PCB Surface Finish Types & Comparison

PCBs form the foundation of all modern electronics. As integrated circuits and other components become progressively smaller and more complex, PCBs must meet increasingly demanding requirements for signal integrity, efficiency, and reliability. One critical aspect impacting these metrics is the PCB surface finish.

The surface finish, or coating, applied to a bare PCB serves multiple functions:

• Protects underlying copper from oxidation and contamination
• Enables and enhances solderability
• Facilitates component bonding and interconnection
• Affects signal transmission and integrity
• Impacts manufacturability and processing

With various options now available, selecting an optimal surface finish entails balancing performance, cost, manufacturability, and application requirements. This article provides a technical overview of the most common PCB surface finishes, including their key characteristics, advantages and disadvantages, applications best suited for each type, and processing considerations.

Driving Factors in Surface Finish Selection

Several interacting considerations drive the selection of a PCB surface finish for a given application, including:

• Solderability and Wettability: The surface coating must readily molten solder, forming a consistent, uniform joint between pads/components and solder without skipped connections or bridging. Wettability represents the spread/adherence of liquid solder across the surface finish.
• Oxidation and Shelf Life: The surface finish prevents airborne oxidation of copper over time prior to soldering to maintain solderability. Shelf life depends on coating robustness.
• Bonding Compatibility: For wire bonding or adhesion to conformal coatings or potting compounds.
• Environmental Friendliness: Compliance with legislation on materials use, waste, effluents, and human/environmental health and safety.
• Reliability and Durability: Continued functioning under electrical, thermal, and mechanical stresses over product lifetime. Includes resistance to thermal or mechanical shock, fatigue, electrical migration, and more.
• Signal Integrity: Clean signals with minimized crosstalk or external emissions for high speed, radio frequency (RF), or sensitive applications.
• Inspectability and Testability: Ease of in-line automated optical inspection and electrical testing. Clear finishes enableinspection; solder masks may interfere.
• Cost: Both material and processing expenses, balanced against performance.
• Manufacturability: Compatibility with fabrication processes like soldering, cleaning, coating, and assembly. Minimized rework rates.

The importance of each factor depends on the product type and operating conditions. For example, shelf life and oxidation resistance may not matter in high volume just-in-time manufacturing with little stockpiling. However, medical or aerospace products may sit for months or years prior to deployment, necessitating longevity.

PCB Fabrication Process Overview

A PCB mechanically supports electronic components while providing the electrical connections between them. The board itself is an insulating fiberglass epoxy laminate clad with a thin copper foil on one or both sides. This foil is photolithographically patterned into traces and pads, forming the conductive routes. Components are soldered to pads, either through hole or on the surface (SMT). Multi-layer boards stack together additional conductive and insulating layers internally.
Bare boards from the PCB fab must undergo several finishing processes before population with components:

Surface Preparation – Thorough washing and microetching roughens the copper and removes oxidation or contamination from storage, shipping or handling.

Surface Finish Application – The coating is applied via wet chemical processes described below for each finish type. Thickness, roughness, and other parameters are closely controlled.

Solder mask coating – The entire board surface gets covered with epoxy ink, excluding contact pads and connectors left exposed. Provides insulation and markings.

Silkscreen application – Printed markings for components, logos, board reference designators, polarity indicators and more aid manufacturing and troubleshooting.

Electrical test – Each board undergoes in-circuit testing to verify electrical continuity compliance and catch defects early. Boards pass/fail based on rigorous quality requirements.

The surface finish forms the outer layer interfacing the pad copper and applied solder, and thus can significantly impact the outcomes of subsequent processing if not properly matched to them. We now examine the most prevalent surface finish options：

Hot Air Solder Leveling (HASL)

The longest standing PCB surface finish, hot air solder leveling (HASL) coats the board with molten solder, forming a solderable tin-lead alloy layer. As its name indicates, heated nitrogen knives smooth the liquid solder as it cools, minimizing uneven buildup. Lead-free HASL substitutes tin-silver-copper solders.

Process: Boards enter the HASL system on a conveyor, passing under an advancing wave of molten solder held in a chamber manifold. Following immersion, air knives blast the boards with heated nitrogen or argon gas to flatten the cooling solder into an even coating with good copper wetting just before the solder solidifies. Edge rails contain the process. The parameters – temperatures, exposure times, gas flows and pressure – require tight control to produce consistent, high quality finishes.

Key Characteristics:

• Low cost process with high throughput
• Mature technology with a 50 year track record
• Solders well to tin-based solders and components
• Good surface wettability and planarity
• High copper coverage resists oxidation
• Lead-free available per RoHS requirements
• Visually inspectable finish

Applications:

With robust performance at low price points, HASL suits cost-sensitive consumer electronics, appliances, automotive electronics, and telecommunications/networking equipment.

Advantages:

• Inexpensive, high-volume capable
• Long shelf life
• Consistent solderability
• High etch/contamination resistance
• Reworkable
• Visible for inspection

Disadvantages:

• Not planar enough for fine pitch ICs
• Thermal shock can warp boards
• Metal dissolution over time
• Bridging risks with dense designs
• Whiskering issues with pure tin
• Debris and dross formation

Organic Solderability Preservative (OSP)

OSP coats the copper traces with an organic protective film composed of imidazole, triazole, or other heterocyclic compounds. This thin polymer layer just 10-100 angstroms thick wards off oxidation for 6-12 months while allowing solder flow once heated. The coating gets applied by dipping or spraying boards after microetching.

Process:

Surface preparation using cleaning and microetching prepares boards for OSP application:

1. boards enter an enclosed chamber
2. OSP mist gets sprayed evenly across boards
3. boards exit chamber and dry
4. final cleaning

OSP solutions contain stabilizing agents and pH buffers for optimal performance. Liquid atomization, temperature, dwell time, chemical concentrations and post-coating handling must be controlled to produce the right coating quality and consistency.

Key Characteristics:

• Ultra-thin transparent coating
• Lead-free and environmentally-safe
• Smoother than HASL with excellent planarity
• Reworkable finish
• Lower first cost than other coatings

Applications:

OSP works well for wireless and portable consumer electronics, IoT products, automotive, avionics, and medical electronics where reliability and leaded content must be minimized. The planar surface aids densely-packed boards.

Advantages:

• Simple low-cost application
• Extremely planar finish
• Long shelf life duration
• Consistent solderability
• Easy rework of OSP-coated boards
• Reduces bridging shorts

Disadvantages:

• Can be scratched off handling if uncoated
• Limited solder masking ability
• Hard to visually inspect coverage
• Short assembly window once opened
• Poor reliability per IPC testing*
• Copper oxidation resumes quickly above 175°C

IPC testing correlated OSP coatings with increased failure rates under thermal, humidity, and vibration exposures. However, many successfully employ OSP regardless.

Immersion Silver

Immersing PCBs in a silver chemical deposition bath plates a thin silver layer onto copper traces, using a galvanic displacement reaction. The coating runs 15-25 microinches (0.4-0.6 microns) thick. It resists tarnishing or oxidation for extended durations.

Process:

Microetching activates the copper surface. Boards get immersed into an acidic silver nitrate or silver fluoborate solution with organic stabilizers and accelerators added. The copper ions swap place with silver ions which plate the board. The parameters – temperature, dwell time, chemical makeup and their concentrations – all require careful control to optimizethickness consistency and uniformly.

Key Characteristics:

• Low cost process with moderate throughput
• Excellent solderability and conductivity
• Resistant to tarnish and oxidation
• Good adhesion strength and reworkability
• Suitable for wire bonding aluminum

Applications:

Immersion silver works for wireless communications, automotive infotainment and ADAS systems, LED lighting, and other reliability-dependent electronics.

Advantages:

• Strong solder joint reliability
• Long-lasting surface protection
• Extremely flat planar surface
• Lower cost than gold finishes
• Lead-free and RoHS compliant

Disadvantages:

• Surface discolors slightly over time
• Can be scratched during handling
• Short working life once exposed
• Difficult for in-circuit testing
• Higher processing temperatures

Immersion Tin

Immersing cleaned PCBs into heated tin solutions deposits a protective tin layer galvanically onto copper traces. Organic additives introduced in the immersion tin bath alter the structure of the satin tin finish from large crystalline to micro-crystalline, averting tin whisker issues.

Process:

Board preparation ensues through typicalcleansing, microetching, water rinsing and then acid dipping prior to entry into the immersion tin bath. Maintaining precise temperature regulation of the alkaline tin solutions along with controlled dwell duration, lift speed, and additive concentrations leads to consistency. The deposition builds up a 4-10 microinch (0.1 – 0.25 micron) thick matte tin layer.

Key Characteristics:

• Matte white tin coating
• Excellent solderability maintained
• Layer thickness uniformity
• Minimal copper dissolution
• Drop-in solution replacement for tin-lead

Applications:

Immersion tin coats intricate, densely-packed boards well. It withstands higher temperature alloys like silver-copper brazing apps. The affordable process also suits high-reliability uses in defense, aerospace, navigation and communications.

Advantages:

• Smooth planar coat for ultra fine pitch ICs
• No whiskering issues
• Reworkable and repairable finish
• Impervious to oxidation in storage
• Flux-less soldering ability
• Lower long term cost

Disadvantages:

• Short working life once exposed
• Prone to staining and dulling
• Termination concerns with matte finish
• Hard to visually inspect coverage
• Temperature sensitive process

Immersion Gold (ENIG)

Electroless nickel immersion gold (ENIG) deposits two metallic layers for robust protection and solderability. First, an auto-catalytic chemical reaction plates nickel across the copper to 3-7 microns thickness without any electrical current. Then a displacement reaction between nickel and solution replaces the nickel surface atoms with a thin gold coating just 0.05-0.15 microns thick.

Process:

The initial PCB microetching prepares the copper. Boards enter an electroless nickel plating tank. Once catalyzed, this autocatalytic deposition process continuously plates nickel without any electrical current. Temperature, pH buffers, chemical concentrations and dwell duration must stay balanced for consistency. Following water rinsing, boards get immersed into a heated gold solution which galvanically displaces nickel atoms, forming a thin gold layer.

Key Characteristics:

• Moderate cost for reliable protection
• 12+ months shelf life
• Gold layer remains largely inert
• Solderable with no preconditioning
• Extremely flat surface
• Reliable but complex process

Applications:

ENIG suits higher value products like enterprise servers, telecom infrastructure, defense electronics, avionics, and medical products where long shelf life and field reliability drive value over cost.

Advantages:

• Long lasting oxidation protection
• Excellent planarity aiding fine pitch ICs
• Consistent uniform thickness
• Highly solderable surface
• Hard gold layer withstands wear
• Ideal for aluminum wire bonding

Disadvantages:

• High material expense (gold)
• Complex process control
• Brittle nickel layer below gold
• Black pad risk over time
• Gold thickness difficult to inspect

Electrolytic Nickel/Gold

Nickel/gold platings apply two metallic layers galvanically using electrical current unlike ENIG. The initial bright nickel underplating runs much thicker at 100-250 microinches (2.5-6 microns) before the gold flash coating goes down at 10-50 microinches (0.25-1.25 microns). This electrolytic process allows thicker, harder gold layers.

Process Overview:

Microetched PCBs get racked into nickel and gold electrolytic plating tanks in sequence where submerged anodes and cathodes allow current to flow depositing metal onto boards. Precisely regulating solution chemistry, temperatures, voltages and waveform parameters ensures consistency. Fixture designs avoid air pockets or density differentials across boards.

Key Characteristics:

• Thicker hard gold layer variants
• Withstands repeated mating cycles
• Lead-free and RoHS compliant
• Reworkable finish

Applications:

Connectors, edge fingers, contact points and bonding pads utilize the durability while moderate cost suits higher volume cost-sensitive applications.

Advantages:

• Harder, more wear resistant gold
• Lower material cost vs. ENIG
• Reliable solder joints
• High throughput process
• Allows probe testing

Disadvantages:

• Highly technique sensitive
• Long process time
• Brittle layers may crack
• Dense PCBs risk plating voids
• Soldermasked areas may need busbars

Electroless Ni Electroless Pd Immersion Au (ENEPIG)

ENEPIG finishes upgrade ENIG, replacing the nickel underlayer with electroless palladium before gold deposition for enhanced surface protection without nickel corrosion or black pad risk. As a newer process, it currently costs more than ENIG however.

Key Characteristics:

• Excellent shelf life 
• Corrosion resistant and inert
• Superior oxidation barrier
• Lead-free and RoHS compliant
• Reworkable finish

Applications:

ENEPIG best suits high-reliability applications with long shelf lives like aerospace, defense, and medical products where reliability guides value over initial cost.

Advantages:

• Highest reliability and longevity
• Eliminates black pad risk
• Ideal for wire bonding needs
• Extremely flat planar surface
• Solderable for 10+ years

Disadvantages:

• Highest cost finish
• Lengthened process time
• Limited industry experience
• Process control challenges
• Hard to visually inspect

Conclusion

This overview of primary surface finishes highlighted how PCB fabrication represents a series of interdependent processes where each step along the way impacts the outcome of those that follow. The surface finish – whether Hot Air Solder Leveled, Organic Solderability Preservative, Immersion Silver, Immersion Tin, Electroless Nickel Immersion Gold, Electrolytic Nickel Gold or related variants – exerts considerable influence.

Engineers factor in shelf life, solderability, bonding suitability, reliability needs, signal integrity, and cost constraints among other determinants when selecting a finish for their particular application and operating environment. Additional considerations include manufacturability, throughput, equipment utilization and ease of quality control.

Striving for an ideal optimization across this matrix of considerations ultimately enables functional, reliable and cost-effective electronic hardware. As applications advance and demands grow more stringent, we can expect continuing innovation in surface finish options and processing techniques to meet emerging needs.