Tin, HASL, ENIG, OSP - types of surface finishes on PCBs

Introduction

Choosing the appropriate surface finish for PCB pads is one of the decisions made during the design or ordering stage of a printed circuit board. This choice has implications for manufacturability and the reliability of solder joints.

The main functions of a surface finish are protecting the copper pads from oxidation during storage and transport, and ensuring adequate solderability required for wetting with solder during the soldering process. In this context, it is important to remember that the selection of a surface finish should be a conscious engineering decision and not merely a matter of cost.

This article covers the main types of PCB pad surface finishes. Enjoy the read.

HASL - Hot Air Solder Leveling

HASL (Hot Air Solder Leveling) is one of the oldest and historically most popular PCB surface finishes. The process involves coating the copper pads with flux and then immersing the board in molten solder. As the PCB is withdrawn from the solder bath, excess alloy (Solder) is leveled using jets of hot air (Hot Air). Hence the acronym HASL, sometimes referred to as "HAL".

The result of the HASL process is a layer of tin-lead or lead-free alloy deposited directly on the copper pads. The solder layer thickness from 1-40µm, typically 5-20µm.

Advantages:

• Low cost.
• Widely used process, broadly available from manufacturers.
• Shelf life of 12 months (provided the minimum required coating thickness is maintained).
• Excellent surface solderability.
• Available in lead-free (LF-HASL) and increasingly rarely in leaded versions.

Disadvantages:

• Uneven surface, making fine-pitch SMT assembly more difficult.
• Thermal stress during the process can lead to micro-cracks in some PCB designs.
• If the HASL coating thickness is too low, the progressively growing SnCu intermetallic compounds can cause a phenomenon known as dewetting

ENIG - Electroless Nickel Immersion Gold

ENIG (Electroless Nickel Immersion Gold) is a two-layer surface finish. The outer surface consists of a thin gold layer with a minimum thickness of 0.04µm [1.6µin] ±4σ from the process mean[1,2], typically 0.075-0.125µm [2.955-4.925µin]. Beneath the gold (between the gold and the copper) lies a nickel layer with a thickness of 3µm–6µm [118.1µin–236.2µin][1,2], which acts as a barrier preventing diffusion between gold and copper. The technology has gained widespread acceptance due to its flat, uniform surface, ideal for fine-pitch SMD component assembly.

The IPC-4552 standard defines requirements for Ni and Au layer thicknesses, corrosion resistance, certain process requirements, and acceptance criteria.

A challenging defect that can occasionally occur is the so-called Black Pad. This is corrosion of the nickel layer (beneath the gold), which leads to defective, mechanically weak solder joints. This phenomenon most commonly occurs during the gold deposition process and is related to process parameters at the PCB manufacturer.

Advantages:

• Flat surface. Ideal for fine-pitch, BGA, QFN, and similar component assembly.
• Shelf life of 12 months.
• Excellent solderability.

Disadvantages:

• Higher cost.
• Risk of Black Pad defects when process issues occur.

OSP - Organic Solderability Preservative

OSP (Organic Solderability Preservative) is an organic coating chemically applied to copper pads, forming a thin layer (0.1-0.6µm) that protects against oxidation. The material decomposes at soldering temperature, exposing the clean copper beneath the solder.

OSP technology is widely used in high-volume consumer electronics manufacturing due to its low cost and surface flatness. However, its limitations include a short shelf life (typically 6 months) and sensitivity to multiple reflow passes.

Advantages:

• Flat surface. Ideal for fine-pitch component assembly.
• Low cost.

Disadvantages:

• Limited suitability for boards requiring multiple soldering passes. Each thermal cycle degrades the OSP finish on pads not soldered during that process.
• Shorter shelf life than HASL or ENIG. Much depends on the OSP supplier. Minimum 6 months; many coatings solder reliably even after 12 months.
• PCB baking degrades the solderability of the surface finish.

Immersion Tin (ImSn)

Immersion Tin (ImSn) is an electroless tin deposition (>1µm) directly onto copper. Requirements are described in IPC-4554, which specifies a minimum thickness of 0.1µm [40µin] at -4σ from the process mean, with typical values in the range of 1.15µm–1.3µm [46µin–52µin][1,4]. Its primary advantage is excellent flatness and full RoHS compliance.

The main challenge is tin-copper interdiffusion, which leads to the formation of intermetallic compounds that, over extended storage, can migrate to the surface and degrade the solderability of the pads.

Advantages:

• Flat surface. Ideal for fine-pitch component assembly.
• Low cost.

Disadvantages:

• Not suitable for processes involving multiple reflow passes.
• Short shelf life, approximately 6 months.
• PCB baking degrades the solderability of the surface finish.

Immersion Silver (ImAg)

Immersion Silver (ImAg) is a thin layer deposited directly onto copper. The thickness ranges from 0.12µm [5µin] to 0.4µm [16µin] at ±4σ from the process mean, with typical values from 0.2µm [8µin] to 0.3µm [12µin][1,3]. Silver provides excellent wettability and surface flatness, making it an alternative to ENIG.

Requirements for this type of finish are covered by IPC-4553, addressing coating thickness, purity, porosity, and inspection methods.

The main weakness of ImAg is its susceptibility to tarnishing in the presence of sulfur compounds in the air or packaging. Tarnishing causes a color change on the surface ranging from slightly yellow to completely black. Improper storage (without dedicated packaging) drastically shortens the usable life. Shelf life is 6–12 months when proper packaging conditions are maintained.

Advantages:

• Flat surface. Ideal for fine-pitch component assembly.

Disadvantages:

• Tarnishing.
• Shelf life of 6-12 months, strongly dependent on storage conditions and silver layer thickness.
• PCB baking slighlty degrades the solderability of the surface finish.

Other Types of Surface Finishes

Beyond the dominant technologies described above, PCB manufacturers offer several other surface finishes:

• ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold). A finish similar to ENIG, but with an additional palladium layer. A nickel layer is deposited on the copper pad, followed by a thin palladium layer, and finally gold. ENEPIG performs well in both soldering and wire bonding with aluminum (Al) or gold (Au) wire. ENEPIG carries no risk of the defect known as "black pad."
• Hard gold. An electrolytically deposited nickel layer followed by an electrolytically deposited gold layer. The hardness of the gold coating is achieved through nickel or cobalt alloying, which significantly increases resistance to friction and mechanical wear. For this reason, it is used as the surface finish for edge connector contacts (gold fingers).
• Flash gold. This can be described as "quick gold plating." This finish is essentially "incomplete hard gold." Technically, it is only the initial stage of the electrolytic hard gold plating process, and this thin gold layer is intended to serve as a substrate for the proper gold coating. In a flash gold finish, however, this thin layer is the final one. If process control is inadequate, the gold layer may not fully cover the underlying nickel substrate, which can lead to degraded solderability over time.
• Soft gold. A copper pad coated with nickel (electrolytically) and gold (electrolytically). Gold is soft when it is relatively pure — without intentional additions of nickel, cobalt, etc. This finish is a good choice for COB (Chip On Board) wire bonding technology. It is worth confirming whether the supplier is not referring to ENIG, in which both nickel and gold are deposited electrolessly and the gold is pure, i.e., soft. The deposition technology and coating composition should be clearly specified to avoid confusing this finish with ENIG.
• Other. The design standard IPC-2221 lists a number of additional surface finishes, such as "N" (nickel for edge connectors), "NB" (nickel on copper soldering pads), etc. Notably, there is also a finish called DIG (Direct Immersion Gold), in which gold is deposited directly onto the copper substrate.[5]

PCB Surface Finish Standards

The selection and verification of PCB surface finishes is governed by a number of industry standards. Below is a summary of key documents:

• IPC-4552 - Requirements for ENIG.
• IPC-4553 - Requirements for Immersion Silver (ImAg).
• IPC-4554 - Requirements for Immersion Tin (ImSn).
• IPC-4555 - Requirements for OSP.
• IPC-4556 - Requirements for ENEPIG.

Summary

There is no single universal PCB pad finish suitable for every application. The decision should be based on an analysis of design requirements, the product's operating environment, assembly process requirements, and the total cost of the product lifecycle.

An engineer designing a reliable electronic product should treat the selection of a surface finish with the same rigor as the selection of components or the definition of the laminate stackup. An error at this stage may only manifest itself as quality problems during assembly or field use.