A Comprehensive Guide to PCB Surface Finishes: Selection, Properties, and Reliability Considerations

Introduction to PCB Surface Finishes

PCB surface finishes play a crucial role in determining the final properties and reliability of the boards. An optimal surface finish provides good solderability, corrosion & oxidation resistance, wear protection, wire bondability, electrical performance, assembly efficiency, and more. This article will provide an in-depth overview of the most common PCB surface finishes, their advantages and limitations, and guidelines for selecting the right surface finish for different applications.

Overview of Common PCB Surface Finishes

There are two major types of PCB surface finishes – metallic and organic. Common metallic finishes include:

• Hot Air Solder Leveling (HASL)
• Immersion Tin (Sn)
• Immersion Silver (Ag)
• Electroless Nickel Immersion Gold (ENIG)
• Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

While common organic finishes are:

• Organic Solderability Preservatives (OSP)
• Liquid Photoimageable Solder Masks (LPISM)

Metallic surface finishes can be further divided into reflowed finishes like HASL and plated finishes like ENIG. Reflowed finishes involve depositing solder on pads, then reflowing to form a coating. Plated finishes use electroplating or chemical deposition to coat the PCB surface with metal layers.

Below is an overview of each type of PCB surface finish:

Hot Air Solder Leveling (HASL)

HASL applies molten solder on PCB pads, which is then leveled by hot air knives. The result is a solder coating of 1-70 μm on pad surfaces. HASL provides excellent solderability and is the most cost-effective finish. However, it has several limitations:

• The thick and uneven deposit can lead to low assembly yields
• Solder masks can become damaged during board fabrication
• Not suitable for fine pitch components due to bridging risks
• The coating wears out over time, reducing solderability

Immersion Tin (Sn)

Immersion tin deposits a thin layer of tin (0.1-0.8 μm) by immersing PCBs into a heated tin-based solution. The tin coating offers superb solderability while being lead-free. It also prevents copper oxidation. But there are concerns over tin whisker growth and poor shear strength.

Immersion Silver (Ag)

Immersion silver involves dipping PCBs into a silver solution to form a thin silver coating (0.1-0.3 μm) on the copper traces. It provides excellent solderability, conductivity and corrosion resistance. However, the coating tarnishes over time. The process also requires careful maintenance of the silver bath.

Electroless Nickel Immersion Gold (ENIG)

ENIG is a dual plating process with electroless nickel (3-6 μm) plated first, followed by a thin immersion gold layer (0.05-0.15 μm). The nickel layer offers a diffusion barrier while gold provides excellent corrosion and wear resistance. But ENIG is relatively expensive. The thickness control of the gold layer is also critical.

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG is a three-layer plating with electroless nickel and palladium layers added below the immersion gold. The palladium acts as a barrier layer to prevent copper diffusion. It also minimizes gold embrittlement issues seen in ENIG. However, the cost is higher than ENIG.

Organic Solderability Preservatives (OSP)

OSP coatings are organic films applied by dipping or spraying PCBs with organic compounds containing azole heterocycles. The thin organic layer (0.1-0.2 μm) protects the copper from oxidation and preserves solderability. OSP is easy to apply and a low-cost process. However, it provides limited protection compared to metals.

Liquid Photoimageable Solder Masks (LPISM)

LPISM are UV-curable solder masks that act as a permanent organic coating on PCBs. It provides insulation, chemical and abrasion resistance while restricting solder flow. LPISM eliminates the need for secondary conformal coatings. But the material and processing costs are relatively high.

Key Properties of Different PCB Surface Finishes

The selection of PCB surface finish influences a range of board properties. Below is a comparison of the key characteristics of common finishes:

Solderability: Immersion Sn and Ag have the best solderability retention. HASL and OSP are short-term options.

Corrosion Resistance: Immersion Ag and Au offers the best protection, followed by Sn and Ni layers.

Oxidation Resistance: Immersion Ag and Au prevents oxidation effectively. OSP has moderate resistance.

Wear/Abrasion Resistance: Hard metal coatings like Au, Ag and Ni-P provide the best wear protection.

Wire Bondability: Ni and Pd surfaces allow high-quality wire bonding.

Solder Leaching/Dissolution: Ni and Pd act as diffusion barriers to minimize leaching.

Cost: OSP and HASL are the most cost-effective finishes. ENIG, ENEPIG and Ag are premium options.

Assemblabilty: HASL can cause issues with lead-free assembly. ENIG, ENEPIG and Ag enable high assembly yields.

So there are always trade-offs to consider based on application requirements and budget constraints.

Guidelines for Selecting PCB Surface Finishes

Here are some key guidelines for choosing an appropriate PCB surface finish:

• Lead vs Lead-Free – Lead-containing HASL is still used widely, while immersion Sn or Ag are lead-free options.
• Fine Pitch Components – For fine pitch ICs, opt for an immersion metal coating to avoid bridging.
• Gold Wire Bonding – ENIG or ENEPIG allow reliable gold wire bonding.
• Corrosion Resistance – If the environment is highly corrosive, silver or gold immersion are the best choices.
• Operating Temperature – At high temps, Sn whiskering and Ni embrittlement can occur. Ag or Au work better.
• Operating Frequency – For microwave and RF boards, silver immersion provides the best conductivity.
• Flex/Rigid-Flex PCBs – Thin and flexible boards require OSP or LPISM finishes.
• Component Density – For high component densities, Ni/Pd or Ni/Au are recommended.
• Component Termination – ENIG works well with gold-plated leads. Sn, Ag and OSP suit tin-based terminations.
• Via-in-Pad Designs – A Ni layer helps prevent solder wicking into vias.
• Hermetic Sealing – Reflowed solders like HASL allow hermetic sealing of components.
• Cost – For prototypes, low volumes or price-sensitive boards, HASL and OSP are the cheapest options.
• Lead Time – Immersion Sn and Ag offer faster turnaround over ENIG or ENEPIG.

So the surface finish influences everything from soldering, bonding, assembly, reworkability to the long-term reliability of PCBs in their envisioned operating environments.

Impact of PCB Surface Finishes on Soldering Processes

PCB surface finishes play a critical role in soldering processes, impacting reflow soldering, wave soldering, and rework. During reflow soldering, finishes like OSP, immersion Ag, Sn, and Au/Ni promote excellent wetting, while HASL can result in incomplete solder paste coalescence. For wave soldering, immersion finishes are preferred to avoid solder beading issues, although certain flux chemistries may attack Ag or Sn coatings. The release characteristics of solder paste during printing vary based on the surface finish, with smooth finishes like Au and Ag enabling easier paste release compared to rougher finishes like HASL.

Smooth surfaces offer advantages in component placement, providing higher accuracy and repeatability for pick and place processes. Additionally, surfaces with smooth, mirror-like finishes like Ag and Au allow for easier optical inspection, enhancing inspectability. However, when it comes to rework, certain finishes can pose challenges. Ni coatings may lead to pad lifting, immersion Ag may cause voiding, and ENIG may experience Au dissolution.

Understanding the specific soldering defects associated with different surface finishes, such as graping with HASL or voiding with ENIG, is crucial for optimizing the soldering process. By selecting the appropriate surface finish, PCB designers can enhance soldering performance, improve yields, and ensure the reliability of their soldered connections.

Reliability Considerations for Various Surface Finishes

The surface finish influences the long-term reliability of solder joints and their resistance to failure mechanisms:

• Intermetallic Compound (IMC) Formation: Thin ImAg formation improves reliability. Thick Cu-Sn IMCs with Sn coating are undesirable.
• Leaching and Dissolution: Ni and Pd act as diffusion barriers to reduce pad leaching into solder.
• Corrosion: Ag, Au and Ni coatings provide the best corrosion resistance. Acidic flux residues can still attack Ag and Sn though.
• Dendrite Growth: Electroless finishes like Ni and Ag are more prone to dendritic growth versus electroplated deposits.
• Thermal Cycling: Finish fracture at pad interfaces is a concern, especially with brittle intermetallics. Ductile Au and Ag withstand cycling better.
• Electromigration: Slower electron transport occurs across interfaces between solder and some finishes (e.g. Ni), aiding electromigration resistance.

So the optimal finish minimizes IMC thickness, inhibits leaching, withstands corrosion, prevents diffusion, reduces voiding sensitivity, avoids dendrite formation, and sustains thermal cycling. This ensures maximum solder joint integrity and reliability under field operation.

Common Defects and Failures Related to Surface Finishes

It is also important to be aware of potential defects introduced by PCB surface finishes during fabrication, assembly and service:

• Dewetting – Ag layers affected by sulfur or contamination can cause poor solder spread.
• Pits and Voids – Immersion Ag is prone to pitting. ENIG may have interfacial voids.
• IMC Spiking – Thick Cu-Sn intermetallics can spike into bulk solder with Sn coating.
• Flaking – Thermal cycling can cause finish flaking at pad edges for films like ENIG and Ag.
• Pink Ring – The pink chromate residue left from ENIG etching can cause solderability issues.
• Gold Embrittlement – Au layers thicker than 0.5 μm in ENIG can become brittle.
• Solder Leaching – Ni layer serves as a diffusion barrier to prevent pad leaching with Cu-dissolving solders.
• Tin Whiskers – Tin coatings can develop tin whiskers over time, creating electrical shorting risks.

So examining PCBs for the presence of such defects is imperative to avoid field failures related to the surface finish. This allows tracking problems to their root cause – whether plating bath issues, board handling or reflow exposure.

Conclusion

In conclusion, selecting the optimal PCB surface finish involves considering multiple factors such as cost, lead time, fine pitch assembly, operating conditions, and reliability requirements. The chosen surface finish affects every stage of the PCB production cycle, from fabrication to soldering, inspection, assembly, and long-term board use. Therefore, it is crucial to balance trade-offs between surface properties, manufacturability, and reliability when making the appropriate finish selection. This article has provided an overview of popular finish options and their effects, empowering PCB designers to make informed decisions and maximize the quality, yield, assembly performance, and operational reliability of their designed boards. In the next article, we will delve deeper into specific application scenarios and further explore the advantages and disadvantages of various PCB surface finishes.

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