PCB Surface Finishes

 

The bare copper on the printed circuit boards (PCBs) would oxidize and compromise the components soldering. In PCB manufacturing, the bare copper, including exposed traces, pads, holes, ground plains and metal edges on the flex circuits, rigid boards and rigid-flex PCBs is coated with a surface finish, which protects the underlying copper against oxidization and therefore increases storability. A surface finish is important for making a reliable connection between the PCB and the electronic component. Applying different surface finish on your boards will vary in shelf life, application and price.

Surface Finish Types

With increasing demands for thin, high-density, fast speeds, and lightweight applications, the surface finish will make all the difference. Nowadays, PCB surface finishes are lead-free, in accordance with Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) directives, and include below 8 types:

  • ENIG (Electroless Nickel Immersion Gold)

  • ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)

  • Hard Gold

  • Soft Gold

  • Immersion Silver

  • Immersion Tin

  • Lead-free HASL (Hot Air Solder Leveling)

  • OSP (Organic Solderability Preservatives)/Entek

 

Final Surface Finish Comparison in Details

Surface Finish

RoHS

Soldering

Alu Wire Bonding

Gold Wire Bonding

BGAs

Flex/Rigid-Flex PCB

Press-Fit Technology

Gold Fingers

Sliding Contacts

Typical Thickness

Shelf Life (Month)

ENIG

Yes

Yes

Yes


Yes

Yes

Yes



Au: 2-3μin

Ni: 120-240μin

12

ENEPIG

Yes

Yes

Yes

Yes

Yes


Yes



Au: 1-2μin

Pad: 4-8μin or 8-15μin

Ni: 100-150μin

12

Hard Gold

Yes







Yes

Yes

Au: 30-50μin

Ni: 100-200μin

12

Soft Gold

Yes

Yes


Yes






Au: 30-50μin

Ni: 100-150μin

12

Immersion Silver

Yes

Yes

Yes


Yes


Yes



6-8μm

6

Immersion Tin

Yes

Yes



Yes


Yes



25-60μin

3-6

HASL with Lead


Yes





Yes



Pad Coverage

12

HASL Lead-free

Yes

Yes





Yes



Pad Coverage

12

OSP /Entek






Yes




4-24μin (Solderable)

3-6

* The shelf life of each PCB surface finish is under controlled conditions.

Moisture Absorption

Printed circuit boards are hygroscopic. They shall absorb moisture to the point of equilibrium. Whereas boards readily absorb moisture they require help to force the moisture back out. IPC-1601 “Printed Board Handling and Storage Guidelines” specification section 3.4 and Table 3-1 provides guidance on the dry baking of printed circuit boards.

Recommendations for Printed Board Baking Profiles

Final Finish

Temperature

Time

Comments

Tin

105-125oC

4-6 Hours

Baking may reduce solderability. See 3.4.1.5

Silver

105-125oC

4-6 Hours

Silver may tarnish. See 3.4.1.4

Nickel/Gold

105-125oC

4-6 Hours

Usually no issue with extended bake. See 3.4.1.2

OSP

See 3.4.1.1

HASL

105-125oC

4-6 Hours

Thickness below 0.77μm [30μin] may turn into pure intermetalics and render the printed board not solderable.

 

Selecting the correct PCB surface finish for your design requires taking factors into account like cost, the final application environment (For example, high heat/thermal, vibration/stability, RF (Radio Frequency)), component density, lead or lead-free requirements, shelf life, shock/drop resistance, production volume, and throughput.

Due to the increased demand for improved performance of electronic applications, surface finishes have also been upgrading. Because of tighter PCB topography, the HASL surface finish is quickly being replaced by gold-based surface finishes, like ENIG, ENEPIC, hard gold and soft gold. The gold-based surface finishes have a lot to offer in terms of features and benefits when used in applications.

 

ENIG (Electroless Nickel Immersion Gold)

ENIG consists of two layers of metallic coating, which are deposited on the copper surface through chemical process. The nickel layer protects the copper from oxidation, and the gold layer protects the nickel layer. This dramatically increases the resistance to corrosion and maintains a flat surface which is crucial for PCB assembly of leadless components. ENIG usage has become very common now due to the accountability for lead-free regulations.

 

ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)

ENIEPIG surface finish is composed of four metal layers, they are copper, Nickel, Palladium and Gold. ENEPIG is probably not new to you, but it is a surface finish that has become increasingly popular recently due to the price reductions in palladium. It also has increased in popularity and use because of its many features and benefits. It also has distinct advantage over the ENIG surface finish.

The difference between ENEPIG and ENIG is the added layer of palladium. The palladium helps protect the nickel layer from corrosion, which helps prevent “black pad” from occurring. This is one of the distinct advantages of ENEPIG over the ENIG surface finish. The immersion gold layer that sits atop the palladium offers nearly complete board protection by protecting and preserving the palladium underneath.

When compared to other surface finishes, like hard gold and soft gold plating, ENEPIG has become more affordable in recent years. The cost of the ENEPIG surface finish decreased because the added palladium layer reduces the required thickness of the more expensive gold layer. The ENEPIG surface finish is capable of majority of the ultra-demanding requirements with multiple package types like, through-holes, SMT, BGA, wire bonding, and even press-fits.

The PCB surface finish with ENEPIG is very thin, making the PCB assembly and soldering process very simple and definitely more reliable when compared to ENIG. ENEPIG also has a long shelf life due to its durability and resistance from tarnishing.

 

Hard Gold vs. Soft Gold

Depending on applications, printed circuit board technologies use gold extensively. Gold has good electrical conductivity, tarnish resistance, solderability after storage, and being excellent etch resist. Electrolytic Ni/Au has a layer of gold plating over a base of electroplated nickel. The purity of the gold plating categorizes this surface finish as either hard gold (98% purity) or soft gold (99.99%). The choice of hard gold versus soft gold depends on the type of application you are looking to create.

 

Hard Gold Surface Finish

Hard gold is a gold alloy with complexes of cobalt, nickel or iron. A low-stress nickel is used between gold over-plating and copper. Hard gold is not suitable for wire bonding. We recommend using the hard gold surface finish for components and applications with heavy usage and high likelihood of wear and tear, like Interconnect Carrier Boards, Edge Connector Fingers, Keypads and Contacts. The thickness of the hard gold surface finish will vary depending on the applications. When hard gold is used for compliance in military applications, the minimum thickness shall be 50-100μinch. Nonmilitary applications require 25-50μinch. Here are the recommended minimum and maximum thickness values:

17.8μin is recommended for IPC maximum solderable thickness

25μin gold over 100μinch nickel for IPC class 1 & Class 2 applications

50μin gold over 100μinch nickel for IPC class 3 applications

 

Soft Gold Surface Finish

A soft gold finish, as the name suggests, contains a higher gold purity on the outer gold plating Soft Gold has a 99.99% purity.

Soft gold finish is used for PCB boards designed mostly for applications that require wire bonding, high solderability and weldability. Soft gold produces a much stronger welded joint when compared to hard gold. Generally speaking, and with all other factors being equal, Electrolytic Nickel/Gold is the most expensive PCB surface finish. However, some applications do require the Electrolytic Nickel/Gold surface finish.

 

Immersion Silver Surface Finish

Immersion silver, also called Immersion Ag, is an ideal choice for flat surface requirements and fine pitch components. It can be sensitive to contaminants found in the air and on some surfaces. This makes it critical to have a highly qualified PCB house to properly control and expedite the PCB packaging process. Immersion Silver surface finish is also subject to tarnishing, which is something to take in account. Common applications include flat surface requirements, which may include Membrane Switches, EMI Shielding, Aluminum Wire Bonding and Very Fine Traces.

 

Immersion Tin (ISn) Surface Finish

Immersion Tin (ISn), also called White Tin, which is a RoHS compliant (lead-free) surface finish that is an ideal choice for flat surface requirement and fine pitch components. It is a deposit of a thin layer of Tin on Copper layer of a PCB board. It’s not used as often as other surface finishes because not all PCB manufacturers offer it. The flatness of this particular coating makes this an ideal surface finish choice for small geometries and components. Although it is an economical choice, it does come with some drawbacks.

There are a few disadvantages to using Immersion Tin surface finish. As you may know, Copper and Tin have a strong affinity for one another. This means, over time, diffusion of one metal into the other will occur. It directly impacts the shelf life of the immersion Tin deposit and ultimately the performance of the finish. For the best results assembly shall run in 30 days.

Tin Whisker is also another downside to using Immersion Tin as a surface finish. The negative effects of Tin whiskers growth are well described in industry related literature and topics of several published papers.

The immersion Tin surface finish is not very durable, therefore, PCB boards with Immersion Tin finish must be handled with caution.

 

Lead-Free HASL (Hot Air Solder Leveling), Sn/Pb

Hot Air Solder Leveling (HASL) is one of the most-commonly used surface finishes in the industry. HASL is divided into two types, and there are two types because one contains tin with lead and the other contains tin without lead. HASL is also one of the least expensive types of PCB surface finishes available. This means access to this surface finish is widely available and it's also very economical.

HASL Surface Finish Process

To create a HASL surface finish, a board is submerged in molten solder (tin / lead). The solder then covers all of the exposed copper surfaces on the board. Upon leaving the molten solder, high pressure hot air is blown over the surface through air knives, this levels the solder deposit and removes the excess solder from the board surface.

HASL provides a very reliable solder joint and shelf life. HASL makes component soldering very effective, but due to the thickness of HASL coating, the planarity of the surface may be unsuitable for fine-pitch components. The HASL deposit is made from a eutectic blend of tin and lead.

If you're using through-hole or larger SMT components, HASL can work well. However, if your board will have SMT components smaller than 0805 or SOIC, it is not ideal. 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.

Due to anti-lead laws in the EU, USA, China and many other locations, demands for other lead-free finishes like ENIG, ENEPIG, and other immersion finishes instead of ENEPIG have become better options. Unfortunately, HASL is also not very environmentally friendly. Compared to ENIG, HASL is not a good environmental choice. ENIG is a better choice because it reduces the use of lead and emission from flux and fusing oil in the manufacturing process. And in terms of cost, it might appear at first that HASL is a cheaper option than other options like ENIG. And in most cases HASL is cheaper when being used for a smaller number of boards. But once you start going through two or more boards the cost of HASL is comparable to others like ENIG. And with the other benefits that ENIG offers there's not much reason to choose HASL as your surface finish.

HASL can still be a good option for a surface finish, but improvements in other surface finishes are making it obsolete. HASL does have a few benefits, but compared to others like ENIG, ENEPIG, and other immersion finishes, it is falling short. For example, ENIG is corrosion resistant, good for aluminum wire bonding, excellent for fine-pitch technology, excellent solder-ability, has a great shelf-life, and so much more. The features and benefits that made HASL a great choice in the past just cannot compare to the improvements of the other surface finishes.

 

OSP (Organic Solderability Preservatives)

OSP is a different kind of surface finish than we have seen so far. OSP is specifically designed to produce the thin, protective, and uniform layer on the copper surface of PCB’s. This coating actually protects the circuitry from oxidation during storage and assembly. It is applied over copper pads to protect the pad before soldering. This finish actually preserves the copper surface from oxidation by providing an organometallic layer protecting the copper prior to the soldering process. It is also a water-based organic surface finish, which means this surface finish is environmentally friendly. Compared to other surface finishes that have high levels of toxicity or consume large amounts of energy, this surface finish is relatively safe and “green” for the environment.

This surface finish is increasing in popularity due to an increased interest in lead-free techniques and fine pitch solutions.

It also requires low equipment maintenance. OSP is the go-to surface finish for copper pad applications. It is a very specific surface finish and as you can see has several cons associated with it. But for the most parts, OSP is a great choice for an organic solution for your surface finish.

This surface finish is very simple and easy to control. It also has an advantage over traditional HASL with regards to coplanarity, and its solderability. However, this finish does require significant changes with the type of flux and number of heat cycles used during assembly process.

Regarding handling of this particular surface finish, there are some definite differences. You cannot touch it with your hands unprotected. Acidic fingerprints will degrade the OSP and leave the copper susceptible to oxidation. Most PCB assemblers prefer to work with metal finishes that are more flexible and endure more heat cycles.

 

MADPCB specializes in helping you choose the right PCB surface finish. You have many factors to think about like cost, application environment, lead/lead-free requirements, shelf life and so much more! Let’s worry about those things and give you the right PCB surface finish.