What are the different types of PCB plating?

Your PCB's performance and lifespan heavily depend on its plating. Choosing the wrong type can lead to frustrating field failures. Getting it right from the start is essential for a robust product.
PCB plating involves depositing a specific metal layer onto the board's copper surfaces, including traces, pads, and through-holes. Common types are HASL, Immersion Tin, Immersion Silver, ENIG, ENEPIG, and Hard Gold, each offering distinct advantages for conductivity, solderability, corrosion resistance, and cost.

Understanding these plating options is more than just a checkbox in your design process; it's about making informed decisions that impact your product's final quality and reliability. As an engineer, I've seen firsthand how a seemingly small detail like plating choice can make or break a project. Let's dive deeper into what these types mean for your designs.

What is PCB plating?

Are your bare copper traces oxidizing and causing solderability nightmares? This exposure leads to poor connections and eventual board failure. PCB plating applies a protective metallic layer, ensuring long-term reliability.
PCB plating is an electrochemical or chemical process that deposits a thin metal layer onto a PCB's copper features. This crucial step enhances solderability, protects the underlying copper from corrosion, improves electrical conductivity, and can increase wear resistance for contact surfaces.

Understanding the PCB Plating Essentials

PCB plating isn't just a superficial coating; it's a fundamental manufacturing step that ensures your printed circuit board functions correctly and lasts as intended. The primary purposes of plating are:

• Protection: Shielding the base copper from oxidation and environmental contaminants. Copper oxidizes quickly when exposed to air, and this oxide layer is not solderable.
• Solderability: Providing a clean, wettable surface for component attachment during assembly. A good plating ensures strong solder joints.
• Conductivity: Enhancing electrical performance, especially for high-frequency signals or specific contact resistances.
• Contact Surface: Creating durable surfaces for connectors, switches, or test points that experience repeated mechanical contact.

There are two main methods used to apply these metallic layers:

1. Electrolytic Plating¹: This method requires an electrical current to deposit metal ions from a solution onto the PCB surface. It's often used for thicker coatings, such as those needed for through-hole plating (PTH) where copper is plated into the drilled holes to connect different layers, or for hard gold on edge connectors.
2. Electroless Plating²: This process uses a chemical reaction (autocatalytic deposition) to deposit metal without an external electrical current. It's excellent for achieving uniform coating thickness, even on complex geometries. ENIG (Electroless Nickel Immersion Gold) is a prime example.

I remember on the Tuxedo Keypad project at Honeywell, we had to be meticulous about the plating for the button contacts. Given the product's use in various global environments, from humid coastal regions to dry inland areas, the plating had to offer excellent corrosion resistance³ and withstand thousands of actuations. We evaluated several options before settling on one that balanced cost and long-term reliability.

What material is used in PCB plating?

Overwhelmed by the various plating materials available for your PCBs? Each material has specific pros and cons. Choosing incorrectly can lead to compromised performance or even project delays.
Common materials for PCB plating include tin, lead (historically, now restricted), gold, nickel, silver, copper, and palladium. The selection depends heavily on the application's demands for solderability, shelf life, cost, fine-pitch compatibility, and whether specialized processes like wire bonding are needed.

Common Materials for PCB Plating

The choice of material for PCB plating is critical and directly impacts the board's functionality, reliability, and cost. Here's a breakdown of commonly used materials:

• Copper (Cu): This is the fundamental conductive material of the PCB traces themselves. It's also used for plating the barrels of through-holes (Plated Through Holes - PTH⁴) to create electrical connections between layers. The initial copper layer is what subsequent plating materials aim to protect and enhance.
• Tin (Sn): Tin is widely used due to its excellent solderability and good corrosion resistance, plus it's relatively inexpensive. It can be applied through Hot Air Solder Leveling (HASL) or as an immersion coating (Immersion Tin - ImSn).
• Lead (Pb): Historically, tin was often alloyed with lead (Sn-Pb HASL) because it improved solder flow and lowered the melting point. However, due to RoHS (Restriction of Hazardous Substances)⁵ directives, lead usage is now heavily restricted in most electronics.
• Nickel (Ni): Nickel typically serves as a barrier layer, usually between the copper and a final gold layer, as seen in ENIG and ENEPIG. It prevents copper migration into the gold, which can degrade solderability. A typical electroless nickel thickness, according to IPC-4552A (for ENIG), is 3 to 6 micrometers (µm) or 120 to 240 microinches (µin).
• Gold (Au): Gold offers superb electrical conductivity, excellent corrosion resistance, and is ideal for contact surfaces and wire bonding. Its thickness varies significantly:
  • Immersion Gold (as in ENIG/ENEPIG): This is a very thin layer, typically 0.05 to 0.125 µm (2 to 5 µin) as per IPC-4552A and IPC-4556. It's primarily for solderability and protection.
  • Hard Gold (Electrolytic Gold): Used for edge connectors and keypads that require high wear resistance. This is a much thicker layer, often ranging from 0.76 to 1.27 µm (30 to 50 µin), and can be even thicker for specific applications.
• Silver (Ag): Immersion Silver (ImAg) provides good conductivity and solderability at a lower cost than gold. However, it can be prone to tarnishing if not handled or stored correctly. IPC-4553A specifies typical thickness for ImAg as 0.12 to 0.40 µm (5 to 15 µin).
• Palladium (Pd): Palladium is used as a diffusion barrier and protective layer in finishes like ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold⁶). It sits between the nickel and gold, offering enhanced protection against nickel corrosion and improving wire bonding reliability. Typical palladium thickness in ENEPIG is 0.05 to 0.15 µm (2 to 6 µin) according to IPC-4556.

The move towards RoHS compliance has significantly influenced material choices, pushing lead-free alternatives to the forefront. At Smiths Medical, when designing components for next-generation infusion pumps, material biocompatibility and the absolute reliability of sensor connections were paramount. ENIG was often our go-to for its flat surface, good solderability, and reliable barrier properties, ensuring consistent performance in critical medical applications.

What is PCB edge plating?

Do your PCB designs require robust connections or shielding right at the board's edge? Standard surface finishes often leave edges exposed. PCB edge plating offers a continuous conductive path.
PCB edge plating, sometimes called castellated plating or border plating, is the process of metallizing the edges of a printed circuit board. This creates a conductive or protective layer that wraps around the board's periphery, used for EMI shielding, edge soldering, or improved grounding.

PCB Edge Plating: A Detailed Overview

Edge plating extends the surface plating over the sides of the PCB, creating a continuous metallic connection from the top and bottom surfaces along the edge. This isn't a standard feature on all PCBs and requires specific design considerations and manufacturing processes.
How it's done:
The process typically involves milling or V-scoring the board edges to create a defined profile before the main plating process (like ENIG or hard gold) that will cover these edges. Careful preparation is crucial to ensure good adhesion and a smooth, continuous plated surface.
Applications of Edge Plating:

• Board-to-Board Connections: Allows PCBs to be soldered directly to each other at right angles or as daughter cards plugging into a motherboard, often seen in module designs.
• Improved Grounding: Provides a continuous ground path along the edge, which can be beneficial for signal integrity and reducing noise.
• EMI/EMC Shielding⁷: A plated edge can help to enclose the circuitry, improving electromagnetic interference (EMI) and electromagnetic compatibility (EMC) performance, especially when the PCB is mounted in a metallic enclosure making contact with the plated edge.
• Increased Current Carrying Capacity: For certain high-power applications, plated edges can contribute to distributing current.
• Mechanical Protection and Housing Integration: Can provide a more robust edge and facilitate better contact when the PCB slides into a chassis or housing.
  Design Considerations:
• Clearance: Internal copper features (planes or traces) must have adequate pullback from the edge to be plated to prevent shorting. A typical recommendation is at least 0.5mm, but this varies by manufacturer.
• Board Thickness: While possible on various thicknesses, manufacturers often specify a minimum, perhaps around 0.8mm, for reliable edge plating, as very thin boards can be challenging to process.
• Plating Material: Usually, the edge is plated with the same finish as the surface pads, such as ENIG or hard gold over nickel.
• Manufacturing Communication: It's vital to clearly specify edge plating requirements in your fabrication notes, including which edges are to be plated and any specific preparation needed.

I recall an aerospace project where we were designing a compact sensor module. To save space and ensure a rugged connection to the main system, we opted for edge plating. This allowed the module to slot directly into a connector on the parent board, providing both mechanical support and electrical connections. The plated edges also contributed to the module's overall EMI shielding, which was a critical requirement.

What is the cheapest material for plating in PCB?

Are you trying to minimize PCB costs without sacrificing essential functionality? The plating material is a significant factor. Knowing the most budget-friendly options can make a big difference.
The traditionally cheapest PCB plating was Tin-Lead HASL. With RoHS restrictions, lead-free HASL (using tin-copper or tin-silver-copper alloys) and Immersion Tin are now common low-cost alternatives. OSP is even cheaper but isn't a metallic plating.

Finding the Most Cost-Effective Plating Solutions

When cost is a primary driver, understanding the relative expense of different plating options is crucial. Keep in mind that "cheapest" doesn't always mean "best value" if it compromises reliability or manufacturability for your specific application.

• Organic Solderability Preservative (OSP)⁸:
  • Process: A very thin, transparent organic coating is applied to the copper pads. It's not a metallic plating.
  • Pros: Extremely low cost (often the cheapest option), provides a very flat surface ideal for fine-pitch components, environmentally friendly (water-based process).
  • Cons: Short shelf life (typically 6-12 months), sensitive to handling (can be easily contaminated), doesn't withstand multiple high-temperature reflow cycles well. Not suitable for probing or wire bonding.
  • Relative Cost: Often 0.8 to 0.9 times the cost of Lead-Free HASL.
• Hot Air Solder Leveling (HASL)⁹ - Tin-Lead (Sn-Pb) and Lead-Free (LF-HASL):
  • Process: The PCB is dipped into a bath of molten solder (tin-lead or a lead-free alloy like Sn-Cu or Sn-Ag-Cu), and then high-pressure hot air knives remove the excess solder, leveling the surface.
  • Pros: Historically very low cost (especially Sn-Pb), excellent solderability, robust finish, long shelf life (especially Sn-Pb).
  • Cons (Sn-Pb): Contains lead, so not RoHS compliant.
  • Cons (General HASL): Surface is not perfectly flat, making it less suitable for very fine-pitch components (e.g., <0.5mm pitch BGAs or QFPs). The process involves thermal stress on the PCB.
  • Relative Cost (LF-HASL): This is often used as a baseline (1.0x) for comparing other finishes. Sn-Pb HASL, where still available, might be slightly cheaper.
• Immersion Tin (ImSn):
  • Process: A thin, flat layer of tin is chemically deposited directly onto the copper.
  • Pros: Flat surface suitable for fine-pitch components, good solderability, RoHS compliant, relatively low cost compared to gold-based finishes.
  • Cons: Shorter shelf life than HASL or ENIG (prone to tin whisker formation over extended periods, though modern processes mitigate this). Can be sensitive to handling and multiple reflow cycles. The tin can diffuse into the copper over time.
  • Relative Cost: Approximately 1.1 to 1.3 times the cost of LF-HASL.

Here's a general comparison table. Note that actual costs can vary significantly based on the manufacturer, volume, board complexity, and market conditions. These are rough estimates for illustrative purposes.

During my time at Honeywell, for the Tuxedo Keypad platform, many of the internal boards that didn't have extremely fine-pitch components or demanding contact surfaces were indeed manufactured with HASL to manage costs for a high-volume product. However, as component densities increased and RoHS became standard, we increasingly shifted to lead-free options like LF-HASL or, for more critical interfaces, ENIG, balancing cost with the necessary performance and regulatory compliance.

Are PCB plating the same as PCB Surface Finish?

Are you getting tangled in PCB terminology? "Plating" and "surface finish" are often used together, but they aren't identical. This can lead to confusion when specifying board requirements.
While closely related, PCB plating and PCB surface finish aren't exactly the same. Plating refers to the deposition of a metallic layer. Surface finish is a broader term for any treatment applied to copper for solderability and protection, including metallic platings and organic coatings.

Plating vs. Surface Finish: Clearing Up the Confusion

It's common for engineers to use "plating" and "surface finish" somewhat interchangeably in casual conversation, but there's a technical distinction that's important for clear communication with manufacturers.
Think of it this way: All metallic platings applied to the surface pads are types of surface finishes, but not all surface finishes involve metallic plating.

• PCB Surface Finish: This is the general term for the coating applied to the copper pads (and other exposed copper features) on a printed circuit board. Its primary purposes are:
  1. To protect the underlying copper from oxidation before assembly.
  2. To provide a solderable surface for attaching components during the assembly process.
• PCB Plating: This specifically refers to the process of depositing a layer of metal (e.g., tin, nickel, gold, silver, palladium) onto the PCB. This can be achieved through electrolytic or electroless methods.
  Let's look at the common types of surface finishes and categorize them:
  1. Metallic Finishes (These involve plating):
     • HASL (Hot Air Solder Leveling) - Tin/Lead or Lead-Free (e.g., Sn/Cu, Sn/Ag/Cu)
     • Immersion Tin (ImSn)
     • Immersion Silver (ImAg)
     • ENIG (Electroless Nickel Immersion Gold)
     • ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)
     • Electrolytic Nickel/Gold (often called "Hard Gold" for edge connectors or "Soft Gold" for wire bonding pads)
     • Electrolytic Tin
  2. Organic Finishes (These do NOT involve metallic plating):
     • OSP (Organic Solderability Preservative)

So, when I specify "ENIG surface finish" for a board, I am referring to a surface finish that is achieved through an electroless plating process involving nickel and then gold. If I specify "OSP surface finish," I am referring to an organic coating, not a metallic plating.
The term "plating" is also used more broadly in PCB manufacturing to describe processes like "through-hole plating" (PTH), where copper is plated into the drilled holes to create electrical vias between layers. This is distinct from the final surface finish applied to the pads, but it is indeed a plating process.
In my daily work, precision in language saves time and prevents errors. When I send out a RFQ (Request for Quotation) to a PCB manufacturer like Magellan Circuits, I clearly state "Surface Finish: ENIG, compliant with IPC-4552A." This leaves no room for ambiguity about the expected metallic coating on the pads.

What are the differences between PCB plating and PCB Surface Finish?

Still finding the distinction between plating and surface finish a bit hazy? This difference is crucial for precise design specifications. Misunderstanding it can lead to incorrect board manufacturing.
PCB plating specifically means depositing a metallic layer (like gold or tin). PCB surface finish is a wider term for all copper protection and solderability coatings, including metallic platings (ENIG, HASL) and organic coatings (OSP). Essentially, plating is a method to achieve certain surface finishes.

Key Differences: PCB Plating vs. PCB Surface Finish

Let's clearly lay out the differences between PCB plating and PCB surface finish to ensure we're on the same page, especially when communicating with fabrication houses.

1. Definition & Scope:
   • PCB Plating: Refers to the actual process of depositing a metallic layer onto a substrate. It’s about the metal and the method of application (e.g., electrolytic gold plating, electroless nickel plating). This also includes processes like plating copper into through-holes (PTH).
   • PCB Surface Finish: This is a broader, more encompassing term. It describes the final condition or treatment of the exposed copper on the PCB, primarily on the pads where components will be soldered. Its main goals are to prevent oxidation of the copper and to ensure good solderability.
2. What they Include:
   • PCB Plating: Is a type of surface finish if the plating is applied to the surface pads. For example, ENIG involves plating nickel and then gold.
   • PCB Surface Finish: Includes various metallic platings as options (like ENIG, HASL, ImAg, ImSn, ENEPIG, Hard Gold). It also includes non-metallic options, most notably OSP (Organic Solderability Preservative).
3. Primary Purpose (in context of pads):
   • PCB Plating (as a surface finish): Provides a solderable metallic surface, protects copper, and can offer additional benefits like wear resistance (hard gold) or act as a barrier layer (nickel in ENIG/ENEPIG).
   • PCB Surface Finish: The overarching goal is to ensure the board is manufacturable (i.e., components can be reliably soldered to it) and that the copper is protected until assembly.
4. Examples to Illustrate:
   • OSP: This is a surface finish, but it is not a plating. It's an organic chemical coating.
   • ENIG: This is a surface finish that is achieved by specific plating processes (electroless nickel plating followed by immersion gold plating).
   • Through-Hole Plating (PTH): This is definitively a plating process (copper plating in the hole barrels), essential for layer interconnection. While vital, it's usually discussed separately from the "surface finish" on the pads, though the plated through-holes will also be covered by the chosen surface finish on their annular rings.

Here's a simple table summarizing the key distinctions:

When I'm working on a design at Magellan Circuits, if a client asks for "gold plating," I'll always clarify: "Are you looking for ENIG for general solderability, ENEPIG for enhanced reliability and wire bonding, or Hard Gold for an edge connector?" Specifying the "surface finish" type (e.g., "Surface Finish: ENIG per IPC-4552A") is the most unambiguous way to ensure the manufacturer produces exactly what's needed for the application.

Conclusion

Selecting the appropriate PCB plating or surface finish is crucial. It impacts your board's performance, long-term reliability, and overall project cost.