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 understand that PCBs have copper finishes on their surface. If they are left unprotected then the copper will oxidize and deteriorate, making the circuit board unusable. The surface finish forms a critical interface between the component and the PCB. The finish has two essential functions, to protect the exposed copper circuitry and to provide a solderable surface when assembling (soldering) the components to the printed circuit board.

Hot Air Solder Leveling (HASL) was once the tried and true method of deliver consistent assembly results. However, the ever-increasing circuit complexity and component density has stretched the capabilities of even horizontal solder levelling systems to their limits.

As component pitches became finer and a need for a thin coating became greater, HASL represented a process limitation for PCB manufacturers. As an alternative to HASL, alternative coatings have been around for several years now, 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 will expose the PCB to temperatures up to 265°C which will identify 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 &#;nish deposited by a chemical displacement reaction that is applied directly over the basis metal of the circuit board, that 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 &#;nish. The negative effects of tin whiskers growth are well described in industry related literature and topics of several 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 finish for circuit boards with EMI shieldingand is also used for dome contacts and wire bonding. The average surface thickness of the silver is 5-18 microinches.

With modern environmental concerns such as RoHS and WEE, immersion silver is environmentally better than both HASL and ENIG. It is popular also due to its lesser cost than 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 ENIGdue 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 usually 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's also extremely green environmentally in comparison with the 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 is the barrier to the copper and is the surface to which the components are actually soldered to. The gold protects the nickel during storage and also provides the low contact resistance required for the thin gold deposits. ENIG is now arguably the most used finish in the PCB industry due the growth and implementation of the RoHs regulation.

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 others: it is bondable. ENEPIG&#;s first crack at a printed circuit board surface treatment fizzled with manufacturing due to its extreme high cost layer of palladium and low demand of use.

The need for a separate manufacturing line was not receptive for these same reasons. Recently, ENEPIG has made a comeback as the potential to meet reliability, packaging needs, and RoHS standards are a plus with this finish. It is perfect 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 on the after-assembly corrosion level.

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
• Is 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, 50 μin gold over 100 μin nickel for Class 3.

Hard gold is not generally applied to solderable areas, because of its high cost and its 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 an 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. If your product does require RoHS, you might consider lead-free HASL. That is only if there are no fine pitch components, since LFHASL cannot be applied perfectly flat. If your design needs to be RoHS compliant and does use fine pitch components, then you'll need to select a flat, lead-free finish, such as Immersion Silver or ENIG. Bear in mind that doing so will necessitate the use of more costly high temperature laminate.

Unsure of what you will need? Consult with a PCB fabricator prior to you making a selection. This will ensure that the combination of the surface finish and material will result in a high-yielding, cost-effective design that will perform 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 the various surface finished from HASL, Immersion Tin, OSP/Entek, ENIG, ENEPIG, or Hard Gold.

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A printed circuit board (PCB) surface finish has two main functions: To prevent the copper from oxidizing and to provide a solderable surface.

Some PCB surface finishes, however, are more popular and versatile than others. As you review your options, you&#;ll likely consider durability, cost, and the materials used within the finish, such as tin-lead, lead-free, various gold finishes, and silver. Additionally, you may have concerns about the finish compatibility with the subsequent termination process &#; such as wire bonding or welding. But how do you know which surface finish is best for your next PCB project?

We&#;ve compiled a comprehensive list of the nine different surface finishes. In this article, we&#;ll explain the purpose and common uses of each surface &#; plus the advantages and disadvantages &#; so you can find the best option for your next project.

The 9 Different PCB Finishes
How to Choose the Best PCB Finish for Your Project

Are you interested in learning more about pcb surface finish comparison? Contact us today to secure an expert consultation!

The 9 Different PCB Surface Finishes

 

Electroless Nickel Immersion Gold (ENIG)

ENIG is the most popular option and is used on about 80% of all PWBs. This finish provides a thin, gold, solderable layer that protects the copper traces with a nickel barrier between it and the copper. ENIG is a good lead-free option that results in a durable, long-lasting finish.

Advantages:

• Protects copper from passivation
• Fast, easy process
• Lead-free
• Long-lasting and durable
• Restriction of Hazardous Substances (RoHS) compliant
• Reasonably low cost
• Very good solderability

 

Hot Air Solder Leveling (HASL)

With HASL, the printed circuit board is held vertically by clamps and dipped into a flux bath and then a molten solder hot vat. The finish, which contains tin/lead, is leveled off with hot air knives and the printed circuit board panel is raised out of the molten solder vat.  This ensures a homogenous thickness over the entire board. It was once one of the most popular finishes but is now mainly used only on military and aerospace applications.

Advantages:

• Meets military requirements
• Low cost

Disadvantages:

• Cannot be used in High Density Interconnect (HDI) structures
• Extremely high temperatures can affect other materials in the PWB
• Process is messy, hot and smelly
• The resulting topography of the HASL finish is not flat enough for many very dense, high pin-count, tight-pitch components surfaces

 

Lead-Free (HASL)

The properties and uses of this finish are the same as HASL &#; except there is virtually no lead in the alloy. Instead, the alloy is a tin and copper, tin and nickel, or tin, copper, nickel and germanium alloy. This finish is not widely used.

Advantages:

• Contains no Lead
• RoHS compliant

Disadvantages:

• Cannot be used in HDI applications
• May not meet military and aerospace requirements
• Possible formation of &#;tin whiskers&#; (thin conductive filaments that can cause short circuits and a variety of other issues)

 

Immersion Tin

With this finish, a layer of tin is deposited onto the copper surface using a chemical process. This is frequently used in high-speed circuits. Immersion tin works well to protect underlying copper from oxidization, but copper and tin in such close proximity can create diffusion from one metal into the other, resulting in challenges.

Advantages:

• No nickel
• Optimal for faster circuitry
• Works well to protect underlying copper from oxidization

Disadvantages:

• Not ideal for two-sided assembly
• Not ideal for long storage periods

 

Organic Solderability Preservative (OSP)/Entek

A thin layer of material is applied over the copper to protect it prior to soldering. Though this water-based organic surface finish was once very popular, it is not widely used today.

Advantages:

• Eliminates metal finishes
• Protects the copper for soldering
• Lead-free
• RoHS compliant

Disadvantages:

• Not commonly used
• Not a long shelf life

 

Hard Gold

Hard gold is plated over the nickel, which holds up well and doesn&#;t break down easily compared to other finishes. Because of its durability, this type of PCB plating is used in applications that involve mechanical contact, such as buttons and keypads.

Advantages:

• Extremely durable
• Long-lasting
• Lead-free
• RoHS compliant

Disadvantages:

• Poor solderability
• Expensive

 

Immersion Silver

Similar to tin, this PCB finish is used in high-speed signals that don&#;t contain any nickel. Immersion silver, however, does require unique storage as it will tarnish if exposed to air.

Advantages:

• Nickel-free
• Faster signal speed than tin
• RoHS compliant and environmentally friendly

Disadvantages:

• Becomes contaminated easily
• Not ideal for two-sided assembly
• More expensive
• Requires the use of sulfur-free paper
• Will tarnish if exposed to air
• Not good for long storage periods

 

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

This three-layer metallic finish is used for printed circuit boards that also require wire bonding. The electroless nickel protects the copper and prevents it from coming into contact with the other metals. Palladium provides another anti-oxidation insurance layer and prevents the nickel from corroding, and the final gold layers adds more of a barrier against oxidation &#; plus it completely covers the PCB.

Advantages:

• Provides a flat pad surface
• Allows more area to land wire as bond pads are flat
• Ideal replacement for soft bondable gold
• Palladium helps provide good solderability
• RoHS compliant
• Nickel corrosion &#; or  &#;black pad syndrome&#; &#; is not an issue

Disadvantages:

• Costs a little more than ENIG

 

Soft Bondable Gold

This PCB finish has been used for a long time and has the same wire bonding process as ENEPIG. The pad is a crowned surface, which means there is less area to land wires.

Advantages:

• Suitable for wire bonding
• RoHS compliant

Disadvantages:

• Difficult solderability

Key Considerations When Choosing Your PCB Finish

It&#;s important to pick the right PCB plating for your product. While there are a lot of available options, you want to consider the size and scope of your project &#; and budget.

For a solid overall option, ENIG offers a high quality finish and low-cost option with good solderability. You may, however, have to choose a finish based on a particular technology or requirement &#; such as HASL, which is military, aerospace, and space approved.

Selecting the right PCB surface finish can be a challenge, but we are here to help. Cirexx is your best-in-class solution for PCB layout, fabrication, and assembly. With more than four decades of experience, we have manufactured PCBs for almost every leading technology company &#; and we&#;ve worked with industries all over the world. Contact us today to learn more about what PCB surface finish you should choose for your next project or to connect with our experienced sales team.

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