Circuit materials rely on high-quality conductors and dielectric materials to interconnect modern complex components and achieve the highest performance. But the copper that serves as most of the conductors on printed circuit boards (PCBs), from DC through millimeter-wave (mmWave) frequencies, needs protection from aging and oxidation effects. That protection can come in the form of electrolytic and immersion coatings. They also provide different degrees of solderability, enabling the formation of solder joints with high integrity even with constantly shrinking components, including miniature surface-mount-technology (SMT) devices. Many types of protective finishes are available for the copper conductors on PCBs, and when making a choice, it helps to know the characteristics and relative costs of each when developing a PCB for the highest performance over the longest lifetime.

Choosing the final plated finish or finishes for the copper conductors on a PCB is not a simple process. It requires consideration of a PCB’s uses and operating conditions. The modern trend for smaller, thinner circuit boards at higher frequencies and speeds with densely packed, closely spaced circuits challenges many PCB fabricators. Manufacturers of circuit materials such as Rogers Corp. regularly ship laminates with a variety of copper weights and thicknesses to PCB fabricators where they are processed into diverse types of PCBs for use in electronic products. Without some form of protection, copper on circuits placed in storage is subject to oxidation. A final plated finish acts as a barrier between the copper conductor and the environment. Not only does it protect a PCB’s copper conductors, but it also provides an interface for solder attachment and wire bonding of circuits and components, including integrated circuits (ICs) constantly growing in complexity and pin counts.

Finding the Finish

A suitable final plated finish should support a PCB’s intended applications as well as its manufacturing processes. Finishes vary in cost due to material costs and the number and types of processing steps required to apply the finish. Some finishes work reliably with high isolation on circuits with closely spaced copper traces while some can form unwanted bridges between the conductors. Some are qualified to military and aerospace requirements, such as temperature, shock, and vibration, while others do not promise the high levels of reliability needed for those applications. Some final plated finishes used for circuits from DC through mmWave frequencies as well as high-speed-digital (HSD) circuits, are:

• electroless nickel immersion gold (ENIG);
• electroless nickel, electroless palladium, immersion gold (ENEPIG);
• hot air solder leveling (HASL);
• immersion silver;
• immersion tin;
• lead-free HASL (LFHASL);
• organic solderability preservative (OSP);
• electrolytic hard gold; and
• electrolytic soft, bondable gold.

ENIG, also known as the immersion gold process, is the most widely used final plated finish for PCB copper conductors. It is a simple, low-cost process which applies a thin layer of solderable gold over a layer of nickel, forming a flat surface with excellent solderability even within densely packed circuits. The ENIG process preserves the integrity of plated thru holes (PTHs) but adds to the loss characteristics of copper conductors at higher frequencies. This RoHS-compliant process has a long shelf life in the time between when it is shipped from a circuit manufacturer to when it is put into a component assembly process or the end-use application, and it provides long-lifetime protection for copper conductors, which has made it a popular finish for many PCB developers.

ENEPIG was developed as an upgrade to the ENIG process, adding a thin coating of palladium between a layer of electroless nickel and top layer of immersion gold. The palladium protects the nickel layer (which is protecting the copper) while the gold protects both the palladium and the nickel. The plated finish is well suited for wire bonding devices to a PCB and can handle multiple reflow solder processes. As with ENIG, it is a RoHS-compliant plated finish.

Immersion silver plating is also a non-electrolytic, chemical process, by immersing a PCB into an ionic silver solution for attachment of silver to the copper surfaces. It provides a more consistent, even finish than ENIG, but lacks the protection and durability provided by ENIG’s nickel layer. It is a simpler finishing process than ENIG and more cost effective than ENIG plated finishes but is not well suited for long storage periods at a circuit fabricator’s location.

Immersion tin plating places a thin layer of tin onto a copper surface by means of a multistep process that includes cleaning, microetching, pre-immersion with an acid solution, immersion in a non-electrolytic immersion tin solution, and final cleanup. An immersion tin plated finish provides excellent protection for copper and copper circuits and supports low-loss performance in HSD circuits. Unfortunately, it is not among the longest-lifetime copper finishes because of the impact of tin on copper over time: the diffusion of one metal into the other can degrade the long-term performance of the circuit’s conductors. As with immersion silver plating, immersion tin plating is applied by a lead-free, RoHS-compliant process.

Organic solderability preservative (OSP) is a nonmetal protective layer which is applied by means of a water-based solution. It is RoHS compliant but does not have a long shelf life for circuit materials stored prior to PCB production. It is best when applied as soon as possible before soldering circuits and devices onto the PCB.

Hard gold finish is applied by an electrolytic RoHS-compliant process and lends long lifetimes to PCBs and their copper conductors. However, because of the high material costs, it is one of the most expensive final plated finishes, with poor solderability. Soft bondable gold finish also has poor solderability but is RoHS compliant and provides an excellent surface for wire-bonding devices to a PCB. More recently, there have been new OSP coatings supplied to the market which are considered to provide permanent protection for the copper. 

Selecting a final plated finish for a PCB involves many factors, including the requirements of an application and the expected operating conditions. Making a choice from among these options is not easy and advice from both a circuit material supplier, such as Rogers Corp. (www.rogerscorp.com), and a circuit fabricator can help ease the selection process. For environmentally sound applications, most of the processes are RoHS compliant. But the various plating processes have different impacts on circuit performance, depending upon analog frequencies and digital speeds. Advice from a material supplier and a circuit fabricator can ensure that a PCB meets performance goals for a long operating lifetime.

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.

Circuit materials rely on high-quality conductors and dielectric materials to interconnect modern complex components and achieve the highest performance. But the copper that serves as most of the conductors on printed circuit boards (PCBs), from DC through millimeter-wave (mmWave) frequencies, needs protection from aging and oxidation effects. That protection can come in the form of electrolytic and immersion coatings. They also provide different degrees of solderability, enabling the formation of solder joints with high integrity even with constantly shrinking components, including miniature surface-mount-technology (SMT) devices. Many types of protective finishes are available for the copper conductors on PCBs, and when making a choice, it helps to know the characteristics and relative costs of each when developing a PCB for the highest performance over the longest lifetime.

Choosing the final plated finish or finishes for the copper conductors on a PCB is not a simple process. It requires consideration of a PCB’s uses and operating conditions. The modern trend for smaller, thinner circuit boards at higher frequencies and speeds with densely packed, closely spaced circuits challenges many PCB fabricators. Manufacturers of circuit materials such as Rogers Corp. regularly ship laminates with a variety of copper weights and thicknesses to PCB fabricators where they are processed into diverse types of PCBs for use in electronic products. Without some form of protection, copper on circuits placed in storage is subject to oxidation. A final plated finish acts as a barrier between the copper conductor and the environment. Not only does it protect a PCB’s copper conductors, but it also provides an interface for solder attachment and wire bonding of circuits and components, including integrated circuits (ICs) constantly growing in complexity and pin counts.

Finding the Finish

A suitable final plated finish should support a PCB’s intended applications as well as its manufacturing processes. Finishes vary in cost due to material costs and the number and types of processing steps required to apply the finish. Some finishes work reliably with high isolation on circuits with closely spaced copper traces while some can form unwanted bridges between the conductors. Some are qualified to military and aerospace requirements, such as temperature, shock, and vibration, while others do not promise the high levels of reliability needed for those applications. Some final plated finishes used for circuits from DC through mmWave frequencies as well as high-speed-digital (HSD) circuits, are:

• electroless nickel immersion gold (ENIG);
• electroless nickel, electroless palladium, immersion gold (ENEPIG);
• hot air solder leveling (HASL);
• immersion silver;
• immersion tin;
• lead-free HASL (LFHASL);
• organic solderability preservative (OSP);
• electrolytic hard gold; and
• electrolytic soft, bondable gold.

ENIG, also known as the immersion gold process, is the most widely used final plated finish for PCB copper conductors. It is a simple, low-cost process which applies a thin layer of solderable gold over a layer of nickel, forming a flat surface with excellent solderability even within densely packed circuits. The ENIG process preserves the integrity of plated thru holes (PTHs) but adds to the loss characteristics of copper conductors at higher frequencies. This RoHS-compliant process has a long shelf life in the time between when it is shipped from a circuit manufacturer to when it is put into a component assembly process or the end-use application, and it provides long-lifetime protection for copper conductors, which has made it a popular finish for many PCB developers.

ENEPIG was developed as an upgrade to the ENIG process, adding a thin coating of palladium between a layer of electroless nickel and top layer of immersion gold. The palladium protects the nickel layer (which is protecting the copper) while the gold protects both the palladium and the nickel. The plated finish is well suited for wire bonding devices to a PCB and can handle multiple reflow solder processes. As with ENIG, it is a RoHS-compliant plated finish.

Immersion silver plating is also a non-electrolytic, chemical process, by immersing a PCB into an ionic silver solution for attachment of silver to the copper surfaces. It provides a more consistent, even finish than ENIG, but lacks the protection and durability provided by ENIG’s nickel layer. It is a simpler finishing process than ENIG and more cost effective than ENIG plated finishes but is not well suited for long storage periods at a circuit fabricator’s location.

Immersion tin plating places a thin layer of tin onto a copper surface by means of a multistep process that includes cleaning, microetching, pre-immersion with an acid solution, immersion in a non-electrolytic immersion tin solution, and final cleanup. An immersion tin plated finish provides excellent protection for copper and copper circuits and supports low-loss performance in HSD circuits. Unfortunately, it is not among the longest-lifetime copper finishes because of the impact of tin on copper over time: the diffusion of one metal into the other can degrade the long-term performance of the circuit’s conductors. As with immersion silver plating, immersion tin plating is applied by a lead-free, RoHS-compliant process.

Organic solderability preservative (OSP) is a nonmetal protective layer which is applied by means of a water-based solution. It is RoHS compliant but does not have a long shelf life for circuit materials stored prior to PCB production. It is best when applied as soon as possible before soldering circuits and devices onto the PCB.

Hard gold finish is applied by an electrolytic RoHS-compliant process and lends long lifetimes to PCBs and their copper conductors. However, because of the high material costs, it is one of the most expensive final plated finishes, with poor solderability. Soft bondable gold finish also has poor solderability but is RoHS compliant and provides an excellent surface for wire-bonding devices to a PCB. More recently, there have been new OSP coatings supplied to the market which are considered to provide permanent protection for the copper. 

Selecting a final plated finish for a PCB involves many factors, including the requirements of an application and the expected operating conditions. Making a choice from among these options is not easy and advice from both a circuit material supplier, such as Rogers Corp. (www.rogerscorp.com), and a circuit fabricator can help ease the selection process. For environmentally sound applications, most of the processes are RoHS compliant. But the various plating processes have different impacts on circuit performance, depending upon analog frequencies and digital speeds. Advice from a material supplier and a circuit fabricator can ensure that a PCB meets performance goals for a long operating lifetime.

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.