smooth soldering surface that aids in component assembly during soldering processes, mitigating the risk of detachment. These two points encapsulate the general functions of PCB surface finish. Furthermore, diverse processes can bestow enhancements in electrical performance, corrosion protection, shielding effectiveness, and other functional attributes.
How Many Types of PCB Surface Finishes
Organic Solderability Preservative (OSP)
OSP finish utilizing organic compounds as a processing method to shield circuits against oxidation is recognized as a more environmentally friendly solution and is widely endorsed by international bodies such as IEC and SEMI. Its popularity stems from its cost-effectiveness, ease of processing, and suitability for fine-pitch components. However, it’s crucial to acknowledge that its protective period is limited. The water-based nature of organic compounds can lead to degradation under high temperatures, rendering them unsuitable for PCB necessitating multiple reflow or wave soldering cycles.
Electroless Nickel Immersion Gold (ENIG)
ENIG offers exceptional surface flatness for circuit boards through a dual-layer coating technology. This method involves a nickel layer acting as a copper barrier and a gold layer safeguarding the nickel from oxidation. ENIG is particularly well-suited for lead bonding and excels in demanding environments due to its use of metallic raw materials. However, it’s important to note that ENIG is less cost-effective when compared to coatings like spray tin. The electroplating process involved carries a potential risk of black pads on PCB, making careful execution crucial.
Hot Air Solder Leveling (HASL)
A conventional PCB surface treatment process highly valued for its exceptional wetting properties, solid electrical conductivity, extended shelf life, and economical processing costs. Despite its traditional nature, it remains widely utilized in contemporary times. With the adoption of the RoHS directive within the electronics manufacturing industry, to align with environmental protection mandates, this process has evolved into HASL-LF. Within this evolved method, a layer of solder is applied to the board’s surface, followed by the passage of hot air to achieve surface uniformity and a robust solderable surface.
Although manufacturers have honed their proficiency in HASL, there remains the potential for uneven surfaces, which could impact assembly accuracy. Furthermore, its performance is subpar with BGAs and SMDs featuring pitches less than 20 microns, thereby limiting its application in high-density circuit boards.
Electroless Nickel Electroless Palladium Immersion Gold(ENEPIG)
From its name, it’s evident that ENEPIG bears resemblance to ENIG. Featuring a three-layer coating structure, it builds upon ENIG by incorporating electroless palladium plating between the nickel and gold layers. This palladium layer acts as a barrier to prevent gold diffusion into the nickel layer. While inheriting all the merits of ENIG, ENEPIG effectively addresses the issue of black pads. However, its drawbacks are quite conspicuous – notably high costs and a complex, multi-step process.
Immersion Silver (ImAg)
Immersion silver surface finish involves depositing a thin layer of silver on the PCB surface to serve as a protective barrier. Silver’s conductivity enhances circuit electrical performance, making it suitable for high-frequency applications. It comes at a reasonable price due to its raw material, and it maintains its quality for over 12 months. However, given silver’s tendency for instability as a metal, it can undergo oxidation and even develop silver whiskers over time. Consequently, it’s not recommended for use in intricate environments.
Immersion Tin (ImSn)
Similar to other treatments, immersion tin involves depositing a thin layer of tin atop the copper layer of the PCB. This technique ensures that the board maintains good solderability even through multiple thermal cycles, making it a lead-free surface finish compliant with RoHS standards. However, it’s important to note that tin is less robust than certain metals, making it more sensitive and prone to damage. Consequently, cautious consideration is advised when applying it in contexts involving severe physical shocks, such as automotive or contact switch applications.
Comparison of PCB Surface Finishes
So far, TechSparks has explained what PCB surface finish is and introduced several types of it. Now, as an electronics manufacturer, how do you make the right decisions when undertaking a project?
- Price: Due to project variations, you can choose based on your actual situation. For example, cost-sensitive clients might opt for cost-effective options like HASL or OSP. High-end electronics requiring quality can go for ENEPIG and ImAg, even if they come with higher costs that the project’s profit can cover.
- Assembly: Consider the project as a whole, not just the bare board. Check if your design involves fine-pitched components, multiple soldering processes, and compatibility with the materials being used.
- Industry Attributes: Look into any special attributes of your industry. For instance, the medical sector has bans on toxic substances, the automotive industry requires robust reliability, and IoT demands signal stability.
Apart from these, there’s more to consider. To facilitate your project’s progress, the table below compiles a comparison of six surface finishes in various attribute aspects.
PS: Smaller numbers indicate better performance in that attribute.
| Surface Treatment | Cost | Conductivity | Durability | Complexity | Shelf Life | Solderability |
|---|---|---|---|---|---|---|
| OSP | 2 | 6 | 5 | 2 | 5 | 5 |
| ENIG | 5 | 2 | 2 | 4 | 2 | 3 |
| HASL | 1 | 5 | 6 | 1 | 6 | 6 |
| ENEPIG | 6 | 3 | 1 | 6 | 1 | 4 |
| ImAg | 4 | 1 | 3 | 5 | 3 | 2 |
| ImSn | 3 | 4 | 4 | 3 | 4 | 1 |
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