Printed Circuit Board Surface Finishes: Advantages and Disadvantages

By Al Wright, PCB Field Applications Engineer
Epec Engineered Technologies

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Anyone involved within the printed circuit board (PCB) industry understands that PCBs have copper finishes on their surface. If left unprotected, the copper will oxidize and deteriorate, rendering the circuit board unusable. The surface finish forms a vital interface between the component and the PCB. It has two essential functions: protecting the exposed copper circuitry and providing a solderable surface when assembling (soldering) components to the printed circuit board.

HASL (Hot Air Solder Leveling) was once the go-to method for ensuring consistent assembly results. However, the rising complexity of circuits and the increase in component density have tested the limits of even advanced horizontal solder levelling systems. As component pitches became finer, the demand for a thinner coating grew, leading to HASL becoming a process limitation for PCB manufacturers. To address this, alternative coatings have become available, both electrolytic and immersion processes.

Listed below are some more common surface finishes used in PCB manufacturing.

HASL / Lead Free HASL

HASL is the predominant surface finish used in the industry. The process consists of immersing circuit boards in a molten pot of a tin/lead alloy and then removing the excess solder by using 'air knives', which blow hot air across the surface of the board.

One of the unintended benefits of the HASL process is that it exposes the PCB to temperatures up to 265°C, identifying any potential delamination issues well before any expensive components are attached to the board.

Printed Circuit Board with HASL / Lead Free HASL Surface Finish

Advantages:

• Low Cost
• Widely Available
• Re-workable
• Excellent Shelf Life

Disadvantages:

• Uneven Surfaces
• Not Good for Fine Pitch
• Contains Lead (HASL)
• Thermal Shock
• Solder Bridging
• Plugged or Reduced PTH's (Plated Through Holes)

Immersion Tin

According to IPC, the Association Connecting Electronics Industry, Immersion Tin (ISn) is a metallic finish deposited by a chemical displacement reaction applied directly over the base metal of the circuit board, which is copper. The ISn protects the underlying copper from oxidation over its intended shelf life.

Copper and tin, however, have a strong affinity for one another. The diffusion of one metal into the other will occur inevitably, directly impacting the shelf life of the deposit and the performance of the finish. The negative effects of tin whiskers growth are well documented in industry literature and numerous published papers.

Printed Circuit Board with Immersion Tin Surface Finish

Advantages:

• Flat Surface
• No Pb
• Re-workable
• Top Choice for Press Fit Pin Insertion

Disadvantages:

• Easy to Cause Handling Damage
• Process Uses a Carcinogen (Thiourea)
• Exposed Tin on Final Assembly can Corrode
• Tin Whiskers
• Not Good for Multiple Reflow/Assembly Processes
• Difficult to Measure Thickness

Immersion Silver

Immersion silver is a non-electrolytic chemical finish applied by immersing the copper PCB into a tank of silver ions. It is a good choice for circuit boards with EMI shielding and is also used for dome contacts and wire bonding. The average surface thickness of the silver is 5-18 microinches.

With modern environmental concerns like RoHS and WEEE, immersion silver is environmentally better than both HASL and ENIG. It is also popular due to its lower cost compared to ENIG.

Printed Circuit Board with Immersion Silver Surface Finish

Advantages:

• Applies more evenly than HASL
• Environmentally better than ENIG and HASL
• Shelf life equal to HASL
• More cost-effective than ENIG

Disadvantages:

• Must be soldered within the day the PCB is removed from storage
• Can be tarnished easily with improper handling
• Less durable than ENIG due to no layer of nickel underneath

OSP / Entek

OSP (Organic Solderability Preservative) or anti-tarnish preserves the copper surface from oxidation by applying a very thin protective layer of material over the exposed copper using a conveyorized process.

It uses a water-based organic compound that selectively bonds to copper and provides an organometallic layer that protects the copper prior to soldering. It is also highly environmentally friendly compared to other common lead-free finishes, which suffer from either being more toxic or substantially higher energy consumption.

Printed Circuit Board with OSP / Entek Surface Finish

Advantages:

• Flat Surface
• No Pb
• Simple Process
• Re-workable
• Cost Effective

Disadvantages:

• No Way to Measure Thickness
• Not Good for PTH (Plated Through Holes)
• Short Shelf Life
• Can Cause ICT Issues
• Exposed Cu on Final Assembly
• Handling Sensitive

Electroless Nickel Immersion Gold (ENIG)

ENIG is a two-layer metallic coating of 2-8 μin Au over 120-240 μin Ni. The nickel acts as a barrier to the copper and serves as the surface to which the components are actually soldered. The gold protects the nickel during storage and provides the low contact resistance required for thin gold deposits. ENIG is now arguably the most used finish in the PCB industry due to the growth and implementation of RoHS regulations.

Printed Circuit Board with Electroless Nickel Immersion Gold (ENIG) Surface Finish

Advantages:

• Flat Surface
• No Pb
• Good for PTH (Plated Through Holes)
• Long Shelf Life

Disadvantages:

• Expensive
• Not Re-workable
• Black Pad / Black Nickel
• Damage from ET
• Signal Loss (RF)
• Complicated Process

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG, a relative newcomer to the circuit board world of finishes, first came on the market in the late 90s. This three-layer metallic coating of nickel, palladium, and gold provides an option like no other: it is bondable. ENEPIG’s initial attempt at becoming a primary surface treatment faltered due to its high cost of palladium and low demand. However, it has made a comeback for its reliability, packaging needs, and RoHS standards compliance, making it ideal for high-frequency applications where spacing is limited.

When compared to the other top four finishes—ENIG, Lead-Free HASL, Immersion Silver, and OSP—ENEPIG outperforms all in post-assembly corrosion resistance.

Printed Circuit Board with Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) Surface Finish

Advantages:

• Extremely Flat Surface
• No Lead Content
• Multi-Cycle Assembly
• Excellent Solder Joints
• Wire Bondable
• No Corrosion Risks
• 12 Month or Greater Shelf Life
• No Black Pad Risk

Disadvantages:

• Still Somewhat More Expensive
• Re-Workable with Some Limitations
• Processing Limits

Gold – Hard Gold

Hard Electrolytic Gold consists of a layer of gold plated over a barrier coat of nickel. Hard gold is extremely durable and is most commonly applied to high-wear areas such as edge connector fingers and keypads.

Unlike ENIG, its thickness can vary by controlling the duration of the plating cycle, although the typical minimum values for fingers are 30 μin gold over 100 μin nickel for Class 1 and Class 2, and 50 μin gold over 100 μin nickel for Class 3.

Hard gold is not generally applied to solderable areas due to its high cost and relatively poor solderability. The maximum thickness that IPC considers to be solderable is 17.8 μin, so if this type of gold must be used on surfaces to be soldered, the recommended nominal thickness should be about 5-10 μin.

Printed Circuit Board with Gold – Hard Gold Surface Finish

Advantages:

• Hard, Durable Surface
• No Pb
• Long Shelf Life

Disadvantages:

• Very Expensive
• Extra Processing / Labor Intensive
• Use of Resist / Tape
• Plating / Bus Bars required
• Demarcation
• Difficulty with Other Surface Finishes
• Etching Undercut can Lead to Slivering / Flaking
• Not Solderable Above 17 μin
• Finish Does Not Fully Encapsulate Trace Sidewalls, Except in Finger Areas

Conclusion

It is important to select the appropriate surface finish for your project by considering the various options while factoring in performance requirements and material costs.

For example, if you are looking for the lowest cost, then Tin-Lead HASL might seem like a good choice, but it is not suitable for RoHS-compliant products. For RoHS-compliant products, lead-free HASL may be considered, but only if there are no fine-pitch components since LFHASL cannot be applied perfectly flat. If your design requires RoHS compliance and fine-pitch components, a flat, lead-free finish like Immersion Silver or ENIG would be necessary, although this will require the use of more costly high-temperature laminate.

Unsure of what you need? Consult with a PCB fabricator before making a selection to ensure that the combination of the surface finish and material results in a high-yielding, cost-effective design that performs as expected.

For more information on circuit board surface finishes, check out our blog posts on why you should bake OSP circuit boards before use and proper circuit board handling with immersion finishes.

Not Sure What Surface Finish Is Best For Your Application?

Our team of experienced engineers is here to help you design a custom PCB solution, utilizing various surface finishes from HASL, Immersion Tin, OSP/Entek, ENIG, ENEPIG, or Hard Gold.

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What is Surface Finish in PCB Manufacturing?

In circuit board manufacturing, the process of implementing the PCB surface finish is critical in enhancing the reliability and shelf life of the PCBA. We will cover the various types of surface finishes the printed circuit boards are given to enhance good inter-metallic bonding and the shelf-life of the assembly.

Current-day PCB assemblies consist mostly of SMT components where pad flatness is a significant consideration. Such PCBAs use electroless nickel and immersion gold surface finish. Based on the customer applications, other surface finishes are also available, like soft gold, sterling silver, and OSP finishes. Sierra Circuits can process various types of surface finishes according to customers' needs, such as hot air solder leveling (HASL), hot air solder leveling lead-free (HASL lead-free), electroless nickel immersion gold (ENIG), and electroless nickel electroless palladium immersion gold (ENEPIG). These different types of surface finishes enhance the board's shelf life, with each surface finish having a different shelf life.

The most commonly used surface finishes are hot air solder leveling (HASL) with tin/lead alloy. Due to RoHS guideline policies, the boards that are RoHS compliant should not contain lead. Hence, in the same HASL process, we are using only tin and its associated alloys for lead-free requirements.

What is a surface finish in PCB?

A PCB surface finish is an intermetallic joint between the bare copper of the solderable area of the printed circuit board and the components. Circuit boards have a base copper surface that is susceptible to oxidation if left without a protective coating, hence the requirement of the surface finish. It is also responsible for preparing the PCB surface for soldering components onto the board during assembly and enhancing the shelf-life of the board.

Significance of a PCB surface finish

It impacts various features such as:

• The quality of the inter-metallic joint
• The process yield
• Rework for production batches along with the scrap rate
• Testing ability
• Cost of the manufacturing process
• The rate at which the board will fail while being used for its intended application

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For more information, please visit lead-free hasl.

What are the types of PCB surface finish?

They can be broadly classified into metallic and organic.

Metallic surface finishes

These surface finishes use metallic layers as a protective coating for the PCB copper layer.

What is HASL PCB finish? (Hot air solder leveling)

HASL is a conventional surface finish applied to circuit boards. The circuit board is generally dipped into a molten solder bath to cover all exposed copper surfaces with solder. Surplus